GB2585882A

GB2585882A - Electrochemical probe

Info

Publication number: GB2585882A
Application number: GB1910362.1A
Authority: GB
Inventors: Racz Romeo-Robert; Bulz Ciprian; Racz Gabriella; Schaefer Andreas
Original assignee: Francis Crick Institute Ltd
Current assignee: Francis Crick Institute Ltd
Priority date: 2019-07-19
Filing date: 2019-07-19
Publication date: 2021-01-27
Anticipated expiration: 2039-07-19
Also published as: GB201910362D0; WO2021014116A1; GB2585882B

Abstract

An electrochemical probe (e.g. 6, Fig. 14) has a plurality of wire electrodes, each having a conductive core and outer insulating layer of no more than 40 μm diameter, arranged alongside each other. The probe is formed by laying the electrodes into channels 5 disposed, maybe by photolithography, on the surface of a sheet of material 4, e.g. a flexible polymer. The sheet is manipulated into a bundle: in a cross-section of the bundle, the sheet traces out a predetermined shape, and the distribution of electrodes depend on that shape and on a predetermined spacing (e.g. 20 μm) of the channels in the sheet. Sheets may be stacked, folded or rolled to form multiple layers in the bundle, perhaps with a hollow channel. The lengths of protruding ends of the wires may be controlled, e.g. tapered. Metal or metal oxide nanostructures may be deposited on the tips of the wires.

Description

ELECTROCHEMICAL PROBE

The present technique relates to the field of electrochemical probes.

Probes for sensing electrical and chemical events of biological systems can be useful for a range of applications, including electrochemical microscopy, a range of in vivo and/or in vitro bioelectrical event recordings, a range of determinations of biologically significant substance/substances (e.g. proteins, neurotransmitters, hydrogen peroxide, calcium, nitric oxide, DNA) or toxicologically relevant substance/substances (e.g.: heavy metals) and electrophysiological, extracellular and intracellular electrophysiological applications, tumor scanning and electrotherapy or cardiovascular scanning, for example.

At least some examples provide a method of forming an electrochemical probe, the electrochemical probe comprising a wire bundle comprising a plurality of wire electrodes arranged alongside each other, and the method comprising: laying a plurality of said wire electrodes into channels disposed across a surface of at least one sheet of material with a predetermined spacing, the wire electrodes each comprising an inner conductive core and an outer insulating layer, and the outer insulating layer having a diameter less than or equal to 40 pm; and manipulating the at least one sheet of material holding the plurality of the wire electrodes to form the wire bundle having a cross-section in which an intersection of said at least one sheet of material with the cross-section of the wire bundle traces out a predetermined shape, wherein a two-dimensional distribution of points at which the wire electrodes intersect the cross-section of the wire bundle is dependent on the predetermined spacing of the channels across the surface of the at least one sheet and the predetermined shape.

At least some examples provide an electrochemical probe comprising: a wire bundle comprising a plurality of wire electrodes arranged alongside each other, the wire electrodes each comprising an inner conductive core and an outer insulating layer, and the outer insulating layer having a diameter less than or equal to 40 pm; wherein the wire bundle is formed of at least one sheet of material holding a plurality of said wire electrodes in channels disposed across the surface of the at least one sheet of material with a predetermined spacing; and wherein the wire bundle has a cross-section in which an intersection of said at least one sheet of material with a cross-section of the wire bundle traces out a predetermined shape, wherein a two-dimensional distribution of points at which the wire electrodes intersect the cross-section of the wire bundle is dependent on the predetermined spacing of the channels across the surface of the at least one sheet and the predetermined shape.

Further aspects, features and advantages of the present technique will be apparent from the following description of examples, which is to be read in conjunction with the accompanying drawings.

Figure 1 schematically illustrates a wire electrode; Figure 2 schematically illustrates a template for holding wire electrodes; Figures 3 to 5 are images showing how wire electrodes may be arranged to provide different front-end profiles for the probe; Figure 6 to 8 are images showing examples of how a template can be manipulated to form a wire bundle; Figures 9 and 10 schematically illustrate further examples of how a template can be manipulated to form a wire bundle; Figure 11 is a flow diagram illustrating a method of forming a wire bundle; Figure 12 is a flow diagram illustrating a method of forming a template and manipulating it to form a wire bundle; Figures 13 to 15 are images showing examples of wire bundles; Figures 16 to 18 schematically illustrate examples of how a template can be manipulated to form a wire bundle; Figure 19A schematically illustrates a technique for manufacturing a mould to be used in manufacturing a template; Figure 19B schematically illustrates a technique for manufacturing a template using the mould of Figure 19A; Figure 20 schematically illustrates an example of an electrochemical probe comprising a bundle of wire electrodes surrounded in insulating material with impedance reducing layers made of gold nano-structures deposited on the tips of the wires at the front and back ends, and an iridium oxide functionalization layer deposited on the gold nano-structures at the front end of the wires; Figure 21 shows an image of the wires before and after depositing the gold nano-25 structures; Figure 22 shows an example of sharpened tips of the wires; Figure 23 is a flow diagram illustrating a method of manufacturing an electrochemical probe; Figure 24 shows an example where a recess is formed in the tips of the wires and the impedance reducing layer and functionalization layer are deposited on the inside of the recess; Figure 25 shows an example of an integrated circuit; Figure 26 shows an example of an apparatus in which the integrated circuit is used to read out and amplify the signals received from corresponding wire electrodes of the electrochemical probe; Figure 27 is an image showing how the wires are packaged into a housing; and Figure 28 schematically illustrates how the wires are packaged into a housing.

Before discussing the embodiments with reference to the accompanying figures, the following description of example embodiments and associated advantages is provided.

An electrochemical probe may comprise a wire bundle, the wire bundle itself comprising a plurality of wire electrodes arranged alongside each other. The wire electrodes may comprise an inner conductive core and an outer insulating layer, the outer insulating layer having a diameter less than or equal to 40 pm. In such an electrochemical probe, it is advantageous to be able to precisely control the arrangement of the wires in the bundle. For example, it may be beneficial to arrange the wires to closely correspond with a biological sample to be tested by the probe. However, precisely arranging the wires for such purposes is difficult using conventional techniques, because the wire electrodes are so narrow and cannot support themselves when standing up on end, so that it is difficult to control the exact positions of the wire electrodes across the cross-section of the bundle when viewed along the axis of the bundle.

According to examples of the present technique, a method of forming an electrochemical probe uses at least one sheet of material as a template for holding the wire electrodes. The at least one sheet of material has a plurality of channels disposed across its surface with a predetermined spacing that can easily be controlled in order to control the arrangement of the wire electrodes in the wire bundle.

To form the wire bundle, the wire electrodes are laid into the channels of the at least one sheet of material, and the at least one sheet of material may be manipulated -for example by folding, rolling or stacking the one or more sheets -to form the bundle. The formed wire bundle has a cross-section in which the at least one sheet of material traces out a predetermined shape. In this way, a two-dimensional distribution of points at which the wire electrodes intersect the cross-section is dependent on the predetermined shape traced out by the at least one sheet of material (which is a controlled shape depending on the way the sheet was manipulated), and also on the predetermined spacing of the channels across the surface of the at least one sheet of material.

