Abstract:
The present invention relates to piezoresistive devices and pressure sensors incorporating such devices. At its most general, the invention provides a piezoresistive device, comprising a piezoresistive material positioned between an upper conductive layer and a lower conductive layer, wherein the piezoresistive material comprises carbon nanoparticles (most preferably graphene nanoplatelets, graphene or carbon nanotubes) dispersed in a polymer matrix material. The invention also relates to methods of manufacturing and using such devices.

Description:
[0001]    The present invention relates to piezoresistive devices, and methods of making and using such devices. 
       BACKGROUND 
       [0002]    Piezoresistive devices find use in a wide-variety of applications, for sensing and quantifying forces. 
         [0003]    One type of commercially-available piezoresistive device includes a flexible upper substrate and a lower substrate separated from one another by an air gap created by a spacer. Two interdigitated sets of linear conductive traces are provided on the lower substrate, and a sheet of conductive carbon is provided opposite this on the upper substrate. When pressure is applied to the flexible upper substrate, the substrate deforms so that the conductive layer is brought into contact with a subset of the conductive traces on the lower substrate. As greater pressure is applied, greater numbers of conductive traces contact the conductive layer, and the amount of resistance measured decreases. Thus, the measured resistance can be related to the pressure applied to the upper substrate, due to the relationship between pressure and curvature of the upper substrate. 
         [0004]    However, such devices suffer from a number of drawbacks. For example, the device is unable to measure small forces, because the applied force must exceed a certain threshold in order to bring the conductive carbon on the upper layer into contact with the conductive traces, and when contact is first established the resistance immediately drops from infinite resistance. In addition, the performance of such devices is highly dependent on the shape and orientation of the object used to apply the force to the top substrate. Furthermore, for such devices to function effectively the spacer must maintain a suitable air gap between the conductive carbon layer and the conductive traces, increasing the bulk of such devices. 
         [0005]    Piezoresistive devices have also been proposed in which upper and lower conducting layers are separated from one another by a piezoresistive material, in which the resistance measured across the piezoresistive material is indicative of the applied force. Advantageously, such devices can be made smaller than the conventional devices described above. 
         [0006]    For example, U.S. Pat. No. 4,734,034 describes a sensor, for use in dental diagnostic procedures, formed from a piezoresistive ink sandwiched between upper and lower sets of linear electrodes provided on upper and lower flexible substrates. The piezoresistive ink, which is overprinted on one or both sets of electrodes, includes graphite and titanium dioxide fillers held together with a vinyl resin binder. The upper and lower sets of linear electrodes are overlaid perpendicularly to one another in a grid-like arrangement, so that the resistance measured between a “drive” electrode on one substrate and a “sense” electrode on the other substrate can be related to the force applied at the point where the electrodes overlap. 
         [0007]    U.S. Pat. No. 4,856,993, which shares inventors in-common with U.S. Pat. No. 4,734,034, reports that the device described in U.S. Pat. No. 4,734,034 does not accurately discriminate between different degrees of applied force. Accordingly, the inventors propose using an improved resistive ink—a carbon-molybdenum disulphide based ink in an acrylic binder—having more linear resistance-to-force characteristics. However, the inventors report that the use of the improved ink necessitates changes to the construction of the device. Specifically, in order to achieve electrical isolation of the “sensed” electrodes, the piezoresistive ink must not bridge adjacent sensed electrodes. To achieve this, the inventors propose individually coating each electrode on one layer of the device with piezoresistive material, so that the piezoresistive material takes the form of stripes separated by electrically-insulating air gaps. This adds significantly to the complexity of manufacture, particularly for devices with closely-spaced electrodes and sensors covering a large area, largely due to the need to accurately register the piezoresistive stripes with the underlying electrodes. Thus, the use of the improved ink in U.S. Pat. No. 4,856,993 comes at the cost of increased manufacturing complexity. 
         [0008]    More recently, alternative carbon-containing piezoresistive ink compositions for use in piezoresistive devices have been proposed. For example, U.S. Pat. No. 8,661,917 describes a piezoresistive ink made by curing a composition containing a conductive carbon material, selected from multi-wall carbon nanotubes, single-wall carbon nanotubes, carbon nanocapsules, graphene, graphite nanoflakes, carbon black and combinations thereof, in specific amounts with specific types of solvent, dispersive agent and unsaturated polyester binder. However, as in U.S. Pat. No. 4,856,993, the device described in U.S. Pat. No. 8,661,917 has the piezoresistive composition printed over the electrodes only, with an air gap between adjacent electrodes on the same substrate to achieve electrical isolation of the electrodes. 
         [0009]    In view of the above, there remains a need for improved piezoresistive devices which are smaller, cheaper and/or easier to manufacture than existing devices, which show equal or improved sensitivity. As should be clear from the above discussion, achieving this combination of interrelated aims is not straightforward, since improving one characteristic can have a negative impact on another. 
       SUMMARY OF THE INVENTION 
       [0010]    At its most general, in a first aspect, the present invention provides a piezoresistive device, comprising a piezoresistive material positioned between an upper conductive layer and a lower conductive layer, wherein the piezoresistive material comprises carbon nanoparticles dispersed in a polymer matrix material. Advantageously, the resistance measured between the upper and lower conductive layers is dependent on the force applied to the piezoresistive material, and thus the device can be used to detect and quantify force applied to the device. Suitably, the lower conductive layer is, or is provided on, a lower substrate and the upper conductive layer is, or is provided on, an upper substrate. 
         [0011]    The skilled reader recognises that the terms “upper” and “lower” are used merely to distinguish between the two conductive layers (i.e. they are synonymous with “first” and “second”), and are not intended to limit the orientation or spatial configuration of the device. 
       Conductive Layers 
       [0012]    Preferably, the lower conductive layer comprises or consists of a plurality of conductive traces which are either discrete (i.e. not in electrical contact with one another) or interconnected (i.e. in electrical contact with one another). For example, the lower conductive layer may comprise a pattern (e.g. an array) of conductive traces, such as an array of linear conductive traces (e.g. spaced, parallel lines). Suitably, in such an arrangement, the spacing between the conductive traces of the lower conductive layer is greater than the spacing between the upper and lower conductive layers. 
         [0013]    When the lower conductive layer takes the form of a plurality of conductive traces, the sensor can be used in two measurement modes. 
