Piezoresponsive textile incorporating graphene

Abstract

To provide a deformable material for use as part of a strain sensing system applicable to strain sensing over large areas.SOLUTION: There is provided an electrically conductive textile containing graphene, which reversibly changes electrical resistance upon deformation, formed from a network of fibers, wherein the textile is arranged in a flat plane and distortion is applied to the textile plane, the textile can undergo elastic deformation within the textile plane, or the textile is subjected to a distortion perpendicular to its plane, the textile can undergo elastic deformation perpendicular to its plane.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to the field of piezoelectric responsive textiles, in particular to conductive textiles that change electrical properties when strained. [Background technology]

[0002] Strain gauges are widely used. When used as pressure gauges, they are very precise and can be made from many materials and by many means. They are usually stand-alone electrical devices that use a material, or arrangement of materials, that undergoes a change in an electrical property when pressure is applied. The change in electrical property is usually resistance, capacitance, or inductance.

[0003] Deformation of a material changes the relative positions of the material's components, resulting in strain. Such deformations can be elastic or inelastic, or a combination of both or partially elastic, with each deformation resulting in some permanent deformation. Deformations can be compressive or extensive and can occur in any or all three physical dimensions. In practice, compressing a sheet of material by applying a force perpendicular to the plane of the sheet thins it in the area of ​​the applied force. Stretching a sheet of material that is compressible in the plane of the sheet often thins it. Various individual or combinations of these deformations can be used to measure strain.

[0004] Elasticity is formally measured as the "modulus of elasticity" (also known as the tensile modulus and Young's modulus). Elastic reversibility is the degree to which an object recovers its original shape after deformation. Generally, elasticity is considered reversible if the object remains fit for purpose after elastic deformation. Strain sensors can compensate for inelastic deformation.

[0005] Strain gauges often use deformable conductors, such as thin metal wires or foils in complex patterns, bonded to flexible or stretchable insulating sheets to maximize sensitivity. When the insulator is stretched, the conductor deforms, changing its resistance. As the conductor stretches, the electrical path becomes narrower and longer, increasing resistance. When compressed, the electrical path becomes shorter and wider, decreasing resistance. This effect can be described as piezoresistance. Such two-dimensional strain gauges can be placed on membranes to function as pressure sensors.

[0006] Piezoresistors can be used to measure variations in strain (and with contained pressure). Semiconductors such as silicon and germanium are well known piezoresistor materials. They undergo large changes in resistance with strain and make excellent high-precision and high-sensitivity pressure sensors.

[0007] Electrical resistance can be reported in many ways. For electrical conduction in thin sheets, the unit "ohms per square" ("ohm / sq" or "ohm / □") is often used and is called "sheet resistance." This unit has the practical advantage of reflecting the desired result regardless of how the material being measured is configured. For example, two sheets of a conductor can have different specific resistances, but can nevertheless give the same, desired sheet resistance when present at different thicknesses. Sheet resistance is usually applied to films of uniform thickness, but can also be applied to non-uniform sheets of conductors, such as textiles.

[0008] Resistance, capacitance, and inductance measurements can be accomplished by many means. Analog strain gauges can use Wheatstone bridges or potentiometers to determine the unknown resistance. Modern digital and semiconductor technology allows for simple, accurate, and relatively low-cost measurement devices to measure one or more electrical parameters.

[0009] Strain sensing over large areas requires robust and relatively inexpensive materials. Semiconductor technology is unsuitable. Strain sensors based on conductive, stretchable polymers, such as rubber, are well known. Velostat® is one commercially available example, where electrically insulating polyolefin is doped with conductive carbon particles (carbon black) to make it conductive. When the sheet is stretched, its resistance changes, making it usable as a strain sensor. The nature of its structure is such that when pressure is applied to the sheet in a direction perpendicular to the sheet, there is a change in resistance in the direction of the applied pressure, but no change in resistance perpendicular to the applied pressure (in-plane or sheet resistance).

[0010] Textiles, also known as fabrics or cloths, are flexible materials consisting of a network of natural or man-made fibers. A wide range of materials are used as fibers depending on the desired properties and application.

[0011] Textiles can be formed from fibers by many methods, including weaving, knitting, knotting, braiding, and nonwoven overlay techniques with additional processes such as inter-tangling (e.g., needlepunching, felting, hydroentangling, spunlacing, water needling). Textiles can also include various processes to improve desired properties, such as carding and heat bonding.