By forming an electrochemical probe according to the method described above, a precise arrangement of the wire electrodes within the bundle can be controlled. The channels in the at least one sheet of material hold the wires in precisely defined positions, enabling an arrangement of the wires in the formed wire bundle to be predicted. Furthermore, by controlling the predetermined spacing of the channels across the surface of the at least one sheet of material, as well as controlling the manipulation of the at least one sheet into the wire bundle, the positions of the wires in the bundle can be precisely controlled. This method enables manufacturing of electrochemical probes with precise arrangements of wire electrodes to be cheaper and hence more scalable commercially to mass production. Also, it allows the electrochemical probe to be designed with precisely controlled electrode arrangements such that it more closely corresponds with a sample to be studied, allowing more accurate measurements to be taken, and allowing the measurements to be more accurately interpreted. This may have a number of potential advantages, one of which may be that the number of animals used in medical research can be reduced.

Although, in the example described above, the wire electrodes are described as having a diameter of up to 40 pm, it will be appreciated that this is an example of an upper limit on the diameter. In some particular examples, the diameter of the wire electrode may be as little as 25 pm or less, or it may be anything less than 40 pm, for example. The core material may be one of gold, platinum, copper, brass, nickel, tin, silver, iron, lead, brass, bronze, platinum-iridium, silver-lead, or any other conductive material. The insulating material may be glass or plastic, for example.

The surface of the sheet of material may be flat at the time the electrodes are laid into the channels, or could be curved (e.g. if the sheet is laid on a curved support). Either way, the three-dimensional problem of positioning the electrodes is transformed into essentially a one-dimensional problem (placing the electrodes into parallel channels at linear positions along a flat or curved surface of the sheet) or a two-dimensional problem (if as well as the position along the surface there is control over the axial position at which the electrodes are laid into the channel, relative to the axis of the channel, e.g. to control the lengths of protruding parts of the electrodes which extend beyond the channel). This makes manufacturing more straightforward than if the electrodes have to be manipulated into positions within the three-dimensional extent of the bundle without use of the template provided by the at least one sheet of material.

The way in which the at least one sheet of material is manipulated to form the wire bundle may depend on the desired arrangement of wire electrodes in the bundle. In some examples, the electrochemical probe has a layered structure, comprising multiple sheet layers of the at least one sheet of material. The cross-section of the wire bundle, in this case, may intersect a plurality of the sheet layers, and at least one of the sheet layers may hold a subset of the wire electrodes. Note that the sheet layers may correspond to entirely separate sheets which are combined to form the wire bundle, or could correspond to different portions of a single sheet which is rolled or folded for example.

In some particular examples, the layered structure mentioned above may be formed by rolling the at least one sheet of material. This demonstrates one particularly simple way of forming a wire bundle from the at least one sheet of material, transforming a linear arrangement of wire electrodes along the surface of the sheet of material into a bundle with a precisely controlled arrangement of electrodes.

A wire bundle formed in this way may, for example, have cross-section which is approximately circular or ovular in shape, although the exact shape formed will depend on exactly how the sheet is manipulated. Where rolling is used, the positions at which electrode positions intersect the cross-section may correspond to a spiral. The at least one sheet of material may, in some examples, be a single sheet of material that is rolled to form the wire bundle, or it may be a plurality of sheets of material.

In some examples, the sheet of material may comprise a plurality of sheets. In such cases the method may comprise manipulating the plurality of sheets of material by stacking the sheets on top of one another to form the multiple sheet layers. This is another simple method for forming a wire bundle from a plurality of sheets. This example could be used to form a probe where the positions at which electrodes intersect the cross-section of the bundle form a grid-like pattern. This technique for forming the wire bundle may be the only manipulation applied to the sheets of material, and the cross-section of the formed bundle may be approximately rectangular. However, it will be appreciated that other shapes of the cross-section might also be formed, instead of a rectangular shape (e.g. if the stacked sheets include sheets of different shapes or different surface area, or if the sheets are stacked with an offset so that one layer extends beyond the end of another layer). It will also be appreciated that the above technique may be applied in combination with the rolling technique previously described. For example, the plurality of sheets may be stacked on top of one another and then rolled.

The manipulation of the at least one sheet of material into the wire bundle may also comprise folding the at least one sheet to form the multiple sheet layers. As with the stacking technique, it is noted that this may be the only manipulation applied to form the bundle, or it may be applied in combination with any of the above-mentioned techniques of rolling and stacking one or more sheets of material. For example, multiple sheets may be folded and then stacked on top of one another, stacked and then folded, or stacked and/or folded and then rolled. In fact, any of the above methods may be used in any combination.

In some examples, the wire bundle may be formed such that it has a hollow channel running along its length. The cross-section of the wire bundle in this case is configured to have a hollow region in its centre, corresponding to the hollow channel.

Arranging the wire bundle such that it has a hollow channel running along its length may be advantageous for any of a number of reasons. For example, the channel may be used to deliver pharmaceutical substances to a patient, to supply a light source to the region being probed (for example by feeding an optical fibre through the channel), to deliver an imaging device such as an endoscope to the area being probed, or for any other purpose. Alternatively, the hollow channel may be present simply to ensure that the diameter of the wire bundle has a desired value, without requiring an excess of the at least one sheet of material to be provided.

In some examples, the hollow channel may be formed simply as a result of the way in which the sheet is manipulated, rather than requiring any specific support structure to be included. For example, when a sheet is rolled into a spiral, a hollow region may be formed at the centre of the spiral, as it may be impractical to ensure the spiral fully fills the cross-sectional area of the bundle with sheet material. Also, the particular way in which the sheet is folded could be controlled to provide a hollow channel.

Alternatively, to form the hollow channel, the at least one sheet of material may be manipulated (such as to be stacked, rolled or folded) to arrange the sheet around a support structure. The support structure can then be taken away to leave the hollow channel in its place or, if the support structure is itself hollow, it may be left in place to provide continuing support for the hollow channel.

As noted above, the predetermined spacing between the channels, and the predetermined shape traced out by the sheet of material in the cross section of the probe, enable the positioning of the wire electrodes in the bundle to be precisely controlled. In some examples, additional control of the positions of the wire electrodes can be provided by controlling the thickness of the at least one sheet of material at predetermined positions. The sheet(s) used to form the bundle could have a uniform thickness, or alternatively one or more sheets of varying thickness (either within a sheet, or from sheet to sheet) could be used to alter the relative spacing of the positions at which electrodes intersect the bundle's cross-section.

This enables the predetermined shape described above to be altered to provide a precise desired arrangement of electrodes in the bundle.

In some examples, the wire bundle is arranged such that at least a subset of the wire electrodes protrude from the edge of the at least one sheet of material. In such examples, the method may comprise controlling the length of the protruding section of each of the subset of the wire electrodes by controlling the positions of the electrodes within the channels (e.g. controlling the axial position of each electrodes relative to the axis of the corresponding channel) -for example by sliding the electrodes along the channels, or by controlling their positions along the channels when laying them into the channels.

