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WO2024211307A1 - Capteurs de contrainte conformes - Google Patents

Capteurs de contrainte conformes Download PDF

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Publication number
WO2024211307A1
WO2024211307A1 PCT/US2024/022669 US2024022669W WO2024211307A1 WO 2024211307 A1 WO2024211307 A1 WO 2024211307A1 US 2024022669 W US2024022669 W US 2024022669W WO 2024211307 A1 WO2024211307 A1 WO 2024211307A1
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WO
WIPO (PCT)
Prior art keywords
strain sensor
electrically conductive
conductive trace
flexible electrically
viscoelastic
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
PCT/US2024/022669
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English (en)
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WO2024211307A8 (fr
Inventor
Conor J. WALSH
Oluwaseun A. ARAROMI
Addison LIU
Robert J. Wood
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Harvard University
Original Assignee
Harvard University
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Filing date
Publication date
Application filed by Harvard University filed Critical Harvard University
Publication of WO2024211307A1 publication Critical patent/WO2024211307A1/fr
Publication of WO2024211307A8 publication Critical patent/WO2024211307A8/fr
Anticipated expiration legal-status Critical
Pending legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B1/00—Measuring instruments characterised by the selection of material therefor
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
    • G01B7/00—Measuring arrangements characterised by the use of electric or magnetic techniques
    • G01B7/16—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring the deformation in a solid, e.g. by resistance strain gauge
    • G01B7/18—Measuring arrangements characterised by the use of electric or magnetic techniques for measuring the deformation in a solid, e.g. by resistance strain gauge using change in resistance
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01L—MEASURING FORCE, STRESS, TORQUE, WORK, MECHANICAL POWER, MECHANICAL EFFICIENCY, OR FLUID PRESSURE
    • G01L1/00—Measuring force or stress, in general
    • G01L1/20—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress
    • G01L1/22—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress using resistance strain gauges
    • G01L1/2287—Measuring force or stress, in general by measuring variations in ohmic resistance of solid materials or of electrically-conductive fluids; by making use of electrokinetic cells, i.e. liquid-containing cells wherein an electrical potential is produced or varied upon the application of stress using resistance strain gauges constructional details of the strain gauges

Definitions

  • Embodiments are related to compliant strain sensors.
  • Soft, compliant force sensors may be used in wearable electronics, wearable robotics, soft robotics, dexterous grippers, and prosthetics.
  • the mechanical compatibility of soft sensors with the human body may also help facilitate interactions between machines and people with minimal mechanical constraints.
  • a strain sensor may include at least one flexible electrically conductive trace including a plurality of sections of the at least one electrical tract that are arranged adjacent to one another, and a non- viscoelastic elastomeric material at least partially encapsulating the at least one flexible electrically conductive trace, wherein the non- viscoelastic elastomeric material is an electrically insulating material, and wherein the non- viscoelastic elastomeric material is configured to bias the plurality of adjacent sections of the at least one flexible electrically conductive trace towards one another.
  • a method of sensing strain may comprise biasing a plurality of adjacent sections of at least one flexible electrically conductive trace of a strain sensor towards one another with a non-viscoelastic elastomeric material, wherein the non- viscoelastic material is an electrically insulating material, applying a strain to the strain sensor, and moving the plurality of adjacent sections of the at least one flexible electrically conductive trace towards a spaced apart configuration to change an electrical resistance of the strain sensor in response to the applied strain, wherein the resistance of the strain sensor is indicative of the applied strain.
  • Fig. 1A depicts a perspective view of a strain sensor, according to one embodiment
  • Fig. IB depicts a cross-section of a strain sensor, according to one embodiment
  • Fig. 2A-2F depicts one method of making a strain sensor, according to one embodiment
  • Fig. 3 depicts a comparison between sensor signal responses for different sensor constructions, according to one embodiment
  • Fig. 4 depicts an application of a strain sensor on a user, according to one embodiment.
  • SCARS anisotropically resistive structures
  • SCARS anisotropically resistive structures
  • these sensors were previously manufactured using pressure sensitive adhesives and elastomers with strong viscoelastic responses.
  • the Inventors recognized that the inclusion of these viscoelastic elastomeric materials in these prior SCARS sensors resulted in significant stress relaxation and hysteretic behavior being exhibited by these prior sensors which may impact the ability of these sensors to be used in applications where high bandwidth and/or long-term signal stability may be desired.
  • the Inventors have recognized that many soft sensor technologies utilize materials which possess non-negligible viscoelastic properties which again may lead to viscosity-induced drift and other non-linear phenomena in the sensor response. These non-linear sensor responses, such as signal hysteresis and/or signal tapering, may lead to inaccurate sensor readouts and may lead to reduced sensor readout stability over time. Furthermore, due to creep, the non-linear behavior of many sensors may worsen over time and eventually progress to an extent where sensor readings can no longer be relied on, which may result in a need for the sensor to be replaced.
  • the strain sensors disclosed herein may include a non- viscoelastic and electrically insulating elastomeric material that is operatively coupled either directly, or indirectly, to one or more corresponding flexible electrically conductive traces.
  • the one or more flexible electrically conductive traces may be formed in a pattern such that a plurality of adjacent portions of the traces may be biased towards one another in an initial unbiased configuration.
  • non-viscoelastic elastomeric material may be provided in the form or one or more layers that at least partially, and in some embodiments fully, encapsulate the one or more flexible electrically conductive traces (e.g., two layers disposed on opposing sides of the one or more flexible electrically conductive traces).