[0012] Conductive textiles can be made from conductive fibers, such as metals; conductive polymers (e.g., polypyrrole); carbon-filled polymer fibers; and metal-filled polymer fibers. Additionally, textiles can be formed from fibers in which a non-conductive polymer (such as a polyolefin or natural fiber) is coated with a conductive layer as described herein, thus incorporating the fibers into a textile. In some cases, conductive textiles are made from a blend of conductive and non-conductive fibers, depending on the desired properties. Alternatively, textiles can be made conductive by coating them with a conductive material.

[0013] Conductive textiles are typically more expensive than non-conductive textiles, limiting the size and breadth of individual applications. Furthermore, most conductive textiles do not have a significant piezo-resistive effect. Even more limited, materials that exhibit a piezo-resistive effect do so only in the direction of applied pressure or strain.

[0014] There are many industrial uses for textiles. Sometimes called "technical textiles," these range from civil engineering and related geotechnical applications to construction, manufacturing, and automotive. Generally, they are considered non-aesthetic and form a component of another piece. For large-scale conductive textiles, there are few cost-effective options.

[0015] Textiles for clothing and medical applications that respond to strain or pressure typically rely on complex electrical components embedded in the textile or attached to the textile after the item is formed. In some cases, sensors are printed onto the textile using conductive ink. In these cases, the sensor is a separate object and not an inherent part of the textile. Summary of the Invention [Problem to be solved by the invention]

[0016] SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide a deformable material for use as part of a strain sensing system that ameliorates at least some of the problems associated with the prior art. [Means for solving the problem]

[0017] According to a first aspect of the present invention, there is provided a conductive textile incorporating graphene that changes electrical resistance upon deformation.

[0018] In particular, conductive textiles incorporating graphene exhibit the piezo-resistive effect in all three dimensions and can be used as pressure and strain sensors.

[0019] Textiles, also known as fabrics or cloths, are flexible materials consisting of a network of natural and / or man-made fibers. A wide range of materials are used as fibers depending on the desired properties and application.

[0020] Textiles can be formed from fibers by many methods, including weaving, knitting, knotting, braiding, and nonwoven overlay techniques with additional processes such as inter-tangling (e.g., needlepunching, felting, hydroentangling, spunlacing, water needling). Textiles can also include various processes to improve desired properties, such as carding, heat bonding, and coating.

[0021] Conductive textiles can be made from conductive fibers, such as metals; conductive polymers (e.g., polypyrrole); carbon-filled polymer fibers; and metal-filled polymer fibers. Additionally, textiles can be formed from fibers in which non-conductive polymers (polyolefins or natural fibers) are coated with a conductive layer as described herein, thus incorporating the fibers into the textile. In some cases, conductive textiles are made from a blend of conductive and non-conductive fibers, depending on the desired properties. Alternatively, textiles can be made conductive by coating them with a conductive material.

[0022] Conductive textiles are typically more expensive than non-conductive textiles, limiting the size and breadth of individual applications. Furthermore, most conductive textiles do not have a significant piezo-resistive effect. Even more limited, materials that exhibit a piezo-resistive effect do so only in the direction of applied pressure or strain.

[0023] There are many industrial uses for textiles. Sometimes called "technical textiles," these range from civil engineering and related geotechnical applications to construction, manufacturing, and automotive. Generally, they are considered non-aesthetic and form a component of another piece. For large-scale conductive textiles, there are few cost-effective options.

[0024] Textiles for clothing and medical applications that respond to strain or pressure typically rely on complex electrical components embedded in the textile or attached to the textile after the item is formed. In some cases, sensors are printed onto the textile using conductive ink. In such cases, the sensor is a separate object and not an inherent part of the textile.

[0025] Graphene is essentially individual layers of graphite and can be formed by many routes, including "top-down" approaches such as mechanical or electrochemical exfoliation of graphite, chemical oxidation of graphite, and partial or complete reduction to graphene after exfoliation as graphene oxide, as well as "bottom-up" approaches such as growth from gases or plasmas on substrates or catalysts. Graphene properties can vary from nearly atomically perfect monolayers, to bilayers, few-layers, and multilayers, up to several layers of macroaggregates resembling ultrafine graphite. Graphene has a high aspect ratio, ultimately being one atomic layer thick (less than one nanometer) and typically spanning hundreds of nanometers to hundreds of microns in the planar direction. Therefore, graphene is referred to as a two-dimensional (2D) material. Graphene is an excellent electrical conductor.

[0026] Preferably, the textile undergoes elastic deformation in the plane of the textile when strained in the plane of the textile and / or elastic deformation perpendicular to the plane of the textile when compressed perpendicular to the plane of the textile. Preferably, the change in resistance is reversible.

[0027] The graphene can be applied to the textile after it is formed. The graphene can be applied to the textile so that it is distributed throughout the thickness of the textile.

