WO2016123651A1 - Deformable sensors and method for their fabrication using ionic liquids - Google Patents

Abstract

A deformable sensor and method of fabrication, the sensor comprising a deformable substrate and a conductive liquid, preferably an ionic liquid.

Description

DEFORM ABLE SENSORS AMD METHOD FOR THEIR FABRICATIO USING IONIC LIQUIDS

FIELD OF INVENTION

[0001] The present invention relates to the field of sensors and fabrication thereof.

[0002] in one form, the invention relates to deformab!e sensors comprising ionic liquids.

[0003] In another form, the invention relates to deformabSe sensors for sensing physical parameters such as pressure, strain etcetera.

[0004] In one particular aspect the present invention is suitable for broad technological application for areas ranging from autonomous artificial intelligence, such as electronic skins on robots to wearable health monitors.

BACKGROUND ART

[0005] If is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is inciuded to explain the context of the present invention. Further, the discussion throughout this specification comes about due to the realisation of the inventor and/or the identification of certain related art problems b the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is inciuded to explain the context of the invention in terms of the inventor's knowledge and experience and, accordingly, any such discussion should not be taken as an admission that any of the material forms part of the prior art base or the common general knowledge in the relevant art in Australia, or elsewhere, on or before the priority date of the disclosure and claims herein.

[0006] in recent times, electronic devices have rapidly evolved from being relatively heavy and bulky to being light, smart and mobile. However, they are still relatively rigid, and this attribute limits the extent to which they can be integrated into appliances and human/appliance interfaces. Accordingly, there is a growing need for flexible and stretdhable electronics-, and electronics that can conform to curvilinear surfaces and soft three-dimensional shapes.

[0007] Sensors are fundamental components of the electronic industr and are used in a wide range of applications, in the past, electronic sensors have often comprised of multiple layers of materials and required metal interconnections which made their manufacturing quite complex,

[0008] In more recent times printing, particularly screen-printing has been used to make sensors based on specialised ceramic materials. However these types of sensors require a sintering process which is not compatible with the incorporation of plastic components due to melting and deformation. Accordingly, sensors have been developed that are based on inks that can be printed directly onto plastic substrates. They offer significant advantages such as ftexibiiity, thinness and light weight and open up new opportunities for sensor use in association with curved surfaces, ultra-thin devices and shock-resistant devices.

[0009] For example, ultra-thin, flexible printed circuits are commercially available as sensors for force measurement from Tekscan, Inc in association with their FlexiForce® trade mark. These sensors are used in many applications to detect and measure a relative change i force or applied load, rate of change in force, to defect contact and/or touch and to identify force thresholds and trigger appropriate action.

[0010] In recent times new types of pressure sensors based on nanotechnology have also been investigated. Such sensors typically contain a number of circuits or complex layered matrix arrays. For example, a flexible, skin-attachable strain-gauge sensor has been developed based on two interlocked arrays of high aspect ratio Pt-eoated poiymeric nanofibres that are supported on thin polydimethylsi!oxane (PDMS) layers. When different sensing stimuli are applied, the degree of interconnection and the electrical resistance of the sensor changes in a reversible, directional manner with specific, discernible strain-gauge factors (Pang et ai, Nature Materials (2012) 1 1 , 795). This type of strain gauge has been used to monitor signals ranging from human heartbeats to the impact of a bouncing water droplet on a superhydrophobic surface. [0011] Stretch sensors are soft pieces of flexible polymer that transmit information about the degree to which they are being stretched. Stretch sensors of the prior art typically comprise elastic capacitors made of laminated polymer structure. The capacitance of the structure changes when the sensor is stretched and this change is measured arid related to deformation.

[0012] Electronic skins (e-skins) are a new class of advanced materials that can be largely pressed, bent, twisted and stretched while maintaining outstanding optoelectronic responses, and will be ke components in future wearable electronics.

[0013] However stretch sensors of the prior art typically have a number of non-ideal characteristics and problems. Stretch sensors often include electrodes that have unpredictable resistance, capacitances are small and susceptible to parasitic effects and there is a complex electromechanical impedanc matching problem at the connections. Sensors based on resistive principles often suffer from drift and rate dependent effects.

