WO2025074119A1

WO2025074119A1 - Hair drying and/or styling apparatus and method

Info

Publication number: WO2025074119A1
Application number: PCT/GB2024/052563
Authority: WO
Inventors: Andrew William NORFOLK; Robert George Milner; Ian Richard Thorp; Aziz TOKGOZ; Kevin Abraham SIBY; Robert Alexander Weatherly; Ben Edward AYSCOUGH
Original assignee: Jemella Ltd
Current assignee: Jemella Ltd
Priority date: 2023-10-04
Filing date: 2024-10-04
Publication date: 2025-04-10
Anticipated expiration: 2026-04-04

Abstract

A hair drying and/or styling apparatus is provided. The apparatus comprises a multilayer heater having a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the apparatus so that during use of the apparatus by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating. The multilayer heater may include: a heater electrode layer comprising a plurality of independently powerable heater electrodes formed of a conductive material that generates heat when a current is passed through them. In this case, the plurality of heater electrodes may be arranged sequentially along a length of the multilayer heater and define a corresponding plurality of heating zones arranged along the length of the hair contacting surface of the multilayer heater. At least one upper dielectric layer is provided over the heater electrode layer to electrically isolate the heater electrode layer. The number of heating zones per centimetre of length of the multilayer heater is preferably between 0.6 and 2.5.

Description

HAIR DRYING AND/OR STYLING APPARATUS AND METHOD

Field of the Invention

The present invention relates to heating apparatus and methods and parts and systems therefor. The heaters can be used for styling hair. Such styling of the hair may be performed by a user in respect of their own hair, for example, or by a hair stylist. The invention has particular, but not exclusive, relevance to a hair drying and/or styling appliance or device comprising at least one heater having a plurality of independently controllable heating zones.

Background to the Invention

Heated hair styling tools use heat to increase the temperature of hair to a desired styling temperature. For example, a hair straightener having a heated plate applies heat directly via conduction to heat the hair, which may be either wet or dry, to achieve the desired temperature for styling. The hair may be heated to a temperature that is particularly suitable for styling hair (for example, to or beyond a hair glass transition phase temperature). At lower temperatures, the user may have to make many passes with the hair straightener over the hair to achieve a desired styling effect, whereas at higher temperatures, there is a risk of causing permanent damage to the hair.

Similarly, a heated brush or hair dryer can also be used to style hair by heating air which in turn heats the hair to a temperature suitable for styling. The hair is typically styled from wet, for example after the user has washed their hair, although the hair could also be styled from dry.

Existing hair styling appliances typically use relatively thick heating plates or heating tubes that provide a certain amount of thermal mass to the hair styling appliance. These heating plates or tubes are heated by a heater that is mounted on an inner surface of the heating plate/tube. As a result of the thermal mass, the heating plates/tubes take time to heat up and, once heated, they can take quite a long time to cool down. This thermal mass makes it quite difficult to control the heating of the hair and over heating or under heating of the hair can result. There has been recent development by the applicant and other companies in developing hair styling appliances that use heaters having a lower thermal mass that can therefore heat up and cool down much more quickly. Such low thermal mass heaters are therefore more responsive and are easier to dynamically vary the temperature with time.

However, there is a need for further improvements to such low thermal mass heaters and to parts and systems used therein. For example, how they can be constructed and mounted within an appliance, as well as how to prevent the formation of high temperature gradients (hot spots) when the heater surface is loaded in a non-uniform manner.

The present invention aims to address or at least partially ameliorate one or more of the above problems. Summary of the Invention

According to one aspect, the present invention provides a hair drying and/or styling appliance comprising a multilayer heater having a plurality of functional layers, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes: a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; and at least one upper dielectric layer over the heater electrode layer to electrically insulate the heater electrode layer; and wherein an upper surface of the dielectric layer and/or a coating applied to the upper surface of the dielectric layer provides the hair contacting surface of the multi-layer heater.

The heater may have a power density that is greater than 2 W/cm² and less than 100 W/cm², preferably greater than 8 W/cm². The upper dielectric layer preferably has a dielectric breakdown strength greater than 500 volts and a thermal impedance between 9.35 x 10’⁴ KW¹cm² and 0.8 KW¹cm².

The dielectric layer may be mounted directly on to an upper surface of the heater electrode layer. Alternatively, the multilayer heater may further comprise a sensor layer having one or more conductive tracks whose resistance varies with temperature, wherein the dielectric layer is mounted directly on an upper surface of the sensor layer. In this case, a second dielectric layer may be provided between the sensor layer and the heater electrode layer.

According to another aspect, the present invention provides a multilayer heater for use in a hair drying and/or styling appliance, the multilayer heater having a plurality of functional layers that are bonded together, wherein the multilayer heater provides a hair contacting surface to heat hair that comes into contact with the multilayer heater, wherein the multilayer heater includes: a heater electrode layer comprising one or more heater electrodes formed of a first electrically conductive material (such as steel) that generates heat when a current is passed through the one or more heater electrodes; a heat spreading layer comprising one or more heat spreaders, each heat spreader being formed of a second electrically conductive material that is different to the first electrically conductive material (for example copper); and at least one dielectric layer sandwiched between the heater electrode layer and the heat spreading layer. The multilayer heater may have a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 30pm and 2mm and preferably between 75pm and 300pm.

In some embodiments, the heater electrode layer comprises a plurality of independently powerable (i.e. independently controllable) heater electrodes formed of an electrically conductive material that each generates heat when a current is passed through them, wherein the plurality of heater electrodes are arranged sequentially along a length of the multilayer heater and define a corresponding plurality of heating zones arranged along the length of the hair contacting surface of the multilayer heater. In this case, the heat spreader layer may comprise a plurality of heat spreaders, at least one positionally aligned with each heating zone.

According to another aspect, there is provided a multilayer heater for use in a hair drying and/or styling appliance, the multilayer heater having a plurality of functional layers that are bonded together, wherein the multilayer heater provides a hair contacting surface to heat hair that comes into contact with the multilayer heater, wherein the multilayer heater includes: a heater electrode layer comprising one or more heater electrodes formed of a first conductive material that generates heat when a current is passed through the one or more heater electrodes; a heat spreading layer comprising one or more heat spreaders, each heat spreader being formed of a second conductive material that is different to the first conductive material; and at least one dielectric layer sandwiched between the heater electrode layer and the heat spreading layer.

The heater electrode layer may comprise a plurality of independently powerable heater electrodes formed of an electrically conductive material that generate heat when a current is passed through them, wherein the plurality of heater electrodes is arranged sequentially along a length of the multilayer heater and define a corresponding plurality of heating zones arranged along the length of the hair contacting surface of the multilayer heater. The heat spreader layer may also comprises one or more heat spreaders.

The multilayer heater will typically have a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 30pm and 2mm and preferably between 75pm and 300pm.

In some examples, the first conductive material comprises steel and the second conductive material comprises copper.

The multilayer heater preferably has an upper dielectric layer provided on a surface of the heater electrode layer that has a dielectric breakdown strength greater than 500 volts and a thermal impedance between 9.35 x 10'⁴ KW¹cm² and 0.8 KW¹cm².

According to another aspect, a hair drying and/or styling appliance is provided comprising a multilayer heater having a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes: a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; at least one upper dielectric layer over the heater electrode layer to electrically insulate the heater electrode layer; wherein the heater is supported within a housing of the appliance by a rigid support; wherein terminals of the one or more heater electrodes are provided on connection tabs that fold under the rigid support, wherein a rigid circuit board is provided under the rigid support that carries drive and control circuitry for controlling the heating of the multilayer heater, and wherein a plurality of spring fingers are provided for making an electrical connection between terminals on the rigid circuit board and the terminals of the one or more heater electrodes provided on said connection tabs.

According to another aspect, a hair drying and/or styling appliance is provided comprising a multilayer heater having a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes: a heater electrode layer comprising a plurality of independently powerable heater electrodes formed of an electrically conductive material that generates heat when a current is passed through them, wherein the plurality of heater electrodes are arranged sequentially along a length of the multilayer heater and define a corresponding plurality of heating zones arranged along the length of the hair contacting surface of the multilayer heater; and at least one upper dielectric layer over the heater electrode layer to electrically insulate the heater electrode layer; and wherein the number of heating zones per centimetre of length of the multilayer heater is between 0.6 and 2.5.

The multilayer heater may have a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 75pm and 300pm.

In some embodiments, the average thermal conductivity of the layers forming the multilayer heater is less than 300 W/m.K (preferably less than 200 W/m.K) and greater than 80 W/m.K. The average thermal conductivity may be averaged through the thickness of the multilayer heater.

The heater may be configured so as to be capable of providing a maximum power density of between 4 WcnT² and 10 WcnT² and in some cases between 4 WcnT² and 25 Wcm’².

In some embodiments, the maximum permitted temperature of a heating zone is less than 250°C.

A heat spreading layer may be provided, the heat spreading layer comprising a plurality of heat spreaders that regularise or homogenise the temperature within the heating zones. Typically, one heat spreader will be provided for each heating zone. Each heat spreader may be formed as an island to reduce heat spreading from one heating zone to an adjacent heating zone. The heat spreaders may be formed as interconnected islands that are electrically interconnected with and thermally decoupled from neighbouring islands (by minimising the area of contact between neighbouring islands) or the adjacent heat spreaders may not touch neighbouring heat spreaders at all. Each heat spreader is typically formed of a metal or other high thermally conductive material.

The heat spreaders may be separated from each other in the plane perpendicular to the thickness by a solid or semi-solid material, whose thermal conductivity is lower than 35 W/m.K and most preferably lower than 0.3 W/m.K.

In some embodiments, the multilayer heater further comprises one or more of: i) a low friction coating an upper surface of which provides said hair contacting surface of the multilayer heater; ii) a lower dielectric layer provided under the heater electrode layer; and iii) an auxiliary heater electrode layer comprising one or more heater electrodes provided below the heater electrode layer and a dielectric layer provided between the heater electrode layer and the auxiliary heater electrode layer.

One or more layers of the multilayer heater may be bonded together using an adhesive or using heat bonding or using physical vapour deposition or using screen printing or another coating process. One or more of the dielectric layers may comprises polyimide.

In some embodiments, the upper dielectric layer may also be a low friction layer that provides the hair contacting surface of the appliance. The upper dielectric layer may be formed as a coating or a wash applied directly over the surface of the heater electrodes.

One or more of the dielectric layers may comprise polyimide or liquid crystal polymer or other high temperature polymer capable of withstanding temperatures over 200°C.

Typically, the multilayer heater is flexible and is bonded (using an adhesive layer) to a rigid structure to provide the multilayer heater with rigidity. Depending on the shape of the rigid support structure, the multilayer heater can be shaped to provide a flat, curved and/or ribbed heating surface. However, embodiments may be provided where the multilayer heater is not rigidified and remains flexible.

In a preferred embodiment (where the hair drying and/or styling appliance is a hair styler), the multilayer heater provides a flat heating surface and has curved edges that provide a curved heating surface. This allows better control of heating the user’s hair during a curling procedure in which the appliance is rotated and the hair is tensioned over the curved edge of the heater.

The multilayer heater may have a flat, curved and/or ribbed heating surface. In some embodiments, the multilayer heater provides a flat heating surface and has curved edges that provide a curved heating surface.

A controller may be provided that is configured to control the application of power to the multilayer heater to control the heat produced by the multilayer heater.

The appliance may take the form of a single arm appliance such as a brush or a hair curler or a two arm device such as a hair styler or straightener or a dual function appliance like the applicant’s “Duet” hair styling device. The appliance is typically a hand held portable device having a handle portion for holding by the user and a hair contacting portion for contacting and heating the hair.

According to another aspect, the invention provides a method of making a hair drying and/or styling appliance, the method comprising: providing a multilayer heater having a plurality of functional layers that are bonded together; mounting the multilayer heater in the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating; wherein providing the multilayer heater includes: providing a heater electrode layer comprising a plurality of independently powerable heater electrodes formed of an electrically conductive material that generates heat when a current is passed through them, wherein the plurality of heater electrodes are arranged sequentially along a length of the multilayer heater and define a corresponding plurality of heating zones arranged along the length of the hair contacting surface of the multilayer heater; and providing at least one upper dielectric layer over the heater electrode layer to electrically insulate the heater electrode layer; and wherein the number of heating zones per centimetre of length of the multilayer heater is between 0.6 and 2.5.

Brief Description of the Drawings

Embodiments of the invention will now be described, by way of example only, and with reference to the drawings in which:

Figure 1a shows an overview of an exemplary hair styling device;

Figure 1 b shows a hair styling device in use;

Figure 2 is a block diagram illustrating the main electronic components of the hair styling device shown in Figure 1 ;

Figure 3a is an exploded view of a heater forming part of the hair styling device shown in Figure 1 ;

Figure 3b is an assembled partially transparent view of the heater shown in Figure 3a;

Figure 4a schematically illustrates the heating zones on the heating surface of the heater shown in Figure 3;

Figure 4b schematically illustrates an alternative arrangement of heating zones;

Figure 5 schematically illustrates a further alternative arrangement of heating zones that are of different sizes and shapes;

Figure 6a illustrates the way in which the heating zones may be formed on a tubular substrate for use in a curling tong or the like;

Figure 6b illustrates the way in which the heating zones may be arranged on a curved substrate which may be used on a heated brush;

Figure 7 illustrates a tress of hair that partly overlaps with zones Z2 and Z4 of a heater;

Figure 8 illustrates a cross-sectional view of a further example of a low thermal mass heater that has curved edges and a supporting substrate onto which the heater is attached with an adhesive or via a diffusion bonding process (e.g. by melting them together) or via an In Mould Labelling process;

Figure 9 is a partially exploded cross-sectional and perspective view of the different layers that form the heater shown in Figure 8;

Figure 10 is a plan view illustrating the form of a heat spreading layer forming part of the heater illustrated in Figure 8;

Figure 11 illustrates a main heating element layer forming part of the heater shown in Figure 8;

Figure 12 is a simplified block diagram illustrating the way in which the heater electrodes of the heater shown in Figure 8 are used to heat the heater and to sense the temperature of the heating zones; Figure 13 shows an overview of an alternative hairstyling device with an alternative heater arrangement;

Figure 14a illustrates two heater assemblies of the alternative hair styling device shown in Figure 13 orientated as they would be when mounted within the housing of the hair styling device;

Figure 14b is a longitudinal cross-sectional view of the top heater assembly shown in Figure 14a;

Figure 14c is an exploded view showing the components of each heater assembly;

Figure 14d is a transverse cross-sectional view of the two heater assemblies when the arms of the hair styling device of Figure 13 are closed by the user with a tress of hair running between hair contact surfaces of the heater assemblies;

Figure 15 is an exploded view illustrating the individual layers of the layered heater used in each of the heater assemblies shown in Figure 14a;

Figure 16 illustrates the form of the main heater electrode layer of the layered heater shown in Figure 15;

Figure 17a illustrates the form of a fuse and connections layer of the layered heater shown in Figure 15;

Figure 17b illustrates in more detail part of the fuse and connections layer shown in Figure 17a;

Figure 18 illustrates a process by which the flexible layered heater is bent over and bonded to a rigid support;

Figure 19a shows a base portion and an upper press portion of a press used to bend the flexible layered heater over the rigid support illustrated in Figure 18;

Figure 19b shows a transverse cross-sectional view of the base portion and the upper press portion during bending of the flexible layered heater around the rigid support;

Figure 19c is a perspective view of the base portion showing the base portion and the recess in which the rigid support and flexible heater are placed during the bending process;

Figure 20 is a partially exploded cross-sectional and perspective view of the different layers that form an alternative heater;

Figure 21 a illustrates a heat spreading and fuse layer forming part of the heater shown in Figure 20;

Figure 21b illustrates the way in which common terminals of the heater electrodes connect to a busbar on the heat spreading and fusing layer shown in Figure 21a;

Figure 22 is a partially exploded cross-sectional and perspective view of another heater assembly;

Figure 23a is an exploded view from above of a flexible heater, an adhesive layer and a heater support;

Figure 23b is an exploded view from below of the flexible heater, an adhesive layer and a heater support shown in Figure 23a;

Figure 24 illustrates a main heating electrode layer forming part of the heater shown in Figure 23;

Figure 25 illustrates a heat spreading a fuse layer forming part of the heater shown in Figure 23;

Figure 26 illustrates in more detail fuse circuitry shown in Figure 25; Figure 27a is a cross-sectional view of the heater assembly shown in Figure 23 illustrating a fuse when intact;

Figure 27b is a cross-sectional view of the heater assembly shown in Figure 23 illustrating when the fuse has melted due to overheating;

Figure 28 is a simplified schematic diagram of drive and control circuitry that can be used to control the heating of the heater shown in Figure 23;

Figure 29 shows an example embodiment of a heater with 3 dielectric layers, a hair contacting layer, and electrode layer;

Figure 30 shows an example of a staircase structure between the dielectric layers;

Figure 31 shows an example of a staircase structure in both the dielectric layers and the hair contacting layer, where each of the dielectric layers has some contact with the hair contacting layer;

Figure 32 shows another example of a staircase structure between the dielectric layers, where each of the dielectric layers has some contact with the hair contacting layer;

Figure 33a illustrates a serpentine heater track with contacts at either end of the track;

Figure 33b illustrates a full coverage heater track with busbar contacts;

Figure 34 illustrates an example curved hair styling device containing a heater;

Figure 35a shows a side view of a hair styling device comprising an active cooling system;

Figure 35b shows a perspective view of the hair styling device of Figure 35a;

Figure 36 illustrates an exemplary use of a hair styling device;

Figure 37 shows an internal view of the hair styling device of Figure 35a and 35b;

Figure 38a shows a perspective view of a further exemplary implementation of a hair styling device comprising an active cooling system;

Figure 38b shows an internal view of the hair styling device of Figure 38a;

Figure 39a shows a perspective view of a yet further exemplary implementation of a hair styling device comprising an active cooling system;

Figure 39b shows an internal view of the hair styling device of Figure 39a;

Figure 40 shows an internal view of a yet further exemplary implementation of a hair styling device comprising an active cooling system;

Figure 41a illustrates a flexible heater prior to being formed over a heater support;

Figure 41 b illustrates the flexible heater shown in Figure 41a after it has been formed around the heater support;

Figure 41c illustrates the use of spring fingers to connect terminals on a rigid PCB and the terminals on connection tabs of a flexible heater; and

Figure 41 d illustrate the spring fingers shown in Figure 41c in a compressed state.