         [0014]    In the first measurement mode, referred to herein as the “different layer measurement mode”, the device is configured to measure the resistance between a conductive trace of the lower conductive layer and the upper conductive layer. 
         [0015]    In the second measurement mode, referred to herein as the “same layer measurement mode”, the device is configured to measure the resistance between two conductive traces forming the lower conductive layer (i.e. electrodes in the same conductive layer). 
         [0016]    In such embodiments, the flow of electrical current does not occur directly between the electrodes, but is instead mediated by the piezoresistive material and the upper conductive layer. More specifically, current from the “drive” electrode flows up through the piezoresistive material, along the upper conductive layer, and back down through the piezoresistive material to the “sense” electrode (i.e. an electron has to flow through the piezoresistive material twice—once “up” and once “down”). Advantageously, this increases the number of passages through piezoresistive material which an electron must make compared to the “different layer measurement mode”. Thus, for a device having a given thickness of piezoresistive material, the resistance measured using the “same layer measurement mode” is higher than that measured for the “different layer measurement mode”, resulting in improved sensitivity for a given thickness. Furthermore, such an arrangement simplifies the manufacture of the sensor, since the electrical connections of resistance measuring equipment only need to interface with the lower conductive layer. 
         [0017]    Optionally, the lower conductive layer comprises two sets of linear conductive traces with are interdigitated with one another (i.e. a first set of interconnected linear conductive traces and a second set of linear interconnected conductive traces, with the first set of conductive traces interdigitated with the second set of conductive traces). In such an embodiment, the first set of conductive traces may form a positive electrode, and the second set of conductive traces may form a negative electrode, which can be used in the “same layer measurement mode”. Advantageously, this provides a compact way of achieving a large area between positive and negative electrodes in the same conductive layer. In particular, it allows accurate measurement of different force “footprints” (e.g. forces applied by objects having different sizes and shapes) over a large area, without the measurement being unduly affected by the particular configuration of the electrodes. 
         [0018]    Similarly to the lower conductive layer, the upper conductive layer may comprise or consist of a plurality of conductive traces. For example, the upper conductive layer may comprise a pattern (e.g. an array) of conductive traces, which may be discrete (i.e. not in electrical contact with one another) or interconnected (i.e. in electrical contact with one another). 
         [0019]    In embodiments in which both the lower conductive layer and upper conductive layer comprise a plurality of conductive traces, the traces of the respective layers may overlay/overlap with one another so as to form a grid-type arrangement. For example, the upper and lower conductive layer may comprise linear conductive traces, with the traces in the upper conductive layer being at an angle (e.g. orthogonal) to those in the lower conductive layer. 
         [0020]    In embodiments in which both the lower conductive layer and upper conductive layer comprise or consist of a plurality of discrete conductive traces, the traces of the respective layers preferably overlay/overlap with one another so as to form a grid-type arrangement. Such an arrangement allows the applied force at a specific location of the device to be measured, because the resistance measured between a trace of the upper conductive layer and a trace of the lower conductive layer is determined by the amount of force applied at the point where those traces overlap. 
         [0021]    The material of the upper and lower conductive layers is not particularly limited. Suitable materials include metals (such as copper and silver), conductive carbon materials (such as carbon black), and indium tin oxide. For example, one or both of the conductive layers may comprise silver, optionally overlaid with a layer of carbon black. 
         [0022]    Suitably, the piezoresistive device further comprises resistance measuring equipment (e.g. an ohmmeter) having positive and negative terminals, wherein one terminal is connected to the lower conductive layer and the other electrode is connected to the upper conductive layer. 
         [0023]    Alternatively, the piezoresistive device comprises resistance measuring equipment (e.g. an ohmmeter) having positive and negative terminals, wherein the positive and negative terminals are connected to different (preferably adjacent) conductive traces in the lower conductive layer. Preferably, the lower conductive layer comprises two sets of linear conductive traces which are interdigitated with one another, with the positive terminal of the resistance measuring equipment connected to one set of traces and the negative terminal connected to the other set of traces. Advantageously, this allows the resistance measuring equipment to be connected to a single layer of the device, which simplifies manufacture. 
       Piezoresistive Material 
       [0024]    Suitably, the piezoresistive material has a high resistance in the absence of an applied force. For example, in the absence of an applied force, the resistance measured between the lower conductive layer and upper conductive layer may be at least 100Ω, at least 1 kΩ, at least 5 kΩ, at least 10 kΩ, 50 kΩ, 100Ω, at least 500 kΩ, at least 700 kΩ, at least 800 kΩ, at least 1 MΩ, or at least 5 MΩ. For a device having a surface area of “A” cm 2 , the resistance at zero applied force may be, for example, at least 100 Ω/cm 2 , at least 1 kΩ/cm 2 , at least 5 kΩ/cm 2 , at least 10 kΩ/cm 2 , or at least 50 kΩ/cm 2 . Advantageously, ensuring that the resistance at zero applied force is large maximises the dynamic range of the device (i.e. the difference between the largest and smallest possible resistance values) and hence improves the sensitivity of the device. 
         [0025]    The piezoresistive material used in the present invention may be a single layer. However, it is preferred that the piezoresistive material comprises multiple layers. Preferably, the multiple layers contact one another in the absence of an applied force. Advantageously, the flow of current across the interface at which layers of piezoresistive material meet is relatively poor, which increases the overall resistance of the multi-layer structure. Thus, all other things being equal, the resistance of a multilayer structure will be greater than that for a single layer structure. This means that increasing the number of layers in the piezoresistive material allows the overall resistance of the piezoresistive material to be increased without significantly increasing the height of the structure, and allows the resistance of the device to be tuned by adjusting the number of layers. 
         [0026]    For the above reasons, the measured resistivity of the deposited piezoresitive material will vary depending inversely on the thickness of layers of piezoresistive material and the number of layers deposited. The resistivity of the piezoresistive material may be, for example, at least 100 Ω/cm, at least 1 kΩ/cm, at least 10 kΩ/cm, at least 100 kΩ/cm, at least 1 MΩ/cm, at least 5 MΩ/cm, or at least 10 MΩ/cm. 
         [0027]    The lower limit for the number of piezoresistive layers forming the piezoresistive material may be one. Preferably, the piezoresistive material comprises two to six layers, most preferably three layers, since the inventors have found that this allows a relatively compact high-resistance material to be produced. 