  • a strain sensor may include at least one, and in some instances a plurality of parallel and/or serially arranged, flexible electrically conductive traces.
  • Each flexible electrically conductive trace may include a plurality of sections that are arranged adjacent to one another when the flexible electrically conductive trace is in an unbiased or unstressed configuration. In some embodiments, this may correspond to the flexible electrically conductive trace forming a meandering pattern moving back and forth within an area to form the plurality of adjacent sections which may be in contact with one another when biased towards each other in a first unstrained condition to provide an electrical path from a first electrical contact of the trace to a second electrical contact of the trace.
  • the flexible electrically conductive trace may be sufficiently elastic such that the contacting sections may be deformed such that they move apart from one another as the sensors move towards a spaced apart configuration such that the contact area between the adjacent sections is reduced as the flexible electrically conductive trace is deformed.
  • the at least one flexible electrically conductive trace may be at least partially, and in some embodiments fully, encapsulated within an electrically insulating and non- viscoelastic elastomeric material.
  • the at least one flexible electrically conductive trace may be disposed between two opposing layers of the non- viscoelastic elastomeric material.
  • the non- viscoelastic elastomeric material may be bonded to the at least one flexible electrically conductive trace in a pre-stretched configuration such that the non-viscoelastic elastomeric material may bias the plurality of adjacent sections of the at least one flexible electrically conductive trace towards one another in the absence of strain being applied to the strain sensor.
  • an applied force may overcome the force biasing the plurality of adjacent sections of the at least one flexible electrical trace towards one another by the pre-stretched non- viscoelastic elastomeric material such that the plurality of adjacent sections may move towards a spaced apart configuration which may change the resistance of the strain sensor in response to the applied strain.
  • This change in resistance may be indicative of the applied strain, and a signal related to this resistance may be output from the strain sensor to one or more associated processors.
  • small strains typically less than 3%, may be transduced as large changes in electrical resistance.
  • an electrical resistance of the strain sensor may be greater in the spaced apart configuration as compared to an electrical resistance of the strain sensor in an unstrained configuration.
  • non-viscoelastic elastomeric material in the strain sensors disclosed herein is desirable to reduce the presence of non-linear behavior in the disclosed sensors, the Inventors have recognized that typical non-viscoelastic elastomeric materials, such as silicone, may not readily bond to materials used for the flexible electrically conductive traces. Poor bonding between these layers may lead to separation between the layers of the strain sensor even with the presence of an adhesive.
  • This intermediate bonding layer may include an intermediate material which exhibits favorable bonding properties to both the selected non- viscoelastic elastomeric material and the at least one flexible electrically conductive trace.
  • the intermediate material may be disposed between the electrically conductive trace and the non- viscoelastic elastomeric material, and the intermediate material may form a stronger bond with the flexible electrically conductive trace and the non-viscoelastic elastomeric material as compared to a bond between the flexible electrically conductive trace and the non-viscoelastic elastomeric material.
  • the intermediate material may also be a non- viscoelastic elastomeric material to again avoid introducing undesirable non-linear effects into the strain sensor. Similar to the non-viscoelastic elastomeric material, the intermediate material may also be electrically insulating.
  • the intermediate material may include polyethylene, polyethylene terephthalate, polyamide, combinations of the forgoing, and/or any other appropriate non- viscoelastic elastomeric material that may bond with the at least one flexible electrically conductive trace and the non-viscoelastic elastomeric material.
  • the intermediate bonding material may have a Young’s modulus between 0.1 MPa and 50 MPa.
  • the intermediate material may have a thickness greater than or equal to 10 nm.
  • the intermediate material may also have a thickness less than or equal to 750 pm thick. In some embodiments, a thickness that is less than 25 pm may be preferred.
  • any appropriate type of material, Young’s modulus, and/or range of thicknesses may be used for the intermediate layer as the disclosure is not limited in this fashion.
  • a flexible electrically conductive trace may refer to any appropriate component exhibiting sufficient electrical conductivity, mechanical strength, flexibility, and elastic recovery to function in the manner described herein.
  • Appropriate materials for a flexible electrically conductive trace may include but are not limited to conductive fiber composites (e.g., carbon fiber composites), conductive polymers, conductive polymer composites, super elastic shape memory metals such as super elastic nickel titanium alloys, and/or metal-coated fiberglass composites.
  • Possible conductive polymer composites include non-conductive polymers such as poly-aramids, e.g., Kevlar, coated and/or impregnated with conductive materials.
  • poly-aramids e.g., Kevlar
  • the flexible electrically conductive trace may comprise a sheet of conductive fibers.
  • these fiber sheets may include unidirectionally- aligned conductive fibers, such as a unidirectionally-aligned carbon fiber composite (CFC).
  • the flexible electrically conductive trace may comprise at least one layer of a sheet of aligned fibers pre-impregnated with an epoxy resin.
  • the sheets of fibers may comprise a weave, mat, and/or any other applicable relatively planar construction of a sheet of conductive fibers.
  • the number of layers of aligned fiber sheets in the flexible electrically conductive trace may be less than or equal to three and/or five, and/or any suitable number of layers maintains desired sensor properties.
  • the layers of unidirectionally-aligned fibers may be arranged such that the alignment directions of the fibers in adjacent layers may be oriented at an angle to each other, e.g., an orthogonal or other appropriate arrangement with an angle difference between the different layers.