This provides a further degree of control of the positions of the electrodes in the wire bundle. In these examples, the precise arrangement of wire electrodes in the bundle can be controlled by controlling the predetermined spacing between the channels, the manipulation of the wire bundle, the positions of the electrodes within the channels and, in some examples, the thickness of the at least one sheet of material in various positions. It will be noted that different levels of control of the arrangement of the wire electrodes can be achieved depending on how many of the above parameters are controlled. For example, controlling more parameters has the advantage of more precise control, while controlling fewer parameters has the advantage of more simplicity. A decision of how many and which of the parameters are controlled may be based on weighing up these two advantages. It should be noted, however, that even controlling just a few of the above parameters provides a marked improvement in the ability to control the arrangement of the electrodes in the wire bundle.

In some examples, the length of the protruding section of each of the subset of wire electrodes is selected in dependence on an object to be probed by the electromechanical probe.

The protruding sections of the subset of wires may take a variety of forms. In some examples, the subset of the wire electrodes that protrude from the sheet are arranged such that the lengths of their protruding sections vary. That is, the ends of the protruding wires are arranged such that they do not all line up horizontal to the edge of the at least one sheet of material.

In some particular examples, the lengths of the protruding sections of the subset of the wire electrodes may arranged to taper. For example, the lengths of the protruding sections of the subset of wire electrodes may successively increase from one side of the cross-section of the wire bundle to the other side of the cross-section of the wire bundle, such that their ends form a diagonal line with respect to the edge of the at least one sheet of material. Alternatively, the length of the protruding section may successively increase from one side of the cross-section of the bundle to an intermediate point, and then again successively decrease from the intermediate point to the other side of the cross-section, such that their ends form a pointed or approximately triangular shape. Each of these arrangements has benefits for certain applications, and the exact arrangement of the protruding sections may be selected in dependence on the particular application.

For wire electrodes for which the front-end profile of the wire is rotationally asymmetric about the axis of the electrode, another factor that can be controlled when placing the wire electrodes into the channels may be the relative rotational position of the wire's front end relative to the axis of the channel. For example, some wire electrodes may have a tapered end surface, which tapers to a sharpened point. The sharpened point can help to facilitate insertion of the electrodes into a tissue sample, as well as influencing the direction in which the electrodes spread when inserted into the tissue sample. Hence, in some probes it may be desired to include electrodes which are at different rotational orientations relative to the axis of the electrodes, e.g. with the tapered surface of the front end of the electrodes pointing in different directions for respective electrodes. This would be relatively difficult to achieve with alternative techniques for manufacturing the probe, but with the technique discussed above, the rotational orientation of each electrode can be controlled when laying the electrode sideways into the corresponding channel, to give a probe front end with a desired pattern of electrode orientations.

As discussed above, the two-dimensional arrangement of the points at which the wire electrodes intersect a cross-section of the wire bundle can be controlled according to the above methods. In some examples, this distribution may be selected in dependence on an object to be probed by the electromechanical probe. For example, the selected distribution may depend on the shape of the surface of the object.

In some examples, the channels disposed across the surface of at least one sheet of material are arranged to be parallel, prior to forming the manipulation.

In some examples, the material of said at least one sheet is a flexible material. This allows the sheet to be easily manipulated into the shapes described above.

The material may comprise a polymer. In particular examples, the polymer may be polydimethylsiloxane (PDMS).

The at least one sheet of material may be formed in any of a number of ways. For example, the at least one sheet of material may be formed from a mould, wherein the mould comprises a pattern corresponding to an arrangement of the channels in the surface of at the least one sheet of material.

The mould used to form the at least one sheet of material may also be produced in any of a number of ways. For example, the pattern corresponding to the arrangement of the channels may be formed in the mould by photolithography.

In some examples, the thickness of the at least one sheet of material is configured to be less than or equal to 500 pm. This may improve the flexibility of the at least one sheet of material. It will be appreciated that 500 pm is an example of an upper limit on the thickness of the at least one sheet of material. In some particular examples, the thickness of the sheet may be less than or equal to 500 pm, less than or equal to 450 pm, less than or equal to 400 pm, less than or equal to 350 pm, less than or equal to 300 pm, less than or equal to 250 pm, less than or equal to 200 pm, less than or equal to 150 pm, or less than or equal to 100 pm. It will be appreciated that although a number of examples have been listed, the thickness of the at least one sheet of material may be limited to a more specific range between any two of the threshold thicknesses listed above.

In some examples, the channels are configured to be separated by at least 20 pm. This reduces the chance that adjacent wire electrodes will interfere with one another.

A separation of 20 pm between the wire electrodes is beneficial, however in some particular examples, the separation may be at least 25 pm, at least 30 pm, at least 35 pm or at least 40 pm. In general, a separation of the order of the diameter of the wire electrodes may be beneficial, although it will be noted that the larger the separation is, the less likely it is that the wire electrodes will interfere.

In some examples, the wire electrodes themselves may comprise a layer of metal nano-structures or metal oxide nano-structures deposited on their tips, at one or both of a first end and a second end of the wire bundle.

Providing the wire electrodes with a layer of metal or metal-oxide nano-structures at the front end and/or the back end of the wires can provide an electrochemical probe with much lower impedance at the front end and/or better connectivity at the back end, increasing the signal/noise ratio of electrochemical signals measured using the probe or of currents transmitted by the probe for neural stimulation purposes. Here the front end refers to the end of the wire electrodes intended for interfacing with the tissue sample, and the back end refers to the end at which the electrodes interface with electronics for reading out signals from the probe or delivering stimulation current to the probe.

In some examples, a probing device may be formed, comprising the electromechanical probe as described in any of the above examples and an integrated circuit. The integrated circuit may comprise a plurality of contact portions, each being configured to receive an electrode signal from one of the plurality of wire electrodes. The integrating circuit may also include an amplifying portion to amplify the electrode signals received at each of the contact portions.

In some particular examples, the connection layers of metal or metal-oxide nano-structures on the tips of the wire electrodes at the contact end of the wire bundle may be arranged to be in contact with the corresponding contact portions of the integrated circuit. This provides a particularly effective connection.

Particular embodiments will now be described with reference to the figures.

An electrochemical probe in accordance with the present technique may comprise a wire bundle, formed of a plurality of wire electrodes held alongside each other. The figures and the following discussion describe the wire electrodes in detail, as well has methods of forming a wire bundle from the wire electrodes, and methods of forming an electrochemical probe from the wire bundle.

Figure 1 shows an example of a wire electrode 1. The wire electrode includes a core 2 made of a conducting material (e.g. a metal or alloy) surrounded by an insulating material 3.

The electrode 1 is an ultramicroelectrode (UME), typically having a diameter less than or equal to 25 pm. It should be noted, however, that the diameter may be anything less than 40 pm; for example, the diameter of the wire electrodes may be less than 40 pm, less than. 35 pm, less than 30 pm, less than 25 pm, less than 20 pm, or less than 15 pm. Here, the wire diameter refers to the diameter of the overall electrode, including both core 2 and insulating material 3. In this example the metal core 1 is made of gold, but other examples of conducting material which could be used include copper, silver, gold, iron, platinum, lead or other metals, as well as crystalline or amorphous alloy compositions such as brass, bronze, platinum-iridium, lead-silver and magnetic alloys such as FeSiB. In this example, the insulating material 3 is glass, but other examples could be used such as plastics or other insulators. The diameter of the core 2 may vary, and could be anything less than the overall electrode diameter, e.g. it could be as small as 10 microns or less, or could be in the nanometre range.