[0014] Efforts have been made to address these drawbacks. Stretch sensors have been described in a number of prior art publications including Jvluth et al (Advanced Materials, 2014, 26, 6307) which describes embedded 3D printing of strain sensors within highly stretchable elastomers. Specifically, a new method of embedded 3d printing is reported for fabricating strain sensors within highly conformal and extensible elastomeric matrices. Creation of soft sensors is described, the sensors having nearly arbitrary planar and 3D motifs in a highly programmable and seamless manner.

[0015] Yoon et al (Advanced Materials, 2014, DOl: 10.1002/adm a.201402588) describes the design and fabrication of novel deformable device arrays on a deformable polymer substrate with embedded liquid-metal interconnections. Active devices attached on stiff islands are electrically connected by an embedded EGaln interconnection, which ensures protection from external damage,

[0016] Sekitani et al {Science, 2012, 321 , 1468) describes a rubberSike stretchable active matri manufactured using elastic conductors. [0017] WO 2012050938 (Kramer et at) describes wearable tactiie keypads with deformable artificial skin. The patent application describes a hyper-elastic, thin, transparent pressure sensitive keypad fabncated by embedding a silicone rubber film with conductive liquid-filled mtcrochannels.

[0018] WO 2013044228 (Wood et a!) describes artificial skin and elastic strain sensors. If relates to an elastic strain sensor which can be incorporated into an artificial skin that can sense flexing by the underlying support structure of the skin to detect and track motion of the support structure. Generally, resistance-type deformable sensors of this type consist of two crucial components: conductive fillers/coatings and polymeric scaffolds. When stretched under a large strain (e.g. ε > 100%), rigid conductors upon/inside soft polymers are easy to break due to their higher Young's modulus (5-6 orders of magnitude larger than elastomers).

[0019] The simultaneous cooperation of mechanical robustness and electronic performance is the key to fabricate reliable deformable sensors. Elastic polymers, with Young's modulus of 106 -107 Pa, are commonly used as the scaffolds due to their reversibiy stretchable ability. To make these elastomers conducting, solid conductive building blocks, such as silicon, gold, carbon or others (high Young's modulus of 1011-1012 Pa), have been filled, deposited or grown upon the polymeric scaffolds. Owing to the considerable difference of Young's modulus (5~€ orders of magnitude) between the elastomers and conductors, the physical deformation of solid conductive components cannot follow the elastic scaffolds, especially under a large mechanical strain (e.g. ε > 100 %). As a result, several cracks would appear upon the conductive parts, yielding poor electronic performances.

[0020] To solve this problem, structure-based approaches have been proposed. Waves, meanders, heiices, spirals or net structured conductive components have been generated to offset the strain by their shape deformation. On the other hand, material- based methods, by utilizing nanoparticles, nanowires or nanosheets to fill the elastic scaffolds, have been exploited to bear strain through their nanoscaie networks. Although dramatic efforts have been invested to reduce the deformation-induced material break, the intrinsic Young's modulus difference between the elastic scaffolds and solid conductors still exist, which indicates a strain limitation of above-mentioned deformable sensors.

[0021] There is therefore a need for low-cost, simple and universal strategy to fabricate deformabie sensors optimally free from materia! delamination or cracking and having long-term durability.

SUMMARY OF INVENTION

[0022] An object of the present invention is to provide an improved sensor using ionic liquids.

[0023] Another object of the present invention is to provide an economical method of fabricating deformable sensors.

[0024] A further object of the present invention is to alleviate at least one disadvantage associated with the related art.

[0025] It is an object of the embodiments described herein to overcome or alleviate at least one of the above noted drawbacks of related art systems or to at least provide a useful alternative to related art systems.

[0026] In a first aspect of embodiments described herein there is provided a deformable sensor comprising a deformable substrate and a conducting liquid, preferably an ionic iiquid.

[0027] The deformable sensor of the present invention is characterised from the deformable sensors of the prior art by the use of conducting liquids, preferably ionic liquids, as the conducting component. Furthermore, the present invention differs from solid-Gonductors/solid substrate (that is, solid-to-solid type) sensor construction, by virtue of the use of a lower Young's modulus conductive material to cooperate with deformable substrates (that is, tiquid-io-solid type sensor construction). 8

[0028] Without wishing to be bound by theory, the present invention typically involves the impregnation or deposition of ionic liquids into or onto eiastomeric structures,

[0029] This ii uid-to-so(id strategy can be widely adapted to various conductive liquids and substrates, preferably elastic substrates. Optimally, mechanica! match between ionic liquids and substrates used in the present invention provides deformable sensors that can work at strain levels up to relatively high values (ε > 600%).