  • the orientation of the fibers of adjacent layers in different directions may be advantageous by preventing and/or decreasing the probability of curling of the flexible electrically conductive trace occurring associated with a curing process for the flexible electrically conductive trace.
  • the sheets with unidirectional alignment of fibers may also provide greater strength in the alignment direction of the fibers when compared to sheets with non-unidirectionally aligned fibers.
  • the aligned fibers in an embodiment, may be greater than or equal to 3 pm in diameter.
  • the diameter of the fiber may also be less than 3 pm in diameter. Of course diameters both less than and greater than those noted above are contemplated as the disclosure is not so limited.
  • the configuration of the stacked layers of conductive fibers may result in increased electrical conductivity in the direction of the aligned fibers of the outer layers of the stack of conductive fibers forming the at least one flexible electrically conductive trace, and may provide the flexible electrically conductive trace with anisotropic properties.
  • Electrical conductivity anisotropy may have a significant effect on the performance of the strain sensor. For example, if the outer layers of the stacked conductive fibers extend lengthwise along the segments of the meander and/or plurality of individual elements, the anisotropy may be oriented such that the sensitivity of the strain sensor may significantly decrease.
  • the strain sensor may be less sensitive with the fibers in a lengthwise configuration as the resistance of the strain sensor changes to a lesser extent.
  • the anisotropy may be oriented such that the sensitivity of the strain sensor may significantly increase.
  • the fiber directions in the various layers may be oriented to be non-parallel to the longitudinal axis of the associated flexible electrically conductive trace. Stated in another way, the fiber directions of the various layers may extend at least partially in an extension (e.g., a length) direction of the sensor.
  • the at least one flexible electrically conductive trace may be a continuous structure.
  • the at least one flexible electrically conductive trace may be a meander having a serpentine-like shape or any elongated pattern that places segments of the flexible electrically conductive trace adjacent to another one.
  • each segment of the meander may be connected to an adjacent segment by the turns at the top and bottom of the serpentine meander.
  • the meander may provide geometric stretchability for the at least one flexible electrically conductive trace.
  • the meander in some embodiments, may be formed by doubling an elongated conductive filament back on itself multiple times, while in another embodiment, may be formed by cutting out the profile of the meander from a planar piece of material, such as via a laser cutting process later described in more detail.
  • the pre-stretch of the non-viscoelastic elastomeric material of the encapsulating layer may be configured to bias the plurality of adjacent segments of the at least one electrically conductive trace towards one another. Segments of the meander which come into contact with adjacent segments may form low resistance pathways which allow current to flow across. Without wishing to be bound by theory, current may flow through contracted portions and thereby across the flexible electrically conductive trace.
  • a flexible electrically conductive trace may have any appropriate thickness and/or shape in the various embodiments disclosed herein.
  • a cross section of a flexible electrically conductive trace may be circular, oval, rectangular, square, and/or any other appropriate shape.
  • the flexible electrically conductive trace may have any appropriate thickness measured in a direction that is perpendicular to a plane the flexible electrically conductive trace lies within when in an unbiased state, or other similar direction, that is greater than or equal to 10 nm, 50 nm, 100 nm, 500 nm, 1 pm, 100 pm, 500 pm, and/or any other appropriate thickness.
  • the thickness may also be less than or equal to 750 pm, 500 pm, 100 pm, 1 pm, 500 nm, 100 nm, and/or any other appropriate thickness. Combinations of the forgoing are contemplated including thicknesses between or equal to 10 nm to 750 pm, or more preferably between or equal to 10 nm and 25 pm. Of course thicknesses both less than and greater than those noted above are contemplated as the disclosure is not so limited.
  • the flexible electrically conductive traces disclosed herein may also have a width, e.g., a transverse dimension perpendicular to the thickness and a longitudinal axis of the trace extending along a length of the different adjacent segments of the trace, greater than or equal to 10 nm, 50 nm, 100 nm, 500 nm, 1 pm, 5 pm, 25 pm, 100 pm, 500 pm, and/or any other appropriate width.
  • the width may also be less than or equal to 750 pm, 500 pm, 100 pm, 25 pm, 5 pm, 1 pm, 500 nm, 100 nm, and/or any other appropriate thickness. Combinations of the forgoing are contemplated including widths between or equal to 10 nm to 750 pm, or more preferably 5 pm to 25 pm.
  • a spacing between adjacent segments of the patterned electrical trace in an unbiased configuration prior to encapsulation may be greater than or equal to 50 nm. Similarly, a spacing between adjacent segments of the patterned electrical trace may be less than or equal to 10 pm. Of course widths and spacings both less than and greater than those noted above are contemplated as the disclosure is not so limited.
  • the at least one flexible electrically conductive trace may also comprise a plurality of individual elements that are arranged adjacent to one another, but not physically connected to one another.
  • such or all of the plurality of individual elements may be brought into contact with adjacent elements by releasing the prestretch of the non-viscoelastic elastomeric encapsulation material to form a contacted portion therebetween.
  • the plurality of individual elements may be arranged parallel or askew to one another. For example, in an embodiment, adjacent individual elements may be angled away from each other by less than or equal to 30 degrees.
  • the plurality of individual elements be presented in any orientation suitable for allowing adjacent elements to come into contact with one another in an unbiased state in the presence of compressive forces applied by the non- viscoelastic elastomeric encapsulation material.