         [0028]    As noted above, as the number of piezoresistive layers is increased the overall resistance of the piezoresistive material increases. Therefore, if the number of piezoresistive layers is large, the overall resistance of the piezoresistive material can become too high to reliably form an effective piezoresistive device (since the material effectively becomes an insulator). The exact number of piezoresistive layers which can be tolerated will depend on the composition of each layer, and on the particular application which the device is being used for. However, the upper limit for the number of layers forming the piezoresistive material may be, for example, fifteen, ten, nine, eight, seven or six. The skilled reader understands that the lower and upper limits for the number of layers given above can be combined to form ranges. For example, the piezoresistive material may comprise one to eight, more preferably two to seven, more preferably still, three to six layers. Advantageously, when the piezoresistive material comprises this number of layers it is possible to achieve a suitable level of resistance in a relatively compact manner at relatively low loadings of carbon nanoparticles. In addition, this can be achieved without requiring an excessive number of steps to build up the piezoresistive material. 
         [0029]    When the piezoresistive material is formed from multiple layers, the composition of each layer may be the same or different. Preferably, all of the piezoresistive layers comprise carbon nanoparticles dispersed in a polymer matrix material. For ease of manufacture, it is preferred that the composition of each of the piezoresistive layers is the same. 
         [0030]    The thickness of the piezoresistive material between the upper conductive layer and lower conductive layer (referred to henceforth as “D V ”) may be, for example, less than 300 μm, less than 200 μm, less than 150 μm, preferably less than 100 μm or less than 75 μm. The lower limit for the thickness of the piezoresistive material may be, for example 1 μm, 3 μm, 5 μm or 10 μm. Preferably, the thickness of the piezoresistive material between the upper conductive layer and lower conductive layer is 1 to 100 μm, more preferably 1 to 75 μm. In instances where the piezoresistive material is formed from multiple layers, each layer may have a maximum thickness of, for example, 50 μm, 25 μm, 15 μm, 10 μm or 5 μm. The minimum thickness may be, for example, 0.5 μm, 1 μm, 3 μm or 5 μm. Preferably, the thickness of each layer is 1 to 15 μm. Advantageously, such layers provide sufficient resistance whilst allowing a relatively thin device to be produced. 
         [0031]    Preferably, the piezoresistive material is applied to the lower conductive layer by overprinting the lower conductive layer with a continuous layer of piezoresistive ink. For example, the ink forms a single block (e.g. it is not printed in stripes). This preference also applies to embodiments in which the lower conductive layer takes the form of a plurality of conductive traces. 
         [0032]    In embodiments in which the lower and/or upper conductive layer comprises a plurality of conductive traces, such overprinting with piezoresistive ink results in piezoresistive material bridging adjacent conductive traces within the conductive layer (i.e. the piezoresistive material extends from one conductive trace to an adjacent conductive trace in the same conductive layer). Thus, the piezoresistive material coats/fills the region between adjacent conductive traces in the same layer. When the piezoresistive material comprises multiple layers, the same piezoresistive layer bridges adjacent traces in the same conductive layer. 
         [0033]    Advantageously, it is simpler to produce a device having piezoresistive material bridging adjacent conductive traces than a device in which each conductive trace is individually overlaid with a separate layer of piezoresistive material, because there is no requirement for careful registration between the electrodes and piezoresisitive material. Instead, the piezoresistive material can be provided by printing a continuous layer of piezoresistive ink over the lower conductive layer. 
         [0034]    In such embodiments, the piezoresistive material fills the channel between adjacent conductive traces. By “channel between adjacent conductive traces”, we mean the volume formed by connecting the opposing surfaces of adjacent conductive traces. More specifically, in embodiments in which the conductive traces are raised features provided on a substrate (e.g. printed traces), the channels occur between the raised traces with the substrate acting as the “base” of the channel and sidewalls of the traces acting as the “sides” of the channel. 
         [0035]    The piezoresistive material may fill, for example, 80% or more of the channel between some or all pairs of conductive traces in the same conductive layer. Preferably, the piezoresistive material fills 90% or more of said channels. Most preferably, the piezoresistive material completely fills the channel between all adjacent conductive traces. Advantageously, this means that the layer of piezoresistive material in contact with the conductive traces can be printed over the conductive traces as a single layer. This can be used to produce a single level surface which can be easily overprinted with further material. 
         [0036]    Differently stated, the device can be considered to comprise a lower part, having a conductive layer, and an upper part, having a conductive layer, wherein the conductive layer of the lower part overlies the conductive layer of the upper part, and the piezoresistive material fills substantially all (e.g. 80% or more, 85% or more, 90% or more, 98% or more, 99% or more) of the volume between the upper and lower parts in the region where the conductive layers overlie one another. The device may be substantially free of air gaps in the region where the conductive layers overlie one another. 
         [0037]    In embodiments in which the upper and/or lower conductive layer is formed of a plurality of conductive traces, the thickness of piezoresistive material between the upper and lower conductive layers (“D V ”) is less than the thickness of piezoresistive material between adjacent conductive traces in the same layer (“D H ”). For example, D H  may be at least four times D V , at least six times D V , at least ten times D V , at least twelve times D V , or at least fifteen times D V . Advantageously, higher ratios of D V :D H  mean that the device has low sensitivity to inhomogeneity in the piezoresistive material and manufacturing process. The lower limit for D H  may be, for example 50 μm, 100 μm, 200 μm, 250 μm or 300 μm. However, the lower limit for D H  is not particularly limited, provided that D H  is greater than D V . In embodiments in which both the lower conductive layer and upper conductive layer comprise a plurality of conductive traces, it is preferred that the piezoresistive material bridges adjacent traces in both the lower conductive layer and the upper conductive layer, as well as the gap between the lower conductive layer and upper conductive layer (i.e. that piezoresistive material fills the channel between conductive traces so as to span adjacent traces). This is in contrast to the devices in U.S. Pat. No. 4,856,993 and U.S. Pat. No. 8,661,917 where the piezoresistive material does not extend between adjacent traces (due to the provision of an electrically insulating air gap). The present inventors have found that the devices of the present invention are extremely sensitive and accurate even in the absence of an air gap between adjacent traces. This is particularly surprising since U.S. Pat. No. 4,856,993 reports that inks which allow sensitive discrimination between different levels of applied force (as in the present invention) require isolation of adjacent traces. Advantageously, such a device is relatively compact and simple to manufacture. 