The wire electrodes may be formed according to a Taylor-Ulitovsky method. The TaylorUlitovsky method is a technique for forming glass sheathed wire electrodes with a very fine diameter, e.g. as small as a few microns. In this process, the metal or alloy conducting material is placed inside a glass tube which is closed at one end and the other end of the tube is heated to soften the glass to a temperature at which the conductor melts. The glass can then be drawn down to produce a fine glass capillary with the metal core inside the glass. Hence, the metal cores can be coated with a glass sheath a few microns thick with this method.

The length of the wire electrodes may depend on the particular application of the electrochemical probe, and may -for example -have lengths as small as 1 cm, or as large as several metres. In some particular examples, the length of the wire electrodes may be greater than or equal to 1 cm. More particularly, the length of the wire electrodes may be greater than or equal to 1.5 cm, greater than or equal to 2 cm, greater than or equal to 2.5 cm, greater than or equal to 3 cm, greater than or equal to 3.5 cm, greater than or equal to 4 cm, greater than or equal to 4.5 cm, or greater than or equal to 5 cm. A probe with electrodes of length 3-5 cm can be particularly useful. In particular applications, the lengths of the wire electrodes may fall outside of the example ranges given above; for example, the wire electrodes may be as long as several metres.

Figure 2 shows an example of a flexible template 4 for holding the wire electrodes 1. The template 4 is a sheet of flexible material, e.g. a polymer material, such as polydimethylsiloxane (PDMS). A number of channels 5 are formed into the surface of the sheet of material 4, and the channels 5 are configured to hold the wire electrodes 1 described in Figure 1. The template may have a thickness of 500 pm or less, which allows it to be more easily manipulated to form a wire bundle and enables more finely spaced arrangements of wire electrodes.

As will be seen from Figures 6 to 10, the spacing between the channels 5 affects the final positions of the wire electrodes within the formed bundle; therefore, the spacing between the channels can be controlled in order to control the arrangement of the wires in the wire bundle. Furthermore, the channels 5 may be separated by a predetermined minimum separation, to reduce the likelihood that the signals carried by adjacent wires will interfere with one another. For example, the inter-channel spacing between the channels 5 may be approximately 25 pm. It will be appreciated that the dimensions mentioned above are just examples, and other dimensions may also be used.

In addition to controlling the spacing between the channels 5, the thickness the template 4 can be varied at specific points, to provide a further degree of control of the positions of the wires in the formed wire bundle.

Figure 3 is an image depicting a number of electrodes 1 held in a template 4, such as the template 4 described in Figure 2. Figure 3 demonstrates how the electrodes 1 can be laid into the channels 5 of the template 4, while the template is laid flat (or approximately flat) on a surface. Thus, it is easy to arrange the wires in a desired arrangement in the template; prior to the template being manipulated to form the wire bundle.

In the example shown in Figure 3, the electrodes 1 are shown protruding from the edge of the template 4. The length of the protruding section of each of the wire electrodes 1 can be controlled by sliding the electrode 1 along the channel 5 in which it is held. In this example, the lengths of the protruding sections of the wire electrodes 1 are arranged such that their ends are approximately level with one another; in other words, their ends are approximately parallel with the edge of the template. In the example of Figure 3, the rotational positions of the electrodes within the channels is also varied. Note how the orientation of the tapered end surface of each electrode varies from electrode to electrode, so that, relative to an axis of the electrode, the circumferential position at which the tip of the tapered front end surface of each electrode varies relative to some reference position about the circumference of the electrode (e.g. the bottom two electrodes shown in Figure 3 have tips of the taper pointing in opposite directions).

Figure 3 demonstrates how the electrodes 1 are laid into the channels 5 of the template 4 while it is laid flat (or approximately flat) on a surface. The sheet of material is then manipulated to form the wire bundle, as discussed below. In this way, the wires 1 can be easily arranged while the template 4 is flat; and that fiat arrangement of electrodes 1 can be transformed into a two-dimensional array of electrodes 1 in the cross-section of the formed bundle by manipulation of the template.

Figure 4 shows an example of a wire bundle 6, in which the wire electrodes 1 protrude beyond a sheet of the template material 4. In this example, the lengths of the protruding sections of the wire electrodes 1 are arranged such that their ends taper from being at their. longest at one edge of the bundle 6 to their shortest at the opposite edge of the bundle 6; that is, their lengths are arranged to successively increase from one side of the bundle 6 (the right-hand side) to the other side of the bundle 6 (the left-hand side). This arrangement may be desirable for particular applications of the electrochemical probe.

Figure 5 shows another example of a wire bundle in which the wire electrodes 1 protrude beyond the template material 4 of the present technique. In this example, the lengths of the protruding sections of the electrodes 1 successively increase from both edges of the bundle 6 towards an intermediate point between the two edges, forming a triangular arrangement.

To provide arrangements such as those depicted in Figures 4 and 5, the arrangement of the wire electrodes 1 in the channels 5 is tailored depending on the desired final arrangement More particularly, the positions of the wire electrodes 1 within the channels 5 may be varied to form a particular pattern while the sheet of material 4 is flat, which maps onto a desired arrangement of the formed wire bundle 6.

There are a number of ways the template 4 can be manipulated to form the wire bundle.

One possibility, as depicted in Figures 6 and 7, is to roll the template 4 to form the wire bundle. Figures 6 and 7 are images showing a sheet of material 4 rolled in this way. For simplicity, the wire electrodes 1 are not shown; it will be appreciated that the spiral shape traced by the sheet of material 4 in the image demonstrates the shape that would be formed by the sheet 4 in the cross-section of a wire bundle formed in this way. As can be seen from the Figure, this shows a particularly simple method of manipulating the sheet of material.

Figure 8 is an image showing a wire bundle 6 formed by rolling the tempiate 4. In this image, the bundle 6 is shown from a side view, in which the wire electrodes 1 are shown protruding from the material 4; as described in previous examples.

Roliing the template material is one example of manipulating the sheet of material 4 to form the wire bundle 6, but other examples are also possible. For example, as shown in Figure 9, a plurality of sheets of the template material 4 may be stacked on top of each other to form the wire bundle. In this case, the positions of the wire electrodes relative to the cross-section of the bundle could follow a grid-pattern, e.g. a square or rectangular grid, triangular grid. Alternatively, the electrode positions can follow a more arbitrary pattern.

Alternatively, as shown in Figure 10, a single sheet of material 4 may be manipulated to form the wire bundle by folding it over itself several times. In Figures 9 and 10, the wire electrodes 1 are not shown. it will be appreciated, however, that this is purely for simplicity, and that in reality the wire electrodes 1 will be arranged in the template 4 before it is manipulated to form the wire bundle.

The above examples of rolling, folding or stacking one or more sheets of the template material 4 to form a wire bundle 6 are just examples. In other examples these techniques may be used in any combination, or other techniques may be used to manipulate the approximately two-dimensional sheet of material into a three-dimensional probe.