  • the material, dimensions, and geometry of the electrically conductive trace may define the initial electrical resistance and the mechanical stiffness of the segments. For example, thinner adjacent segments, for a given overall area of the at least one flexible electrically conductive trace, may result in a higher initial resistance and also a reduced mechanical stiffness. For such an embodiment, a comparatively lower prestretch may be required in the non-viscoelastic elastomeric encapsulation material to produce a desired segment deflection for a given spacing of thinner segments. Conversely, for thicker segments, a comparatively higher pre-stretch in the non-viscoelastic elastomeric encapsulation material may be required to achieve the same desired segment deflection for the same given spacing. In some embodiments, the spacing between segments may also contribute to the amount of deflection required for adjacent segments to make electrical contact. For example, a greater amount of pre- stretch may be required to bring adjacent segments with greater spacing between adjacent segments into contact, with all other variables being held constant.
  • a flexible electrically conductive trace in any of the embodiments disclosed herein may be made from a material with sufficient elasticity and stiffness to permit it to be repeatedly deformed between a spaced apart configuration and unbiased configuration.
  • a Young’s modulus of the flexible electrically conductive trace may between 50 MPa and 50 GPa.
  • the flexible electrically conductive trace may exhibit a recoverable elastic strain range that is less than or equal to 10%, 5%, or any other appropriate strain.
  • the recoverable strain may be greater than or equal to 1%, 5%, and/or any other appropriate strain. Combinations of the above are contemplated including recoverable strains between or equal to 1% and 10%.
  • the flexible electrically conductive trace may also exhibit a loss factor less than or equal to 0.02, 0.01, 0.005 and/or any other appropriate loss factor.
  • the loss factor may also be greater than or equal to 0.001, 0.005, 0.01, and/or any other appropriate range. Combinations of the forgoing are contemplated including, for example, a loss factor between or equal to 0.001 and 0.02.
  • a loss factor also known as a dissipation factor
  • a flexible electrically conductive trace may also capable of withstanding a torque of at least 10 Nm or other appropriate torque.
  • a Young’s modulus, elastic strain range, and a loss factor may be measured using a stress strain response of an appropriate material sample subjected to a tensile test, such as a Dynamic Material Analysis (DMA).
  • DMA Dynamic Material Analysis
  • any appropriate type of non-viscoelastic elastomeric materials may be used including, but are not limited to, silicone, thermoplastic polyurethane, natural rubber, latex, poly-aramid resins, polyimides, and other appropriate elastomeric materials with sufficiently low viscoelastic properties. It should be understood that a degree of cross linking, molecular weight, and/or other appropriate material parameters may affect the properties of the polymer. Thus, any of the above noted materials may be appropriately selected to provide the desired material properties disclosed herein.
  • a non- viscoelastic elastomeric material layer for an encapsulant may have any appropriate thickness measured in a direction that is perpendicular to a plane the layer lies within when in an unbiased state, or other similar direction, that is greater than or equal to 50 pm, 100 pm, 250 pm, 500 pm, 1 mm, 2.5 mm and/or any other appropriate thickness.
  • the thickness may also be less than or equal to 5 mm, 2.5 mm, 1 mm, 500 pm, 250 pm, and/or any other appropriate thickness. Combinations of the forgoing are contemplated including thicknesses between or equal to 50 pm and 5 mm, or more preferably 50 pm to 500 pm. Of course thicknesses both less than and greater than those noted above are contemplated as the disclosure is not so limited.
  • a non- viscoelastic elastomeric material may be used at least in part to construct an encapsulant and/or encapsulating layer.
  • the non-viscoelastic elastomeric encapsulant may be pre-stretched before being attached to the at least one flexible electrically conductive trace and/or intermediate bonding layer.
  • the pre-stretch of the encapsulating layer may determine an amount of stored elastic energy present within the strain sensor in an unbiased state, and consequently may determine the amount of shear stress transmitted to the at least one flexible electrically conductive trace and the resistance range of the strain sensor.
  • the encapsulant may be pre-stretched to greater than or equal to 1%, 10%, 20%, and/or any other appropriate strain.
  • the encapsulant may also be pre-stretched to less than or equal to 50%, 40%, 30%, 20%, 10%, 5% or any other appropriate strain. Combinations of the forgoing are contemplated including, for example, strains between or equal to 1% and 50%. It is noted that increasing the pre-stretch beyond 50% may decrease the sensitivity of the strain sensor and/or the overall linearity of the sensor response. Additionally, excessive pre-stretch may result in a saturation zone at the beginning of the sensor response as at a certain point, as the electrical resistance between adjacent traces in the unbiased configuration approaches a minimum as the contact area reaches a maximum.
  • the encapsulant may preferably be pre-stretched between or equal to 1% to 5% though other strains may also be used.
  • compressive strains may be induced within the encapsulating layer by thermal shrinkage.
  • materials such as thermoplastic polyurethane (TPU) and/or polyester TPU may experience thermal shrinkage when heated at high temperatures.
  • TPU thermoplastic polyurethane
  • the at least one flexible electrically conductive trace and/or intermediate bonding layer may be bonded to an unstretched encapsulating comprising a thermally shrinkable material and may be subsequently exposed to heat to induce thermal shrinking.
  • Compressive strains may be induced within the strain sensor after thermal shrinking and cause at least a portion of adjacent segments and/or elements of the at least one flexible electrically conductive trace to come into contact. Therefore, thermal shrinkage may have a similar effect to encapsulating the flexible electrically conductive and/or intermediate bonding layer in a pre-stretched non- viscoelastic elastomeric encapsulating layer.