         [0038]    When the piezoresistive material is formed from multiple layers, it is preferred that all of the layers constituting the piezoresistive material are in contact with one another in the absence of an applied force (i.e. there is no gap between layers). In other words, the layers do not contain an intervening gap filled by an insulating material, such as air. Some, preferably all, of the layers of the piezoresistive material may be bonded together. When the multiple piezoresistive layers are built up by printing piezoresistive ink, the constituents of the layer may be bonded together by the constituents of the ink without the need for a separate adhesive layer. 
       Carbon Nanoparticles 
       [0039]    The piezoresistive material used in the present invention comprises carbon nanoparticles. Suitably, the carbon nanoparticles are conductive particles. 
         [0040]    Suitably, the carbon particles have a high aspect ratio. Advantageously, the resistance of the piezoresistive material is particularly sensitive when high aspect ratio particles are used. It is believed that this is due to the fact that the probability of particles forming an electrical connection between two spaced points decreases more rapidly as the distance between the points is increased for particles having a high aspect ratio (due to the influence of the orientation of such particles). 
         [0041]    Piezoresistive materials comprising high aspect ratio carbon nanoparticles are particularly useful when used in embodiments where the upper and/or lower conductive layer comprise a plurality of discrete conductive traces, and piezoresistive material fills the gaps between said traces. This is because relatively closely-spaced conductive traces can be effectively electrically isolated from one another without the need for an intervening electrically insulating material, such as air (in contrast to the devices described in U.S. Pat. No. 4,856,993 and U.S. Pat. No. 8,661,917). 
         [0042]    The carbon nanoparticles may comprise or consist of, for example, graphene, graphene nanoplatelets, or carbon nanotubes (multi-wall carbon nanotubes and/or single-wall carbon nanotubes). Advantageously, these forms of carbon nanoparticles provide extremely high aspect ratio conductive particles. 
         [0043]    The carbon nanoparticles may comprise or consist of graphitic or graphene platelets (nanoplatelets), preferably having a platelet thickness less than 100 nm and a major dimension (length or width) perpendicular to the thickness. The platelet thickness is preferably less than 70 nm, preferably less than 50 nm, preferably less than 30 nm, preferably less than 20 nm, preferably less than 10 nm, preferably less than 5 nm. The major dimension is preferably at least 10 times, more preferably at least 100 times, more preferably at least 1,000 times, more preferably at least 10,000 times the thickness. The length may be at least 1 times, at least 2 times, at least 3 times, at least 5 times or at least 10 times the width. 
         [0044]    The loading of carbon nanoparticles in the polymer matrix material may be less than 50 wt. %, less than 40 wt. %, less than 30 wt. %, less than 20 wt. %, less than 15 wt. %, less than 10 wt. %, less than 6 wt. %, less than 5 wt. %, less than 4 wt. %, less than 3 wt. %, less than 2 wt. %, or less than 1 wt. %, and for some uses more preferably less than 0.5 wt. %, less than 0.2 wt. % or less than 0.1 wt. % of the total weight of the piezoresistive material. Conversely, the loading may be at least 0.001 wt. %, at least 0.005 wt. % or at least 0.01 wt. %, and for some uses at least 0.1 wt. %. 
         [0045]    It is important that the carbon nanoparticles are uniformly dispersed throughout the polymer matrix material, since aggregates (clumps) of material increase the chances of localised electrical connections forming between conductive layers and decrease the conductivity of the rest of the piezoresistive material (resulting in a less uniform response across the device). It is particularly important to minimise the number of aggregates in embodiments where the upper and/or lower conductive layer comprise a plurality of conductive traces, to minimise the probability of electrical connections occurring between adjacent traces. However, it is not straightforward to achieve a suitably uniform dispersion of graphene, graphene nanoplatelets and carbon nanotubes, since such particles have a powerful tendency to agglomerate, and are difficult to disperse in solvents and polymer materials. 
         [0046]    Preferably, the carbon nanoparticles are chemically functionalised carbon nanoparticles. That is, the carbon nanoparticles incorporate functional groups which improve the affinity of the nanoparticles for the solvents and/or polymer matrix material used to form the piezoresistive material, thus allowing a more uniform distribution of particles to be achieved. For example, the carbon nanoparticles are preferably functionalised carbon nanotubes (MWCNTS or SWCNTs), functionalised graphene, or functionalised graphene nanoplatelets. 
         [0047]    The inventors have found that when carbon nanoparticles are prepared using agitation in low-pressure plasma, such as described in WO2010/142953 and WO2012/076853, they are readily obtained in a format enabling dispersion in solvents and subsequently in polymer matrices, or directly in polymer melts, at good uniformity and at levels more than adequate for the purposes set out above. This is in contrast to conventional processes for separating and functionalising carbon particles, which are extreme and difficult to control, as well as damaging to the particles themselves. 
         [0048]    Specifically, the starting carbon material—especially carbon nanotubes or graphitic carbon bodies—is subjected to a particle treatment method for disaggregating, de-agglomerating, exfoliating, cleaning or functionalising particles, in which the particles for treatment are subject to plasma treatment and agitation in a treatment chamber. Preferably the treatment chamber is a rotating container or drum. Preferably the treatment chamber contains or comprises multiple electrically-conductive solid contact bodies or contact formations, the particles being agitated with said contact bodies or contact formations and in contact with plasma in the treatment chamber. 
         [0049]    The particles to be treated are carbon particles, such as particles which consist of or comprise graphite, carbon nanotubes (CNTs) or other nanoparticles. 
         [0050]    Preferably the contact bodies are moveable in the treatment chamber. The treatment chamber may be a drum, preferably a rotatable drum, in which a plurality of the contact bodies are tumbled or agitated with the particles to be treated. The wall of the treatment vessel can be conductive and form a counter-electrode to an electrode that extends into an interior space of the treatment chamber. 
         [0051]    During the treatment, desirably glow plasma forms on the surfaces of the contact bodies or contact formations. 
         [0052]    Suitable contact bodies are metal balls or metal-coated balls. The contact bodies or contact formations may be shaped to have a diameter, and the diameter is desirably at least 1 mm and not more than 60 mm. 
         [0053]    The pressure in the treatment vessel is usually less than 500 Pa. Desirably during the treatment, gas is fed to the treatment chamber and gas is removed from the treatment chamber through a filter. That is to say, it is fed through to maintain chemical composition if necessary and/or to avoid build-up of contamination. 