Figure 11 is a flow diagram illustrating a method of producing a wire bundle 6 using a template 4 and a number of wire electrodes 1. The method includes a step 10 of laying the wire electrodes 1 into the channels 5 of the template 4 (the at least one sheet of material). Then, in step 12, the sheet of rnaterial 4 is rolled, folded or stacked as described in the previous examples to form the wire bundle 6. In this way, a two-dimensional array of electrodes 1 with a desired arrangement (in the cross-section of the wire bundle 6) can be created simply by arranging the electrodes 1 into a flat sheet of material 4 and manipulating that sheet into a bundle 6.

As described above, the final arrangement of the of the wire electrodes 1 in the bundle 6 can be precisely controlled by adjusting the spacing between the channels 5 in the surface of the sheet of material 4, and by controlling the method used to manipulate the sheet of material 4 into the wire bundle 6. Further control of the positions of the wire electrodes 1 in the formed bundle 6 can be provided by controlling the thickness of the sheet of material at specific positions, to control the final spacing of the electrodes 1 in the wire bundle 6. Figure 12 shows an example of forming a wire bundle 6 in accordance with this technique. The method includes a step 14 of controlling the thickness of the sheet of material 6, depending on the desired arrangement of the wire electrodes 1 in the formed wire bundle 6. At step 16, the channels 5 are formed into the sheet of material 4 with a predetermined spacing, wherein the predetermined spacing will also affect the final arrangement of the wire electrodes 1 in the wire bundle 6. Once the template material 4 is formed according to the above steps, the wire electrodes 1 are laid into the channels 5 of the material 4 in step 10. Finally, in step 12, the one or more sheets of material 4 are rolled, folded or stacked in order to form the wire bundle 6.

Figure 13 is an image showing a cross-section of a wire bundle 6, in which a plurality of the wire electrodes 1 are held in a sheet of the template 4. In this arrangement, the cross-section of the bundle 6 shows a hollow region 18 in the centre of the wire bundle 6. The hollow region 18 corresponds to a hollow channel that runs along the length of the wire bundle 6, and may have any of a number of uses. For example, the hollow channel 18 can be used for delivering pharmaceutical substances (e.g. drugs) to a patient, for retrieving samples from a patient, for providing a source of light to the subject being probed, or for any other use. In this example, a rolled arrangement of the probe is shown, however a channel can be formed in a probe of any example.

Figures 14 and 15 are images of a similar example of a rolled wire bundle 6 comprising a hollow channel in its centre, shown from different angles. Figure 14 shows the wire bundle 6 from its side, and Figure 15 shows a cross-section of the wire bundle 6.

As noted above, folded and stacked arrangements of the template material 4 into a wire bundle 6 may also be arranged to have a hollow channel. The hollow channel may be formed in a number of ways. For example, the channel may be formed by roiling the sheet around a support structure and removing the support structure. Alternatively, the support structure may remain in place. The support structure can also be used to form a hollow channel in the stacked or folded arrangements of the wire bundle 6. For example, Figure 16 shows how sheets of the material 4 can be stacked around a support structure 20 in order to form a wire bundle 6 which has a hollow channel.

Similarly, a single sheet of material 4 may be folded around a support structure 20 as shown in Figure 17.

In some examples, a support structure 20 is not needed. One example may be a rolled arrangement, as shown in Figures 13 to 15. However, another example may be in the stacked arrangement, in which the sheets of material 4 may be cut to predetermined lengths prior to being stack, in order to ensure that when they are stacked a hollow channel is formed. This is shown in Figure 18.

The template sheet 4 can be formed in a number of ways; however, one particular example of a technique for manufacturing the template material is to spin-coat a master mould with a thin layer of the material (for example, PDMS). Figures 19A and 198 demonstrate an example of this technique.

Figure 19A shows an example of forming a mould 30, from which the template 4 can be formed. To form the mould, a salinized silicon wafer 22 is coated with a thin film of epoxy-based negative photoresist 23. The coated Si water is then exposed to UV light from a light source 24 in a pattern 26 corresponding to the desired arrangement of channels 5 in the final template 4.

The UV light may be applied in a direct write, rnaskless photolithography machine, for example. The sections of the negative photoresist 23 that were exposed to light become insoluble. Therefore, a solvent can then be applied to the coated Si wafer to dissolve the non-exposed regions, leaving raised areas 28 corresponding to the pattern of the light on the surface in the photolithography step. This forms a mould 30, which can be used to form the template 4, as discussed below.

Figure 198 schematically illustrates the forming of a template 4 by spin-coating the mould 30 formed in accordance with Figure 19A. First, a polymer mixture 32 comprising PDMS is applied to the mould 30. For example, the polymer mixture may be a mixture of PDMS and a cross-linker in a 10:1 ratio, which is centrifuged and degassed before being applied to the mould 30. The mould 30 is then spun, to produce a thin, even layer of the polymer mixture 32 on the mould 30. Finally; the polymer layer 32 is cured (for example, at approximately 80°C for approximately 30 minutes) and then peeled off the mould 30 to form the final template 4. The template 4 has channels 5 corresponding to the raised areas 28 of the mould 30.

It will be appreciated that the above-described method for producing the template material is just one example, and that other examples are also applicable to the present technique.

Figure 20 shows an example of an electrochemical probe 340. In the example of Figure 20, the template material 4 is not shown, in order to more clearly illustrate the electrodes 1. However, it will be appreciated that in reality the electrochemical probe 340 illustrated in Figure 20 would also comprise a template 4 as described above. Also, the diameter of the insulating layer has been exaggerated for understanding, but in practice it may not be as thick relative to the inter-electrode spacing as is shown in the example of Figure 20.

An electrochemical probe may be a probe for current and/or voltage measurements or injection in biological samples and a range of electrophysiological applications, or a probe for determination of the presence and/or quantity of one or more biologically and/or toxicologically significant substances in biological and/or liquid samples. The electrochemical probe 340 comprises a wire bundle including a number of wire electrodes 1 arranged alongside each other (for example the wire electrodes may be arranged parallel to each other or nearly parallel). The wires could be arranged in the bundle in a regular pattern (such as a square/rectangular lattice or stack arrangement, or a hexagonal packed arrangement), or in an irregular pattern, according to the above-described techniques. Each wire electrode 1 includes a core 2 made of a conducting material (e.g. a metal or alloy) surrounded by an insulating material 3. The electrodes 1 are ultramicroelectrodes (UMEs) having a diameter of, for example, less than or equal to 25 pm. In this example the metal core 2 is made of gold, but other examples of conducting materials which could be used include copper, silver, gold, iron, platinum, lead or other metals, as well as crystalline or amorphous alloy compositions such as brass, bronze, platinum-iridium, lead-silver and magnetic alloys such as FeSiB. In this example, the insulating material 3 is glass, but other examples could use plastics or other insulators.

The probe 340 has a front end, which is the end of the probe for interfacing with the sensing target, and a back end, which is the end of the probe for transmitting the signals measured from the sensing target to the signal read out electronics or data processing equipment. At the front end, the wire electrodes 1 each have an impedance reducing layer of gold nano-structures deposited on the tips of the wire electrodes, and an iridium oxide (IrOx) functionalisation layer comprising a layer of iridium oxide nano-structures deposited on top of the gold nano-structures. At the back end, the tips of the wire electrodes have a connection layer for connecting to an electrical connector or the read out electronics. The connection layer in this example is also made of gold nano-structures, but the back end does not have the additional functionalization layer.