  • a non- viscoelastic elastomeric material such as the non- viscoelastic elastomeric material and non- viscoelastic intermediate material, and other similar terms in any of the embodiments disclosed herein may refer to a material exhibiting a sufficiently low viscoelastic response, elasticity, and stiffness to provide the disclosed sensor performance characteristics during repeated deformations between spaced apart and unbiased configurations. In some embodiments, this may include elastomers exhibiting a Young’s modulus between 0.1 MPa and 50 MPa. The elastomeric material may also exhibit a loss factor less than or equal to 0.05, 0.02, 0.01, 0.005 and/or any other appropriate loss factor.
  • the loss factor may also be greater than or equal to 0.001, 0.005, 0.01, and/or any other appropriate range. Combinations of the forgoing are contemplated including, for example, a loss factor between or equal to 0.001 and 0.05.
  • the non- viscoelastic elastomeric material may also be capable of elastically recovering strains that are less than or equal to 150%, 100%, 50%, 20%, 10%, or any other appropriate strain.
  • the elastically recoverable strain may also be greater than or equal to 10%, 20%, 50%, 100%, and/or any other appropriate strain. Combinations of the forgoing are contemplated including elastically recoverable strains between or equal to 10% and 150%, 20% and 50%, and/or any other appropriate strain.
  • An appropriate test for measuring these material properties may include tensile strain tests and/or DMA.
  • a compliant strain sensor corresponding to any of the embodiments disclosed herein may exhibit a range of desired operating properties.
  • a strain sensor may have a working strain sensing range that is less than or equal to 10% strain and/or any other appropriate range.
  • the strain sensor may also be able configured to mechanically withstand and/or recover from less than or equal to 50%, 100%, or other strain as noted above.
  • Another property for strain sensor characterization is settling time, also referred to as stress relaxation time, which may be an amount of time over which the strain sensor output signal may settle within a certain range of the final output value after being subjected to strain.
  • stress relaxation time also referred to as stress relaxation time, which may be an amount of time over which the strain sensor output signal may settle within a certain range of the final output value after being subjected to strain.
  • a desirable sensor response may have a relatively steady signal after static and/or quasistatic loading.
  • the undesirable sensor response may include a sharp drop in output signal followed by a constant decline such that the signal does not approach a steady state sensor reading, e.g., flat, signal response.
  • Acceptable ranges to determine a settled signal may depend on the desired application.
  • a sensor constructed according to the embodiments disclosed herein may exhibit a settling time less than or equal to 10 seconds, 5 seconds, 1 second, or other appropriate time response.
  • the settling time may also be greater than or equal to 100 nanoseconds, 1 ps, 1 ms, 500 ms, 1 s, and/or any other appropriate time response. Combinations of the above are contemplated including, for example, a settling time between or equal to 100 nanoseconds and 10 seconds.
  • a desirable sensor response may also include minimal hysteresis phenomenon during loading, as seen in Fig. 3, such that the signal output by the strain sensor may remain relatively consistent at the same states during compression compared to relaxation during sequential loading cycles.
  • sensors including pressure sensitive adhesives and elastomeric materials exhibiting larger amounts of viscoelastic properties may exhibit increased viscous behavior which may be associated with increased stress relaxation and hysteresis phenomena as indicated in the figure.
  • the non-viscoelastic compliant strain sensor may be used for human physiological monitoring, as described in more detail below.
  • the non- viscoelastic strain sensors disclosed herein may be used for any application which comprises flexible strain measurements, including but not limited to soft robotics, microfluidics, arteries and/or veins, textile deformations.
  • the small and customizable sizes of the strain sensors they may be adapted for use in a multitude of technologies.
  • the disclosed compliant strain sensor may offer a range of benefits including, but not limited to flexibility, elastic deformation, creep-resistance, improved signal settling times, and small sizing.
  • the disclosed sensors may also offer increased comfort, adaptability, adjustability, and unobtrusiveness when implemented for sensing applications in wearable technologies.
  • embodiments in which other benefits different from those listed above are provide are also possible as the disclosure is not so limited.
  • Fig. 1A shows a perspective view of one embodiment of a compliant strain sensor 100, where a flexible electrically conductive trace 102 is disposed between two encapsulating layers 104 of non-viscoelastic elastomeric material in the form of a top layer 106 and a bottom layer 108 that at least partially encapsulate the one or more traces of the device.
  • the flexible electrically conductive trace 102 may include a plurality of adjacent segments 110 that may be positioned adjacent to and extend at least partially in the same direction as other adjacent segments.
  • the depicted segments are provided in the form of a continuous serpentine pattern though other meandering patterns with adjacent segments may also be used.
  • a spacing between adjacent segments 110, and the corresponding contact area may be used to control the overall resistance of the strain sensor 100 as described previously above.
  • the strain sensor 100 rests in an unbiased configuration, where compressive forces within the strain sensor (e.g., from a pre-stretched bias of the non-viscoelastic elastomeric material) biases the adjacent segments 112 towards one another, such that at least a portion of the segments contact each other and form a contacting portion 112.
  • Fig. IB depicts a cross-section of the strain sensor 100 shown in Fig. 1A.
  • a non-viscoelastic intermediate material 114 may be disposed between the non-viscoelastic elastomeric encapsulating layer 104 and the flexible electrically conductive trace to improve a bond between the non-viscoelastic intermediate material and the associated one or more flexible electrically conductive traces.