         [0054]    The treated material, that is, the particles or disaggregated, deagglomerated or exfoliated components thereof resulting from the treatment, may be chemically functionalised by components of the plasma-forming gas, forming e.g. carboxy, carbonyl, OH, amine, amide or halogen functionalities on their surfaces. Plasma-forming gas in the treatment chamber may be or comprise e.g. any of oxygen, water, hydrogen peroxide, alcohol, nitrogen, ammonia, amino-bearing organic compound, halogen such as fluorine, halohydrocarbon such as CF 4 , and noble gas. Oxygen-functionalised materials, plasma-processed in oxygen, or oxygen-containing gas, are particularly preferred. 
         [0055]    Any other treatment conditions disclosed in the above-mentioned WO specifications may be used, additionally or alternatively. Or, other means of functionalising and/or disaggregating carbon particles may be used for the present processes and materials, although we strongly prefer plasma-treated materials. 
         [0056]    For the present purposes the type and degree of chemical functionalisation of the particles is selected for effective compatibility at the intended loadings with the selected polymer matrix material. Routine experiments may be effective to determine this. 
       Polymer Matrix Material 
       [0057]    Suitably, the polymer matrix material is an elastic material. The particular choice of elastic material is not particularly limited, provided that it is sufficiently elastically deformable at the required measurement conditions (pressure and temperature), and holds the carbon particles in position (so that the distribution of carbon particles does not change over time). Suitable materials include, for example, vinyl polymers (including polymers or copolymers of vinyl chloride, vinyl acetate and vinyl alcohol), polyester polymers, phenoxy polymers, epoxy polymers, acrylic polymers, polyamide polymers, polypropylenes, polyethylenes, silicones, elastomers such as natural and synthetic rubbers including styrene-butadiene copolymer, polychloroprene (neoprene), nitrile rubber, butyl rubber, polysulfide rubber, cis-1,4-polyisoprene, ethylene-propylene terpolymers (EPDM rubber), and polyurethane rubber. The polymer matrix material may be, for example, a copolymer of vinyl chloride, vinyl acetate and/or vinyl alcohol. 
       Substrate 
       [0058]    Suitably, the upper conductive layer is provided on an upper substrate and the lower conductive layer is provided on a lower substrate. Preferably, at least one of the substrates is relatively flexible. The degree of flexibility required will depend on the particular application which the sensor is intended for use in. For example, for applications where it is necessary to specifically localise the position at which a force is applied to the substrate, a relatively flexible substrate may be used. In contrast, when the exact position of the force is unimportant, a relatively more rigid substrate may be suitable. 
         [0059]    Suitable materials for the substrate include, for example, polymers (such as PET), glass, fabrics, metals and composite materials such as fiberglass. 
       Preferred Embodiments 
       [0060]    In a particularly preferred embodiment, the present invention provides a piezoresistive device comprising a piezoresistive material positioned between an upper conductive layer and a lower conductive layer, wherein the piezoresistive material comprises 50 wt. % or less carbon nanoparticles selected from carbon nanotubes and graphene nanoplatelets dispersed in a polymer matrix material (the weight percentage being relative to the total weight of the piezoresistive material). 
         [0061]    In another particularly preferred embodiment, the present invention provides a piezoresistive device comprising:
       a lower substrate, having a conductive layer (“lower conductive layer”);   an upper substrate, having a conductive layer (“upper conductive layer”); and   a piezoresistive material positioned between the upper and lower substrate, wherein the piezoresistive material comprises carbon nanoparticles (preferably selected from carbon nanotubes and graphene nanoplatelets) dispersed in a polymer matrix material, and
 
wherein conductive layers on the lower and upper substrate overlay one another, and the piezoresistive material fills substantially all of the volume between the upper and lower substrates in the region where the conductive layers overlie one another. Advantageously, such a device allows particularly sensitive measurement of applied force and is relatively simple to manufacture (as explained above).
       
 
         [0065]    In another particularly preferred embodiment, the present invention provides a piezoresistive device comprising a piezoresistive material positioned between an upper conductive layer and a lower conductive layer, wherein the piezoresistive material comprises carbon nanoparticles selected from carbon nanotubes, graphene and graphitic nanoplatelets dispersed in a polymer matrix material, and wherein the upper and lower conductive layers each comprise a plurality of spaced conductive traces with piezoresistive material filling the channel between conductive traces on both the upper and lower conductive layers. Advantageously, such a device is relatively simple to manufacture because the upper and lower parts of the device can be identical, and hence it is possible to form the device from a single type of sheet produced by printing a repeating pattern of conductive traces on a substrate and completely overlaying the conductive traces with piezoresistive material. This is in contrast to the device in U.S. Pat. No. 4,856,993 and U.S. Pat. No. 8,661,917 where the piezoresistive material must be in registration with the conductive traces and, in the case of U.S. Pat. No. 4,856,993, where the upper and lower parts of the device are different. 
         [0066]    These preferred embodiments may have any of the optional and preferred features discussed above. 
       Applications 
       [0067]    Advantageously, the high sensitivity, large dynamic range (in terms of the range of forces which can be measured) and small size of the piezoresistive device of the present application mean it is suitable for use in a wide range of applications. In particular, a single embodiment of the device is particularly versatile, unlike the “conventional” device described in the background section above where devices must be carefully tailored to a specific application (e.g. tailoring the “threshold force” required, and adapting the device to the particular shape and orientation of the object being measured). 
         [0068]    For example, the device may find use as a pressure sensor. Suitable applications for such a sensor may be found in a range of areas including sports (e.g. jump height testing), medicine (e.g. gait analysis), dentistry (e.g. dental occlusion measurement), retail (e.g. footfall analysis in a shop), packaging, furniture and clothing. In addition, the device may find use as a strain gauges. Suitable applications for such a strain gauge may include, for example, detection of distortions in pipes. 
       Manufacturing Method 
       [0069]    In a further aspect, the present invention provides a method of manufacturing a piezoresistive device according to the first aspect. 
         [0070]    Such a method may include the steps of: 
         [0000]    (i) providing a first conductive layer;
 
(ii) depositing one or more layers of piezoresistive material, as defined above, over the first conductive layer; and
 
(iii) bringing a second conductive layer into contact with the piezoresistive material.