Overview of Hair Styling Device

Figure 1a illustrates a hand held (portable) hair styler 101. The hair styler 101 includes a first movable arm 104a and a second movable arm 104b, which are coupled at proximal ends thereof to a shoulder or hinge 103. The first arm 104a bears a first heater 106a at its distal end, and the second arm 104b bears a second heater 106b at its distal end. The first and second heaters 106a, 106b oppose one another and are brought together as the first and second arms 104a, 104b are moved from an open configuration to a closed configuration. As shown in Figure 1 b, during use, a tress of hair 140 is sandwiched between the two arms 104 so that the user’s hair is in contact with, and therefore heated by, outer heating surfaces of the heaters 106a, 106b. Therefore, as the user pulls the hair styler 101 along the tress of hair 140, the tress of hair 140 is heated by conductive heating to a suitable temperature to facilitate styling.

A user interface 111 is provided to allow the user to set user defined parameters and for the device to output information to the user. For example, a desired operating temperature may be set via the user interface 111. The user interface 111 may have a dial, button or touch display for allowing the user to input information to the device 101 and the user interface 111 may have an indicator light, display, sound generator or haptic feedback generator for outputting information to the user. In this embodiment, the user interface 111 also comprises a control button or switch 114 to enable the user to turn the device 101 on or off; and an indicator light 115 to show whether the power is on.

A printed circuit board assembly (not shown) may be provided at any suitable location within the housing of the device 101 and carries the control circuitry for controlling the operation of the device 101 and for controlling the interaction with the user via the user interface 111. In this example, electrical power is provided to the device 101 by means of a power supply located at an end of the device, via a power supply cord 105. The power source may be AC mains or a DC power supply. However, in an alternative embodiment the power supply may comprise one or more DC batteries or cells (which may be rechargeable, e.g. from the mains or a DC supply via a charging lead), thereby enabling the device 101 to be a cordless product. Where the device 101 is a cordless product, the power supply may be incorporated within the device 101 .

In use, the device 101 is turned on, enabling power to flow through the heaters 106 to cause them to heat up. The user then opens the first and second arms 104a, 104b and, normally starting from the roots of the hair (i.e. near the scalp), a length or tress of hair 140 (which may be clumped) is introduced between the arms 104a, 104b, transversely across the heaters 106a, 106b. The user then closes the arms 104a, 104b so that the length of hair 140 is held between the first and second arms 104a, 104b and then the user pulls the hair through the closed arms (as illustrated in Figure 1 b). The outer (hair contacting) surface of the heaters 106 is flat in this embodiment and so the hair styler 101 can be used to straighten the user’s hair. The hair styling device 101 shown in Figure 1 can also be used to curl the hair by turning the device 101 through approximately 180 degrees or more after clamping the hair between the arms 104a, 104b and before moving the device 101 along the tress of hair 140.

Hair has a relatively high thermal mass and when in contact with the heating surface of the heater 106 the hair absorbs a significant amount of the heat energy. The heaters 106 must quickly supply the lost heat energy back to the heating surface otherwise the temperature of the heating surface will drop and potentially impact on the quality of the thermal styling. If the temperature of the heaters 106 fall below that required to raise the hair temperature above the glass transition temperature of the hair, the hair will not retain the styled shape. However, if the hair is heated to a temperature that is too high, the hair can undergo significant damage. As such, the device 101 must be able to control the temperature so that the heating surface of the heaters 106 remains within a particular temperature range. Furthermore, it must maintain the temperature range both when hair is frequently and quickly loaded and unloaded onto the heating surface, and when hair is held on the heating surface for a prolonged period of time.

Control Circuitry

Figure 2 is a simplified block diagram of control circuitry 216 that controls the operation of the hair sty I er device 101 shown in Figure 1 . As shown, the control circuitry 216 comprises a power supply 221 that, in this embodiment, derives power from a battery power source (not shown). A power supply input may be provided to charge the battery via an AC to DC converter (not shown), which may be external or internal to the device 101. Alternatively, the power supply 221 may derive power from an AC mains input.

In this example, power is provided to the heaters 206 for heating the user’s hair. The power supplied to the heaters 206 is controlled by a controller 228 having a microprocessor 229. The power supplied to the heaters 206 is controlled by drive circuitry 223 (which may include one or more power semiconductor switching devices) which controls the application of an AC mains voltage, or a DC voltage derived from AC mains via a power supply or from a battery, to the heaters 206 in accordance with instructions from the microprocessor 229. The microprocessor 229 is coupled to a memory 230 (which is typically a non-volatile memory) that stores processor control code for implementing one or more control methods that control the heating of the heaters 206 in accordance with a desired operating temperature of the heaters 206 and sensed temperatures of the heaters obtained from temperature measurement circuitry 225. The temperature measurement circuitry 225 may be temperature sensors such as thermistors or they may use circuitry that senses the resistance of heater electrodes that are used to heat the heaters 206, which resistance depends on the temperature of the heater electrode.

Figure 2 also shows that the user interface 211 is coupled to the microprocessor 229, for example to provide one or more user controls and/or output indications such as a visual indication or an audible alert. The output(s) may be used to indicate to the user, for example, if they have inserted too much hair between the heaters 206 or if they are moving the device 101 too quickly along the hair tress 140.

Finally, the control circuitry includes communications circuitry 227 to allow the device to communicate with a remote sensor, a remote server, or a remote application (e.g. on a mobile telephone). The communications circuitry 227 may use, for example, Bluetooth, Wi-Fi and/or 3GPP communication protocols to communicate with the remote device.

Heaters

The heaters 306a, 306b are low thermal mass heaters and can therefore heat up and cool down quickly. Figures 3a and 3b show an exemplary embodiment of such heaters 306a, 306b, which comprise a stack of thin layers. Referring in particular to Figure 3a, the heaters 306a, 306b include an upper dielectric (electrically insulating) layer 362, an electrode layer 363 that has a plurality of separate heater electrodes 367, and a lower dielectric layer 366 which electrically insulates the heater electrodes 367 from other components mounted behind the heater 306a, 306b. The three layers 362, 363 and 366 are bonded together either through an adhesive layer (pressure set or thermoset) or through diffusion bonding of the contacting materials (e.g. melting them together) and define a heater 306 that is very thin (the three layers have an overall thickness of between 30pm to 1000pm (preferably between 75 pm and 300 pm) in the case of low voltage operation (less than about 40 Volts) and 0.8mm to 2.0mm in the case of AC operation) and with very low thermal mass. The upper surface of the layer 362 provides the hair contacting surface of the heater 306, although a non-stick coating may be applied to the upper surface of the layer 362 to facilitate the passage of the user’s hair over the heating surface if the layer 362 does not itself have such non-stick properties. The hair contacting surface of the heater is a single smooth surface over which the hair can pass. The bonded layers 362, 363 and 366 define a flexible heater 306 and rigidity of the heater is provided in the illustrated embodiment by mounting the heater layers 362, 363 and 366 into a rigid support 368 which forms a base. These layers may be mounted onto the rigid support after the layers themselves have been bonded together or they may be bonded one at a time (or multiple at a time) onto the rigid support 368. If a flexible heater is desired, then there is no need for the rigid support 368 or if a support is used, this may be a non-rigid support. Thus, in this embodiment, there is no heater plate or tube that is heated by the heaters 306, and instead, the heaters 306 directly heat the user’s hair. This provides a hair sty I er 101 having a very low thermal mass which can therefore heat up and cool down much more quickly than prior art stylers.

In the illustrated embodiment, there are ten heater electrodes 367 that each snake across and back across the width of the heater 306, folding twice such that they each cross the width three times. The ends of each of the heater electrodes 367 are electrically connected through the lower dielectric layer 366 to electrical connections within the rigid support 368, which connect to an electrical connector 370. Drive circuitry 323 that is mounted within one of the arms 104 connects to the heater electrodes 367 via the electrical connector 370 and applies electrical power to the individual heater electrodes 367 to control the heat generated by each heater electrode 367. The electrical connector 370 extends from a surface of the rigid support 368 facing away from the surface layer 362 (shown in Figures 3a and 3b as extending directly away from the upper layer 362, but it could also be provided as extending in a perpendicular direction).

Each of the heater electrodes 367 thus creates an individual heating zone 467 on the hair contacting surface of the heater 306, which spans the width (which we shall refer to as the x-direction) of the heater 306 and the heater electrodes 367 are arranged sequentially one after the other along the length (the y-direction) of the heater 306. Figures 4a and 4b show schematic views of different arrangements of such heating zones 467. Figure 4a shows an arrangement corresponding to that of Figures 3a and 3b, in which the heating zones 467-1 to 467-10 are arranged along the y-direction only. Figure 4b shows an alternative arrangement, in which heating zones 467-1 to 467-16 are arranged in both the x- and y- directions. Such an arrangement of heating zones 467 can be provided by arranging two sets of heater electrodes 367 like those shown in Figure 3a side by side in the width (x-) direction. The heaters 406 may be separated in this way into any number of heating zones 467 and may comprise any number of heating zones along the x- and y-directions. In particular, whilst Figure 4b shows two zones along the x-direction, a greater number of zones in the x-direction could also be provided. The heating zones 467 of the heaters 406a, 406b can be operated (heated) independently, which can help to reduce hot/cold spots when using very low thermal mass heaters 306 such as those shown in Figure 3.

The heating zones illustrated in Figure 4 are all the same size. Of course, different sized heating zones 467 may be provided, as illustrated in Figure 5, which shows a heater 506 having seven different sized heating zones (labelled 5Z1 to 5Z7). The way in which the heater electrodes 367 would be arranged to define these different sized zones would be understood by the skilled reader and will not be described in detail here.

The heating zones 467 described above form part of a heater having a flat hair contacting surface. The heater is not limited to flat hair contacting surfaces and can be configured for use in a tubular form, with heating zones labelled 6Z1 to 6Z4, (as illustrated in Figure 6a) for example for use in a hair curler device or in a curved form, with heating zones labelled 6Z1 to 6Z6, (as illustrated in Figure 6b) for example for use in a heated hair brush. The heater surface may have a corrugated or ribbed shape to provide a hair crimping device.

The temperature of each heating zone 467 is independently controllable. Each heating zone 467 can be set to a target temperature. The target temperature of each heating zone 467 may be different. A separate temperature sensor may be provided for sensing the temperature of each heating zone 467 which is fed back to the microprocessor 229 to allow the microprocessor 229 to control the delivery of power to the heater electrode 367 of the corresponding heating zone 467. Alternatively, if the heater electrodes 367 are formed of a material having a Positive Temperature Coefficient (PTC) or a Negative Temperature Coefficient (NTC) (such that its resistance varies with its temperature), then the temperature of each heating zone 467 can be determined by determining the resistance of the corresponding heater electrode 367. The microprocessor 228 may thus control the heating on a per-zone basis in order to reduce the difference between the actual temperature of the heating zone 467 and the target temperature for that heating zone 467.

Heating Zone Sizing

One issue with low thermal mass heaters 306 is the regulation of hair contacting surface temperature in the locally hair loaded regions of the heater within desired temperature limits, without causing overheating of the unloaded regions at the same time. Specifically, when the user loads a tress of hair 140 onto the heaters 106, some parts of the heater will be loaded with hair whilst other parts will not be loaded with hair. Upon loading with hair, more power is supplied to the heater 306 to ensure that all regions on the hair contacting surface can be retained within and/or recovered back to the desired operating temperature limits. The low thermal mass heaters 306 described above are relatively thin and the dielectric layers are formed of materials with relatively low thermal diffusivities. If there was just a single heating zone, and hence a single continuous heater electrode 364 running across the whole length and whole width of the heater 306, then when more power is supplied to the heater 306 to recover the temperature drop in the locally hair loaded regions, the unloaded regions would undergo overheating, which could cause the heater materials to exceed their maximum operating temperatures, or cause the overheated regions to burn relatively small bundles/strands of hair that come into contact with them. This overheating can be prevented by using materials with higher thermal diffusivities in the layers that constitute the heater, and/or by increasing the thicknesses of the layers that constitute the heater and/or by dividing the heater 306 into multiple separately powered and controlled heating zones 367 across its length or its length and width. Increasing the thickness of the layers increases the thermal mass of the heater 306 which is undesired and there are limited materials that have the required dielectric strength and high thermal diffusivity (and which are available for use in mass produced consumer products). Therefore, the inventors have divided the heaters 306 up into plural heating zones. These heating zones can be equally and/or unequally sized and can be arranged regularly and/or irregularly across the width and length of the heater.

However, overheating can still occur within a single heating zone. For example, if half of the heating zone is loaded with hair (which is assumed to be the realistic worst case scenario during operation) and the other half is not loaded with hair, then the half that is loaded with hair will cause the temperature of that part of the heating zone to drop which will cause more power to be applied to that heating zone in its entirety. That applied power will bring the average temperature of the heating zone back up to the desired operating temperature, but the unloaded part of the heating zone will be above the average temperature of the heating zone. This temperature increase may be sufficient to cause the unloaded part to overheat. At the same time the loaded part of the heating zone will be below the average temperature causing a reduction in heat transfer and reduced styling performance. This situation is illustrated in Figure 7, which shows a tress of hair 740 overlying heating zones 7Z2, 7Z3 and 7Z4, with heating zone 7Z3 being fully loaded with hair and heating zones 7Z2 and 7Z4 being only partially loaded with hair. This problem can be reduced by making the heating zones very small - but that is costly due to all the connections needed to connect each heater electrode 367 for each heating zone back to the drive circuitry 223 as well as the number of control switches in the drive circuitry 223 needed to control the powering of each heater electrode 367. The inventors have found that for a given permitted maximum temperature within the heater, a maximum size of the heating zones can be defined which depends on the maximum power density to hair that can be extracted from the heating zone and the material characteristics and thicknesses of the layers forming the heating zone.

Specifically, if it is assumed that only one half of a heating zone 467 is loaded with hair, upon loading with hair, the maximum temperature that occurs in the unloaded half of a heating zone 467 can be defined with the equation below: q. W² T ¹ max = T ¹ Tar 4 '- - _ . r

16t. k where,

T_Max = maximum temperature (°C) on the surface of the heater which would occur in the unloaded half (worst case) of an individual heating zone; T_Tar = target operational temperature or average temperature (°C) of individual heating zones; q = power density (Wm’²) required to heat hair passing over the surface to the desired temperature for styling;

W = width of a heating zone measured perpendicular to the motion of hair over the surface; t = total thickness of the layers that constitute the heating zone; and k = the thickness averaged thermal conductivity of the thin layers that constitute the heating zone.

If it is assumed that the thickness averaged thermal conductivity of the constituent layers of a heating zone 467 and the total thickness of the layers that form the heating zone 467 are known and fixed (for any given device), then the above equation can be used to determine the required zone width (W) and hence a number of divisions along the length of the heater that will prevent overheating of the unloaded halves, when their other halves are loaded with hair, and more power is supplied to maintain and/or recover the hair contacting surface temperatures back to the desired operating limits. Consequently, for a given surface area that must be covered with the considered heater technology, the equation above can be used to determine the number of heating zones that should be positioned along the length of the given surface area, so that each heating zone 467 can be operated without exceeding the maximum operating temperature of the heater materials and without causing the temperature of the unloaded part of a heating zone 467 to exceed the maximum temperature ( T_max) that could cause burning of relatively small bundles/strands of hair that come in contact with such overheated regions of the heating zone.

Specifically, the required divisions along the length can be determined from:

Where,

L = length of the heater plate (perpendicular to the direction that hair typically travels across the surface); n_L = number of zonal divisions along the length of the heater plate;

T_Max ⁼ maximum permitted temperature (°C) on the surface of the heater (which would occur in the unloaded half (worst case) of an individual heating zone) needed to avoid damage to hair or the heater;

T_Tar = target operational temperature or average temperature (°C) of individual heating zones; k = the thickness averaged thermal conductivity (Wm’^{1 o}C’¹) of the layers that constitute the heating zone; t = total thickness of the layers that constitute the heating zone; and q = maximum power density (Wm’²) required to heat hair passing over the surface to the desired temperature for styling.

For a hair styling device, the inventors have found the following suitable ranges for these parameters:

- Peak power density required for styling dry hair (<j) is typically greater than 4 W/cm² and less than 50 W/cm² and preferably greater than 8 W/cm² and less than 25 W/cm²

- The average thermal conductivity of the layers forming the heating zone (fc) (averaged through the depth of the various layers) is between 15 and 300 W/m.K and preferably between 80 and 200 W/m.K.

- The maximum permitted temperature of a heating zone to manage (ideally avoid) hair damage is less than 250°C, more preferably less than 220°C and most preferably less than 200°C.

- The total thickness of the layers (t) which make up the heater is less than 300pm but no less than 75pm due to manufacturing limitations.

- The target operational temperature of the heater (T_Tar) is between 150°C and 230°C.

Operating within these ranges, the inventors have found that the required number of heating zones per unit length (cm) along the length of the heater is between 0.6 and 2.5 per cm which is equivalent to a zone width (in the lengthwise direction of the heater) of between 0.4 cm and 1 .7 cm.

Of course, this is for the case of there not being multiple zones in the width direction of the heater as well (e.g. this is for the single row case shown in Figure 4a). If multiple rows of heating zones 467 are provided along the length of the heater (such as is shown in Figure 4b), then each row of heating zones 467 should meet the limits defined above if the above described overheating problem is to be avoided.

Alternative Heater Arrangements

A first alternative flexible heater 806 is illustrated in Figure 8, which shows on the left hand side an exploded cross-sectional view of the heater 806 and substrate 868 and on the right hand side a perspective view of the heater 806 and substrate 868. As shown in Figure 8, the heater 806 has curved edges 872-1 and 872-2 that are shaped to match the shape of an upper surface 874 of the rigid support substrate 868 so that the flexible heater 806 can be bonded securely using an adhesive, or diffusion bonding (thermoforming) of the underlying materials to the upper surface of the rigid substrate 868, or by overmoulding in which the carrier is injection moulded over the back of the flexible heater within the mould. The curved edges of the heater 806 can be formed, for example, using a heat forming process. Figure 8 also illustrates that one or more surface mounted electronic components 876 may be attached to an underside of the heater 806. These components may be, for example, thermistors for sensing the temperature of the heating zones 867 of the heater 806 or fuses that can cut power to the heater electrode of each zone or all zones in the case of a zone overheating. Figure 8 also shows a control printed circuit board (PCB) 878 that carries the drive and control electronics 216 illustrated in Figure 2 that controls the heating of the different heating zones 867 of the heater 806.

As before, the heater 806 is formed from a number of discrete layers that are mechanically or chemically bonded together. Each layer has a thickness between about 1 pm and 150 pm and preferably between 1 pm and 100 pm, more preferably between 10 pm and 70 pm or between 2 pm and 10 pm. The different layers forming part of the heater 806 are shown in exploded cross-sectional and perspective views in Figure 9. A description of each layer is given below.