  • the intermediate material may exhibit an increased bonding strength to the encapsulating layer and to the flexible electrically conductive trace than a bonding strength between the encapsulating layer and the flexible electrically conductive trace.
  • a non- viscoelastic intermediate material 114 may be bonded between the non- viscoelastic elastomeric encapsulating layer 104 and the flexible electrically conductive trace 102.
  • the strain sensor may optionally comprise one or more inner non-viscoelastic elastomeric layers 116 and 118 disposed between the non- viscoelastic intermediate layer 114 and the outer non- viscoelastic elastomeric encapsulating layer 104.
  • the inner non-viscoelastic elastomeric layers may be deposited as an uncured material (e.g., an uncured resin) between a cured intermediate material layer and corresponding outer encapsulating layer to bond the outer encapsulating layer thereto once the one or more inner layers are cured. This process may facilitate bonding between the non- viscoelastic intermediate material layer and the associated non-viscoelastic elastomeric encapsulating layer.
  • a semi-solid film of uncured resin may be deposited as a first layer 116 directly on the intermediate material layer 114 and a second uncured liquid resin layer may be deposited onto the first layer 116.
  • the strain sensor may only have a single inner non- viscoelastic elastomeric material layer disposed between a non- viscoelastic intermediate layer and the non-viscoelastic elastomeric encapsulating layer as the disclosure is not so limited.
  • any appropriate type of non- viscoelastic elastomeric materials may be used for the various non- viscoelastic elastomeric material layers.
  • either the same or different materials as the non- viscoelastic elastomeric encapsulating layer may be used.
  • These inner non-viscoelastic elastomeric material layer for the second non- viscoelastic elastomeric material layer may have any appropriate thickness measured in a direction that is perpendicular to a plane the layer lies within when in an unbiased state, or other similar direction, that is greater than or equal to 10 pm, 25 pm, 50 pm, 100 pm, 250 pm, 500 pm, and/or any other appropriate thickness.
  • the thickness may also be less than or equal to 750 pm, 500 pm, 250 pm, 100 pm, 50 pm, 25 pm, and/or any other appropriate thickness. Combinations of the forgoing are contemplated including thicknesses between or equal to 10 pm to 750 mm, or more preferably between or equal to 10 pm and 25 pm. Of course thicknesses both less than and greater than those noted above are contemplated as the disclosure is not so limited.
  • the flexible electrically conductive trace 102 may comprise multiple layers of unidirectionally-aligned fibers, e.g., five layers of unidirectionally-aligned conductive fiber composites.
  • the layers of the flexible electrically conductive trace may be impregnated with a resin, and activation and/or curing of the preimpregnated resin may bond the layers of the electrically conductive member together as well as bond the intermediate layer to the flexible electrically conductive trace.
  • the surface of the intermediate layers not bonded to the electrically conductive member may optionally be treated with oxygen plasma or otherwise treated with an appropriate coating and/or surface treatment to improve bonding with the one or more subsequently deposited layers of non- viscoelastic elastomeric material.
  • the Inventors recognized the need to create a new fabrication method for strain sensors to accommodate for the common unsatisfactory bonding properties between non- viscoelastic elastomeric materials and the flexible electrically conductive traces.
  • adhesive films were used to bond the flexible electrically conductive trace to previous encapsulation materials.
  • the encapsulation material was changed to silicone to improve the non- viscoelastic properties of the SCARS sensor and the new silicone encapsulation material did not bond well with the previously-used adhesive sheets.
  • the flexible electrically conductive trace 102 e.g., carbon fiber composite sheets
  • a non-viscoelastic intermediate material layer 114 e.g., polyethylene (PE) films of 25 pm thickness, as shown in Fig. 2A.
  • the non-viscoelastic intermediate material layer 114 may prevent separation between layers within the strain sensor as the non- viscoelastic intermediate material can provide improved bonding to the non-viscoelastic encapsulating material layer 104 and to the flexible electrically conductive trace 102, as compared to the bonding directly between the flexible electrically conductive trace and the non- viscoelastic encapsulating material.
  • the non-viscoelastic intermediate material layer 114 may be placed on the outermost surfaces of the flexible electrically conductive material 102 in the plane of the strain sensor, and the combination of the non- viscoelastic intermediate material and the flexible electrically conductive material may be hereby referred to as an intermediate assembly 120, as seen in Fig. 2B.
  • the intermediate assembly 120 comprising the electrically conductive material and the intermediate layer may be subjected to heat and pressure, such as in a heat press, which may bond the flexible electrically conductive trace 102 to the non-viscoelastic intermediate material 114.
  • a resin may be pre-impregnated in the layers of the flexible electrically conductive trace. Additionally, as seen in Fig.
  • the asymmetric stress throughout the intermediate assembly 120 which may induce curling due to heat applied during the curing process may be minimized by stacking the layers within the flexible electrically conductive trace 102 such that the direction of alignment of the fibers in each layer may be orthogonal, or otherwise angled, relative to fibers in adjacent layers.
  • the intermediate assembly 120 may be cut into a serpentine or other meandering pattern to form a plurality of adjacent portions of an electrical trace.
  • the cutting process, or micro-patterning process may be performed on a diode-pumped solid-state (DPSS) laser, as shown in Fig. 2B.