 
         [0071]    Preferably, the first conductive layer comprises a plurality of conductive traces provided on a substrate, and the one or more layers of piezoresistive material are all deposited as continuous layers (so that piezoresistive material fills the channel between conductive traces). As explained above, depositing a continuous layer of piezoresistive material over several traces is simpler than depositing stripes of material. In addition, when all layers of piezoresistive material are deposited as continuous layers, the deposition can be repeated without requiring process changes between each layer (e.g. changes in the pattern of deposited material). 
         [0072]    More preferably, the first conductive layer comprises a first set of interconnected linear conductive traces and a second set of interconnected linear conductive traces, and the traces of the first and second set of traces are interdigitated. Advantageously, such a device can be operated both in the “different layer measurement mode” and the “same layer measurement mode” described above. 
         [0073]    The step of providing a first conductive layer (step (i) above) may involve depositing a conductive ink on a substrate. Suitable printing techniques include, for example, screen printing, flexography, rotogravure, inkjet, and offset lithography. 
         [0074]    Suitably, the step of depositing one or more layers of piezoresistive material (step (ii) above) involves printing one or more layers of a piezoresistive ink. Suitable printing techniques include, for example, screen printing, flexography, rotogravure, inkjet, and offset lithography. The piezoresistive ink comprises the carbon nanoparticles dispersed in a solvent and polymer matrix material. 
         [0075]    When multiple layers of piezoresistive ink are printed, each layer is dried before a subsequent layer is added. The device may be heated after the application of each piezoresistive ink layer to speed up drying of the ink. 
         [0076]    When using a piezoresistive ink, the method preferably involves a step of preparing the ink for printing. This preparation step may involve mixing or homogenising the ink to evenly distribute the carbon nanoparticles in the ink&#39;s polymer binder. Preferably, the preparation step involves homogenising the ink, since the inventors have found that this ensures a uniform distribution of carbon nanoparticles and can help to break up agglomerates of nanoparticles in the ink. This homogenisation is particularly useful in embodiments in which the first and/or second conductive layers comprise or consist of a plurality of conductive traces, and the conductive layers are overprinted with a continuous layer of piezoresistive ink which bridge the gaps between adjacent traces, because the homogenisation helps to ensure that the lateral resistance (i.e. the resistance between adjacent traces) remains high and uniform through the device. Suitable homogenisation can be achieved using, for example, a three roll-mill or rotor-stator homogeniser. 
         [0077]    Suitably, the step of bringing a second conductive layer into contact with the piezoresistive material (step (iii)) involves overlaying a second substrate, having the second conductive layer, onto the first substrate. 
         [0078]    The first conductive layer may be identical to the second conductive layer. For example, the device can be created by:
       preparing a lower part by providing a conductive layer and depositing one or more layers of piezoresistive material, as defined above, over the conductive layer; and   bringing an upper part into contact with the lower part, wherein the upper part is identical to the lower part.       
 
         [0081]    Advantageously, this simplifies construction of the device, since it requires only a single type of substrate to be produced. For example, both parts of the device can be obtained by dividing (e.g. cutting) a single multilayer structure. 
         [0082]    The piezoresistive device produced using methods of the present invention may have any of the preferred and optional features referred to above. 
       Methods of Operating a Sensor Device 
       [0083]    In another aspect, the present invention provides a method of using a pressure sensor of the first aspect, as defined above. For example, the present invention provides a method of using a pressure sensor of the first aspect according to the “same layer measurement mode” described above. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0084]    Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which: 
           [0085]      FIG. 1A  shows a conventional commercially available piezoresistive pressure sensor; 
           [0086]      FIG. 1B  shows the behaviour of the sensor shown in  FIG. 1A  upon application of a force; 
           [0087]      FIG. 2  is a cross-sectional view of a known piezoresistive pressure sensor described in U.S. Pat. No. 4,856,993; 
           [0088]      FIG. 3  shows a top view of the electrode arrangement for the device of  FIG. 2 ; 
           [0089]      FIG. 4  is a cross-sectional view of a piezoresistive device of the present invention set up for measurement according to the “different layer measurement mode” described above; 
           [0090]      FIG. 5  is a cross-sectional view of a piezoresistive device of the present invention set up for measurement according to the “same layer measurement mode” described above; 
           [0091]      FIG. 6  is a top view of the lower electrode arrangement of the device of  FIG. 5 ; 
           [0092]      FIG. 7  is a bottom view showing the lower electrodes overlaying the upper electrodes in the device of  FIG. 5 ; 
           [0093]      FIG. 8  is a schematic diagram showing the flow of current in the device of  FIG. 5 ; 
           [0094]      FIG. 9  is a logarithmic plot showing measured resistance vs applied force for a device according to the present invention, having a single upper and lower conductive layer and six piezoresistive layers; 
           [0095]      FIG. 10  is a plot showing the effect of the number of layers of piezoresistive ink on measured resistance; 
           [0096]      FIGS. 11 and 12  are logarithmic plots showing measured resistance vs applied force for devices incorporating piezoresisitve ink with different loadings of carbon nanoparticles. The device in  FIG. 11  had a higher loading than that in  FIG. 12 ; and 
           [0097]      FIG. 13  is a logarithmic plot showing measured resistance vs applied force for a device having the configuration shown in  FIG. 5 , having four layers of piezoresistive ink. 
       
    
    
     DETAILED DESCRIPTION 
       [0098]      FIG. 1A  shows a conventional commercially-available sensor  1 , consisting of a lower substrate  3  and flexible upper substrate  5  separated by an air gap created by a ring-shaped spacer  7 . The lower substrate includes interdigitated positive and negative electrodes  9 , positioned opposite to a conductive layer  11  on the upper substrate. The electrodes of the same polarity are interconnected, and terminate in a common terminal (not shown). As shown in  FIG. 1B , when a force is applied to upper substrate  5 , the substrate deforms so as to bring the conductive layer  11  into contact with a subset  13  of the electrodes  9 . This allows current to flow between positive and negative electrodes via the conductive layer  11 . The overall resistance measured between the positive terminal and negative terminal depends on the number of electrodes in contact with the conductive layer. Thus, the measured resistance can be related to the deformation of the upper substrate, which will depend on the applied force. 