Low Friction Coating 981 (Optional)

This is an optional layer and can be added to create a smooth, low friction surface to enhance the user experience by making the heater 806 feel less grippy against the hair. This layer would be as thin as possible (for example, between 10 and 50 pm and preferably between 1 and 3 pm) to reduce the thermal resistance from the heater 806 to the hair, whilst still being sufficiently durable and scratch resistant.

This layer would typically be applied last, possibly as a spray coating (e.g. Cerasol), after the rest of the heater 806 has been produced and assembled around the rigidifying substrate 868. This may be needed with Cerasol because this coating is prone to cracking when flexed, and once applied the coating will reduce the natural flexibility of the heater, and so it should be applied once the heater 806 has been formed into its final shape.

Alternatively, this coating may comprise multiple layers including, for example, a primer layer (of about 6pm), a base coat layer (of about 25pm) and a top coat layer (of about 10pm).

This layer could also have combined functionalities with other proposed layers (e.g. more than just offering low friction) such as dielectric strength I electrical separation provided the material is sufficiently electrically insulative.

Heat Spreading Layer 982 (optional)

This is also an optional layer and, when provided, helps to spread the heat within each heating zone 467 to ensure that the temperature of individual heating zones 467 is able to maintain an acceptable degree of homogeneity during typical use. As discussed above, if a heating zone 467 was to be partially loaded with hair and was sufficiently large, the unloaded portion of the heating zone 467 could develop an unacceptably high temperature, whereas the loaded region would be too cold, as heat could not adequately flow from the hot region to the cold region. This problem is exacerbated by the anisotropic thermal characteristics of the serpentine like heater electrodes 367, and by the fact the control electronics 216 would typically work to maintain an “average” temperature within the heating zone 467 based on the overall resistance of the heater electrode that forms the heating zone 467 - from the perspective of the control electronics 216, the heating zone 467 would be at the “correct” temperature despite having hot and cold regions. Each heating zone 467 would have its own heat spreader, which is thermally separated (there is a high thermal impedance/low thermal conductivity) from the heat spreaders for adjacent zones. This is desirable to prevent heating zones 467 from heating neighbouring heating zones 467 which might otherwise increase power consumption, reduce warm up time and complicate algorithms based on zonal power consumption by adding crosstalk. Figure 10 illustrates an example form of the heat spreader layer 1082. As shown, in this example there are 20 heat spreaders 1091-1 to 1091-20, each formed of a relatively high thermal conductivity material (such as copper). Each heat spreader 1091 is separated from its neighbouring heat spreaders 1091 and in effect forms an island of thermally conductive material over the corresponding heating zone. The heat spreaders 1091 may be separated from each other by a solid material having a thermal conductivity lower than 35 W/mK or they may be separated by air. The heat spreaders 1091 may be formed, for example, by taking a planar layer of metal (such as a layer of copper) that is bonded onto the layer below and then etching this layer of copper to physically separate the individual heat spreaders 1091 (so that they do not touch each other). Provided there is a break between neighbouring heat spreaders 1091 , it is difficult for heat from one heating zone 467 to pass into neighbouring heating zones 467. The solid material (dielectric and/or scratch resistant low friction material(s)) that is provided in the gap between adjacent heat spreaders 1091 may be provided by a PVD (Physical Vapour Deposition) DLC (Dimond Like Carbon), bond film, coating or a wash that is applied to the heat spreading layer 1082 after the etching process has formed the gaps between adjacent heat spreaders 1091 and may be the coating layer 981 described above. Alternatively other suitable methods may be used to provide solid material in the gap between adjacent heat spreaders, such as masking and vapour deposition etc.

This heat spreader layer 1082 can provide mechanical integrity to the overall heater 806, providing some protection from damage to the hair contacting surface that might otherwise expose the underlying heater electrodes 367, which in turn could lead to short circuits or loss of functionality.

Polyimide Separator layer 983

The polyimide separator layer 983 provides electrical insulation between the hair contacting surface of the heater 806 (which may be the upper surface of this layer 983 if the optional layers 981 and 982 are not provided) and the main heater electrode layer. This layer 983 would have as low thermal impedance as possible whilst still achieving the dielectric requirements of the layer. As the name suggests, this layer is formed of polyimide, although other dielectric materials could be used. Because this layer is relatively thin, it does not significantly impede heat transfer between the main heater electrodes and the hair contacting surface, despite its low thermal conductivity (less than 0.2 W/mK). However, in-plane, it is able to prevent heat spreading from one heating zone 467 to an adjacent heating zone 467.

Main Heater Electrode & Sensing Layer 984

This layer 984 is where heat is created by dissipating electric power from the power source (e.g. a power supply unit (PSU) or one or more batteries). This layer 984 comprises a number of independently controllable heater electrodes 367 each defining a corresponding heating zone 467. Independently controllable means that each heating zone can be heated to any desired target temperature (or switched on/off) independently of the other heating zones. So, the set point temperature of a heating zone may, if desired, be different from the set point temperature of other heating zones. Figure 11 illustrates in more detail the form that this layer 984 takes in this example heater 806. As shown, in this example, there are twenty independently controllable heater electrodes 1164-1 to 1164-20 that each defines a corresponding heating zone 467. Each heater electrode 1164 is formed of a track of resistive material, whose geometry (track width, thickness, length) and material is specified in order to achieve the desired resistance and peak power density (W/cm²) requirements based on the voltage output of the relevant power source.

Each heater electrode 1164 is formed into a serpentine pattern using, for example, chemical etching as a manufacturing process. In more detail, a solid layer of electrically conductive material is provided and then etched to form the different heater electrodes 1164. The straight lines shown in Figure 11 are the etched parts of the layer 984 and the white parts of the figure show the serpentine conductor paths that form the heater electrodes 1164. Other processes such as printing, thick film printing, physical vapour deposition and the like could be used to form the heater electrodes 1164. In this illustrated example, adjacent heater electrodes 1164 share a common positive terminal (although in other embodiments they may share a common ground terminal) to reduce the number of electrical connections needed to be made between the drive and control board 878 and the heater 806. This common positive terminal is connected to the different heater electrodes at suitable vias 1165-1 to 1165-5, which connect through to connection circuitry below (not shown) that connects to the drive and control board 878. The other end of each heater electrode connects through a respective switch (not shown) to the drive and control board 878 to allow independent control of current flow through each heater electrode 1164. As those skilled in the art will appreciate, it is not essential to have such a common positive (or ground) terminal, each heater electrode 1164 may be physically separate from all other heater electrodes 1164 in which case, each end of each heater electrode 1164 would be connected separately back to the drive and control board 878.

As schematically illustrated in Figure 11 , the end of each heater electrode 1164 that is connected to the switch is provided at the edge of the heater and the direction of the serpentine tracks changes in this edge portion (which corresponds to the portion of the heater which is curved over the upper surface 874 of the rigid support substrate 868). The inventors have found that this arrangement helps heat generated in the heater electrodes 1164 in these edge portions to pass up to the top surface of the heater which is more likely to come into contact with the user’s hair. However, if the device is twisted in use such that the user’s hair comes into contact with this curved edge portion, then the hair will still be heated as this curved edge portion is heated.

The conductive material used in the layer 984 is preferably a PTC or an NTC material (such as stainless steel or copper) so that the resistance of the heater electrode 1164 depends upon its temperature - and so the temperature of the heating zone 467 can be determined by measuring a parameter that varies with the resistance of the corresponding heater electrode 1164. This removes the need for separate temperature sensors for each heating zone as a single sensor can be used to measure the temperature of each heating zone (as discussed below with reference to Figure 12).

Figure 12 is a schematic view of the way in which the heater electrodes 1264 may be connected together and to the drive circuitry 1223 and the power supply 1221 . As shown in Figure 12, each heater electrode 1264 is connected at one end to the power supply 1221 and at the other end to a respective switch (in this case a MOSFET switch) 1295-1 to 1295-20. The switches 1295 are controlled by the microprocessor 1229. When a heater electrode 1264 is to provide heat, the corresponding switch 1295 is closed thereby connecting the heater electrode 1264 to ground through the resistor 12R. As a result, current flows from the power supply 1221 to ground causing the heater electrode 1264 to heat up. The microprocessor 1229 can control the position of each switch 1295 independently thereby allowing each heater electrode 64 to be powered independently.

When the temperature of a selected heating zone 467 is to be determined, the switch 1295 of the corresponding heater electrode 1264 is closed and all other switches 1295 are opened. In this way, the selected heater electrode 1264 is provided in series with the resistor 12R. Since the heater electrodes 1264 are formed of a PTC or an NTC material whose resistance changes with the temperature of the heater electrode 64, by measuring the voltage dropped across the resistor 12R (using the operational amplifier 1297), the microprocessor 1229 can determine the resistance of the selected heater electrode 1264 and hence can determine the temperature of the corresponding heating zone 467. If the determined temperature is above the desired temperature for that heating zone 467, then the microprocessor 1229 can reduce the power applied to that heater electrode 1264; or if the heating zone 467 is at a lower temperature than that desired, then the microprocessor 1229 can increase the power applied to the corresponding heater electrode 1264. Any suitable ON/OFF control or PWM (pulse width modulation) control can be used to vary the power applied to the different heater electrodes 1264. The microprocessor 1229 can select each heater electrode 1264 in turn in order to determine the temperature of each heater electrode 1264/heating zone 467.

Polyimide Separator (Optional) 985

When an auxiliary heater electrode layer is provided, this layer is required to provide the required electrical separation (insulation) between that auxiliary heater electrode layer and the main heater electrode layer 984 described above. This polyimide layer 985 would have a low thermal resistance in the thickness direction whilst still achieving the dielectric requirements. Due to this layer being relatively thin, it will have a low thermal conductivity in the plane perpendicular to its thickness of less than about 35 W/mK. Other dielectric materials could be used instead of polyimide.

Auxiliary Heater Electrode Layer (Optional) 986

Some embodiments of the heater 806 may benefit from the presence of an additional heating element layer 986. This additional layer 986 could be used to dissipate power (create heat) from a secondary power source that operates at a different voltage to the main power source 221 , for example the main power source could be a power supply and the power source for the auxiliary heater electrode layer 986 could be one or more batteries or supercapacitors. In other embodiments the primary source could be one or more batteries and the auxiliary one or more supercapacitors. Alternatively still, the conductors on this auxiliary layer 986 could become the primary heaters, and those on the main heater electrode layer 984 would just be used for temperature sensing or vice versa.

The heater electrodes on the auxiliary layer 986 will typically have the same form as the heater electrodes 1164 used in the main heater electrode layer 984 - so that they will define the same heating zones 467 as the heating zones 467 defined by the heater electrodes 1164 on the main heater electrode layer 984. The path taken by the heater electrodes on the auxiliary layer 986 does not need to follow the same path as the corresponding heater electrodes 1164 formed on the main heater electrode layer 984. For example, whilst the main part of each heater electrode 1164 on the main heater electrode layer 984 (ignoring the edge part of each heater electrode 1164) serpentines in the longitudinal direction of the heater 806 in Figure 11 , the corresponding heater electrodes of the auxiliary heater electrode layer 986 could be arranged to serpentine in the width direction of the heater 806. Such an arrangement would reduce the anisotropic thermal conductivity caused by tracks mostly facing one direction, and may help to spread the heat flow within the heating zone 467 particularly if the heating zone 467 is only partially loaded with hair.

Polyimide backing 987

This layer encapsulates and electrically insulates the bottom heating layer (either the main or the auxiliary heating layer) so as not to allow it’s accidental exposure and to prevent moisture ingress. This backing layer 987 electrically separates the bottom heating layer from any surface mounted components that are present on the surface mounting layer 988 (discussed below) on the bottom of the heater 806. If desired, this dielectric layer 987 can be made thicker than the upper dielectric layers to provide enhanced structural integrity of the flexible part of the multilayer heater. As with the other dielectric layers, this backing layer 987 does not need to be a polyimide layer and other dielectric materials could be used.

Rear Side Surface Mount Components (Optional) 988

This layer is used to mount components on to the rear of the flexible heater 306. These components may be temperature sensors (e.g. thermistors) or other components involved in providing fusing functionality for the heater (e.g. solder links).

This layer may be produced using standard chemical etching methods from the PCB manufacturing process. Additional surface mount components would be added later.

This layer may be treated during manufacturing to provide a rough copper surface (e.g. “Brown Oxide” or “Black Oxide”). This enables better bonding of the flexible heater 806 to the underlying support structure 868 when using an adhesive film 989. High Temperature Adhesive / Bonding Layer 989 (Optional)

The function of this layer is to enable bonding of the flexible heater 806 to the rigid substrate 868 (shown in Figure 8) that forms the final shape of the overall heater. Various types of adhesive could be used such as a pressure activated adhesive (PAA), heat activated adhesive (HAA), or thermosetting epoxy films (prepregs and B-stage films). It could also be a thermoplastic bonding film which sets after heat and pressure have been applied in a forming tool.

Another method of joining the flexible heater 806 to the support carrier 868 is to mount the heater in its final shape and overmould (a form of injection moulding) the carrier directly onto the back. In this case, layer 989 may be a material chosen for moulding compatibility, ensuring the plastic that the support carrier 868 is made from fuses to the adhesive/bonding layer 989 providing a strong bond between the heater and carrier.

A second alternative heater arrangement will now be described that is used in the hair styling device 1301 shown in Figure 13. As shown, in the device of Figure 13, each heater 1306 has eight heating zones 1367 along the length of the heater 1306 and two heating zones 1367 across the width of the heater 1306. The skilled reader will appreciate that components of this embodiment are largely the same to those described in the previous embodiments.

Figure 14a shows the two heater assemblies 1432a and 1432b that carry the heaters 1306a and 1306b of the hair styling device 1301 shown in Figure 13. The orientation of the heater assemblies 1432 shown in Figure 14a is the same as when they are mounted within the housing of the hair styling device 1301 shown in Figure 13. Thus, heating assembly 1432b is shown with the hair contacting surface facing down whilst heating assembly 1432a is shown with the hair contacting surface facing upwards. It can be seen from heating assembly 1432b, that each heating assembly 1432 has a projection 1436-1 and 1436-2 at each end of the assembly 1432, which hold the heating assembly 1432 within the housing 1310 of the hair styling device 1301 shown in Figure 13. It can also be seen from heating assembly 1432b, that each heating assembly has resilient feet (in this case two) 1433 on the internal surface of the heater assembly 1432 that faces the internal surface of the housing 1310. These resilient feet 1433 help to allow the heater assemblies 1432 to move slightly relative to the housing 1310 during use of the hair styling device 1301.

Figure 14b is a longitudinal cross-sectional view of the top heater assembly 1432b shown in Figure 14a and Figure 14c is an exploded view showing more clearly the components of each heater assembly 1432. Starting from the bottom of Figure 14c, the heater assembly 1432 includes: a layered heater 1406 similar to the one described above with reference to Figures 9 to 11 ; a rigid support 1468 to which the layered heater 1406 is attached; a printed circuit board 1478 which carries the drive and processing circuitry used to control the heater 1406 and a heater carrier 1480 which is used to secure the heater 1406 within the housing 1310 of the hair styler. Figure 14d is a transverse cross-sectional view of the two heater assemblies 1432 when the user has closed the arms of the hair styling device and a tress of hair 1440 is sandwiched between the hair contacting surfaces of the two heater assemblies 1432a and 1432b. The housing 1310 of the hair styling device 1301 is not shown in Figure 14d for simplicity.

Figure 15 is an exploded view showing the individual layers of the layered heater 1406 used in this embodiment. As before, the heater 1406 is formed from a number of discrete layers that are mechanically or chemically bonded together. Each layer has a thickness between about 1 pm and 150 pm. The layers provided in the heater 1406 are similar to the layers provided in the heater 806 described above. A description of each layer is given below.

Heat Spreading Layer 1582

This layer helps to spread the heat within each heating zone 467 (some of which are labelled in Figure 15) to ensure that the temperature of individual heating zones 467 is able to maintain an acceptable degree of homogeneity during typical use. As discussed above, if a heating zone 467 was to be partially loaded with hair and was sufficiently large, the unloaded portion of the heating zone 467 could develop an unacceptably high temperature, whereas the loaded region would be too cold, as heat could not adequately flow from the hot regions to the cold regions.

Each heating zone 467 in this embodiment has its own heat spreader 1591 , which is thermally separated (there is a high thermal impedance/low thermal conductivity) from the heat spreaders of adjacent heating zones. This is desirable to prevent heating zones 467 from heating neighbouring heating zones 467 which might otherwise increase power consumption and reduce warm up time, as well as complicate signal processing that relies on power per zone data. As shown in Figure 15, the heat spreader layer 1582 has sixteen heat spreaders 1591-1 to 1591-16, each formed of a relatively high thermal conductivity material (such as copper). Each heat spreader 1591 is physically separated from its neighbouring heat spreaders 1591 and in effect forms an island of thermally conductive material over the corresponding heating zone. The heat spreaders 1591 may be separated from each other by a solid material having a thermal conductivity lower than 35 W/mK or they may be separated by air. The heat spreaders 1591 may be formed, for example, by taking a planar layer of metal (such as a layer of copper) that is bonded onto the layer below and then etching this layer of copper to physically separate the individual heat spreaders 1591 (so that they do not touch each other). Provided there is a break between neighbouring heat spreaders 1591 , it is difficult for heat from one heating zone 467 to pass into neighbouring heating zones 467. The solid material that is provided in the gap between adjacent heat spreaders 1591 may be provided by a coating or a wash that is applied to the heat spreading layer 1582 after the etching process has formed the gaps between adjacent heat spreaders 1591 . Alternatively other suitable methods may be used to form substantially or fully physically separated individual heat spreaders 1591 and any suitable method may be used to provide solid material in the gaps between individual heat spreaders 1591.

This heat spreading layer 1582 can also provide mechanical integrity to the overall heater 1406, providing some protection from damage to the hair contacting surface that might otherwise expose the underlying heater electrodes, which in turn could lead to short circuits or loss of functionality.

Dielectric Separator layer 1583

The dielectric separator layer 1583 provides electrical insulation between the heat spreading layer 1582 and the main heater electrode layer 1584 below. This dielectric separator layer 1582 would have as low thermal resistance as possible whilst still achieving the dielectric requirements of the layer. This layer may be formed of polyimide, although other dielectric materials could be used. Because this layer is relatively thin, the in-plane thermal impedance is high relative to the out of plane thermal impedance. The thermal conductivity of this layer is quite low (less than 35 W/mK). This helps to prevent heat spreading from one heating zone 467 to an adjacent heating zone 467.

Main Heater Electrode & Sensing Layer 1584

This layer 1584 is where heat is created by dissipating electric power from the power source (e.g. a power supply unit (PSU) or one or more batteries).