Substrate

[0030] Typically the substrate comprises natural substances, synthetic substances or combinations thereof that individually, or in combination are deformable. For exampie the substrate may be siretchafole, bendable, twistable, malleable or ductile.

[0031] The substrate may comprise an ordered structure, such as woven fibres, or be of random or amorphous structure such as a sponge. Typically the substrate is chosen from the group comprising natural substances such as wool, cotton, skin or hide; synthetic polymers such as polyester, polyethylene, nylon o artificial skin; natural polymers such as rubber. Eiastomeric polymers are particularly preferred and provide a useful scaffold for supporting the ionic liquid.

[0032] in a preferred embodiment the substrate comprises a fabric, such as a fabric comprising woven synthetic or natural fibres or combinations thereof. Fabric-based deformable sensors according to the present invention are sensitive, stable and exhibit a long life-time. While conductive liquids have been used in pressure sensitive matrixes of the prior art it has not hitherto been known to use the combination of an ionic liquid with a deformable substrate such as a fabric to form a sensor.

[0033] Preferably the sensitivity of deformable sensor according to the present invention has a lower operating limit of about 0,05 % deformation,

[0034 Preferably deformable sensors according to the present invention are sufficiently stable that there are negligible loading-unioading signal changes over at least 10,000 cycles. [0035] Preferably deformable sensors according to the present invention have a lifespan of more than two months. tonic Liquid

[0038] The ionic liquid should exhibit the properties of (i) low evaporation at ambient temperature and pressure, and (is) adherence to the elastic substrate,

[0037] Ionic liquids are also colloquially referred to as 'liquid electrolytes¹, 'ionic melts', 'ionic fluids', fused salts', 'liquid salts' or Ionic glasses'. Ionic liquids comprise ions or ion-pairs forming salt-like materials that are liquid below an arbitrary temperature, such as near ambient temperature, ionic liquids typically have a low vapour pressure, preferably evaporating at much iower rates than water, indicating a long lift-time in ionic liquid state. Furthermore, the typically low Young's modulus, high viscosity and low surface tension (typically -15-35 mN-m^"1, more typically ~2Q-30 mN-m^"1) of ionic liquids contribute to their advanced ability to adhere to substrates such as elastomers under a 100 % strain.

[0038] Preferably the ionic liquid of the present invention comprises salts having a tow melting point or low eutectic temperature. Preferably the melting point or eutectic temperature is less than ambient temperature, nominally less than about 25 °C, so that they remain fluid at room temperature.

[0039] In a preferred embodiment the ionic liquid is a eutectic system. A eutectic system is a homogeneous solid mix of atomic and/or chemical species that forms a joint super-lattice, having a unique atomic percentage ratio between the components (as each pure component has its own distinct bulk lattice arrangement}. It is only in this aiornic mofecuiar ratio that the eutectic system melts as a whole, at a specific temperature (the eutectic temperature) the super lattice releasing at once all its components info a liquid mixture. The eutectic temperature is the lowest possible melting temperature over all of the mixing ratios for the relevant component species.

[0040] For example, a metallic liquid such as eutectic gallium-indium (EGa!n) has a lower Young's modulus than that of elastoraeric supports and can be used as a ionic liquid in the present invention to provide a iargs-strain-avaiiabie current device. However EGaln, like most metallic liquids exhibits the drawback of showing poor adhesion to elastic substrates due to its large surface tension {hundreds of mN m-1) leading to poor spreading and adhesion to the elastic scaffolds. Nonetheless, EGain can be used for the present invention if, for example it is sealed inside pre-generated microchannels.

[0041] in a particularly preferred embodiment the ionic liquid of the present invention is chosen from the group comprising

tetrabuiyJphosphonium methanesulphonate

1 -methyi-S-Qctylimidazolium chloride

1 -buty!-3-methyiimidazolium bromide

• 1-(3-cyanopropyi)-3-methyl!midazolium bis(trifiuoromethyisui honyl) amide

* l-butyl-2,3-dimeihy1-imida2ol!.M^'m teirafluorQborate.

[0042] In a second aspect of embodiments described herein there is provided a method of fabricating a deformable sensor according to the present invention including the step of wetting a substrate with a ionic liquid.