[0028] The graphene may be applied to one side of the textile such that only a portion of the thickness of the textile contains the graphene.

[0029] Graphene may be applied to the fibers comprising the textile after the fibers are formed, or alternatively, graphene may be incorporated into the fibers comprising the textile.

[0030] Preferably, the fibers are conductive and the textile is conductive. The textile may be formed so that it is not uniformly conductive. The proportion of conductive fibers may be 100%, or more than 50%, or more than 10%, or more than 1%.

[0031] The present invention alternatively provides a textile having a first side including at least one region comprising electrically conductive graphene and a second side including at least one region comprising graphene, whereby an electrical resistance is formed by at least one of the regions on the first side and at least one of the regions on the second side, and the electrical resistance is variable when the textile is deformed.

[0032] The textile may be configured such that each of the conductive first regions on the first and second sides is connected to a conductive second and third region on each side, and the conductive second and third regions may be used to connect an electrical device to the first region.

[0033] The textile may be configured to include a repeating pattern including first, second, and third regions, the second and third regions not being electrically connected to one another except where the first region is interposed between the second and third regions.

[0034] An electrical circuit can be formed through the electrical connection of the second and third regions, where the connection points of the circuit form an electrical path having a first resistance in the first first region and a second resistance in the second first region, the first resistance and the second resistance being different.

[0035] Preferred embodiments of the invention will now be described by way of specific, non-limiting examples and with reference to the drawings. [Brief explanation of the drawings]

[0036] [Figure 1]10 is a graph showing the change in electrical resistance of a rectangular textile-sensor made from nonwoven polyester coated on one side with graphene when compressed perpendicular to the plane of the sheet.

[0037] [Figure 2] Two graphs show an elastic textile made from elastane coated on one side with a flexible graphene coating, showing the change in electrical resistance (left) and elongation (right) with time for three cycles of elongation. DETAILED DESCRIPTION OF THE INVENTION

[0038] Various forms of graphene exist. Ideal graphene is pure carbon and is the best electrical conductor in the graphene family, one of the best conductors discovered so far. It is free of defects and other chemical elements such as oxygen. Graphene oxide (GO) is a highly oxidized form of graphene that is an electrical insulator.

[0039] Intermediate species include, among other things, partially reduced graphene oxide (prGO) or functionalized graphene. Various chemical functional groups are attached to the edges and / or basal planes of graphene. This functionality allows for tuning of graphene's electrical and physical properties, facilitating its incorporation into or onto materials such as plastics to form composites. The incorporation of heteroatoms, where carbon atoms are replaced by other atoms such as nitrogen or other covalently bonded atoms, can also be used to tune graphene's properties.

[0040] Graphene can also be single or multilayer and of various dimensions. Various terms are used to describe the structural arrangements, and attempts have been made to standardize the terminology. Regardless of the terminology, these single and multilayer structures of graphene have useful electrical conductivity and give rise to properties in polymers, fibers, and textiles, as described herein.

[0041] These various permutations of graphene are generalized herein as "graphene" unless otherwise specified and their properties are described in detail. The continuum from electrically conductive to electrically insulating means that many forms of graphene can be used as electrical conductors, and even graphene with low conductivity can serve a purpose, especially when other properties are desirable.

[0042] Graphene can be produced by many methods, including anodic bonding, carbon nanotube cutting, chemical exfoliation, chemical synthesis, chemical vapor deposition, electrochemical exfoliation, electrochemical intercalation, growth on silicon carbide, liquid-phase exfoliation, micromechanical cutting, microwave exfoliation, molecular beam epitaxy, photoexfoliation, precipitation from metals, and thermal exfoliation. Some of these methods have produced materials known as: chemically converted graphene, few-layer graphene, GO, graphene oxide, graphene nanoflakes, graphene nanoplatelets, graphene nanoribbons, graphene nanosheets, graphite nanoflakes, graphite nanoplatelets, graphite nanosheets, graphite oxide, LCGO, liquid crystalline graphene oxide, multilayer graphene, partially reduced graphene oxide, partially reduced graphite oxide, prGO, rGO, reduced graphene oxide, and reduced graphite oxide.

[0043] Incorporation of graphene into textiles can be achieved by many methods, but in each case, the properties of the fiber and textile will influence the method of incorporation. The incorporation method will depend on the fiber and textile chemistry, graphene chemistry, graphene form, the process used to incorporate graphene into or onto the fiber, and the process for forming the textile. For synthetic or composite fibers, a preferred method involves blending graphene into a polymer or composite material prior to forming the synthetic fiber. Both natural and synthetic fibers can be coated with graphene to make conductive fibers, and textiles and textile intermediates can be coated to provide electrical conductivity to the textile.