         [0099]      FIG. 2  shows another prior art sensor  101 , as described in U.S. Pat. No. 4,856,993. Sensor  101  comprises a lower substrate  103  and upper substrate  105  each bearing conductive traces  107  and  109  separated by layers of piezoresistive material  111  and  113 . The conductive traces of the upper and lower substrates overlay one another so as to form a grid arrangement, as shown in  FIG. 3 . The piezoresistive material comprises carbon-molybdenum disulphide in an acrylic binder. Resistance is measured between “drive” trace  109  and one of the “sensed” traces  107 . However, to produce a functioning device with this material it is necessary for adjacent “sensed” traces to be electrically isolated from one another. Thus, resistive material  111  takes the form of individual stripes separated by an air gap. 
         [0100]      FIGS. 4 to 7  show piezoresistive devices according to the present invention.  FIG. 4  shows a piezoresistive sensor  1001 , consisting of a lower substrate  1003  and upper substrate  1005  each bearing discrete silver traces  1007  and  1009  (again, a single trace is shown, in cross section, for the upper substrate) separated by layers of piezoresistive material  1011  and  1013 . The distance between silver traces in the same layer D H  is greater than the distance between conductive traces in different layers D V  (although, note that the device in  FIG. 4  is not to scale). 
         [0101]    In contrast to the prior art device of  FIG. 2 , the piezoresistive material of the device in  FIG. 4  includes high aspect ratio carbon nanoparticles, in this case 5 wt. % functionalised graphene nanoplatelets in a vinyl polymer matrix. The use of this particular material in the context of the device having D H  greater than D V  means that adjacent conductive traces are effectively electrically isolated from one another without an intervening insulating material (e.g. air). Thus, piezoresistive ink layers  1011  and  1013  are completely overprinted on both the upper and lower substrates, meaning that the ink fills the channel between adjacent silver traces (illustrated for the lower layer by cross-hatched region  1011   a ). This makes the device particularly simple to manufacture, since the upper and lower parts of the device can be manufactured in the same way. For example, a single starting substrate can be overprinted with silver traces and one or more layers of the piezoresistive ink, and subsequently cut in half and overlaid to form the device shown in  FIG. 4 . 
         [0102]    Similarly to the device shown in  FIG. 2 , the pressure across the device in  FIG. 4  can be determined by measuring the resistance between a silver trace  1007  on the lower substrate and a silver trace  1009  on the upper substrate. 
         [0103]    The device in  FIG. 5  is similar to that shown in  FIG. 4 , but in this case the resistance is measured between adjacent silver traces on the same substrate. In the device of  FIG. 5 , the lower substrate has two electrodes—positive electrode  1015  and negative electrode  1017 —as shown in  FIG. 6 . The positive electrode is formed from interconnected linear conductive traces  1007   a  having a common positive terminal  1019 . The negative electrode has a similar arrangement of silver traces  1007   b  and terminates in a negative terminal  1021 , with the silver traces of the negative electrode interdigitating and alternating with those of the positive electrode. In this embodiment, the upper substrate has a single silver layer  1009  overlying all of the conductive traces of the lower substrate (as shown in  FIG. 7 ). However, the form of the silver layer  1009  is not limited in this embodiment, provided that the layer overlays the silver traces  1007   a  and  1007   b  of the positive and negative electrodes, and could take the form of rows of silver traces (e.g. as shown in  FIG. 3 ) or any other regular or irregular pattern. 
         [0104]    In this device, resistance is measured between the positive terminal  1019  and negative terminal  1021 . Electron flow between adjacent electrodes occurs via the silver layer  1009 , with direct current flow between adjacent conductive traces being negligible (or even prevented) due to the use of the high aspect ratio carbon nanoparticle-based ink and the spacing of the traces. More specifically, as shown in  FIG. 8 , electrons flow between adjacent traces by flowing “up” through the piezoresistive material, across the silver layer  1009  and “down” through the piezoresistive material. Advantageously, this means that electrons must travel through a longer path of piezoresistive material in the device of  FIG. 5  compared to the device of  FIG. 4 , meaning the resistance values measured in  FIG. 5  are higher than those for  FIG. 4  (all other things being equal). Since a higher starting resistance helps to improve the dynamic range of the device, this increased electron path length helps to improve sensitivity without increasing height. 
       EXPERIMENTAL RESULTS 
     Example 1 
       [0105]    A piezoresitive device was produced and measured according to the different layer measurement mode described above, in order to assess the piezoresistive properties of a high aspect ratio carbon nanoparticle ink. 
         [0106]    Two 10 mm by 9 cm strips of a conductive silver ink (AG 500, Conductive Compounds PE) were screen printed onto a PET substrate (175 μm thickness) using a DEK 248 screen printer. Each strip was then overprinted with three layers of piezoresistive ink containing carbon nanoparticles including functionalised GNPs (Haydale Graphene Industries plc) in a polymer matrix and solvent, whilst leaving a small area of the conductive silver exposed at one end. Each piezoresistive ink layer was dried before the application of subsequent ink layers. The assembly was then cut in half (each bearing a silver strip), and the two halves overlaid with the piezoresistive ink layers facing one another, so as to form a piezoresistive sensor. 
         [0107]    To measure the resistance behaviour of the device, a multimeter (Agilent RMS) was attached to the exposed silver on each half of the device, and pressures of between 1 and 3000 N were applied using a Housfield extensometer. To achieve an even distribution of force, the sensor was attached to flat acrylic blocks and a spacer with the same area as the active sensor area was placed on top of the acrylic block. Measurements were repeated four times. 
         [0108]    The results of these experiments are shown in  FIG. 9 . As is clear from  FIG. 9 , the results for the second, third and fourth run are in excellent agreement, but differ from those obtained for the first run. It is believed that this could be due to irreversible physical changes occurring within the sensor the first time force is applied. In this regard, it is interesting to note that after a force of 2000 N is applied the measured resistance for run 1 falls into agreement with that of runs 2 to 4. This suggests that, in this case, a force of at least 2000 N is required to achieve the irreversible physical changes which occur between 1 and 3000 N. 
         [0109]    It is noted that there is a slight divergence in the measured resistance values for runs two, three and four at forces below around 40 N—this is probably due to artefacts caused by the extensometer since forces lower than 50N are close to instrument limits (hence there is an increased instrument error in this range). 