This layer 1584 comprises a number of independently controllable heater electrodes 1664 each defining a corresponding heating zone 467. Figure 16 illustrates in more detail the form that this layer 1584 takes in this example heater 1406. As shown, in this example, there are sixteen independently controllable heater electrodes 1664-1 to 1664-16 that each defines a corresponding heating zone 467. Each heater electrode 1664 is formed of a track of resistive material, whose geometry (track width, thickness, length) and material are specified in order to achieve the desired resistance for a specific power source voltage, therefore providing a desired peak power for a given heating zone.

Each heater electrode 1664 may be formed into a serpentine pattern using chemical etching as a manufacturing process (although, as discussed above, other manufacturing processes can be used to form the heater electrodes 1664). In more detail, a solid layer of conductive material is provided and then etched to form the different heater electrodes 1664. The dark regions shown in Figure 16 are the boundaries between the etched parts of the layer 1584 between the white serpentine parts of the figure that are the serpentine conductor paths that form the heater electrodes 1664. In this illustrated example, the adjacent heater electrodes 1664 connect to a common positive terminal (although in other embodiments they may connect to a common ground terminal) to reduce the number of electrical connections needed to be made between the drive and control board 1478 and the heater 1406. This common positive terminal is connected to the different heater electrodes at the vias 1665-1 to 1665-4, which connect through to connection circuitry below in layer 1590, that then connects to the drive and control board 1478. The other end of each heater electrode 1664 connects to ground through a respective switch mounted on the drive and control board 1478 to allow for independent control of current flow through each heater electrode 1664. As those skilled in the art will appreciate, it is not essential to have such a common positive (or ground) terminal, each heater electrode 1664 may be physically separate from all other heater electrodes 1664 in which case, each end of each heater electrode 1664 would be connected separately back to the drive and control board 1478. As illustrated in Figure 16, both lengthways edges of this layer have five tabs extending outward from it. Eight of the ten total tabs (excluding the middle pair) each contain the ground terminals for two heater electrodes 1664 that bend round the upper surface of the rigid support substrate 1468 to connect to the drive and control board 1478.

As can be seen in Figure 16, each heater electrode 1664, starting from its ground terminal, is formed in a serpentine pattern running along the edge of its corresponding heating zone, only reaching a small way into the width of said heating zone. This arrangement of the heater electrode along the edge helps to encourage heat transfer into the centre of each heating zone. On reaching the other end of the length of the heating zone it then forms a serpentine pattern perpendicular to the first, finishing at a corner of its heating zone which can be found along the principal axis of the layered heater 1406 where 3 neighbouring heater electrodes 1664 converge at one of the shared vias 1665 where they connect to the shared positive terminal. The sixteen ground terminals are on the tabs that connect to the drive and control board 1478. The serpentine paths in the central area of each heating zone extends along the length of the heater. This is to encourage heat flow along that direction to account for hair only partially being loading across a zone (as shown in Figure 7).

The conductive material used in the layer 1584 (to form the heater electrodes 1664) is preferably a PTC or an NTC material (such as stainless steel or copper) so that the resistance of the heater electrode 1664 depends upon its temperature - and so the temperature of the heating zone 467 can be determined by measuring a parameter that varies with the resistance of the corresponding heater electrode 1664. The heater electrodes 1664 shown in Figure 16 may be connected to the circuitry shown in Figure 12 that is carried on the drive and control board 1478.

Fuse and connections layer 1590

This layer 1590 carries an electrical fuse for each heating zone 467 as well as the common positive terminal that connects to the positive tail of each heater electrode 1664 through the vias 1665. As can be seen in Figure 15, each fuse is located adjacent a corresponding one of the heating zones 467 and the fuses are electrically connected together in series and then to the circuitry on the drive and control board 1478. If any one of the heating zones 467 overheats, then the corresponding fuse 1734 closest to that heating zone 467 will blow which breaks the series connection to the control board 1478. This causes power to be cut immediately to all the heater electrodes 1664.

Figures 17a & 17b show in more detail the fuse and connections layer 1790 that carries the fuses 1734 and the busbar 1735. Specifically, Figure 17b shows the common positive terminal 1735 to which each of the heater electrodes 1664 connects at the vias 1665. The positive terminal 1735 connects to the drive and control board 1478 via the tab which extends out of the centre of one of the long sides of the connection circuitry layer (identical in location along the length of the heater 1406 to the central tab shown in Figure 16 that has no earth terminals attached to it). This circuit board connection tab also carries the two ends of the fuse circuit which are connected to the drive and control board 1478. In the case of solder links being the fuse solution, there will be a small amount of solder resist I solder mask applied in the space between the “arrow heads” shown 17b. This ensures a clean disconnection by repelling melted solder in the event of an overheated zone.

Dielectric backing 1587

This layer electrically insulates the fuse and connections layer so as not to allow its accidental exposure and to prevent moisture ingress. If desired, this dielectric layer 1587 can be made thicker than the upper dielectric layers to provide enhanced structural integrity of the flexible part of the heater 1406. As with the other dielectric layers, this dielectric layer 1587 may be a polyimide layer or it may be formed from another dielectric material.

Figure 18 illustrates how the flexible layered heater 1406 is bent over and bonded to the rigid support 1468 pre and post bonding. Figures 19a to 19c illustrates the forming equipment and process that is used to bend the layered heater 1406 over the rigid support 1468. The forming equipment includes a base portion 1937 and an upper press portion 1938 (shown in Figures 19a and 19b). The base portion 1937 includes a recess in which the rigid support 1468 is inserted followed by the flexible heater 1406. The recess of the base portion 1937 is shaped to match the outline of the flexible heater 1406 so that there is a close fit between the recess and the flexible heater 1406. In this way, the flexible heater 1406 cannot move around during the pressing operation. The upper portion 1937 has press features 1939 that push down on the layered heater 1406 so that the sides and the tabs of the layered heater 1406 are pushed down and bend around the rigid support 1468 - as shown in the cross-sectional view shown in Figure 19b. An adhesive layer may be provided between the layered heater 1406 and the rigid support 1468 to securely bond the heater 1406 to the rigid support 1468. Once bonded the upper press portion is removed from the base portion 1937 and the bonded heater and rigid support subassembly is ready for connection to the drive and control board 1478 and the other components that make up the heater assembly 1432 shown in Figure 14.

In some of the above described embodiments, a heat spreading layer 982 was provided above the heater electrodes to help spread the heat within each heating zone. In other embodiments, the heat spreading layer 982 may be provided below the heater electrode layer 984 as the heat spreaders can still perform their function of spreading the heat within an individual zone regardless of whether it is above or below the heater electrode layer 984. However, positioning the heat spreading layer 982 above the heater electrode layer (i.e. closer to the hair contacting surface) can be advantageous as this layer can provide a scratch resistance function to the heater 106. It is also possible to include a heat spreading layer both above and below the heater electrode layer(s). Figure 20 illustrates the possible layers (not all of which are needed as discussed above with reference to Figure 9) of a heater assembly that has a heat spreading layer 988’ provided under the heater electrode layer 984. The reference numerals used in Figure 20 are the same as those used in Figure 9 for corresponding layers. In this example, the heat spreading layer 988’ that is located under the heater electrode layer 984 includes heat spreaders as well as fusing elements that were mounted on layer 988 in Figure 9. Figure 21a and 21 b illustrates in more detail the form of the optional heat spreading layer 988’. This heat spreading layer has sixteen heat spreaders 2191-1 to 2191-16. The heat spreaders 2191 are formed of a thermally and electrically conducting material like copper. Each heat spreader 2191 is electrically connected to at least one neighbouring heat spreader 2191 by a fuse (not shown) that sits between the neighbouring heat spreaders. A dashed circle 2134 shows one of the locations where a fuse is installed to electrically connect neighbouring heat spreaders 2191-9 and 2191-10. The heat spreaders are arranged so that when the fuses (not shown) are in place, there is an electrical connection from a positive fuse connection 2136 that is coupled to heat spreader 2191-1 , through the heat spreaders 2191-1 to 2191-16 and back to a negative fuse connection 2138 that is coupled to heat spreader 2191-16. If one of the heating zones overheats and the corresponding fuse connection breaks, then that current path breaks and the controller (to which the positive and negative fuse connections 2136, 2138 are coupled) can detect this break in the current path (for example by applying a voltage across the two fuse connections 2136 and 2138 and detecting the presence of a current (fuse circuit working correctly) or the absence of a current (meaning that one or more fuses have blown)) and can take the appropriate control action - such as stopping or preventing power being applied to the heater electrodes. Like the fuse and connections layer 1790 shown in Figure 17, the layer 988’ also includes a central busbar 2135 to which the positive tails of the heater electrodes connect through the vias 2165 - illustrated by the dashed lines 2165 in Figure 21 b.

In the above embodiments that have a heat spreading layer, the individual heat spreaders 1091 (see Figure 10) were formed as islands that do not touch neighbouring heat spreaders, in order to minimise the ability of heat to transfer from one heating zone to a neighbouring heating zone. This helps signal to noise for sensing and the independent control of the different heating zones. In some embodiments, it may be desirable to provide an electrical connection between neighbouring heat spreaders - for example to ground the heat spreading layer. In this case the individual heat spreaders may have some conducting material connecting them with at least some of their neighbouring heat spreaders. Even though an electrical connection is provided between adjacent heat spreading elements, as long as the connection is relatively small (for example less than 1/10^th of the length/width of the heat spreader), there will still be, in effect, a thermal break or thermal decoupling between immediately adjacent heat spreaders. In one possible implementation, each heat spreader may be electrically connected to the vias 1665/1965 that couple to the common terminal of the heater electrodes. This will prevent the buildup of unwanted static in the heat spreading layer and may also obviate the need for the bus bar 1735/2135 shown in Figures 17 and 21 - as the connection to the electronics can then be made by connecting to the heat spreader(s) closest to the edge of the flexible heater. However, since there is minimal physical connection between adjacent heat spreaders they can still perform the desired function of spreading the heat within the respective heating zones whilst minimizing the spread of heat from one zone to an adjacent zone or zones. Figure 22 shows an exploded cross-sectional and perspective view of another heater assembly which shows on the left hand side an exploded transverse cross-sectional view of the heater 2206 and substrate 2268 and on the right hand side a perspective view of the heater 2206 and substrate 2268. As before, the heater 2206 is formed from a number of discrete layers that are mechanically or chemically bonded together. These layers include:

Layer 2281 is a low friction coating that also provides electrical insulation. This layer may be formed, for example, from a ceramic coating or wash and is directly applied on to the heater electrode layer 2284. The layer 2281 is designed to give 500 volts of dielectric breakdown strength and have thermal impedance between 9.35 x 10'⁴ KW¹cm² and to 0.8 KW¹cm². This provides the required electrical insulation between the hair contacting surface of the heater (the upper surface of the layer 2281 ) and the heater electrodes whilst minimising the temperature drop that will occur through this coating layer 2281. Minimising the temperature drop through the layer 2281 is important when the heater electrodes are being used to sense temperature, as this will make the determined temperature closer to the actual temperature of the hair contacting surface. Ceramic based coatings, such as Cerasol with a thickness of about 30 to 45 pm, can provide this dielectric breakdown strength and have a thermal impedance of about 0.5 KW¹cm² to 0.6 KW¹cm². Other materials such as Aluminium Nitride can provide the required dielectric breakdown strength whilst providing even lower thermal impedances. For example, a 30 pm layer of Aluminium Nitride can provide the require dielectric breakdown strength of 500 volts and has a thermal impedance of just 9.35 x 10'⁴ KW¹cm². However, for a mass- produced device such as a hair styler, the cost of an Aluminium Nitride layer may be too high in practice.

Layer 2284 is the heater electrode layer that carries the heater electrodes for heating the different heating zones of the heater. The electrodes may be formed from any suitable conducting material, although stainless steel is preferred.

Layer 2287 is an insulation layer (made for example from polyimide) that provides electrical insulation between the electrode layer 2284 and the heat spreading layer 2288 underneath. Polyimide is a good option for this insulation layer 2287.

Layer 2288 is the heat spreading layer that carries the heat spreaders and the fusing elements discussed above.

Layer 2292 is an adhesive layer that is used to bond the flexible heater 2206 (formed by layers 2281 , 2284, 2287 and 2288) to the rigid support 2268.

Figure 23a is a perspective view from above showing the flexible heater 2206 (with the layers 2281 , 2284, 2287 and 2288 bonded together), the adhesive layer 2292 and the rigid support 2268. Figure 23b is a view from below of the flexible heater 2206, adhesive layer 2292 and the rigid support 2268. As shown in Figure 23b, the rigid support 2268 has honeycomb struts 2353 to provide rigidity whilst keeping the weight down. The rigid support 2268 also includes eight vent holes 2355 (the ones at the two ends being obscured by the honeycomb struts 2353). These are positioned opposite eight thermal fuses that are mounted on layer 2288 of the heater 2206. In this embodiment, there are sixteen heating zones and each thermal fuse provides overheat protection for two neighbouring heating zones. Holes 2357 are provided in the adhesive layer 2292 around the vent holes 2355 to ensure that the vent holes 2355 are not blocked with adhesive.

Figure 24 is a plan view of the independently controllable heater electrodes 2464-1 to 2464-16 on the heater electrode layer 2284 that define the sixteen heating zones 467 provided in this example. Each heater electrode 2464 is formed of a track of resistive material, whose geometry (track width, thickness, length) and material is specified in order to achieve the desired resistance for a specific power source voltage, therefore providing a desired peak power for a given heating zone. Each heater electrode 2464 may be formed into a serpentine pattern using chemical etching as a manufacturing process (although, as discussed above, other manufacturing processes can be used to form the heater electrodes 2464). In more detail, a solid layer of conductive material is provided and then etched to form the different heater electrodes 2464. The dark regions shown in Figure 24 are the boundaries between the etched parts of the electrode layer between the white serpentine parts of the figure that are the serpentine conductor paths that form the heater electrodes 2464. In this illustrated example, each heater electrode 2464 serpentines from an edge of the heater 2206 to a centre line of the heater 2206 before returning in a serpentine path back to the starting edge of the heater 2206.

Adjacent heater electrodes 2464 share a common positive terminal (although in other embodiments they may connect to a common ground terminal) to reduce the number of electrical connections needed to be made between the drive and control circuitry (not shown) and the heater 2206. The common positive terminal for pairs of adjacent heater electrodes 2464 are connected back from the edge of the heater 2206 to the drive and control circuitry (described in more detail below). Thus, in this example, no central vias 1665 or busbars are needed to connect to the positive tails of the heater electrodes 2464. The other end of each heater electrode 1664 connects to ground through a respective switch forming part of the drive and control circuit to allow for independent control of current flow through each heater electrode 2464. As those skilled in the art will appreciate, it is not essential to have such a common positive (or ground) terminal, each heater electrode 2464 may be physically separate from all other heater electrodes 1664 in which case, each end of each heater electrode 2464 would be connected separately back to the drive and control circuitry. As illustrated in Figure 24, both lengthways edges of this electrode layer 2284 have twelve tabs extending outward from it that bend round the upper surface of the rigid support substrate 2268 to connect to the drive and control circuitry. Sixteen of the twenty-four total tabs each contain the ground terminal for a heater electrode 2464 and eight contain the common positive terminal for a pair of adjacent heater electrodes 2464. As can be seen in Figure 24, in this example and unlike the examples shown in Figures 11 and 16, each heater electrode 2464 does not change direction of the serpentine path at the edge portion of the heater 2206.

The conductive material used in the layer 2284 (to form the heater electrodes 2464) is preferably a PTC or an NTC material (such as stainless steel or copper) so that the resistance of the heater electrode 2464 depends upon its temperature - and so the temperature of the heating zone 467 can be determined by measuring a parameter that varies with the resistance of the corresponding heater electrode 2464.

Figure 25 shows in more detail the form of the heat spreading layer 2288 used in this example (as viewed from below the heater 2206). As shown, this heat spreading layer 2288 includes sixteen heat spreaders 2591 -1 to 2591 -16 that are positionally aligned with the corresponding heater electrodes 2464-1 to 2464-16. The heat spreaders 2591 are formed of a thermally and electrically conducting material like copper. Each heat spreader 2591 (except for heat spreaders 2591-1 and 2591-16) is electrically connected to a neighbouring heat spreader at a corner portion thereof. Each heat spreader 2591 is also electrically connected to another neighbouring heat spreader 2591 by a fuse that sits between the neighbouring heat spreaders. A dashed circle 2534 shows one of the locations where a fuse is installed to electrically connect neighbouring heat spreaders 2591-3 and 2591-4. The heat spreaders 2591 are arranged so that when the fuses are in place, there is an electrical connection (and therefore a current path represented by the dashed arrows) from a positive fuse connection 2536 that is connected to heat spreader 2591-1 , through the heat spreaders 2591-1 to 2591-16 and back to a negative fuse connection 2538 that is coupled to heat spreader 2591-16. If one of the heating zones overheats and the corresponding fuse connection breaks, then this current path breaks and the controller or control circuitry (to which the positive and negative fuse connections 2136, 2138 are coupled) can detect this break in the current path (for example by applying a voltage across the two fuse connections 2536 and 2538 and detecting the presence of a current (fuse circuit working correctly) or the absence of a current (meaning that one or more fuses have blown)) and can take the appropriate control action - such as stopping or preventing power being applied to the heater electrodes. The operation of preferred control circuitry to detect a fuse melting and to take a control action will be described in more detail later. Unlike the fuse and connections layers 1790 and 2190 shown in Figures 17 and 21 , the layer 2288 does not include a central busbar to which the positive tails of the heater electrodes 2464 connect through vias.

Figure 26a and 26b are zoomed perspective views of a fuse 2634 used in this example to connect adjacent heat spreaders 2591-3 and 2591-4. In this example, the fuse 2634 is formed of an electrically conductive solder material that electrically connects the adjacent heat spreaders 2591-3 and 2591-4. The fuse material sits on and electrically bridges across a layer of solder resist 2641. Figure 26a shows the fuse when intact, such that current can flow between adjacent heat spreaders 2591-3 and 2591-4; and Figure 26b shows what happens if a heating zone next to the fuse overheats and melts the solder material of the fuse 2634. In particular, when the solder material melts, it is repelled off the solder resist 2641 and beads up to the side where it will cool (once power is removed from the heaters) and solidify again. The solder resist 2641 is not electrically conductive, and so when the solder material melts and moves off the solder resist 2641 (as shown in Figure 26b), the adjacent heat spreaders 2591-3 and 2591-4 are electrically separated from one another thereby breaking the electrical connection between the two fuse connections 2536 and 2538. As discussed above, this break in the electrical connection is detected by the control circuitry and used to control (typically stop) the power delivery to the heater electrodes 2464.