[0044] For dispersion of graphene in polymers for synthetic or composite fibers, graphene can be present as a powder or as a dispersion in a fluid. Pre-dispersion of graphene in a suitable fluid facilitates dispersion of graphene in the polymer. Graphene coating is preferred from a dispersion of graphene in a fluid. Methods for incorporating graphene into polymers include melt compounding of graphene into the polymer, in-situ polymerization of graphene and polymer, and solution mixing. Regardless of the technique used, sufficient dispersion of graphene is desirable to obtain electrical conductivity with minimal graphene.

[0045] In some cases, additives are required to reduce phase separation between graphene and polymer.

[0046] In a preferred method, textiles are formed from fibers containing graphene. The fibers are formed by melt extrusion from polymer pellets or powder. Graphene is added to the melt extrudate in a concentrated form dispersed in a carrier polymer. The carrier polymer may be the same as or different from the bulk polymer. The concentrated form of the graphene polymer dispersion is mixed and diluted in the melt extrusion process to obtain the desired concentration of graphene in the fibers. In another embodiment, the concentrated form of graphene is dispersed in a fluid such as oil, solvent, or water.

[0047] Electrical measurements depend on the conductivity of the circuit. Sufficient conductivity depends on the size and length of the conductive path and the conductivity of the conductive medium. This combination of variables provides a wide range over which measurements are valid. The measurement method must be tailored to the desired results and conditions. This allows the textile's conductivity to be tailored to the desired application and measurement method. In some cases, such as when the measurement voltage is high, the resistance change is large, and the circuit path is short, the conductivity of a conductive textile can be very low.

[0048] In some embodiments, the resistance of the circuit is measured, while in other embodiments, the capacitance or inductance is measured.

[0049] In one embodiment, natural cotton, woven, non-elastic garment textiles were coated with a dispersion of graphene from a carrier solvent. After drying, the coated areas were conductive. The conductivity can be adjusted to provide a desired conductivity and a desired piezoresistive response to compression and extension. The conductivity and piezoresistive response can be controlled by the amount of graphene applied and the infiltration of the graphene into the textile. The greater the thickness of the graphene infiltration into the textile, the greater the piezoresistive response.

[0050] It can be hypothesized that the three-dimensional textile structure provides a scaffold that, when coated with appropriate graphene particles, provides a mechanism by which compression of the textile perpendicular to the direction of the textile sheet increases fiber-to-fiber contact across the thickness of the textile in the direction of compression, thereby leading to a greater number of conductive paths for current flow and, therefore, a lower measured resistance. This change in resistance can be measured both in the direction of the applied compression (across the thickness of the textile) and in the plane of the textile (perpendicular to the direction of applied compression).

[0051] In another embodiment, a thick, low-density, nonwoven polyester felt textile was coated with an aqueous polymer dispersion of graphene to form a conductive layer within the top of one side of the textile. When compressed, the low-density felt textile deformed significantly, resulting in a piezoresistive response measured across the entire sheet (in-plane) of the textile. As predicted by theory, the piezoresistive response (measured as a change in voltage and converted to resistance) showed a decrease in resistance with increasing applied force, with the decrease in resistance being greater for larger areas of applied force proportional to the area of ​​the textile.

[0052] In another embodiment, a thin coating of graphene was applied as an aqueous polymer dispersion to one side of a thin, elastic woven textile. In some cases, the textile was elastic in both directions; in other cases, the textile was elastic in only one direction. The graphene coating was applied only to the surface of the textile without significant penetration of the graphene within the thickness of the textile. Stretching the textile in the stretchable direction produced an increase in resistance proportional to the degree of stretch. Compressing the thin textile in a direction perpendicular to the plane of the textile produced a slight decrease in resistance. In this case, the graphene coating behaved like a two-dimensional strain gauge.

[0053] The invention will now be described with reference to the following non-limiting examples. [Example]

[0054] Example 1: Approximately 140 g / m 2 Approximately 10cm of melt-spun, nonwoven, needle-punched polyester 2 A square of 1000 mm was coated with a 0.05 wt% dispersion of graphene in xylene by repeatedly dipping the textile into the dispersion until it turned black. After air drying, the measured conductivity was approximately 2000 Ω / sq.

[0055] Example 2: Approximately 140 g / m 2 Approximately 5 cm x 2 cm strips of melt-spun, nonwoven, needle-punched polyester were coated with a graphene oxide dispersion in water by repeatedly dipping the textile by hand into the dispersion and leaving it immersed until the geotextile turned dark brown. The coated textile was then treated with citric acid as a reducing agent to convert the graphene oxide to graphene. After rinsing and air drying, the measured conductivity was 870 Ω / sq.