  • Other potential methods of cutting the intermediate assembly 120 may include waterjet, electrical discharge machining (EDM), lithographic cutting (electron beam lithography, soft lithography), femtosecond lasers micromachining, and/or any applicable manufacturing methods capable of machining appropriate feature sizes on a desired size scale, e.g., the micron scale.
  • additive manufacturing methods may also be used to create the desired pattern.
  • Potential additive manufacturing methods may comprise micro-molding, printing with conductive media, 3D printing conductive filament, and any other applicable additive manufacturing methods capable of machining appropriate feature sizes on the micron scale.
  • the width of the individual portions of the trace of the intermediate assembly 120 were approximately 200 pm in width, with spacing between adjacent segments and/or elements of approximately 10 pm, though other dimensions may also be used as previously discussed.
  • the combined intermediate assembly 120 may be then cleaned, e.g., with isopropyl alcohol in an ultrasonic cleaner, before the surface of the intermediate assembly may be treated with oxygen plasma, as shown in Fig. 2C.
  • Oxygen plasma treatment, as well as other surface treatments may improve bonding properties between the functionalized surface and a non-viscoelastic elastomeric material. Any methods which can improve the bonding properties of the intermediate assembly may be suitable, including but not limited to surface coating, functional layers, and mechanical roughening (e.g., sand blasting).
  • a first non- viscoelastic elastomeric material layer 116 was placed on the outermost surfaces of the intermediate assembly 120, as shown in Fig. 2D.
  • the first non-viscoelastic elastomeric material layers 116 was placed in a semi-cured form, such as a non- viscoelastic liquid elastomer adhesive thin film.
  • the first non-viscoelastic elastomeric layers 116 may be bonded and cured to the intermediate assembly 120 using minimal pressure, to ensure that liquid non-viscoelastic elastomeric material does not enter the spacing between the segments of the meander and/or plurality of individual elements of the intermediate assembly and interfere with the sensitivity and conductive properties of the strain sensor.
  • the first non-viscoelastic elastomeric material layers 116 may act as a mechanical barrier, ensuring that any subsequent layers do not enter the spacing between the segments and/or elements of the intermediate assembly.
  • the cured first non-viscoelastic elastomeric material layers 116 may also act as an adhesion substrate to any subsequent layers.
  • Electrical contacts 122 which may electrically connect the strain sensor to a resistance measurement apparatus, may then be added to ends of the intermediate assembly 120, as shown in Fig. 2E.
  • the electrical contacts may use any appropriate flexible conductive materials, for example, in one embodiment, the electrical contacts may be 1.5 mm strips of copper-coated polyimide films.
  • the electrical contacts 122 may be connected to the intermediate assembly 120 using a conductive epoxy, e.g., a silver conductive epoxy, or other appropriate type of bonding method at connections 124.
  • a four-point resistance measurement approach comprising four connection 124 locations, may be used to eliminate the effects of contact resistance and changes in the electrical conductivity of the conductive epoxy over time. Then, as shown in Fig. 2F, the assembly from Fig.
  • the second non-viscoelastic elastomeric material layers 118 and the non-viscoelastic elastomeric encapsulating layers 104 may be at least partially encapsulated by the second non-viscoelastic elastomeric material layers 118 and the non-viscoelastic elastomeric encapsulating layers 104.
  • the second non- viscoelastic elastomeric material layers may be applied in a liquid form and act as an adhesive between the flexible electrically conductive trace and/or the first non- viscoelastic elastomeric material layers 116 (acting as a mechanical barrier to prevent the adhesive from entering the flexible electrically conductive trace), and the non- viscoelastic elastomeric materials.
  • the second non- viscoelastic elastomeric material layers may be applied in a liquid form and act as an adhesive between the flexible electrically conductive trace and/or the first non- viscoelastic elastomeric material layers 116 (acting as a mechanical
  • the non- viscoelastic elastomeric encapsulating layers may be pre-stretched to ensure that upon release, adjacent portions of the flexible electrically conductive trace are biased towards each other to place the separate portions into electrical contact through the side portions of the traces oriented towards each other in a neutral unbiased (i.e., unstretched) configuration of the sensor.
  • the pre-stretch may be performed by acrylic stretching jigs, and/or any suitable mechanism capable of inducing and maintaining a strain within a material during a curing process.
  • the second non- viscoelastic elastomeric material layers 118 may be cured after the non- viscoelastic elastomeric encapsulating layer has been applied to the sensor assembly.
  • the second non-viscoelastic elastomeric material layers may be cured using any appropriate resin curing pressures and/or temperatures as the first non- viscoelastic elastomeric material layers may help to prevent the liquid second non-viscoelastic elastomeric material layers from entering the spacing between the segments and/or elements of the at least of flexible electrically conductive trace.
  • the resulting strain sensor from the above-described fabrication method may exhibit a low profile with an overall thickness of 0.75 mm.
  • the above described compliant strain sensors were used in a wearable sensor arrangement for physiological monitoring. Applied to the body, the strain sensor measured strains due to local deformations and muscle bulging, which may be correlated to muscle force output.
  • the sensitivity and size of the developed sensors can be easily adjusted and/or tuned according to a set of parameters, allowing for easy customization of the sensors for various monitoring needs.
  • the set of parameters may comprise at least one from a group of elastic modulus, settling time, recoverable strain, sensor dimensions, pre-stretch, sensitivity, and any other applicable material properties.
  • the sensors may be used to monitor muscle stiffness, fatigue, joint torque and/or degradation over time, which may be useful for monitoring diseases such as Parkinson’s.