[0043] Since ionic liquids are softer than typical elastomeric support and can conformably 'wet' complex topological^ structures at a range of size scales, the inherent mechanical mismatch issues in conventional methodologies can be circumvented.

[0044] Where used herein, 'wetting' is intended to refer to the ability of the ionic liquid to maintain contact with the substrate surface, resulting from intermolecuiar interactions when the two are brought together. This may be achieved, for example, by Immersing or dipping the substrate in the ionic liquid, or pouring or dropping the ionic liquid on the substrate.

[0045] The method of fabrication based on use of ionic liquids is of a generalised, platform nature, being applicable to any type of hydrophiiic/hydrophobic ionic liquid species, and capable of turning virtually any soft elastomeric materials/supports (such as force-spun fibre mats, rubbers, clothes and sponges) into sensors in a simple and rapid manner. Thus, using the simple yet efficient ionic liquid wetting approach, any elastomeric objects with dimensions from microscaie to macrosca!e could be converted into a highl deformable, piezoresistive strain sensors in a small experimental timeframe.

[0048] For example wetting can be carried out by contacting the elastic substrate with the ionic liquid at least once and preferably multiple times.

[0047] The method of fabrication or sensors according to the present invention is relatively uncomplicated, importantly, the fabrication time for the sensors of the present invention may be rapid - typically less than half a minute (which corresponds to the time taken for the ionic liquid to wet the substrate).

[0048] In addition, sensors according to the present invention can be patterned by any convenient method such as simple, direct pen writing, 'ink-jet' or stamp printing. Despite such simplicity in fabrication, the sensors of the present invention are capable of functioning at ultra-large strains (ε * - 00%); high sensitivity down to a Sow-strain of approximately 0.05 %; high durability with negligible loading-unloading signal changes over about 10,000 cycles; washable without the need of sealing; long-term stability after exposing a naked sensor to ambient conditions for >2 months.

[0049] in addition, sensors according to the present invention can be attached to skin or integrated with cloth to enable true wearability, allowing a wide range of body motion tracking and wrist pulse monitoring. The fabrication method of the present invention opens a new powerful route to synthesize piezoresistive sensors with a myriad of applications in future wearable electronics. The present invention thus provides a sensor for attachment to the skin or hide of a human or animal for sensing of physical parameters associated with physiological phenomena.

[0D60] The present invention thus provides a liquid ionic !ayer strategy to prepare deformabie sensors that are relatively simple yet can accommodate high levels of strain and exhibit long lifespan.

[0051] Other aspects and preferred forms are disclosed in the specification and/or defined in the appended claims, forming a part of the description of the invention .

[0052] in essence, embodiments of the present invention stem from the realization that ionic liquids can be used as the conducting component in. a deformabie sensor. More particularly, the invention of the present application is based on the realisation that a mechanical match between tonic liquids and substrates can be used to achieve deformabie sensors that can work at relatively high strain. More specifically it has been realised that mechanical mismatch (where the Young's moduli of inorganic conductors are many orders of magnitude larger than that of the soft elastomers) leads to poor long- term durability due, for example to material delamination and/or local fracturing in inorganic components.

[0053] Advantages provided by the deformabie sensors of present invention comprise the Following: * can work at relatively high strain levels, up to at least 600%;

* relatively low operating limit of about 0.05% deformation; « sensitive at relatively low strain levels;

♦ relatively long lifespan;

* stable;

* useful for a wide range of applications, particularly i the biomedicai field;

« can be incorporated into skin or wearable fabrics.

[0D54J Advantages provided by the fabrication method of present invention comprise the following:

» can be carried out rapidly - less than 30 seconds in some embodiments;

• simple to cany out;

♦ economical;

• can be applied to a wide range of substrates.

[0055] Stretch sensors of the present application are suitable for a wide range of applications including, but not limited to, the following:

• animation/motion capture;

• autonomous artificial intelligence - such as electronic skins on robots;

• interacting with electronic or augmented reality devices; * bringing a skin-like perception in response to external stimul in prostheses an peripheral nervous system interface technologies;

« wearable health monitors far monitoring vital signs during exercise, sports training, medical procedures and early warning;

• non-vital, but nonetheless important, physiological parameters for health and rehabilitation purposes.