         [0110]    The large dynamic range, repeatability, and interpolatable nature of the pressure response of the device means that it is particularly well-suited to use as a sensitive pressure sensor. In particular, the sensor appears to work effectively at high pressures, in contrast to prior art pressure sensors based on piezoresistive carbon-based inks where resistance plateaus at relatively low pressures. 
       Example 2 
       [0111]    Experiments broadly following a similar protocol to that described for Example 1 were carried out using piezoresistive ink containing carbon nanoparticles including functionalised GNPs in a vinyl chloride copolymer based binder. 
         [0112]    In this case silver traces of 20 mm width, 150 mm length and ˜8 μm height were screen printed with a 54/64 mesh onto a 330 μm thick PET substrate using a DEK 248 screen printer. Three layers of piezoresistive ink were printed as continuous blocks over the silver traces using a 54/64 mesh to give a total height of ˜9 μm. A piezoresistive device was then formed following the same approach as in Example 1, and the resistance measured over a circular area with a diameter 15 mm (1.77 cm 2  area of compression). Three different devices were produced having “high”, “medium” and “low” loadings of carbon nanoparticles in the piezoresistive material. The devices having medium and low loadings had four and eight times less carbon than the high loading respectively. 
         [0113]    The device made using the piezoresistive material with high carbon nanoparticle loading displayed resistances of less than 1Ω at all applied pressures, demonstrating that this ink is not piezoresistive and hence not suitable for measuring applied pressures. In contrast, devices produced using piezoresistive materials having medium and low loadings of carbon nanoparticles showed a repeatable and interpolatable variation in measured resistance with applied pressure, and a suitably high resistance at zero applied force to be useful pressure sensors. The device having low carbon nanoparticle loading performed better than the device having medium carbon nanoparticle loading. 
       Example 3 
       [0114]    Experiments were carried out to determine the effect of increasing the number of layers of piezoresistive ink on resistance, with results shown in  FIG. 10 . 
         [0115]    A series of lower substrates were produced by screen printing an indium tin oxide sheet with one, two or three layers of a piezoresistive ink containing 3.5 wt. % functionalised graphene nanoplatelets (with negligible content of other types of carbon particle) dispersed in vinyl chloride copolymer based binder and solvent (15 parts binder to 85 parts solvent). The dried ink had a GNP content of ˜20 wt. %. Piezoresistive devices were formed by combining the lower substrates with an upper substrate, consisting of a further indium tin oxide sheet optionally bearing a single layer of the same piezoresistive ink as the lower substrate. The resistance of these devices under an applied pressure of 2000 N was measured using a Housfield extensometer with a circular area of compression of 1.77 cm 2  (diameter 15 mm) 
         [0116]    As can be seen in the results shown in  FIG. 10 , the resistance of the devices increased with the number of layers, and the devices having a piezoresistive ink layer on the upper substrate (the square data points in  FIG. 10 ) had a higher resistance than those lacking a layer of piezoresistive ink (the diamond data points in  FIG. 10 —i.e. devices in which the upper substrate did not have a piezoresisitve layer deposited on the silver). 
         [0117]    The resistance values measured in these tests were particularly high. It is thought that this is due to the use of inks incorporating low loadings of functionalised GNPs with negligble content of other carbon particles. 
       Example 4 
       [0118]    A piezosensitive device was produced and measured according to the “same layer measurement mode”. In this example, a lower substrate was produced by overprinting a PET sheet (330 μm thickess) with interdigitated silver positive and negative electrodes having the pattern shown in  FIG. 6 . The silver traces were printed using a 100/34 mesh, to produce interconnected linear traces of width ˜600 μm wide, length ˜15 mm and height ˜7 μm, with a separation of ˜400 μm between adjacent traces of the positive and negative electrodes. 
         [0119]    An upper substrate was produced by screen printing a continuous sheet of silver (15 mm length, 16 mm width, ˜8 μm height) on a further PET substrate using a 54/64 mesh, and subsequently coating this with two continuous layers of a piezoresistive ink containing carbon nanoparticles including functionalised GNPs in a liquid medium containing 15:85 vinyl chloride copolymer based binder:solvent (corresponding to the “low” loading ink from Example 2). A piezoresistive device was created by bringing the electrodes of the lower substrate into contact with the piezoresistive ink of the upper substrate. 
         [0120]    A second device incorporating three layers of piezoresistive on the upper substrate was produced following the same procedure described above. 
         [0121]    The resistance of the devices was then measured by measuring the resistance between the positive and negative electrodes on the lower substrate. The results of measurements for the two layer and three layer devices are shown in  FIGS. 11 and 12  respectively. 
       Example 5 
       [0122]    A further piezoresistive device was produced and measured according to the “same layer measurement mode”. In this example, a lower substrate was produced by overprinting a PET sheet with interdigitated silver positive and negative electrodes following the method described in Example 4, and subsequently coating this with two continuous layers of a piezoresistive ink containing carbon nanoparticles including functionalised GNPs in a liquid medium containing 15:85 vinyl chloride copolymer based binder:solvent (corresponding to the “low” loading ink from Example 2) using a 100/34 mesh to give a piezoresistive material of height ˜4 μm. The resistance between the positive and negative electrodes was measured and found to be 220 kΩ in the absence of an applied force. 
         [0123]    An upper substrate was produced by screen printing a silver electrode on a further PET sheet, and subsequently coating this with two layers of the same piezoresistive ink used for the lower substrate. A piezoresistive device was created by bringing the piezoresistive ink layers of the lower substrate into contact with the piezoresistive ink of the upper substrate. Resistance between the positive and negative electrodes was measured under varying applied forces, producing the results shown in  FIG. 13 . 
         [0124]    The resistance of the lower substrate in the absence of the upper substrate was significantly greater than that measured for the device incorporating the upper substrate. This difference is such that the contribution of electron flow directly between adjacent conductive traces (i.e. not mediated by the upper conductive layer) is negligible in the piezoresistive device incorporating the upper and lower substrates. 
         [0125]    In respect of numerical ranges disclosed in the present description it will of course be understood that in the normal way the technical criterion for the upper limit is different from the technical criterion for the lower limit, i.e. the upper and lower limits are intrinsically distinct proposals. 
         [0126]    For the avoidance of doubt it is confirmed that in the general description above, in the usual way the proposal of general preferences and options in respect of different features of the piezoresistive device and methods described above constitutes the proposal of general combinations of those general preferences and options for the different features, insofar as they are combinable and compatible and are put forward in the same context.