Figure 21 shows an image of the gold nano-structure hemispheres formed on the back ends of the wires and the back end before hemisphere deposition. Each individual single crystal of the hemisphere may have a unit size in the nanometre scale, e.g. smaller than 300, 100 or 50 nm for example. On the other hand, the overall hemisphere of nano-structures on the back-end may have a width at the micrometre scale, e.g. around 10-20 pm in this example, and on the front end the hemisphere may have a width not exceeding 20% of the wire core's diameter. As can be seen from Figure 20, the hemisphere may extend over the insulating sheath of the wires as well as the core material on the back-end to facilitate contact with the integrated circuit providing the electronics for reading out signals from the probe. It will be appreciated that the gold-nanostructure layers formed at the front and back ends of the wire electrodes need not be perfectly hemispherical -in general any mound or bump formed on the tip of the wire electrodes may be sufficient.

As shown in Figure 22, the tips of the wire electrodes at the front end can be sharpened to provide a tapered surface that is angled to a point, to facilitate insertion into the brain or other sample material. Different electrodes of the bundle may have the angled surface in different orientations so that when the bundle is inserted into the sample, the angled surface pushes against the sample and is deflected sideways (towards the "pointy side" of the electrode -the side of the tip surface where the point of the tip is located -e.g. in the lower window in section a of Figure 22 the pointy side would be the lower side of the tip surface). For example, by arranging the bundle so that the pointy side of the electrodes are arranged towards the outside of the bundle, then when the bundle is inserted into the sample the free ends of the wire electrodes can diverge and the electrodes can spread out to reach different target areas within the sample, which can be particularly useful for brain stimulation or neuronal recording for example.

Figure 23 is a flow diagram illustrating a method of manufacturing an electrochemical probe. At step 33 wire electrodes are formed using the Taylor-Ulitovsky method. The Taylor-Ulitovsky method is a technique for forming glass-sheathed wire electrodes with a very fine diameter, e.g. as small as a few microns. In the process, the metal or alloy conducting material is placed inside a glass tube which is closed at one end and the other end of the tube is heated to soften the glass to a temperature at which the conductor melts. The glass can then be drawn down to produce a fine glass capillary with the metal core inside the glass. Hence, metal cores of diameters in the range 1 to 133 microns can be coated with a glass sheath a few microns thick with this method. In particular, wires with a core in the range 1-10 pm surrounded in 10-40 pm of glass can be useful for electrical and electrochemical sensing. The metal used can include copper, silver, gold, iron, platinum, lead or other metals as well as crystalline or amorphous alloy compositions such as brass, bronze, platinum-iridium, or magnetic alloys.

At step 35, the electrodes are formed into a bundle or stack with the wires running parallel to each other. For example, the microwires can be machine wrapped into bundles of 10s, 100s, 1000s, 10000s 100000s, tens of millions or hundred millions of wire electrodes, to provide multiple channels for recording or cover the available contact portions on an integrated circuit. The bundling can be through the procedure shown in the earlier examples, by laying the electrodes into the channels in the template sheet(s) of material, and manipulating the sheets.

At step 38, a connection layer comprising metal nano-structures is deposited on the tips of the wire electrodes at the back end of the probe. The connection layer can be deposited by electrodeposition, in which the bundle of electrodes is held in a bath of electrolyte and a voltage difference is applied between the wire bundle and another electrode to cause ions in the electrolyte to be attracted to the wire electrode bundle, depositing a coating of metal nanostructures on the tip of each wire.

In one particular example, gold micro-hemispheres are deposited from a two-part aqueous cyanide bath containing 50 g1:1 potassium dicyanoaureate(I) (K2[Au(CN)2]) and 500 9121 KH2PO4 dissolved sequentially in deionized water (18 MOhm) (Tech, UK) at 60°C. All reagents were supplied by Sigma-Aldrich (RIM) UK, and were used without further purification. Prior to electrodeposition the polished substrate was washed with ethanol (90%), rinsed with deionized water, wiped with a lint-free cloth {Kromer:8 (rFPO). Kanter:a OLIM} lat) and dried at 50°C for 1 hour in an autoclave. The electrodeposition protocol was carried out with a VSP 300 potentiostat-galvanostat (Bio-Logic (RTM) France) controlled with EC-1.ab (RTM) (Bio-Logic (RTM) France) in a three-electrode cell setup composed of a gold UME bundle as working electrode (WE), a coiled platinum wire (99.99%, GoodFerlow (RI ra) , US) as counter electrode (CE) and a Ag/AgClIKCl/3.5m reference electrode (REF) supplied by BASi, USA (E vs. NHE = 0.205V). The REF was kept separated from the bath by a glass tube containing the support electrolyte and a porous Vycor (RIM) glass separator. During gold deposition the WE potential was kept at Ered =-1.1V vs. REF for a time determined according to the desired size of the gold hemisphere to be formed. During electrodeposition the bath was thermostated at 60°C under vigorous (500 rpm) stirring. This technique has been successful for many different types of metal conductor material, including gold, platinum, tin, copper, brass, bronze, silver and lead.

Gold can be a particularly useful material for the back end connection layer. In contrast with their applications for the front end sensing, the connection of individual or high-count bundled UMEs to integrated circuitry is poorly examined and represents a significant drawback towards their usability in biomedical applications. Literature offers little or no documentation regarding reversible interfacing methods of individual or UMEs to macroscopic conductors or integrated circuitry, the main practices being based on soldering, conductive silver-epoxy bonding or mercury-dip. Although applied, these methods can easily increase the RC cell time constant at high frequencies given the stray capacitance at the glass-mercury/conductive epoxy junctions and are not relevant for reversible contacting individual or bundled UME assemblies; scaling such practices to high-count UME bundles (up to 1 million, for example) are a considerable engineering challenge. The state-of-the-art indium bump bonding developed for pixelated sensor and read-out chip interconnection employing photolithography, sputtering and evaporation or later electrodeposition could be a suitable processing practice, however due to indium's tensile and ductile properties, mechanical properties and overall tribological behaviour it cannot be applied as a reversible interconnection material in UME interfacing. Copper bumps as interconnects could be considered from a mechanical point of view, however given their possible diffusion into SiO in the presence of an electric field, breaking down transistor reliability, and affinity towards oxidation, make Cu a less attractive candidate as an interconnect material in physiological environments. In contrast, gold is a promising contact material in medium wear conditions which can seamlessly enable reversible, scalable, low-cost, ultra-fine pitch and high yield bumping for interconnection purposes.

At step 39 of Figure 23, an impedance reducing layer of metal or metal oxide nano- structures is deposited on the tips of the wire electrodes at the front end. This can be done by the same electrodeposition protocol as described above for step 38 for the back end. The material used for the nano-structures at the front end can be the same or different to the material used for the nano-structures at the back end, but in one example both use gold nano-structures.

At step 40 a functionalization layer is deposited on the impedance reducing layer at the front end. Again, this can be deposited by electrodeposition (although other techniques such as spraying could also be used). For example, a layer of metal oxide (e.g. iridium oxide) can be deposited on top of the gold nano-structures at the front end.