[0056] Further scope of applicability of embodiments of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, white indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the disclosure herein will become apparent to those skilled in the art from this detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Further disclosure, objects, advantages and aspects of preferred and other embodiments of the present application may be better understood by those skilled in the relevant art by reference to the following description of embodiments taken in conjunction with the accompanying drawings, which are given by way of illustration only, and thus are not limitative of the disclosure herein, and in which:

[0058] FIG. 1 is a schematic illustrating a facile and rapid liquid-to-solid strategy to generate deformabie sensors by simply dropping ionic liquids (6.) upon the network of fibres (2) making up the fabric. Owing to their lower Young's modulus (5 to 6 orders of magnitude smaller than that of elastomers), the ionic liquids typically remain as a thin yet continuous liquid layer (3) even when the fabric has been stretched (4) under a significant strain and recovered (5), yielding a long-stra in-available deformabie sensors.

[0059] FIG. 2 illustrates current-time characteristics of the fabrication process of ionic liquid infused polymeric fibres during a non-conducting phase (8), fabrication (10), adjustment (12) and establishment of the deformabie sensor (14). Taking l-butyl-2,3- dimethyl-imidazolium tetrafluoroborate (conductive component) and polyoiefin elastomer (POE) fibre network (elastic substrate) as an example, the ionic liquid was directly dropped onto the fibre network by a pipette. After several cycles of stretching, the ionic liquid uniformly spread out upon the fibre surfaces, yielding noise-free, stable and continuous electrical responses after 47 seconds. The fabrication is rapid, adjustment is just 24 seconds and the process is equipment-free.

[0060] 3D laser scanning confocai microscope observations were made of ionic liquid infused POE fibre sheet under 0 % strain (ε ~ 0) (width 644.40 micron, height 644.40 micron, depth 79.48 micron) and at 100 % strain (ε = 100%). Rhodamine B was dissolved inside ionic liquid to enable it show red fluorescence when exposed to the laser irradiation. No matter before o after stretching (in the direction of the arrows), continuous red fluorescence could be observed along the fibres, indicating the existence of ionic liquid layer upon fibre surfaces. In this case, the fibre sensors could remain conductive even at a large strain, yielding a long-strain-available deformable sensor. (Width 844.40 micron, height 644.40 micron, depth 79.48 micron)

[0061] FIG. 3 illustrates the application of the liquid-to-soiid strategy and method of the present invention. Specifically, FIG. 3 is a digital image of a fibre sheet (20) with an inset scanning electron microscopy (SEM) image of the fibre sheet showing 3D structure. After it has been infused with ionic liquid the fibre sheet becomes a deformable sensor capable of changing from an unstrained resting condition (t - 0%, f - -21.5 uA, E=5.005V) to working under large strains of 600% (ε - 600%, ί = -848.9 μΑ, E=5.005V) when stretched from 1cm to 7 cms.

[0062] FIG. 4 is a digital image of a commercial cotton T-shirt (25) with SEM observations of the cotton fabric showing 3D structure. After being infused by ionic liquid, the cotton fibre piece becomes a deformable sensor capable of changing from an unstrained resting condition (ε = 0%, i ~ -168.3 μΑ, E=5.005V) to working under large strains of 100% (ε = 100%, i = -81.71 μΑ_» E= 5.005V).

[0063] FIG. 5 is a digital image of a commercial poiyurethane washing sponge (30) with SEM images of the sponge showing 3D structure. After being infused by ionic liquids, the sponge becomes a deformable sensor capable of changing from an unstrained resting condition (ε - 0%, i = -248.9 μΑ, E=5.005V)of working under large strains of 120 % (ε - 120%, I - -100.4 μΑ, E - 5 005V).

[0064] FIG. 6 illustrates the stable electrical performance of ionic liquid infused fibre sheets. Specificall Fig. 6 is a plot illustrating the dependence of electrical resistance on the inducing time by using a plastic plate. The inset image is schematic of plate inducing process,

[0065] FIG. 7 is a plot illustrating the dependence of electrical resistance on the weight ratio of ionic-liquid/fabric. Error bars in FIG. 6 and FIG. 7b, represent standard deviation from testing results for five times. The inset image is schematic of dropping process. Based on the investigation in FIG. 6 and FIG. 7b, the following sample preparations have been fixed at ionic-liquid/fibres weight ratio around 200 %, and underwent the plate inducting treatment for at feast 5 times.