Figure 27a and 27b are cross-sectional views through the heater 2206 (showing the electrode layer 2284, the insulation layer 2287 and the heat spreader and fuse layer 2288), the adhesive layer 2292 and the support 2268, showing the placement of a fuse 2634 and the corresponding vent hole 2355 discussed above. In particular, Figure 27a is a cross- sectional view when the fuse 2634 is intact and Figure 27b is a cross-sectional view when the fuse 2634 has melted and moved off the solder resist 2641 . As those skilled in the art will appreciate from Figure 27, the vent hole 2355 prevents pressure build up due to the heated air. An air pocket 2744 is provided within the support 2268 to house the fuse 2634.

Drive & Control Circuitry

Figure 28 is a schematic diagram of the way in which the heater electrodes 2464 may be connected together and to the drive circuitry 2823 and the power supply 2821. As shown in Figure 28, each heater electrode 2464-1 to 2464-16 is connected at one end to the power supply 2821 through a master switch 2851 and at the other end to a respective switch (in this case a MOSFET switch) 2895-1 to 2895-16. The switches 2895 are controlled by the microprocessor 2829. When a heater electrode 2464 is to provide heat, the corresponding switch 2895 is closed thereby connecting the heater electrode 2464 to ground through the resistor 28R. As a result, current flows from the power supply 2821 to ground causing the heater electrode 1264 to heat up (provided the master switch 2851 is closed). The microprocessor 2829 can control the position of each switch 2895 independently thereby allowing each heater electrode 64 to be powered independently to attain its own desired set point temperature. Typically, the set point temperatures for the different heater electrodes 2464 will be the same.

When the temperature of a selected heating zone 467 is to be determined, the switch 2895 of the corresponding heater electrode 2464 is closed and all other switches 2895 are opened. In this way, the selected heater electrode 2464 is provided in series with the resistor 28R. Since the heater electrodes 2464 are formed of a PTC or an NTC material whose resistance changes with the temperature of the heater electrode 2464, by measuring the voltage dropped across the resistor 28R (using the operational amplifier 2897), the microprocessor 2829 can determine the resistance of the selected heater electrode 2464 and hence can determine the temperature of the corresponding heating zone 467. If the determined temperature is above the desired temperature for that heating zone 467, then the microprocessor 2829 can reduce the power applied to that heater electrode 1264; or if the heating zone 467 is at a lower temperature than that desired, then the microprocessor 2829 can increase the power applied to the corresponding heater electrode 2464. Any suitable ON/OFF control or PWM (pulse width modulation) control can be used to vary the power applied to the different heater electrodes 2464. The microprocessor 2829 can select each heater electrode 2464 in turn in order to determine the temperature of each heater electrode 2864/heating zone 467. Figure 28 also shows that the voltage supplied to the heater electrodes 2464 may also be provided to the microprocessor 2829 (through suitable scaling or conversion circuitry (not shown) if at a voltage greater than can be accepted by the microprocessor 2829). This voltage input allows the microprocessor 2829 to adjust the driving of the heater electrodes 2464 in the event that, for example, the power is supplied by batteries and the batteries are becoming depleted. The voltage applied across the heater electrodes may drop for other reasons, including voltage drops along cables during high loads, tolerances in the outputs of the power supply etc. By measuring the applied voltage, the microprocessor 2829 can use this information to calculate more accurately the resistance of each heater electrode (and hence the temperature of that heater electrode) given the present circuit conditions. For example, the microprocessor 2829 can use the measured voltage across resistor 28R to work out the current flowing through the heater electrode 2464 (by dividing the measured voltage across resistor 28R by the known resistance of resistor 28R). The microprocessor 2829 can then determine the resistance of the heater electrode 2464 by subtracting the voltage across resistor 28R from the sensed voltage applied to the heater electrode 2464 and dividing that by the determined current. The calculated resistance can then be equated, if desired, to the temperature of the heater electrode 2464 through an appropriate look up table.

The inventors have found that sensing the temperatures of the heater electrodes in the above manner only requires about 5% of the overall time available - which does not therefore interfere with the powering of the heater electrodes.

Figure 28 also shows how the eight fuses 2634-1 to 2634-8 used in this example are connected to the control circuitry and can automatically remove power from the heater electrodes 2464. In particular, as shown in Figure 28, the gate of the master switch 2851 is connected to the power supply through a potential divider circuit 2856 that connects to ground through the fuses 2634 and an optional test switch 2858. In normal operation, when the fuses 2634 are intact, the voltage at the gate of the master switch 2851 will be at a lower voltage than at the source. This means that the switch is closed and current can flow from the power supply 2821 through the master switch 2851 to the heater electrodes 2464. However, in the event that one or more of the fuses 2634 melts and breaks the electrical connection between the potential divider circuit 2856 and ground, then the voltage on the gate of the master switch 2851 will become the same as the voltage on the source of the master switch 2851 , and this will cause the master switch 2851 to open, thereby isolating the heater electrodes 2464 from the power supply 2821 .

The optional test switch 2858 is provided to allow the microprocessor 2829 to test the circuitry for faults. Specifically, it is possible for the master switch 2851 to fault into a permanently closed position, in which case, in the event one or more of the fuses 2634 melts and breaks the connection to ground, the master switch 2851 will not break the connection between the power supply 2821 and the heater electrodes 2464. However, by providing the test switch 2858, which can be opened and closed by the microprocessor 2829, the microprocessor 2829 can check that the master switch 2851 has not failed into a permanently closed state. In more detail, when the microprocessor 2829 opens the test switch 2858, this simulates a break in one of the fuses 2634, which should open the master switch 2851 . The microprocessor 2829 can then monitor the temperature of one or more of the heater electrodes 2464 (using the op-amp 2897) in the manner discussed above. If the master switch 2851 is operating correctly, then the temperature of the or each monitored heater electrodes 2464 should drop (rapidly because the heater has a low thermal mass). If the temperature of any of the monitored heater electrodes 2464 remains above a threshold temperature after the test switch has been opened, then the microprocessor 2829 can assume the master switch 2851 has faulted in its closed state) and open all the switches 2895 to prevent further heating of the heater electrodes 2464.

As shown in Figure 28, the test switch 2858 and the switches 2895 are n-channel MOSFETs and the master switch 2851 is a p-channel MOSFET. The advantage of using n-channel switches is that they will go into an open state in the event of power being removed from the control circuit, which should remove all power to the heater electrodes 2464. Of course, other switches could be used.

Mains Operated Heater

In the above embodiments, a DC power source was used to provide electrical power for heating the heater electrodes 1664. This DC power source will typically be one or more batteries, although DC supplies that derive their power from a mains power AC signal may be used. Where DC operation is used, the DC voltage preferably complies with the Safe Extra Low Voltage (SELV) regulation requiring voltages of less than 42.4 Volts to be used. When AC power is directly supplied to the heater electrodes, thicker or more dielectric layers are typically used between the heater electrodes 1664 and the hair contacting surface of the hair styler.

In order to allow the product testing authorities to be able to test the dielectric strength of each layer, the applicant has used dielectric layers in the form of adhesive tapes that can be peeled apart for product testing. However, such dielectric tape layers are still relatively thick which means that there is still scope to reduce the thickness of the dielectric layers and hence reduce further the thermal mass of the heater. Further, the adhesive part of such dielectric tape creates an undesirable thermal barrier between each dielectric layer and the hair contacting surface. Therefore, thinner dielectric layers can be provided that can be applied using a coating, other deposition processes, or other direct bonding or forming processes. However, the inventors have realised that this creates challenges for the testing authorities as they can no-longer peel apart the individual layers and test each layer separately.

The following disclosure aims to provide a layered heater having a structure that can allow testing authorities to test the dielectric properties of multiple dielectric layers of the heater.

Figure 29 shows an embodiment of an AC powered multi-layer heater 2906 comprising a stack of thin layers. The heater 2906 includes an electrode layer 2984, a hair contacting layer 2981 , and dielectric layers 2983-1 , 2983-2, and 2983-3 to electrically insulate the electrode layer 2984 from the hair contacting layer 2981 . Further dielectric layers may be provided, or in some cases there may only be two dielectric layers. As defined in the International Electrotechnical Commission (IEC) document relating to household and similar electrical appliances safety requirements, if only a single layer of dielectric is provided (when operated with AC mains power), then it must be able to withstand (without breakdown) at least 3 kV, and if multiple dielectric layers are provided, then each dielectric layer must be able to withstand (without breakdown) at least 1.75 kV. As will be appreciated, the requirements of the dielectric layer(s) as defined by the IEC, or indeed by any other official entity, may change over time.

Regarding the layers 2981 , 2984, 2983-1 , 2983-2, and 2983-3, each layer will generally have a first and second surface, or an upper and lower surface. In the context of Figure 29: the lower surface of the electrode layer 2984 is in contact with the upper surface of the dielectric layer 2983-3, the lower surface of the dielectric layer 2983-3 is in contact with the upper surface of the dielectric layer 2983-2, the lower surface of the dielectric layer 2983-2 is in contact with the upper surface of the dielectric layer 2983-1 , and the lower surface of the dielectric layer 2983-1 is in contact with the upper surface of the hair contacting layer 2981. However, the usage of the wording “upper” and “lower” does not define the orientation of the heater, where the upper surface must always be pointing upwards. Instead, this wording is to emphasise how the layers are positioned relative to one another. Indeed, as the heaters 2906 may be used in all manner of hair styling appliances such as curlers, the heaters may be in any orientation, and as will be discussed in more detail below, the heaters 2906 may be flexible and set up in a curved apparatus.

The layers 2981 , 2984, 2983-1 , 2983-2, and 2983-3 of the heater 2906 may be bonded together through an adhesive layer (pressure set or thermoset), through diffusion bonding of the contacting materials (e.g. melting them together), or other bonding method and define a heater 2906 that is intended to be very thin with the dielectric layer(s) having a total thickness of 600 microns or less. The hair contacting layer 2981 may be a substrate layer that the dielectric is formed on, however this may not necessarily be the case, and other manufacturing methods and materials will be described later. The heater defined by layers 2981 , 2984, 2983-1 , 2983-2, and 2983-3 may also be flexible and where rigidity of the heater is required, this may be provided by a rigid support to which the heater 6 is attached.

Dielectric Testing

When it comes to testing the dielectric breakdown voltage of each dielectric layer, the testing authorities must be able to test each dielectric layer. Therefore, various arrangements that allow a top surface of each dielectric layer to be accessible (either directly or via the electrode layer) will now be described with references to Figures 30 to 32. Each described arrangement may be used in isolation or in combination with other embodiments.

In the example illustrated in Figure 30, there is a hair contacting layer 3081 , electrode layer 3084, and dielectric layers 3083-1 , 3083-2, and 3083-3. In this embodiment in order to enable testing of the dielectric breakdown voltage of each dielectric layer 3083-1 , 3083- 2, and 3083-3, each dielectric layer 3083 has an exposed surface in the direction of the electrode layer 3084. In other words, part of the upper surface of each dielectric layer 3083-1 , 3083-2, and 3083-3 has no layer directly contacting it. In this case, and generally in this application, upper simply means in the direction from the hair contacting layer 3081 to the electrode layer 3084. In the example shown in Figure 30, the exposed surfaces are provided in close proximity to each other and form a staircase structure at an edge of the heater 3006. The staircase structure of the dielectric layers illustrated in Figure 30 may be provided along one or more sides of the heater or even just part of a side or a corner of the heater. These exposed portions of the dielectric layers allow the breakdown voltage of each dielectric layer to be tested with electrical probes. For example, the breakdown voltage of dielectric layer 3083-1 can be tested with one probe contacting the upper surface of layer 3083-1 (for example at point A) and another probe contacting a point on the lower surface of the hair contacting layer 3081 . The voltage applied between the two probes is then increased until it is above a defined threshold (e.g. 1.75 kV) or until voltage breakdown occurs and current flows between the two probes. Similarly, the breakdown voltage of dielectric layer 3083-2 can be tested with one probe contacting the upper surface of layer 3083-2 (for example at point B) and the other probe contacting the upper surface of layer 3083-1 below (for example at point A); and the voltage applied between the two probes is then increased until it is above a defined threshold (e.g. 1 .75 kV) or until voltage breakdown occurs and current flows between the two probes. The breakdown voltage of dielectric layer 3083-3 can be tested with one probe contacting the upper surface of layer 3083-3 (for example at point C) and the other probe contacting the upper surface of layer 3083-2 below (for example at point B); and the voltage applied between the two probes is then increased until it is above a defined threshold (e.g. 1 .75 kV) or until voltage breakdown occurs and current flows between the two probes. In the case of layer 3083-3, the probe may either be placed in contact with the upper surface of the electrode layer 3083-3 or anywhere on the upper surface of the electrode layer 3084 (because of the relatively high conductivity of the electrode layer 3084 compared to that of the dielectric layer 3083-3, placing the probe on the electrode layer 3084 would not significantly impact the dielectric breakdown voltage of layer 3083-3).

An example capable of testing the breakdown voltage of each dielectric layer with respect to the hair contacting layer is illustrated in Figure 31 . Similar to the previous example, the heater 3106 has a hair contacting layer 3181 , electrode layer 3184, and dielectric layers 3183-1 , 3183-2, and 3183-3. In order to measure the dielectric breakdown voltage of each dielectric layer 3183-1 , 3183-2, and 3183-3, the dielectric layers are staggered from each other so that again each dielectric layer 3183 has an exposed upper surface in the direction of the electrode layer 3184. In this example, the hair contacting layer 3181 is also arranged to directly contact part of each dielectric layer at a location where the dielectric layer has the exposed surface. This means that both the dielectric layers 3183-1 , 3183- 2, and 3183-3 and the hair contacting layer 3181 define a staircase structure in the region where testing is to be performed. It should be noted that in locations directly under the electrode layer 3184, three dielectric layers 3183-1 , 3183-2 and 3183-3 will be provided between the dielectric layer and the hair contacting layer 3181. In this example, the dielectric breakdown voltage can be tested by applying the test voltages between the upper surface of each dielectric layer 3183-1 , 3183-2, 3183-3 and the lower surface of the hair contacting layer 3181. Thus for example, the breakdown voltage of dielectric layer 3183-1 can be tested with one probe contacting the upper surface of layer 3183-1 (for example at point D) and another probe contacting a point on the lower surface of the hair contacting layer 3181. The voltage applied between the two probes is then increased until it is above a defined threshold (e.g. 1 .75 kV) or until voltage breakdown occurs and current flows between the two probes. Similarly, the breakdown voltage of dielectric layer 3183- 2 can be tested with one probe contacting the upper surface of layer 3183-2 (for example at point E) and the other probe contacting a point on the lower surface of the hair contacting layer 3181 ; and the voltage applied between the two probes is then increased until it is above a defined threshold (e.g. 1.75 kV) or until voltage breakdown occurs and current flows between the two probes. The breakdown voltage of dielectric layer 3183-3 can be tested with one probe contacting the upper surface of layer 3083-3 (for example at point F) and the other probe contacting a point on the lower surface of the hair contacting layer 3181 ; and the voltage applied between the two probes is then increased until it is above a defined threshold (e.g. 1.75 kV) or until voltage breakdown occurs and current flows between the two probes. Unlike in the example shown in Figure 30, the upper surface of dielectric layer 3183-3 cannot be tested by contacting a probe to the electrode layer 3184, as the electrode layer 3184 is laterally displaced from the portion of the dielectric layer 3183-3 that is contacting the hair contacting layer 3181 .

A further example capable of testing each dielectric layer with respect to the hair contacting layer is illustrated in Figure 32. As before, the heater 3206 has a hair contacting layer 3281 , electrode layer 3284, and dielectric layers 3283-1 , 3283-2, and 3283-3. As before, in order to allow measurement of the dielectric breakdown voltage of each dielectric layer 3283-1 , 3283-2, and 3283-3, each dielectric layer 3283 has an exposed surface in the direction of the electrode layer 3284. The dielectric layers 3283-1 , 3283-2 and 3283-3 are arranged to overlap with each other so that each layer has an exposed portion that directly contacts the upper surface of the hair contacting layer 3281 . In this embodiment, the hair contacting layer 3281 has a uniform thickness and the dielectric layers are arranged in two stair cases - one going from left to right in the figure, with dielectric layer 3283-3 ending before dielectric layer 3283-2 which in turn ends before dielectric layer 3283-1 ; and one going from top to bottom of the figure, with dielectric layer 3283-2 extending beyond dielectric layer 3283-1 and with dielectric layer 3283-3 extending beyond dielectric layer 3283-2.

With this arrangement, the breakdown voltage of dielectric layer 3283-1 can be tested with one probe contacting the upper surface of layer 3283-1 (for example at point G) and another probe contacting a point on the lower surface of the hair contacting layer 3281. The voltage applied between the two probes is then increased until it is above a defined threshold (e.g. 1 .75 kV) or until voltage breakdown occurs and current flows between the two probes. Similarly, the breakdown voltage of dielectric layer 3283-2 can be tested with one probe contacting the upper surface of layer 3283-2 (for example at point H) and the other probe contacting a point on the lower surface of the hair contacting layer 3281 ; and the voltage applied between the two probes is then increased until it is above a defined threshold (e.g. 1 .75 kV) or until voltage breakdown occurs and current flows between the two probes. The breakdown voltage of dielectric layer 3283-3 can be tested with one probe contacting the upper surface of layer 3283-3 (for example at point I) and the other probe contacting a point on the lower surface of the hair contacting layer 3281 ; and the voltage applied between the two probes is then increased until it is above a defined threshold (e.g. 1 .75 kV) or until voltage breakdown occurs and current flows between the two probes. In the examples described above, the dielectric layers were arranged into a staircase arrangement to provide an exposed surface for each dielectric layer to allow the testing thereof. As those skilled in the art will appreciate, it would also be possible to have other structures that allow at least a portion of the top surface of each dielectric layer to be exposed. For example, one or more wells or blind bores may be provided through the dielectric layers so that the one or more wells or blind bores expose the upper surface of all the dielectric layers. For example, one well or blind bore may be provided down to the upper surface of each dielectric layer. So, if there are three dielectric layers, then two wells or blind bores may be provided with one well or blind bore going down to the middle dielectric layer and one going down to the lower dielectric layer. Alternatively, one well or blind bore may be provided that has a larger outer well or bore that extends down to the middle dielectric layer and a smaller sized well or bore that extends further down to the upper surface of the lower dielectric layer.