In one particular example, the electrodeposition protocol was carried out from a modified electrolyte solution based on a formulation reported by Meyer et al. (2001, "Electrodeposited iridium oxide for neural stimulation and recording electrodes", Neural Systems and Rehabilitation Engineering, IEEE Transactions on, 9(1), pp.2-11.), containing 10 gL-1 iridium (IV) chloride hydrate (99.9%, trace metal basis, Siefed-Aldnfe(RTNI) 1, Germany), 25.3 gL-oxalic acid dihydrate (reagent grade, Sigma-Alerted (RTM), Germany), and 13.32 gL-1 potassium carbonate (99.0%, BioXtra, Sigme-Aldrich (RIM) , Germany). Reagents were added sequentially to 50% of the solvent's volume first by dissolving IrCI in the presence of oxalic acid followed by the addition of K2CO3 over a 16 hour period until a pH=12 was reached. The electrolyte was aged for approximately 20 days at room temperature in normal light conditions until the solution reached a dark blue colour. I rOx was electrodeposited using a multichannel VSP 300 (. (104-09le (RTM), France) potentiostat-galvanostat in 3 electrode cell setup comprising a glass-ensheathed Au wire bundle as working electrode (WE), a platinum rod (0.5mm diameter, 99.95%, Goodfellow (RIM), LIS) as counter electrode, and AglAgClIKCl/3.3m (Bioanalytical Systems, US) as a reference electrode (REF). The electrochemical protocol was composed of three consecutive stages combining galvanostatic polarisation (GP), cyclic voltammetry (CV) and pulsed potentiostatic protocols (PP). Between protocols open circuit voltage (OCV) of the WE was monitored for 180 second and represents the steady-state period. During galvanostatic deposition the WE potential was set to 0.8V vs. REF for 500 seconds. During CV deposition the WE potential was swept from - 0.5V to 0.60 V vs. REF at 100mVs in both anodic and cathodic direction. During the pulsed potentiostatic deposition the WE potential was stepped from OV to 0.60V vs. REF with 1 seconds steps for 500 seconds.

It will be appreciated that, while steps 38, 39 and 40 are shown in Figure 23 as taking place after the wire electrodes 1 are arranged into the wire bundle 6, these steps could alternatively be carried out either before the wire electrodes 1 are placed into the channels 5 of the template 4. Hence, in other examples each electrode could be formed individually, and the connection layer, impedance reducing layer and functionalization layer could be deposited on each wire individually, before the electrodes are bundled. In this case, the electrodes may already have the nano-structure layers and functionalization layer deposited on them at the point when they are laid into the channels of the template material to form the wire electrodes into a bundle.

Also, the functionalization layer may be optional and some examples of probes may not be provided with any functionalization layer.

Also, the bundling method shown above using the template sheet 4 may be used in probes which do not provide any nano-structure layers at all, so in other examples steps 38 and 39 may be omitted and the functionalization layer could be the only layer at the tips of the electrodes.

The method of Figure 23 may include an additional recess forming step 34 between steps 33 and 35. In step 34, part of the tips of the electrode is dissolved using a solvent to form a recess 41 in the end surface of the electrode 1 as shown in part (a) of Figure 24. For example, the recess can be formed by an electrochemical leaching step (e.g. by dissolving into an electrolyte in the presence of electrical current). The parts of the electrode 1 which are not to be dissolved may be masked by covering them with a mask material, so that only the portion at the end of the tip is dissolved. The subsequent steps of Figure 23 are then performed on the wire electrodes having the recess in their tips. Therefore, as shown in part (b) of Figure 24, when the impedance reducing layer is subsequently deposited at step 38 of Figure 23, the nano-structures 42 are deposited on the inside of the recess 41. The nano-structures 42 may also extend onto the surface of the electrode tip outside the recess. When the functionalization layer (e.g. IrOx) is then deposited on top of the impedance reducing layer at step 40, the functionalization layer 44 is deposited inside the recess. The functionalization material may also protrude out of the recess beyond the end of the electrode tip as shown in part (c) of Figure 24.

The approach shown in Figure 24 provides several advantages. Firstly, providing a recess means that a greater volume of iridium oxide or other functionalization material can be deposited at the end of the electrode, which can improve the electrochemical properties of the probe. For example, given the available space, the charge capacity of the iridium oxide layer can be improved up to a 1000 times. Also, this approach provides robustness against mechanical deterioration of the electrode tips. During the working life of the probe, the electrodes may repeatedly be inserted into a sample and removed, and so the tips of the electrodes may gradually be worn away by contact against the sample, which can cause deterioration of the signals measured by the probe. By including the recess and depositing the surface layers inside the recess, then even if the end of the probe is worn down (e.g. so that the surface now is at the position indicated by the line 46 in Figure 13), then there will still be a layer of the impedance reducing nano-structures and a layer of the functionalization material at the end.

Figure 25 shows an example of an integrated circuit (IC) 50 which can be used to read out and amplify signals measured using the electrochemical probe. The IC 50 may be a multi electrode array (MEA), a CMOS based potentiostat or a pixelated photodetector. All of these are already available commercially and therefore it is not necessary to design a bespoke circuit for this purpose, which reduces the cost of implementing an apparatus for electrochemical measurements. As shown in Figure 25, the IC 50 includes a number of pixel read out circuits 52 arranged in a square or rectangular grid pattern, with each pixel readout circuit including a contact region 54 made from a conductive material (e.g. platinum, gold, indium) connected to an amplifier read out circuit 56. The amplifier circuit may be formed according to any known semiconductor (e.g. CMOS-based) circuit design. The signals amplified by each pixel readout circuit may then be output to a processor, memory or external apparatus for storage or analysis.

Figure 26 shows how the electrochemical probe 340 may be interfaced with the integrated circuit 50. As shown in Figure 26, the gold contact bumps 58 at the back ends of the respective wire electrodes 1 can simply be pressed directly against the contact bumps 54 of the respective pixel readout circuits of the IC 50 to provide the electrical connection between the wires and corresponding pixels (without any interposing connector unit between the wire bundle and the IC 50). Hence, the integrated circuit provides a multi-channel amplification and readout system for reading and writing the electrode signals from the respective wires. It is not essential that every pixel readout circuit of the IC 50 interfaces with a corresponding wire electrode. Depending on the arrangement of the wires within the bundle, it is possible that some pixel readout circuits may not contact a corresponding wire.

Figure 27 shows an alternative technique for interfacing the probe with read out electronics or a data processing apparatus. In this example, the wire bundle forming the electrochemical probe 340 can be packaged into an enclosure, but the enclosure does not include an integrated circuit as discussed above for amplifying the signals from the probe.

Instead, each wire is individually bonded or soldered to a corresponding channel of a connector (e.g. a socket or plug). When the probe is in use, the connector can be coupled to an external amplifier or other electronic device for processing the outputs of the electrodes. Hence, it is not necessary for the probe itself to include circuitry for amplifying or processing the signals read by each electrode. A free end of the wire bundle (for inserting into the sample) may extend beyond the end of the probe housing/packaging. Figure 27 shows examples with different lengths of the free end portion of the wire electrodes, which is useful for allowing localized recordings from different regions of the sample. For example, for neuronal recording in a mouse brain, the shorter probe shown in the left hand part of Figure 27 was used for probing the olfactory bulb while deeper structures such as the piriform-cortex, thalamus, hippocampus or brainstem were probed using the longer probes shown on the right hand side.