  • the efficacy of the non-viscoelastic strain sensor 100 in human physiological monitoring was demonstrated in a laboratory setting. As shown in Fig. 4, a non- viscoelastic strain sensor 100 and a previously-developed TPU-PSA sensor 200 were placed on the forearm of an adult male participant, approximately on top of the belly of the extensor carpi ulnaris muscle 204. The placement of the strain sensor was used to determine palpation of the muscle and/or curvature of the muscle (muscle bulge), during wrist flexion. During the human physiological monitoring experiment, a participant was instructed to perform wrist extension, i.e., elevation of the hand, onto a force plate 202 which has been instrumented with a load cell.
  • wrist extension i.e., elevation of the hand
  • a two-dimensional laser profiler such as a LJ-X8200 from Keyence, was positioned over the non- viscoelastic strain sensor 100 and the measurements of the laser profile were used as a ground truth for any sensor deformation during the experiment.
  • the participant’s relative force production was tracked via the force plate 202.
  • the participant was instructed to produce a force and hold it for four seconds (with the assistance of a metronome) and subsequently release the force as quickly as they were able.
  • the results of the experiment show that the sensor response follows the general trend of the force plate signal.
  • the results show that the non- viscoelastic strain sensor is as mechanically resilient as its TPU-PSA counterpart for real-world use cases and as such, the sensor holds promise for applications in physiological monitoring.
  • sensors were tested experimentally to evaluate the mechanical properties. The responses to both axial strain as well as curvature configurations were demonstrated, representing the two modes of operation previously demonstrated with the SCARS mechanism.
  • axial strain mode sensors were secured to acrylic sample holders using bolts, nuts, and cyanoacrylate (CA) glue.
  • cyclic testing samples were moved through 0-10% strain cycles ten times at a constant displacement rate of 83 pm/s.
  • curvature mode sensors were adhered to tempered 125 pm-thick sheet steel with CA glue, then mounted to a rigid support on one end. On the other end, an anvil was attached to a mechanical tester which moved vertically to deflect the sheet metal, mimicking the bending conditions that may be encountered in wearable applications.
  • the zero position was taken to be where the anvil contacted the sheet metal in its undeformed, flat state.
  • the anvil was displaced 1mm downwards at a rate of 10 mm/s and held at that displacement for 300 seconds.
  • the anvil was moved through 5 mm of downward displacement at a rate of 0.4mm/s.
  • the linear response of the non- viscoelastic strain sensor was be compared to the prior SCARS architecture manufactured using thermoplastic polyurethane (TPU) and pressure sensitive adhesives (PSA), with each sensor having a R2 > 70% to a linear fit.
  • TPU thermoplastic polyurethane
  • PSA pressure sensitive adhesives
  • the observed response to static changes in curvature showed that the currently disclosed non-viscoelastic strain sensor construction exhibited a more stable response with considerably less drift over time where the sensor signal stabilized within approximately 2 seconds as compared to the TPU-PSA construction where the sensor signal did not stabilize within the testing period of 300 seconds. It should be noted that both sensors exhibited a small change in signal magnitude during the testing period - less than 6% for the TPU-PSA sensor and less 2% for the non- viscoelastic strain sensor.
  • Cyclic testing of the sensor in curvature mode was also performed and the drift in the sensor signal for the TPU-PSA sensor was notably larger than for the non-viscoelastic strain sensor, with the peak sensor value decreasing by approximately 3% for the TPU-PSA sensor compared with less than 1% for the non- viscoelastic strain sensor.

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  • Physics & Mathematics (AREA)
  • General Physics & Mathematics (AREA)
  • Measurement Of Length, Angles, Or The Like Using Electric Or Magnetic Means (AREA)

Abstract

L'invention concerne un capteur de contrainte comprenant un élément conducteur ayant une pluralité d'éléments agencés adjacents les uns aux autres, et un matériau élastomère non viscoélastique encapsulant l'élément conducteur, et un adhésif non viscoélastique, dans un état non polarisé, des forces de compression amenant au moins l'un de la pluralité d'éléments à entrer en contact avec au moins une partie d'un élément adjacent et, lorsqu'une contrainte est appliquée, une déformation élastique résultante amenant au moins l'un de la pluralité d'éléments à s'écarter d'un élément adjacent de sorte que la partie en contact diminue ou disparaît. L'invention concerne également un procédé de fabrication dudit capteur de contrainte.
PCT/US2024/022669 2023-04-03 2024-04-02 Capteurs de contrainte conformes Pending WO2024211307A1 (fr)

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Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20170059426A1 (en) * 2014-03-20 2017-03-02 The University Of Akron Flexible tactile sensors and methods of making
US20210215554A1 (en) * 2018-05-21 2021-07-15 President And Follows Of Harvard College Ultra-sensitive compliant strain sensors
WO2022026651A1 (fr) * 2020-07-29 2022-02-03 Bend Labs, Inc. Électrode et systèmes de blindage et procédés pour capteurs conformes

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US20170059426A1 (en) * 2014-03-20 2017-03-02 The University Of Akron Flexible tactile sensors and methods of making
US20210215554A1 (en) * 2018-05-21 2021-07-15 President And Follows Of Harvard College Ultra-sensitive compliant strain sensors
WO2022026651A1 (fr) * 2020-07-29 2022-02-03 Bend Labs, Inc. Électrode et systèmes de blindage et procédés pour capteurs conformes

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