[0066] FIG. 8 is a strain-response plot for the ionic liquid infused POE fibre sensor. The sensor exhibit a gauge factor (GF) of 1 .11 during 0 % and 600 % strain.

[0067] FIG. 9 is a plot of resistance change of the sensor as a function of time (input frequency: 0,5 Hz) for the applied strain in the range of 1 % - 200 %: 1 % (21 ), 3% (23), 5% (25), 10% (28), 20% (30), 30% (32), 50% (34), 100% (36), 200% (38).

[0068] FIG. 10 illustrates the lifespan test under a strain of 10 % at a frequency of 1 Hz. The resistance change curves were recorded after each 2,000 cycles and 200 cycles of data were presented in each recording. The other part of the figure is a magnified view of the part of the AR R0-t curve after 0,000 loading-unloading cycles,

[0069] FIG. 1 1 is a plot of resistance change of the sensor as a function of time (input strain: 1%) for diverse frequencies including 9 Hz (Fig.11 a), 4 Hz (Fig.1 1 ) and 1.4 Hz (F^'jg.11c), The applied voltage in all the electrical tests was 5 V.

[0070] FIG. 12 illustrates monitoring of human behaviours using a sensor according to the present invention. The ionic liquid infused POE fibre sheets have been cut Into suitable sizes that were closely attached to the human body. To connect the electronic system as well as physical integration to cloth, commercial conductive threads were directly sewed upon the two ends of sensors as the source-drain electrodes.

[0071] Due to the long-range sensitive feature (strain from 0.05 % to 800 %) of this liqutd-to-s^'oi^'id type sensor, human behaviours such as one finger touching (40), two finger touching (42), one finger scanning (44), hand touching (46), swiping (48), head moving (50), increased bending angles (52), fist gripping (54), trembling (56) twisting (58), small bending (60) large bending (62) and walking: (64) can be detected by sensors at. the shoulder (66), trapezius (68), elbow (70), arm muscle (72) and knee (74). They can all be monitored by the upward and downward slopes of the relative resistance associated with extemai-force-induced deformation of fibre sensors.

[0072] FIG. 13 illustrates the potential of ionic liquids to adhere to elastomers even under a large strain. FIG. 13a is a photograph of a 4 μ!_ ionic liquid (1-butyl-2,3-dimethyl- imidazolium tetrafluoroborate) droplet on a flat polyoiefin elastomer (POE) substrate before stretching with a strain of 100 % and having a contact angle of 84.2 ± 1.3°. FIG. 13b is a photograph of a 4 pL ionic liquid (1 -butyl-2,3-dimethyl-i^'mida2oli^'um tetrafluoroborate) droplet on a flat polyoiefin elastomer (POE) substrate after stretching with a strain of 100%. The droplet deformed following the stretching of bottom elastomer, yielding the contact angle decrease from 84.2 ± 1.3° to 47.2 ± 2.8°. This indicates that the ionic liquid could adhere to the elastomers well even under a large strain.

[0073] FIG. 14 illustrates stable electrical responses of ionic liquid infused cotton fabric or pofyurethane sponge based deforrnabie sensors.

[0074] FIG. 14a is a plot of resistance change of a cotton fabric based sensor as a function of time (input frequency: 0.5 Hz) for the applied strain in the range of 1% - 50%, that is, 1 % (80), 5% (82), 10% (84), 15% (86), 20% (88), 25% (90), 30% (92), 40% (94) and 50% (96),

[0075] FIG. 14b is a plot of resistance change of a polyurethane sponge based sensors as a function of time (input frequency: 0.5 Hz) for the applied strain in the range of 1% - 50%. The sample preparations have been fixed at ionic-liquid/fabric o sponge weight ratio around 200 %, and underwent the plate inducting treatment at least 5 times. The applied voltage in all the electrical tests was 5 V.

[0076] FIG. 15 illustrates steady response to static stretching of ionic liquid infused POE fibre sensors. The sample preparation has been fixed at ionic-iiquid/fibres weight ratio around 200 %, and underwent the plate-inducing treatment for at feast 5 times. The resistance of sensor under each stretching was constant. The detailed l~V curve shows mechanical loads unde various strains: 0% (100), 1 % (106), 20% (1 12), 40% (116), 100% (120), 200% (124).