Figure 28 schematically illustrates an example of how wire electrodes 1 an be connected to an enclosure 60 as shown in Figure 27. As shown in the figure, the wire electrodes 1 may be fed through a spacer 62 and soldered to contact portions 64.

For wire bundles with relatively low channel count (e.g. less than 1000 wires in the bundle), either the approaches shown in Figures 26 and Figures 27 to 28 can be used. However, when the channel count is higher (e.g. greater than 1000 wires), then it becomes increasingly impractical to individually bond each wire to the connector, and in this case the approach shown in Figure 26 may be more useful, whereby the bumps on the end of each wires are simply pressed against the contact portions of a pixelated integrated circuit.

The examples described above show how an electrochemical probe can be formed, such that the wire electrodes in the electrochemical probe have a precisely controlled arrangement. Using a template material comprising a plurality of channels formed into its surface, the electrodes can be laid into the channels and the template can be manipulated -for example, by rolling, folding and/or stacking -to form a wire bundle with a desired arrangement of wire electrodes in its cross-section.

Although illustrative embodiments of the invention have been described in detail herein with reference to the accompanying drawings, it is to be understood that the invention is not limited to those precise embodiments, and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope and spirit of the invention as defined by the appended claims.

Claims

CLAIMS1. A method of forming an electrochemical probe, the electrochemical probe comprising a wire bundle comprising a plurality of wire electrodes arranged alongside each other, and the method comprising: laying a plurality of said wire electrodes into channels disposed across a surface of at least one sheet of material with a predetermined spacing, the wire electrodes each comprising an inner conductive core and an outer insulating layer, and the outer insulating layer having a diameter less than or equal to 40 pm; and manipulating the at least one sheet of material holding the plurality of the wire electrodes to form the wire bundle having a cross-section in which an intersection of said at least one sheet of material with the cross-section of the wire bundle traces out a predetermined shape, wherein a two-dimensional distribution of points at which the wire electrodes intersect the cross-section of the wire bundle is dependent on the predetermined spacing of the channels across the surface of the at least one sheet and the predetermined shape.
2. The method of claim 1, wherein: the electrochemical probe has layered structure comprising multiple sheet layers, wherein the cross-section of the wire bundle intersects a plurality of the sheet layers; each sheet layer comprises a portion of said at least one sheet of material; and at least one of the sheet layers holds a subset of the wire electrodes.
3. The method of claim 2, wherein: the method comprises manipulating the at least one sheet of material holding the plurality of the wire electrodes to form the wire bundle by rolling the at least one sheet to form the multiple sheet layers.
4. The method of any of claims 2 to 3, wherein: the at least one sheet of material comprises a plurality of sheets of material; and the method comprises manipulating the plurality of sheets of material holding the plurality of the wire electrodes to form the wire bundle by stacking the plurality of sheets on top of each other to form the multiple sheet layers.
5. The method of any of claims 2 to 4, wherein: the method comprises manipulating the at least one sheet of material holding the plurality of the wire electrodes to form the wire bundle by folding the at least one sheet to form the multiple sheet layers.
6. The method of any preceding claim, comprising: manipulating the at least one sheet of material holding the plurality of the wire electrodes to form the wire bundle having a hollow channel along its length; wherein the cross-section of the wire bundle is configured to have a hollow region in its centre corresponding to the hollow channel.
7. The method of claim 6, comprising: manipulating the at least one sheet of material holding the plurality of the wire electrodes to form the wire bundle by manipulating the at least one sheet to arrange the at least one sheet around a support structure, to provide the hollow channel at a location corresponding to the support structure.
8. The method of any preceding claim, comprising: controlling the thickness of the at least one sheet of material at predetermined positions, wherein the predetermined shape is dependent on the thickness of the at least one sheet of material at the predetermined positions.
9. The method of any preceding claim, wherein: the wire bundle comprises at least a subset of the wire electrodes each having a protruding section extending beyond the at least one sheet of material; and the method comprises controlling the length of the protruding section of each of the subset of the wire electrodes, by controlling the positions of the electrodes within the channels.
10. The method of claim 9, wherein: the length of the protruding section of each of the subset of the wire electrodes is selected in dependence of an object to be probed by the electrochemical probe.
11. The method of any of claims 9 and 10, wherein the subset of the wire electrodes include wire electrodes with different lengths of the protruding section. 30
12. The method of any of claims 9 to 11, wherein lengths of the protruding sections of the subset of the wire electrodes are tapered to provide one of: the length of the protruding section successively increasing from one side of the cross-section of the wire bundle to the other side of the cross-section of the wire bundle; and the length of the protruding section successively increasing from said one side of the cross-section of the wire bundle to an intermediate point of the cross-section and then successively decreasing from said intermediate point to the other side of the cross-section of the wire bundle.
13. The method of any preceding claim, wherein: the two-dimensional distribution of points at which the wire electrodes intersect the cross-section of the wire bundle is selected in dependence of an object to be probed by the electrochemical probe.
14. The method of any preceding claim, wherein: the channels disposed across the surface of at the least one sheet of material are parallel, prior to performing the manipulation.
15. The method of any preceding claim, wherein the material of said at least one sheet is a flexible material.
16. The method of any preceding claim, wherein: the material comprises a polymer.
17. The method of claim 16, wherein: the material comprises polydimethylsiloxane (PDMS).
18. The method of any preceding claim, comprising: forming the at least one sheet of material from a mould, wherein the mould comprises a pattern corresponding to an arrangement of the channels in the surface of at the least one sheet of material.
19. The method of claim 18, wherein said pattern is formed in the mould by photolithography.
20. The method of any preceding claim, wherein: a thickness of the at least one sheet of material is less than or equal to 500 pm.
21. The method of any preceding claim, wherein: the channels are separated by at least 20 pm.
22. The method of any preceding claim, wherein: the wire electrodes comprise a layer of metal nano-structures or metal oxide nano-structures deposited on tips of the wire electrodes at a first end or second end of the wire bundle.
23. The method of any preceding claim, comprising: forming a probing device comprising the electrochemical probe and an integrated circuit, the integrated circuit comprising a plurality of contact portions, each contact portion being configured to receive an electrode signal from one of the plurality of wire electrodes, and an amplifying portion to amplify the electrode signal received at the contact portions.
24. The method of claim 23, comprising: arranging connection layers of metal nano-structures on the tips of the wire electrodes at a contact end of the wire bundle to be in contact with corresponding contact portions of the integrated circuit.
25. An electrochemical probe comprising: a wire bundle comprising a plurality of wire electrodes arranged alongside each other, the wire electrodes each comprising an inner conductive core and an outer insulating layer, and the outer insulating layer having a diameter less than or equal to 40 pm; wherein the wire bundle is formed of at least one sheet of material holding a plurality of said wire electrodes in channels disposed across the surface of the at least one sheet of material with a predetermined spacing; and wherein the wire bundle has a cross-section in which an intersection of said at least one sheet of material with a cross-section of the wire bundle traces out a predetermined shape, wherein a two-dimensional distribution of points at which the wire electrodes intersect the cross-section of the wire bundle is dependent on the predetermined spacing of the channels across the surface of the at least one sheet and the predetermined shape.