Electrode Layer

As those skilled in the art will appreciate, the electrode layers 2984, 3084, 3184, 3284 have been shown in schematic form in Figures 29 to 32. The exact form that the electrodes take is not important for the explanation of this aspect of the disclosure. The electrode layer may, for example define one or more heater elements that heat up when an electrical current passes through. The heater elements may take the form of meandering track arranged over the area of the heater to be heated or they may be defined by a busbar electrode arrangement. Figure 33a is an example of a heater track 3384a that extends between two contact points 3371a and 3371 b in a meandering fashion across the surface of the dielectric layer 3375a below. Figure 33b is an example of a busbar electrode arrangement having two busbar electrodes3373a and 3373b that are electrically connected together by an electrically conductive portion 3384b. For optimal heating, the busbar electrodes 3373 should span the longer two sides of the rectangular electrically conductive portion 3384b. As shown in Figure 33b, both the busbar electrodes and the electrically conductive component 3384b are mounted on top of a dielectric layer 3383a and 3383b. The dielectric layers 3375a and 3375b may correspond to the upper dielectric layer of a three dielectric layer arrangement like those shown in Figures 29 to 32 described above.

It should be noted that there may be multiple heating elements in the same electrode layer. For example, on the same electrode layer 3384 there may be multiple serpentine tracks 3384a with their own contacts 3371a and 3371b and control systems, or multiple busbar type electrode arrangements 3373 with their own control systems. Indeed, a combination of serpentine tracks 3384a and full coverage tracks 3384b could be used on the same electrode layer, each track having its own control system, or a single control system that can control each electrode. When multiple heating elements are used, they may be laterally spaced next to each other so that each heating element defines a separate heating zone of the heater. In otherwords, there may be multiple heater elements creating their own heating zones, with each heating zone having its own independently controlled heating element (as described in the embodiments above). As above, each heating element may be in the same electrode layer on top of the same dielectric insulation layer on the same hair contacting surface. Additionally or alternatively, each or some of the multiple heater electrodes may be provided on separate heater electrode layers with dielectric layers between the electrode layers. In any case, the heating zones may be of the same or different sizes, and the temperature of each zone may be independently controllable.

Flexible and curved heaters

As discussed above, the multilayer heaters described herein are very thin and have a low thermal mass. The heaters themselves are therefore flexible which is desirable if the heater is to be curved for heating a curved hair contacting surface. Figure 34 schematically illustrates part of a curved head portion of a heated hair brush type of hair styling device that has a curved housing 3477, an electrode layer 3464, and top dielectric layer 3475 (two or more other dielectric layers (not shown) are also provided between the electrode layer 3464 and the hair contacting layer). The outer surface of the housing 3477 may be the hair contacting layer, or the hair contacting layer may be thermally connected to the housing 3477, essentially making the housing an extension of the hair contacting layer. The heater may be rigidly formed into a curved shape prior to mounting in the housing 3477, or a flexible heater may be rigidly contained and gain rigidity when mounted within the housing 3477. In the example shown in Figure 34, a serpentine heater track 3464 is illustrated, but a busbar type of heater electrode arrangement may be used instead.

Additional (optional) layers

It may also be desired to electrically insulate the heater tracks from other parts of the styling apparatus or housing (other than the hair contacting surface). Therefore, there may also be dielectric insulation layers on top of an electrode layer. There may also be other layers or coatings on the hair contacting layer to improve certain functions of the styling apparatus. A description of a number of optional layers is given above with reference to Figures 9 and will not be repeated here.

Materials and Fabrication

The following is a list of appropriate materials and possible manufacturing processes for each material for each type of layer described above.

Hair Contacting Layer

The hair contacting layer may be made of a polymer such as polyimides, polyamides, nylon, liquid crystal polymers, polyphenylene sulfide, or glass filled polyphenylene sulfide. If they are sheet polymers, appropriate manufacturing processes include hot pressing, thermoforming, lamination, CNC machining, and stamping. If they are liquid polymers, they can be manufactured through casting, extrusion, or liquid moulding, and cured by oven curing, vacuum curing, or curing under applied stress. For a liquid/resin polymer, manufacturing processes include injection moulding, over moulding, resin casting, and die casting, and curing by the same as previously. The hair contacting layer may also be made of a ceramic material such as silicon dioxide, a-aluminium oxide, y-aluminium oxide, K- aluminium oxide, zirconium dioxide (zirconia), zirconia toughened alumina, aluminium nitride, magnesium aluminate, magnesium oxide (magnesia), magnesia stabilized zirconia, silicon nitride, mica. These can be manufactured by ceramic moulding, slip casting, die casting, dry pressing, isostatic pressing, and extrusion, and cured by oven curing, vacuum curing, or curing under applied stress. The hair contacting layer may also be made of a metal such as copper, copper alloys, copper nickel alloys, steel, steel alloys, nickel, nickel alloys, nickel chromium alloys, iron, iron chromium alloys, iron chromium aluminium alloys, aluminium, or aluminium alloys. Such metals could be manufactured by extrusion rolling, roll forming, die forming, laser cutting, forging, CNC machining, stamping, or hydroforming.

Dielectric layer

A dielectric layer may be made of a polymer such as polyimides, polyamides, nylon, liquid crystal polymers, polyphenylene sulfide, or glass filled polyphenylene sulfide. If these are sheet polymers they can be applied by hot pressing, diffusion bonding, thermoforming, lamination, adhesive liquid, or adhesive tape. If these are liquid polymers then they can be coated by casting, doctor blade coating (screen printing), dipping, spin coating, spraying, roller coating, and cured by oven curing, vacuum curing, or digital thermal processing. If they are a liquid/resin polymer then they can be applied by injection moulding, over moulding, resin casting, die casting, or die coating. A dielectric layer may also be a ceramic such as silicon dioxide, a-aluminium oxide, y-aluminium oxide, K- aluminium oxide, zirconium dioxide (zirconia), zirconia toughened alumina, aluminium nitride, magnesium aluminate, magnesium oxide (magnesia), magnesia stabilized zirconia, silicon nitride, mica. If these are sheet ceramics, then they can be applied by hot pressing, diffusion bonding, thermoforming, lamination, adhesive liquid, or adhesive tape. If they are a ceramic coating then they can be applied by aerosol deposition coating, casting, doctor blade coating (screen printing), dipping, spin coating, spraying, or roller coating, and cured by oven curing, vacuum curing, or digital thermal processing. Physical vapor deposition, chemical vapor deposition, and plasma assisted chemical vapour deposition is also possible with certain materials such as diamond-like carbon.

A dielectric layer may also be formed directly on a metal, or otherwise oxidisable, substrate or hair contacting layer through Plasma Electrolytic Oxidation (PEO) or Electro Chemical Oxidation (ECO). For example, performing PEO or ECO on an aluminium substrate will grow a dielectric layer of crystalline aluminium oxide.

Heater Track

A heater track in an electrode layer may be made of copper, copper alloys, copper nickel alloys, steel, steel alloys, nickel, nickel alloys, nickel chromium alloys, iron, iron chromium alloys, iron chromium aluminium alloys, aluminium, aluminium alloys, silver, silver alloys, gold, gold alloys, carbon, or graphite. These can be manufactured by stamping, etching (including reactive ion etching, splutter/sputter etching (ion milling), deep reactive ion etching, isotropic wet etching, anisotropic wet etching, wet etching with the dipping method, and wet etching with the spin-and-spray method), wire forming by wire drawing or wire extrusion, or a deposition method such as physical vapor deposition, chemical vapor deposition, and plasma assisted chemical vapour deposition. A heater track may also be printed by micro-dispenser, precision fluid dispensing with a valve and/or jet valve system, thick film printing, thin film printing, or inkjet printing. Materials appropriate for such printing methods include silver conductor pastes, silver-palladium conductor pastes, silver-platinum conductor pastes, gold conductor pastes, platinum conductor pastes, copper conductor pastes, carbon conductor pastes, and graphite conductor pastes. A heater track may also be a ceramic such as molybdenum disilicide, silicon carbide, barium titanate, lead titanate. These materials would be manufactured by ceramic moulding, slip casting, die casting, dry pressing, isostatic pressing, or extrusion, and could be cured by oven curing, vacuum curing, or curing under applied stress.

Bus Bar

A bus bar as required for a full coverage heater element could be made of copper, copper alloys, copper nickel alloys, steel, steel alloys, nickel, nickel alloys, nickel chromium alloys, iron, iron chromium alloys, iron chromium aluminium alloys, aluminium, aluminium alloys, silver, silver alloys, gold, gold alloys, carbon, or graphite. These can be manufactured by stamping, etching (including reactive ion etching, splutter/sputter etching (ion milling), deep reactive ion etching, isotropic wet etching, anisotropic wet etching, wet etching with the dipping method, and wet etching with the spin-and-spray method), wire forming by wire drawing or wire extrusion, or a deposition method such as physical vapor deposition, chemical vapor deposition, and plasma assisted chemical vapour deposition. A bus bar may also be printed by micro-dispenser, precision fluid dispensing with a valve and/or jet valve system, thick film printing, thin film printing, or inkjet printing. Materials appropriate for such printing methods include silver conductor pastes, silver-palladium conductor pastes, silver-platinum conductor pastes, gold conductor pastes, platinum conductor pastes, copper conductor pastes, carbon conductor pastes, and graphite conductor pastes.

In order to fabricate a multi-layer heater with a staircase structure, various fabrication methods may be used as specified above. Specifically, to fabricate layers of different sizes, it may be convenient to use masking techniques, positive masks, or negative masks, to create a staircase structure, or other structures as required to fabricate a multilayer heater with multiple dielectric layers with an exposed top surface.

Another technique that may generally be used to fabricate a multi-layer heater is in-mould electronics (I ME) manufacturing. This process involves assembling some layers inside a mould (i.e. around the edges of the mould, with a space in the middle), and injecting material into the mould to otherwise fill the gaps.

Hair Cooling

Referring to the hair sty ler 3501 shown in Figure 35, when heat-styling hair, the process of cooling the hair after it has been heated can be very important for improved styling. If the hair is cooled whilst being held in a curled style, then it has been found that the curl compression (the tightness of the curl that remains) is improved. For all styling types, not just curling, hair typically may also hold the style better if cooled in a desired style. For example, users can use a hair straightening appliance, such as the styler 3501 , to curl hair by wrapping the hair around one arm 3504a of the styler 3501 such that it wraps around and passes over the heater 3506a (which may be any of the heaters described above) - and then running the styler 3501 over the tress of hair. This is illustrated in Figure 36, which shows a user curling a tress of hair 3640 using the styler 3501 . The tress of hair 3640 is wrapped around the sty ler 3501 such that it touches the case 3502a and 3502b of each arm and, when closed, will also touch the heater 3506a and 3506b of each arm. The hair can then be curled by passing the sty ler 3501 over the tress 3640 in this position, which means it will pass between the heaters 3506a, 3506b and over the case 3502a, 3502b. If the case 3502a, 3502b of the sty ler 3501 cools the hair 3640, then a tighter (and typically longer-lasting) curl can be achieved as the hair is cooled while in a curled position. This is referred to as improved curl compression.

For hair stylers with conventional heaters (i.e. ceramic heating plate), heat typically leaks into the surrounding components, such as the casing, due to the nature of its heat generation and materials. This means the case is typically already somewhat heated while the user curls the hair, and typically the temperature of the hair increases the temperature of the case even further. This can lead to a sub-optimal curl. The conventional heater technology also fills the space inside the caseworks of the styler, making it difficult to include any cooling components without making the styler too large. By contrast, the very low thermal mass heaters 3506, as described above, firstly lead to a much lower level of heat leakage into the surround casing in comparison to conventional heating technology. Secondly, the very low thermal mass heaters 3506 occupy less space within the casework, which facilitates active cooling components to be provided as well. This can be useful as, although the heater 3506 will heat the casing to a lesser degree than a conventional ceramic heater might, the heat of the hair itself can create an increase in the temperature of the casing. Active cooling components can therefore be provided within the styler 3501 to facilitate cooling of the hair in addition to heating.

Fan cooling

Figures 35a and 35b show a schematic side view and perspective view, respectively, of an embodiment of the styler 3501 comprising active cooling components. In particular, the two arms 3504a and 3504b, each comprising a heater, 3506a and 3506b respectively, and a case, 3502a and 3502b respectively, are connected by and moveable relative to a shoulder 3503, within which is provided a fan cooling mechanism. The small fan causes air to be circulated within the case 3502a, 3502b, thereby actively cooling the case via a heat exchange. The fan causes air to flow from the shoulder 3503 along at least one, but typically both, of the arms 3504a, 3504b of the styler 3501. In some implementations, the air flows wholly within the case 3502a, 3502b while in other implementations the air flows out of the case 3502a, 3502b.

Figure 37 shows a schematic perspective view of one arm 3504a of the styler, in which the end of the arm 3504a has been removed to show the inner components within the case 3502a below the thin heater 3506a. Immediately below the heater 3506a is a carrier 3768 for the heater, which supports the heater 3506a. Below and connected to the carrier 100 is a structure arranged to support the heater 3506a and carrier 3768, while still allowing air to flow within the case 3502a. As such, the structure has an open structure which comprises resilient feet 3733 at the bottom of the carrier 3768 which are in contact with the case 3502a to allow the heater 3506 to move slightly relative to the case 3502a. The structure also includes a projection 3736 at each end of the carrier 3768 that engages with the case 3502a to hold the heater 3506a and the carrier 3768 in position within the arm 3504a. The cavity 3779 in which the air can flow is adjacent to the majority of the surface area of the case 3502a.

Figures 38a and 38b show an exemplary implementation of the fan cooling arrangement, in which air from the fan is used to cool the case via a heat exchange cooling mechanism. Figure 38a shows a schematic perspective view of the whole sty ler 3501 , with the ends of the casing removed such that the inner components within the casing 3502a, 3502b can be seen. Figure 38b shows a magnified view of just the open end of one arm 3504a, showing the inner components within the casing 3502a. As described above, this arrangement comprises the carrier 3768, and support structure comprising resilient feet 3733 and projection 3736. In addition to this, the casing 3502a comprises inwardly protruding ribs 3893 that are arranged on the inner surface of the casing 3502a. The ribs 3893 run parallel to the length of the arm 3504a and act to guide the flow of air from the fan, located within the shoulder 3503, along the length of the case 3502a. The ribs 3893 can help to create a smooth flow of air within the cavity 3779. This can improve the efficiency with which the air flows over the surface of the case 3502a and removes heat. The ribs 3893 also increase the surface area of the case 3502a over which the air can flow, cooling the case 3502a via a heat exchange cooling effect.

Figures 39a and 39b show an alternative cooling mechanism, in which the air from the fan is used to cool the hair directly. Figure 39a shows a schematic perspective view of the styler 3501 , with heaters 3506a, 3506b on each arm 3504a, 3504b and a fan located within the shoulder 3503. Figure 39b shows an enlarged perspective view of an arm with the end removed such that the internal components can be seen within the casing 3502a. As for the previous implementation, the internal components comprise the carrier 3768 below the heater 3506a, and the support structure comprising the resilient feet 3733 and the projection 3736. In this implementation, the case 3502a comprises apertures 3996, which facilitate air flow from the cavity 108 within the case 3502a to outside of the case 3502a. This air flow can therefore directly cool hair located on the external surface of the case 3502a. The apertures 3996 as illustrated are formed as elongate slots running parallel to the length of the arm 3504a of the styler 3501 . However, other arrangements and configurations could also be used.

The extent of the cooling may be changed by altering the flow throughput of the fan. This may be altered in dependence on the temperature of the heater 3506 - which may be the desired setpoint temperature, a measured temperature (as measured by the temperature measurement circuitry 225) and/or may be in dependence on the thermal load of the heater 3506.

Liquid cooling

In an alternative implementation, cooling could be performed using a heat exchange cooling mechanism using a fluid. In such an implementation, a conduit containing a fluid is provided adjacent to the case 3502 such that it can be cooled by the liquid via a heatexchange mechanism. The conduit is fl uidically sealed to prevent fluid from leaking out of the styler 3501 or onto the internal components. The fluid may be water or may be a specialized coolant fluid. The conduit may be configured such that it covers a continuous spread across the inner surface of the case 3502, or it may be arranged as a series of conduits passing over the inner surface of the case 3502 (these may be connected, effectively forming a long conduit snaking across the inner surface), or indeed an alternative arrangement. There may additionally (and optionally) be provided a pump and/or agitator for enhancing the flow of the fluid through the conduit(s).

As for air flow cooling systems, the cooling effect may be altered by increasing the throughput of fluid through the conduit, for example by changing the settings of a pump and/or agitator. This may be performed in dependence on the setpoint temperature of the heater, the measured temperature and/or the thermal load on the heater.

Thermoelectric cooling

In a yet further implementation, the cooling of the case 3502 can be implemented using a thermoelectric cooling system. This is illustrated in Figure 40, which shows a schematic perspective view of an arm 3504a of the sty ler with the end removed so that the internal components can be seen. As for the previous implementations, the inner components comprise a carrier 3768 below the heater 3506a, and a support structure comprising resilient feet 3733 and projection 3736. In addition, on the inner surface of the case 3502 is provided a thermoelectric cooling system 4098. This is configured to act as a solid-state heat exchanger to cool the case 3502 via thermoelectric cooling. As illustrated in Figure 40, the thermoelectric cooling system 4098 may be arranged such that it spans across the available inner surface area of the case 3502. However, alternative arrangements of the thermoelectric cooler may be used, for example in which the cooling system is arranged in a pattern configuration.

Thermoelectric cooling systems offer the advantage of good control over the cooling (i.e. the cooling temperature). The cooling can therefore be controlled, for example, in dependence on the temperature of the heater 3506 (based on the desired operating temperature, thermal load and/or on measurements made by the temperature measurement circuitry 225). Additionally, as thermoelectric coolers can be made to very small sizes, an array of thermoelectric coolers may be provided within the case 3502. Each cooler of the array may be arranged to correspond to one heating zone of the heater 3506a, or to a number of heating zones of the heater 3506a. This can allow the cooling to be defined in dependence on the loads of individual heating zones. For example, a lower cooling temperature may be used when it is determined that adjacent heating zones have been loaded with hair.

In some implementations of the styler 3501 in which the case 3502a, 3502b comprises apertures 3996 (as illustrated in Figures 39a and 39b), a styling product can be distributed through the apertures 3996 while a user is styling their hair. Such a styling product may for example be a hair spray, which could be used to help set a style, or a conditioning spray, etc. In such an implementation, the styler 3501 may comprise a reservoir for holding the product and/or may be configured to receive a cartridge containing a hair product. The styler 3501 may then further be configured to disperse the product into a mist. The cooling mechanism has been described above in reference to a hair straightening appliance comprising two arms; however, active cooling mechanisms such as those described above may also be implemented in other hair drying and styling appliances. For example, a wand-shaped curling iron may comprise heating and cooling portions.