[0077] FIG. 8 illustrates detection of a 0.05 % strain by ionic liquid infused POE fibre sensors according to the present invention. FIG. 16a is a plot of resistance change of fibre based sensors as a function of time (input frequency: 0.5 Hz) for the applied strain in the range of 1 % - 10 %, specifically 10% (130), 9% (132), 8% (134), 7% (136), 6% (138), 5% (140), 4% (142), 3% (144), 2% (146), 1 % (148). FIG. 15b is a piot of resistance change of fibre based sensors as a function of time (input frequency: 0.5 Hz) for the applied strain in the range of 0.1 % (150) and 0.05% (152). The applied voltage in aii the electrical tests was 5 V.

[0078] FIGS. 17a and 17b illustrates how an ionic liquid infused POE fibre sensor fabricated according to the present invention can respond diverse frequencies from 0.1 to 9 Hz, Plots of resistance change of the sensor as a function of time (input strain: 1%) for diverse frequencies including 9.0 Hz, 8.5 Hz, 7.1 Hz, 5.9 Hz, 9 Hz, 4.0 Hz, 3,2 Hz, 2.9 Hz, 1.3 Hz, 1.0 Hz, 0.3 Hz, 0.2 Hz and 0.1 Hz. The applied voltage in all the electrical tests was 5 V.

[0079] FIG. 18 illustrates how nodding head behaviour can be monitored b the ionic liquid infused POE fibre sensor. Current-time characteristics of the volunteer nodding her head regularly. The inset images are representative digital images to show the human behaviour during the test. The applied voltage in the electrical test was 5 V,

[0080] FIG. 19 illustrates plots of resistance change as a function of time for diverse kinds of ionic liquids suitable for use in the present invention using an input frequenc of 0.5hz for the applied strain in the range of 1 % to 50%, specifically 50% (160), 30% (162), 10% (184), 5% (166), 1% (188). The sample preparations were fixed at a sonic liquid/substrate weight ratio around 200%. The applied voltage in all the electrical tests was SV. The ionic liquids used in the tests were as follows:

FIG. 19a - tetrabuty!phosphonium methanesulphonate

FIG. 19b - 1-methyi-3-octylimidazoiium chloride

FIG. 19c - 1 -butyi-3-methyitmidazolium bromide

FIG. 19d - 1-(3-cyanopropyJ)-c-methylimjdazolium bis(trifluoromethyisulphonyl) amide

DETAILED DESCRIPTION

[0081] Preferably the ionic liquids of the present invention are room temperature ionic liquids, such as saits with a low melting point (<25°C.) such that they are liquid at room temperature. Typically the evaporate at much lower rates than water, indicating a long lift-time in ionic liquid state, and exhibit high viscosity and low surface tension (**2O-30 rnN-m^*1}, which contributes to their ability to adhere to a substrate under a 100 % strain (FIG, 13). The ionic liquids can be elongated in response to deformation, such as stretching, of the substrate. The steps of coating, then integration of the ionic liquid onto the substrate is also relatively simple.

[0082] Taking 1-butyl-2,3-dimethyl mtdazoliurn tetrafluofoborate (conductive component) and potyolefin elastomer (POE) fibre network (elastic scaffold) as example s of a suitable ionic liquid and substrate, the ionic liquid can be directly dropped onto the fibre network by a pipette. After several cycles of stretching, the ionic liquid could uniformly spread out upon the fibre surfaces, yielding noise-free, stable and continuous electrical responses after 47 seconds. Owing to reduced thickness and increased length of the ionic liquid layers under a certain strain, the resistance of the sample would increase accordingly. This trend reverses when the fibres returned to thei unstretched state, resulting in the generation of liquid-to-solid type deformabie sensors. The time for fabrication and adjustment using this method is comparatively, short - 24 seconds - and the process requires minimal equipment The present invention thus provides a relatively simple and straightforward method for fabricating deformabie sensors.