[0056] Example 3: Graphene nanoplatelets (GNPs) were prepared by thermal exfoliation of expandable graphite at 1050°C in argon, followed by ultrasonic exfoliation in water. Scanning electron microscopy (SEM) showed that the platelets averaged approximately 1 micron in diameter and ranged from a single layer to over 10 layers. The GNPs were mixed with an aqueous acrylic binder to obtain a 2 wt% graphene dispersion, approximately 190 g / m 2 A melt-spun, nonwoven, needle-punched polyester fabric was blade-coated on one side to provide a coating of approximately 2 wt% graphene on the textile. Electrical resistance was measured on both sides of the textile, ranging from approximately 3400 Ω / sq on the coated side to infinite (>20 MΩ) on the uncoated side. Figure 1 shows the pressure response curve of a 4 cm x 14 cm sample of the coated textile when a 4 cm x 4 cm area was compressed across the width of the specimen. It is observed that the resistance exhibits a predictable relationship with the applied force.

[0057] Example 4: Commercially available elastic textiles made from elastane fibers (also known as spandex and Lycra) were blade-coated on one side with a 2 wt. % graphene dispersion in an aqueous acrylic binder using a cellulosic thickener. The coated area was approximately 20 cm x 2 cm. Once dried at 120°C, the graphene coating became flexible and partially elastic. The two-point resistance measured along the length of the coated area was approximately 3.5 kΩ. The highly elastic nature of elastane meant that the starting points for any separate series of resistance measurements were different. Small amounts of elastane stretch (<5%) produced a nearly reversible change in resistance, with an increase in resistance of approximately 300 Ω per 1% stretch. Figure 2 shows the observed relationship between stretch and resistance change, and it is observed that the relationship between the amount of stretch and electrical resistance appears predictable.

[0058] As will be appreciated by those skilled in the art, the above-described embodiments are merely examples of how the inventive concept may be implemented, and other embodiments are contemplated that, although differing in their details, fall within and represent the same inventive concept.

Claims

1. Conductive textiles containing graphene that change electrical resistance when deformed.

2. The textile is arranged in a plane; 10. The textile of claim 1, wherein the textile is adapted to undergo elastic deformation in the plane of the textile when a strain is applied to the plane of the textile.

3. 10. The textile of claim 1, wherein the textile is adapted to undergo elastic deformation normal to the plane of the textile when subjected to a strain normal to the plane of the textile.

4. 4. The textile of claim 2 or claim 3, wherein the change in resistance is reversible.

5. 10. The textile of claim 1, wherein the graphene is applied to the textile after formation of the textile.

6. 6. The textile of claim 5, wherein the graphene is applied to the textile such that the graphene is distributed throughout the thickness of the textile.

7. 6. The textile of claim 5, wherein the graphene is applied to one side of the textile such that only a portion of the thickness of the textile comprises the graphene.

8. 10. The textile of claim 1, wherein the graphene is applied to fibers comprising the textile after the fibers are formed.

9. 10. The textile of claim 1, wherein the graphene is incorporated within fibers comprising the textile.

10. 10. The textile of claim 8 or claim 9, wherein the fibers are electrically conductive and the textile is electrically conductive.

11. 11. The textile of claim 10, wherein the fibers are not uniformly conductive.

12. 12. The textile of claim 11, wherein about 100% of the fibers are conductive.

13. 12. The textile of claim 11, wherein greater than 50% of the fibers are conductive.

14. 12. The textile of claim 11, wherein greater than 10% of the fibers are conductive.

15. 12. The textile of claim 11, wherein greater than 1% of the fibers are conductive.

16. the textile includes one or more regions comprising graphene that is electrically conductive; The textile of claim 1 , wherein the one or more regions change electrical resistance when deformed.

17. the conductive first region is electrically connected to the conductive second and third regions; the conductive first region changes resistance with deformation; 17. The textile of claim 16, wherein the second and third regions are adapted for use in connecting an electrical device to the first region.

18. the first, second, and third regions include a repeating pattern; 18. The textile of claim 17, wherein the second and third regions are electrically connected to one another only where the first region is disposed between the second and third regions.

19. an electrical circuit can be created by electrically connecting a plurality of first regions to two or more of the second and third regions; connection locations of the circuit within the regions are selected to create an electrical path having a first resistance to one of the first regions and a second resistance to another of the first regions; 20. The textile of claim 18, wherein the first resistance and the second resistance incorporate different patterns.

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