The cooling may be implemented in dependence on the heating of the heaters; for example, there may be particular combinations of heating temperature and cooling temperature may be used. In some implementations, a user may be able to choose this combination. Additionally or alternatively, the combinations may be predefined for each heating temperature.

Additionally, the very thin nature of the heaters can facilitate the heaters and the cooling regions being provided very close to one another and/or in some implementations the heater itself being cooled by the cooling system. As the heaters have very low thermal mass, they can cool down very quickly, and then can undergo active cooling, which is transmitted across to the hair. This could facilitate heating and cooling the hairwith a same or adjacent portion of the styler.

Electrical Connections to Heater

As discussed above, the drive and control circuitry is typically mounted on a printed circuit board (PCB) and connections need to be made between this printed circuit board and the tails of the heater electrodes and the fuse circuitry. Figure 14 shows an example where the PCB 1478 is mounted under the heater support 1468 and the tails of the electrodes and fusing circuitry are bent round behind the heater support 1468. The conductive layers on the PCB are made from copper and, as discussed above, the conductive tracks that form the heater electrodes are preferably made from stainless steel. The connections that are made between the copper on the PCB and the stainless steel on the heater need to be able to provide a reliable and consistent connection over a range of temperatures (between 10°C and 150°C). The applicant has tried directly soldering or using an FFC type connector to connect to the tracks of the heater. However, solder connections onto stainless steel are not easy mass manufacturable. The acid-based fluxes that are used are highly toxic and can delaminate the flexible heater 1406 from the heater support 1468, without adequate protection. Additionally, the soldered joints are subject to thermal fatigue, leading to cracking and potentially failing with time, especially when the solder connections are on the flexible heater 1406, which is the hottest component in the assembly. FFC connectors, while simpler, are prohibitively expensive for a mass- produced product such as a hair styling device.

The applicant has therefore developed a simpler and cheaper way of establishing electrical continuity between the stainless steel terminals of the flexible heater 1406 and the copper terminals on the rigid PCB 1478. This entails the use of a spring finger to establish a direct mechanical connection between the terminals on the flexible heater 1406 and the terminals on the rigid PCB 1478, that can maintain a consistent electrical contact resistance between the two parts as they expand and contract due to temperature variances. Figure 41a illustrates a heater 4106 (which may be any of the multilayered flexible heaters described above) sitting on the upper surface of the heater support 4168 before the flexible heater 4106 is formed over the upper surface of the heater support 4168 using, for example, the forming process described above with reference to Figures 18 or 19. Once formed, the resulting assembly (as shown in Figure 41 b) has the connection tabs 4150-1 and 4150-2 of the flexible heater 4106 (carrying the terminals of the heater electrodes and the fusing circuitry) folded under the heater support 4168. The connection tabs 4150 are preferably adhered to the underside of the heater support 4168. The rigid PCB 4178 (carrying the drive and control circuitry) has a plurality of surface mount spring fingers 4152 - one for each electrical connection to be made between the circuitry on the rigid PCB 4178 and the terminals of the heater electrodes and the fusing circuitry that are on the connection tabs 4150. The rigid PCB 4178 with spring fingers 4152 is then attached to the bottom of the heater support 4168 (as illustrated in Figure 41c) such that the spring fingers 4152 are aligned and in contact with the corresponding terminals on the connection tabs 4150. The rigid PCB can be attached to the heater support 4168 using screws, retention clips, friction fit etc. The attachment of the rigid PCB 4178 preferably partially compresses the spring fingers 4152 (as shown in Figure 41 d), that enables them to absorb any dimensional variations (piece to piece tolerancing, or thermal expansion and contraction) while maintaining a consistent contact pressure (and hence contact resistance) on the terminals of the heater electrodes and the fusing circuitry. Preferably ingress protection would be provided around the rigid PCB and spring fingers to prevent dirt or liquids from influencing the stability of the contact.

As shown in Figures 42c and 41 d, the spring fingers have a general “V” shape and are made of an electrically conductive material such as steel. The shape and thickness of the material forming the spring fingers 4152 provides the spring fingers 4152 with “spring” or resilience. Other shapes of spring fingers 4152 could of course be used.

The use of such spring fingers 4152 offers a number of advantages:

- they are an order of magnitude less expensive than FCC connectors;

- they are easy to incorporate in a mass-produced product such as a hair sty ler as they may be “pick and placed” on the rigid PCB with solder paste (a post process in a reflow oven ensures secure contact between the rigid fingers and the terminals on the rigid PCB);

- Having the only solder connection in the assembly on the rigid PCB offers some isolation between the heat generating heater 4106 and the solder (which aids in longevity);

- The flexibility of the spring fingers 4152 should absorb the dimensional variations caused by thermal expansion and contraction of the rest of the assembly; and

- Assuming there is no dirt or moisture ingress, the spring fingers 4152 should maintain a connection of constant force across a range of temperatures, maintaining a constant contact resistance. Modifications and alternatives

Detailed embodiments and some possible alternatives have been described above. As those skilled in the art will appreciate, a number of modifications and further alternatives can be made to the above embodiments whilst still benefiting from the inventions embodied therein. It will therefore be understood that the invention is not limited to the described embodiments and encompasses modifications apparent to those skilled in the art lying within the scope of the claims appended hereto.

The invention has been described above by way of implementation in a hair styling device for straightening hair (‘hair straighteners’) which employ flat hair styling heaters 106. However, it could alternatively be implemented in any form of hair styling device, such as (but not limited to) crimpers, curlers or heated brushes. The heaters 106 may define a heating surface that is flat, curved, ridged or in the shape of a barrel. The hair styling device may have two arms like the device illustrated in Figure 1 or it may be a single armed device. The heaters described above may also be used in hair dryers or in combination devices that use conductive heating and air to dry and style the user’s hair (such as those described in the applicant’s earlier PCT application WO 2021/019239). In embodiments where air is used, the heaters 106 may be perforated so that air passes through the heater and is warmed by the heater as the air passes through.

In the above embodiments, Metal Oxide Semiconductor Field Effect T ransistor (MOSFET) switches were used to control powering and sensing of the heater electrodes. As those skilled in the art will appreciate, other switches could be used instead. For example, Field Effect Transistors (FETs) could be used, such as Gallium Nitride FETs or bipolar junction transistors (BJTs).

In the examples described above, the heater electrodes were described as following a serpentine path. As those skilled in the art will appreciate, the use of the word serpentine is intended to encompass any path within the heating zone, that may be a tortuous path, that achieves a desired resistance within the heating zone whilst maintaining as far as possible a consistent current density. The paths may be designed to have a substantially uniform thickness in the width (and depth) direction so as to maintain consistent current density and thereby reduce the occurrence of hot spots.

In the above examples, a heat spreading layer was provided having individual heat spreaders corresponding to the individual heater electrodes provided in the heater electrode layer. As those skilled in the art will appreciate, this is not essential. In examples where the heater electrodes are arranged in side by side rows of heater electrodes each extending along the length of the heater (like those shown in, for example, Figure 13), one heat spreader may be provided for the heater electrodes in each row.

In the preferred control circuitry described above, the fuses were coupled between the master switch and a reference potential (ground). As those skilled in the art of circuit design will appreciate, the fuses may be coupled to the master switch in many different ways. For example, the fuses could be coupled between a supply reference potential (e.g. 5V) and the control gate of a control switch, the output of which is connected to the control gate of the master switch. In this case, when one of the fuses melts, this breaks the connection between the control switch and the reference potential causing the control switch to change state, which change of state causes the master switch to also change state, thereby preventing power from being supplied to one or more of the heater electrodes. Other arrangements are of course possible.

In the preferred heater arrangement described above, eight fuses were provided to protect sixteen heating zones. As those skilled in the art will appreciate, one fuse may be provided for each heating zone or indeed, one fuse may be provided for three or more heating zones. With heating zones arranged along the length and the width of the heater, one fuse for four heating zones works well.

Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “containing”, means “including but not limited to”, and is not intended to (and does not) exclude other components, integers or steps.

The expressions “to dry hair”, “drying hair” or “decrease a moisture level of hair” and the like, as used in the present disclosure, can refer both to the removal of “unbound” water that exists on the outside of hair when wet, or the removal of “bound” water, which exists inside individual hairs, and which can be interacted with when heat styling hair. The “bound” water need not necessarily be removed when drying hair, although removal of some bound water may occur during a drying or styling process.

Various other modifications will be apparent to those skilled in the art and will not be described in further detail here.

Various examples have been described above. The following numbered clauses summarise one or more aspects of some of those examples:

Aspect 1

1. A hair drying and/or styling appliance including a multilayer heater comprising a plurality of layers that are bonded together, wherein the multilayer heater includes: a hair contacting layer; a heater electrode layer comprising a heater electrode formed of a conductive material that generates heat when a current is passed through the heater electrode; and a plurality of dielectric layers interposed between the heater electrode layer and the hair contacting layer, wherein at least one dielectric layer is in contact with the heater electrode layer, and at least a portion of each dielectric layer that is not in contact with the heater electrode layer is exposed in the direction of the heater electrode layer.

2. The appliance according to clause 1 , wherein the plurality of dielectric layers form a staircase structure.

3. The appliance according to clauses 1 or 2, wherein the hair contacting layer forms a staircase structure.

4. The appliance according to any of the preceding clauses, wherein at least a portion of each dielectric layer is in contact with the hair contacting layer.

5. The appliance according to any of the preceding clauses, wherein the hair contacting layer further comprises a coating layer.

6. The appliance according to any of the preceding clauses, wherein at least a portion of the hair contacting surface is flat, curved, and/or ribbed.

7. The appliance according to any of the preceding clauses, wherein at least a portion of the multilayer heater is flexible.

8. The appliance according to clause 7, wherein the multilayer heater is held within a curved housing.

9. The appliance according to any of the preceding clauses, wherein the heater electrode layer comprises one or more heater elements.

10. The appliance according to clause 9, wherein each of the one or more heater elements comprises a serpentine heater track with heater track contacts provided at either end of the serpentine track, or wherein the heater element comprises a busbar heater arrangement having a pair of busbar electrodes and an electrically conductive portion extending between the busbar electrodes.

11 . The appliance according to any of clauses 9 or 10, wherein the heater electrode layer comprises a plurality of independently powerable heater elements that define a corresponding plurality of heating zones on a heating surface of the multilayer heater.

12. The appliance according to any of the preceding clauses, wherein the plurality of dielectric layers have a total thickness of less than 0.6mm.

13. The appliance according to any of the preceding clauses, wherein the plurality of dielectric layers each have a dielectric breakdown voltage greater than 1.75kV at a leakage current of 100mA.

14. The appliance according to any of the preceding clauses, wherein at least one dielectric layer is directly bonded with at least one of the hair contacting layer, another dielectric layer of the plurality of dielectric layers, or the heater electrode layer.

15. The appliance according to any preceding clause, wherein the multilayer heater is flexible and is mounted to a rigid support, wherein terminals of the one or more heater electrodes are provided on at least one connection tab that folds under the rigid support. 16. The appliance according to clause 15, wherein a rigid circuit board is provided under the rigid support and wherein a plurality of spring fingers are provided for making an electrical connection between terminals on the rigid circuit board and the terminals of the one or more heater electrodes provided on said connection tabs.

17. A method of making a hair drying and/or styling appliance having a multilayer heater comprising a plurality of layers that are bonded together, wherein the method comprises: providing a hair contacting layer; providing a heater electrode layer comprising a heater electrode formed of a conductive material that generates heat when a current is passed through the heater electrode; and interposing a plurality of dielectric layers between the heater electrode layer and the hair contacting layer such that at least one dielectric layer is in contact with the heater electrode layer, and arranging the dielectric layers so that at least a portion of each dielectric layer that is not in contact with the heater electrode layer is exposed in the direction of the heater electrode layer.

Aspect 2

1 . A hair drying and/or styling appliance comprising: a heater for providing heat for drying and/or styling hair; a case; and a cooling component; wherein the heater is a multilayer heater comprising a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes: a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; and at least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer; wherein the multilayer heater has a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 30 pm and 2 mm; and wherein the cooling component is configured actively to cool the case during styling.

2. The appliance of clause 1 , wherein the case is arranged at least partially surrounding the heater.

3. The appliance of clause 1 or 2, wherein the case is arranged to support the heater. 4. The appliance of any preceding clause, wherein the heater is arranged on a first face of the appliance and the case is located on at least one further face of the appliance.

5. The appliance of any preceding clause, wherein the appliance comprises an arm, and the heater and the case are both provided on the arm; preferably wherein the heater is arranged on a first surface of the arm and the case is located on at least one further face of the appliance.

6. The appliance of any preceding clause, wherein the cooling component is configured to cool the case via heat exchange.

7. The appliance of any preceding clause, wherein the cooling component comprises a fluid within a conduit adjacent an inner surface of the case.

8. The appliance of any preceding clause, wherein the cooling component comprises a fan device for moving gas, preferably wherein the gas is air.

9. The appliance of clause 8, wherein the appliance comprises two arms joined by a shoulder, and wherein the fan is provided in the shoulder and configured for moving gas along one or both the arms.

10. The appliance of clause 8 or 9, wherein the fan device is configured for moving gas within an inner cavity of the case.

11 . The appliance of clause 10, wherein the case comprises apertures arranged such that air can flow from the inner cavity of the case to outside of the case.

12. The appliance of clause 11 , wherein the apertures are formed as slots, preferably wherein the slots are arranged parallel to the length of the appliance, more preferably parallel to the length of an arm of the appliance.

13. The appliance of any of clauses 8 to 12, further comprising ribs protruding from a surface of the case, preferably protruding radially inwards from an inner surface of the case.

14. The appliance of clause 13, wherein the ribs are arranged such that they run along a direction parallel to the length of the appliance, preferably the length of an arm of the appliance.

15. The appliance of clause 7, wherein the fluid is a liquid, preferably a coolant liquid.

16. The appliance of any preceding clause, wherein the cooling component comprises a thermoelectric cooling element.

17. The appliance of any preceding clause, further comprising at least one support within the case to support the heater, preferably wherein the support is arranged to stabilize the heater relative to an or the inner cavity of the case. 18. The appliance of any preceding clause, wherein the heater comprises a plurality of independently controllable heating zones, and preferably wherein the cooling component comprises a plurality of independently controllable cooling zones.

19. The appliance of any preceding clause, wherein a combined thermal conductivity of the multilayer heater in a plane perpendicular to the thickness is less than 15 W/m.K and greater than 0.1 W/m.K.

20. The appliance according to any preceding clause, wherein the multilayer heater is flexible and is mounted to a rigid support, wherein terminals of the one or more heater electrodes are provided on at least one connection tab that folds under the rigid support.

21. The appliance according to clause 20, wherein a rigid circuit board is provided under the rigid support and wherein a plurality of spring fingers are provided for making an electrical connection between terminals on the rigid circuit board and the terminals of the one or more heater electrodes provided on said connection tabs.

22. A method of operating a hair drying and/or styling appliance comprising a heater configured to heat hair for styling, the heater being arranged in a case, wherein the method comprises heating the heater while simultaneously cooling the case.

23. The method of clause 22, wherein the cooling the case comprises operating an active heat exchange mechanism, preferably provided within an interior of the appliance.

24. The method of clause 23, wherein the active heat exchange mechanism comprises at least one of: a fan for facilitating flow of a gas, preferably air; a liquid cooling system; and a thermoelectric cooling system.

25. The method of any of clauses 22 to 24, further comprising directly cooling the hair by facilitating flow of a gas, preferably air, out of the case.

26. The method of any of clauses 22 to 25, wherein the hair drying and/or styling appliance is the hair drying and/or styling appliance of any of clauses 1 to 21 .

27. A computer program product comprising computer implementable instructions for causing a programmable device to carry out the method of any of clauses 22 to 26.

Aspect 3

1 . A hair drying and/or styling appliance comprising a multilayer heater comprising a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes: a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; and at least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer; wherein the multilayer heater has a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 30pm and 2mm; and wherein a combined thermal conductivity of the multilayer heater in a plane perpendicular to the thickness that is less than 300 W/m.K and greater than 15 W/m.K.

2. The appliance according to clause 1 , comprising a fan for creating an air flow that passes by or through apertures in the multilayer heater and is heated by the multilayer heater.

3. The appliance according to any of clauses 1 or 2, wherein the heater has a power density that is greater than 2 W/cm² and less than 100 W/cm², preferably greater than 8 W/cm².

4. The appliance according to any of clauses 1 to 3, wherein the heater electrode layer comprises a plurality of independently powerable heater electrodes that define a corresponding plurality of heating zones on a heating surface of the multilayer heater.

5. The appliance according to clause 4, wherein the combined thermal conductivity of the multilayer heater in the plane perpendicular to the thickness is measured along a line that passes through adjacent heating zones.

6. The appliance according to clause 4 or 5, wherein a maximum size of each heating zone depends upon the power density of the multilayer heater, the thickness of the multilayer heater and a lateral conductivity of the multilayer heater.

7. The appliance according to clause 6, wherein the maximum size of each heating zone further depends on a maximum permissible temperature difference between different parts of the heating zone in the case where a heating zone is partially loaded with hair.

8. The appliance according to any of clauses 4 to 7, wherein the multilayer heater further comprises at least one heat spreading layer provided over the upper dielectric layer and/or under the heater electrode layer, the heat spreading layer comprising a plurality of heat spreaders that regularise the heating provided within the heating zones.

9. The appliance according to clause 8, wherein each heat spreader is formed as an island that does not touch neighbouring heat spreaders to reduce heat spreading from one heating zone to an adjacent heating zone.

10. The appliance according to clause 8 or 9, wherein each heat spreader is formed of a metal.

11 . The appliance according to any of clauses 8 to 10, wherein the heat spreaders are separated from each other in the plane perpendicular to the thickness by a solid or semi- solid material, whose thermal conductivity is lower than 35 W/mK and most preferably lower than 0.3 W/mK.

12. The appliance according to any of clauses 1 to 11 , wherein the layers of the multilayer heater are bonded together to have a peel strength of at least 0.35 Newtons per mm.

13. The appliance according to any of clauses 1 to 12, wherein the multilayer heater further comprises one or more of: i) a low friction coating an upper surface of which provides a hair contacting surface of the multilayer heater; ii) a lower dielectric layer provided under the heater electrode layer; and iii) an auxiliary heater electrode layer comprising one or more heater electrodes provided below the heater electrode layer and a dielectric layer provided between the heater electrode layer and the auxiliary heater electrode layer.

14. The appliance according to any of clauses 1 to 13, wherein one or more layers of the multilayer heater are bonded together using an adhesive or using heat bonding or using physical vapour deposition or using screen printing or another coating process.

15. The appliance according to any of clauses 1 to 14, wherein one or more of the dielectric layers comprises polyimide.

16. The appliance according to any of clauses 1 to 15, wherein the multilayer heater is flexible and is bonded to a rigid structure to provide the multilayer heater with rigidity.

17. The appliance according to any of clauses 1 to 16, wherein the multilayer heater has a flat, curved and/or ribbed heating surface.

18. The appliance according to any of clauses 1 to 17, wherein the multilayer heater provides a flat heating surface and has curved edges that provide a curved heating surface.

19. The appliance according to any of clauses 1 to 18, further comprising a controller configured to control the application of power to the multilayer heater to control the heat produced by the multilayer heater.

20. The appliance according to any of clauses 1 to 19, wherein the appliance is a single arm or a two arm device.

21 . The appliance according to any preceding clause, wherein the multilayer heater is flexible and is mounted to a rigid support, wherein terminals of the one or more heater electrodes are provided on at least one connection tab that folds under the rigid support.

22. The appliance according to clause 21 , wherein a rigid circuit board is provided under the rigid support and wherein a plurality of spring fingers are provided for making an electrical connection between terminals on the rigid circuit board and the terminals of the one or more heater electrodes provided on said connection tabs.

23. A method of making a hair drying and/or styling appliance, the method comprising: providing a multilayer heater having a plurality of functional layers that are bonded together; mounting the multilayer heater in the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating; wherein providing the multilayer heater includes: providing a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; and providing at least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer from the hair contacting surface; wherein the multilayer heater has a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 30pm and 2mm; and wherein a combined thermal conductivity of the multilayer heater in a plane perpendicular to the thickness that is less than 300 W/m.K and greater than 15 W/m.K.

Aspect 4

1. A hair drying and/or styling appliance comprising a multilayer heater having a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes: a heater electrode layer comprising a plurality of independently powerable heater electrodes formed of an electrically conductive material that generates heat when a current is passed through them, wherein the plurality of heater electrodes are arranged sequentially along a length of the multilayer heater and define a corresponding plurality of heating zones arranged along the length of the hair contacting surface of the multilayer heater; and at least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer; and wherein the number of heating zones per centimetre of length of the multilayer heater is between 0.6 and 2.5.

2. The hair drying and/or styling appliance according to clause 1 , wherein the multilayer heater has a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 75pm and 300pm.

3. The hair drying and/or styling appliance according to clause 1 or 2, wherein an average thermal conductivity of the layers forming the multilayer heater is less than 300 W/m.K and greater than 80 W/m.K. 4. The hair drying and/or styling appliance according to clause 3, wherein the average thermal conductivity is averaged through the thickness of the multilayer heater.

5. The hair drying and/or styling appliance according to any of clauses 1 to 4, operable to provide a power density of between 4 WcnT² and 25 Wcm'² to heat hair passing over the hair contacting surface.

6. The hair drying and/or styling appliance according to any of clauses 1 to 5, wherein the maximum permitted temperature of a heating zone is less than 250°C.

7. The appliance according to any of clauses 1 to 6, wherein the multilayer heater further comprises a heat spreading layer, the heat spreading layer comprising a plurality of heat spreaders that regularise the heating provided within the heating zones.

8. The appliance according to clause 7, wherein each heat spreader is formed as an island to reduce heat spreading from one heating zone to an adjacent heating zone.

9. The appliance according to clause 8, wherein the heat spreaders are formed as interconnected islands that are electrically interconnected with and thermally decoupled from neighbouring islands or wherein adjacent heat spreaders do not touch neighbouring heat spreaders.

10. The appliance according to any of clauses 7 to 9, wherein each heat spreader is formed of a metal.

11 . The appliance according to any of clauses 7 to 10, wherein the heat spreaders are separated from each other in the plane perpendicular to the thickness by a solid or semisolid material, whose thermal conductivity is lower than 35 W/mK and most preferably lower than 0.3 W/mK.

12. The appliance according to any of clauses 1 to 11 , wherein the multilayer heater further comprises one or more of: i) a low friction coating an upper surface of which provides said hair contacting surface of the multilayer heater; ii) a lower dielectric layer provided under the heater electrode layer; and iii) an auxiliary heater electrode layer comprising one or more heater electrodes provided below the heater electrode layer and a dielectric layer provided between the heater electrode layer and the auxiliary heater electrode layer.

13. The appliance according to any of clauses 1 to 12, wherein one or more layers of the multilayer heater are bonded together using an adhesive or using heat bonding or using physical vapour deposition or using screen printing or another coating process.

14. The appliance according to any of clauses 1 to 13, wherein one or more of the dielectric layers comprises polyimide. 15. The appliance according to any of clauses 1 to 14, wherein the multilayer heater is flexible and is bonded to a rigid structure to provide the multilayer heater with rigidity.

16. The appliance according to any of clauses 1 to 15, wherein the multilayer heater has a flat, curved and/or ribbed heating surface.

17. The appliance according to any of clauses 1 to 16, wherein the multilayer heater provides a flat heating surface and has curved edges that provide a curved heating surface.

18. The appliance according to any of clauses 1 to 17, further comprising a controller configured to control the application of power to the multilayer heater to control the heat produced by the multilayer heater.

19. The appliance according to any of clauses 1 to 18, wherein the appliance is a single arm or a two arm device.

22. The appliance according to any of clauses 1 to 21 , wherein the upper dielectric layer has a dielectric breakdown strength greater than 500 volts and a thermal impedance between 9.35 x 10'⁴ KW¹cm² and 0.8 KW¹cm².

23. A method of making a hair drying and/or styling appliance, the method comprising: providing a multilayer heater having a plurality of functional layers that are bonded together; mounting the multilayer heater in the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating; wherein providing the multilayer heater includes: providing a heater electrode layer comprising a plurality of independently powerable heater electrodes formed of a conductive material that generates heat when a current is passed through them, wherein the plurality of heater electrodes are arranged sequentially along a length of the multilayer heater and define a corresponding plurality of heating zones arranged along the length of the hair contacting surface of the multilayer heater; and providing at least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer; and wherein the number of heating zones per centimetre of length of the multilayer heater is between 0.6 and 2.5.

Aspect 5

1. A hair drying and/or styling appliance comprising a multilayer heater having a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes: a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; and at least one upper dielectric layer over the heater electrode layer to electrically insulate the heater electrode layer; wherein the multilayer heater has a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 30pm and 2mm; and wherein an upper surface of the dielectric layer and/or a coating applied to the upper surface of the dielectric layer provides the hair contacting surface of the multi-layer heater.

3. The appliance according to clause 1 or 2, wherein the heater has a power density that is greater than 2 W/cm² and less than 100 W/cm², preferably greater than 8 W/cm².

4. The appliance according to any of clauses 1 to 4, wherein the dielectric layer is mounted directly on to an upper surface of the heater electrode layer.

5. The appliance according to any of clauses 1 to 3, wherein the multilayer heater further comprises a sensor layer comprising a conductive track whose resistance varies with temperature, wherein the dielectric layer is mounted directly on an upper surface of the sensor layer.

6. The appliance according to clause 5, wherein a second dielectric layer is provided between the sensor layer and the heater electrode layer.

7. The appliance according to any of clauses 1 to 6, wherein the heater electrode layer comprises a plurality of independently controllable heater electrodes that define a corresponding plurality of heating zones on the hair contacting surface of the multilayer heater. 8. The appliance according to clause 7, wherein the multilayer heater further comprises at least one heat spreading layer provided under the heater electrode layer, the heat spreading layer comprising a plurality of heat spreaders that regularise the heating provided within the heating zones.

9. The appliance according to clause 8, wherein each heat spreader is formed as an island that does not touch neighbouring heat spreaders to minimise heat spreading from one heating zone to an adjacent heating zone.

11 . The appliance according to any of clauses 8 to 10, wherein the heat spreaders are separated from each other in the plane perpendicular to the thickness by a solid or semisolid material, whose thermal conductivity is lower than 35 W/m.K and preferably lower than 0.3 W/m.K.

13. The appliance according to any of clauses 1 to 12, wherein the multilayer heater further comprises one or more of: ii) a lower dielectric layer provided under the heater electrode layer; and iii) an auxiliary heater electrode layer comprising one or more heater electrodes provided below the heater electrode layer and a dielectric layer provided between the heater electrode layer and the auxiliary heater electrode layer.

18. The appliance according to any of clauses 1 to 17, wherein the multilayer heater provides a flat heating surface and has curved edges that provide a curved heating surface. 19. The appliance according to any of clauses 1 to 18, further comprising a controller configured to control the application of power to the multilayer heater to control the heat produced by the multilayer heater.

20. The appliance according to any of clauses 1 to 19, wherein the appliance is a one arm or a two arm device comprising a handle.

23. A method of making a hair drying and/or styling appliance, the method comprising: providing a multilayer heater having a plurality of functional layers that are bonded together; mounting the multilayer heater in the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating; wherein providing the multilayer heater includes: providing a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; and providing at least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer from the hair contacting surface; wherein the multilayer heater has a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 30pm and 2mm; and wherein an upper surface of the dielectric layer and/or a coating applied to the upper surface of the dielectric layer provides the hair contacting surface of the multi-layer heater.

Claims

1. A hair drying and/or styling appliance comprising a multilayer heater having a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes: a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; and at least one upper dielectric layer over the heater electrode layer to electrically insulate the heater electrode layer; wherein an upper surface of the dielectric layer provides the hair contacting surface of the multi-layer heater.

2. The appliance according to claim 1 , wherein the at least one dielectric layer is formed as a coating on an upper surface of the heater electrode layer.

3. The appliance according to claim 2, wherein the coating is applied as a spray, a wash, physical vapour deposition, sputtering or evaporation.

4. The appliance according to any of claims 1 to 3, wherein the heater has a power density that is greater than 2 W/cm² and less than 100 W/cm², preferably greater than 8 W/cm².

5. The appliance according to any of claims 1 to 4, wherein the dielectric layer is mounted directly on to an upper surface of the heater electrode layer.

6. The appliance according to any of claims 1 to 5, wherein one or more of the heater electrodes are formed of a conductive material whose resistance varies with the temperature of the heater, whereby the temperature of the hair contracting surface can be determined by measuring the resistance of the one or more heater electrodes.

7. The appliance according to any of claims 1 to 6, wherein the heater electrode layer comprises a plurality of independently controllable heater electrodes that define a corresponding plurality of heating zones on the hair contacting surface of the multilayer heater.

8. The appliance according to claim 7, wherein the plurality of independently controllable heater electrodes are arranged in a two dimensional array along the length and width of the heater.

9. The appliance according to claim 8, wherein the plurality of independently controllable heater electrodes are arranged in two rows extending along the length of the heater.

10. The appliance according to any of claims 7 to 9, wherein the multilayer heater further comprises at least one heat spreading layer provided under the heater electrode layer.

11 . The appliance according to claim 10, wherein a second dielectric layer is provided between the heater electrode layer and the heat spreading layer.

12. The appliance according to claim 10or 11 , wherein the heat spreading layer comprises a plurality of heat spreaders that regularise the heating provided within the heating zones.

13. The appliance according to claim 12 when dependent on claim 9, wherein one heat spreader is provided for each row of independently controllable heater electrodes.

14. The appliance according to claim 12, wherein at least one heat spreader is provided for each heater electrode.

15. The appliance according to any of claims 12 to 14, wherein each heat spreader is formed as an island that does not touch neighbouring heat spreaders to minimise heat spreading from one heating zone to an adjacent heating zone.

16. The appliance according to any of claims 12 to 15, wherein each heat spreader is formed of a metal.

17. The appliance according to any of claims 12 to 16, wherein the heat spreaders are separated from each other in the plane perpendicular to the thickness by a solid or semisolid material, whose thermal conductivity is lower than 35 W/m.K and preferably lower than 0.3 W/m.K.

18. The appliance according to any preceding claim, wherein the multilayer heater further comprises an auxiliary heater electrode layer comprising one or more heater electrodes provided below the heater electrode layer and a dielectric layer provided between the heater electrode layer and the auxiliary heater electrode layer.

19. The appliance according to any of claims 1 to 18, wherein the multilayer heater is flexible and is bonded to a rigid structure to provide the multilayer heater with rigidity.

20. The appliance according to any of claims 1 to 19, wherein the multilayer heater has a flat, curved and/or ribbed heating surface.

21. The appliance according to any of claims 1 to 20, wherein the multilayer heater provides a flat heating surface and has curved edges that provide a curved heating surface.

22. The appliance according to any of claims 1 to 21 , further comprising a controller configured to control the application of power to the multilayer heater to control the heat produced by the multilayer heater.

23. The appliance according to any of claims 1 to 22, wherein the appliance is a one arm or a two arm device comprising a handle.

24. The appliance according to any preceding claim, wherein the multilayer heater is flexible and is mounted to a rigid support, wherein terminals of the one or more heater electrodes are provided on at least one connection tab that folds under the rigid support.

25. The appliance according to claim 24, wherein a rigid circuit board is provided under the rigid support and wherein a plurality of spring fingers are provided for making an electrical connection between terminals on the rigid circuit board and the terminals of the one or more heater electrodes provided on said connection tabs.

26. The appliance according to any of claims 1 to 25, wherein the upper dielectric layer has a dielectric breakdown strength greater than 500 volts and a thermal impedance between 9.35 x 10'⁴ KW¹cm² and 0.8 KW¹cm².

27. A method of making a hair drying and/or styling appliance, the method comprising: providing a multilayer heater having a plurality of functional layers that are bonded together; mounting the multilayer heater in the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating; wherein providing the multilayer heater includes: providing a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; and providing at least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer from the hair contacting surface; wherein an upper surface of the dielectric layer provides the hair contacting surface of the multi-layer heater.

28. A hair drying and/or styling appliance comprising a multilayer heater having a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes: a heater electrode layer comprising a plurality of independently powerable heater electrodes formed of an electrically conductive material that generates heat when a current is passed through them, wherein the plurality of heater electrodes are arranged sequentially along a length of the multilayer heater and define a corresponding plurality of heating zones arranged along the length of the hair contacting surface of the multilayer heater; and at least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer; and wherein the number of heating zones per centimetre of length of the multilayer heater is between 0.6 and 2.5.

29. The appliance according to any of claim 28, wherein the upper dielectric layer has a dielectric breakdown strength greater than 500 volts and a thermal impedance between 9.35 x 10’⁴ KW¹cm² and 0.8 KW¹cm².

30. The appliance according to claim 28 or 29, wherein the multilayer heater has a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 75pm and 300pm.

31. The appliance according to any of claims 28 to 30, wherein an average thermal conductivity of the layers forming the multilayer heater is less than 300 W/m.K and greater than 80 W/m.K.

32. The appliance according to claim 31 , wherein the average thermal conductivity is averaged through the thickness of the multilayer heater.

33. The appliance according to any of claims 28 to 32, wherein the heater electrodes are configured to provide a power density of between 4 WcnT² and 25 Wcm'²to heat hair passing over the hair contacting surface.

34. The appliance according to any of claims 28 to 33, wherein the maximum permitted temperature of a heating zone is less than 250°C.

35. The appliance according to any of claims 28 to 34, wherein the multilayer heater further comprises an auxiliary heater electrode layer comprising one or more heater electrodes provided below the heater electrode layer and a dielectric layer provided between the heater electrode layer and the auxiliary heater electrode layer.

36. A method of making a hair drying and/or styling appliance, the method comprising: providing a multilayer heater having a plurality of functional layers that are bonded together; mounting the multilayer heater in the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating; wherein providing the multilayer heater includes: providing a heater electrode layer comprising a plurality of independently powerable heater electrodes formed of a conductive material that generates heat when a current is passed through them, wherein the plurality of heater electrodes are arranged sequentially along a length of the multilayer heater and define a corresponding plurality of heating zones arranged along the length of the hair contacting surface of the multilayer heater; and providing at least one upper dielectric layer over the heater electrode layer to electrically isolate the heater electrode layer; and wherein the number of heating zones per centimetre of length of the multilayer heater is between 0.6 and 2.5.

37. A multilayer heater for use in a hair drying and/or styling appliance, the multilayer heater having a plurality of functional layers that are bonded together, wherein the multilayer heater provides a hair contacting surface to heat hair that comes into contact with the multilayer heater, wherein the multilayer heater includes: a heater electrode layer comprising one or more heater electrodes formed of a first conductive material that generates heat when a current is passed through the one or more heater electrodes; a heat spreading layer comprising one or more heat spreaders, each heat spreader being formed of a second conductive material that is different to the first conductive material; and at least one dielectric layer sandwiched between the heater electrode layer and the heat spreading layer.

38. The multilayer heater according to claim 37, wherein the heater electrode layer comprises a plurality of independently powerable heater electrodes formed of an electrically conductive material that generates heat when a current is passed through them, wherein the plurality of heater electrodes are arranged sequentially along a length of the multilayer heater and define a corresponding plurality of heating zones arranged along the length of the hair contacting surface of the multilayer heater.

39. The multilayer heater according to claim 38, wherein the heat spreader layer comprises a plurality of heat spreaders.

40. The multilayer heater according to any of claims 37 to 39, wherein the multilayer heater has a thickness, as measured across all of the plurality of layers of the multilayer heater, which is between 30pm and 2mm and preferably between 75pm and 300pm.

41. The multilayer heater according to any of claims 37 to 40, wherein the first conductive material comprises steel and the second conductive material comprises copper.

42. The multilayer heater according to any of claims 37 to 41 , comprising an upper dielectric layer provided on a surface of the heater electrode layer and having a dielectric breakdown strength greater than 500 volts and a thermal impedance between 9.35 x 10'⁴ KW¹cm² and 0.8 KW¹cm².

43. A hair drying and/or styling appliance comprising a multilayer heater having a plurality of functional layers that are bonded together, wherein the multilayer heater is mounted within the appliance so that during use of the appliance by a user, hair contacts a hair contacting surface of the multilayer heater and is heated by conductive heating, wherein the multilayer heater includes: a heater electrode layer comprising one or more heater electrodes formed of a conductive material that generates heat when a current is passed through the one or more heater electrodes; at least one upper dielectric layer over the heater electrode layer to electrically insulate the heater electrode layer; wherein the heater is supported within a housing of the appliance by a rigid support; wherein terminals of the one or more heater electrodes are provided on connection tabs that fold under the rigid support, wherein a rigid circuit board is provided under the rigid support that carries drive and control circuitry for controlling the heating of the multilayer heater, and wherein a plurality of spring fingers are provided for making an electrical connection between terminals on the rigid circuit board and the terminals of the one or more heater electrodes provided on said connection tabs.