[0083] In the native state A, the resistance of the sensor can be described as:

" <■^■■'■ ^■» (Equation 1) where R is the total resistance of the sensor, p is the specific electrical resistance of ILs, Sn is the cross-sectional area of ionic liquids along each fiber/structure. When the sensor was stretched (state B), its resistance become

L - il + s )

S. /.- ;

λ^' ··-< ^ι<ί» ^' ' * (Equation 2) 10 where ' is the resistance of the sensor under a certain strain, ε is the strain, <5n is the modified factor of each cross-sectional area of ionic liquids along each fibre/structure under the strain. The resistance switching between state A and state B was fully reversible. Remarkably, such stretch-induced responses were highly stable and could be reproduced after 2 months of sample aging under ambient conditions

[0084] To investigate the existence state of ionic liquid layers upon fibre surfaces before and after stretching, laser scanning confoca! microscope was used. High reproducibility and durability could be attributed to strong adhesion of ionic liquids to POE fibres, and delarnination/crack-free in ionic liquids, POE and ionic liquid/POE interface. Strong adhesion before and after stretching was clearly demonstrated.

[0085] Rhodamirte B was dissolved in ionic liquid to facilitate observations. Rhodamine B exhibits red fluorescence when exposed to a laser beam having an excitation wavelength at 561.3nm. Before stretching, red fluorescence appeared inside fibre gaps (the fibres have no fluorescence), indicating that the ionic liquid had fully permeating inside the fibre network. When stretched under a 100% strain, several aligned fibre structures can be found parallel to the stretching direction. Notably, red fluorescence covered the whole fibre structure, indicating the existence of continuous ionic liquid layer upon stretched fibre surfaces. The ionic liquid layers became thinned under a strain, leading to the increase of resistance, as described in the Equation 2.

[0086] Since the Young's modulus of the ionic liquid is smaller than that of elastomers, the strain limitation of this liquid-to-solid type deformabie sensor depend on the elastic scaffolds themselves rather than conductive components. In other words, this stretchabfe sensor can work until the mechanical break of elastomers due to severe deformation. For the fibre network in this study, the as-prepared fibre-based sensors can work even at the strain more than 600 % (FIG. 3). After stretching, this sensor can return with stable mechanical robustness as well as electronic performance,

[0087] The method of the present invention is general and can be used for a wide range of substrates, and is applicable to a wide range of elastomeric scaffolds. Besides POE fiber mats, other kinds of scaffolds, including ID rubber bands, 2D commercial cotton fabric (FIG. 4), or 3D porous sponges (FIG. 5), could also be wetted by the ionic liquids to generate stretchable sensors within half a minute.

[0088] Owing to the existence of continuous ionic liquid layers upon the fabric or porous networks, these fabric/porosity-based sensors showed large-strain-conducing property (e of 100% for cotton fabric and ε of 120% for the sponge, and stable electrical responses according to diverse strain values (FIGS. 14 & 15), in short, a liquid-to-solid strategy can be adapted to a wide range of different elastic scaffolds in the fabrication of deformab!e electronics.

[0089] The ionic liquids used in the method of the present invention can also be applied to a substrate in any appropriate pattern, simply by a pen delivery. In addition to displaying aesthetic patterns they could simultaneously serve as highly deformable and wearable piezoresistive strain sensors, !n addition, the piezoresistive sensors of the present invention may be washable, particularly if hydrophobic ionic liquids are used, without the need of additional sealing steps.

[0090] In one exemplary experiment 1-{3-cyanopropyi)-3-methyiimidazolium bis(trifiuoromethylsuifonyi)amide was used impregnate a piece of commercial cotton fabric (2.5x2.5 cm²). The hydrophobic ionic liquid molecules became trapped in the cotton fibers, and were not removed during vigorous washing steps. After being completely blow dried with air, the sensors recovered full sensing performance.

[0091] In additional to visual demonstrations of this liquid-to-solid strategy, a quantifiable investigation was performed. Owing to the high viscosity feature of ionic liquids, they require a certain time to wet then spread upon the elastic scaffolds. Therefore, a plate was used to induce the ionic liquid spreading along the fibres, and recorded the dependence of resistance on the inducing times (FIG. 6).

[0092] At the beginning, ionic liquid spread randomly upon the fibres, thus, the resistance was around 0.8 Mo-cm^"1. After plate-inducing 5 times, the resistance value of ionic liquid infused fibres remained at 0.2 ΜΩ-errf¹. The weight ratio of ionic-liquid/fibres also played an important role in deciding their resistance (FIG. 7). By gradually dropping the ionic liquid upon the fibre network, the resistance decreased from 8.16 D-cm^"1 (ratio of 31 wt%) to 0.15 MO-cm^"1 (ratio of 294 wt%). Thus, the following sample preparations were fixed at ionic-liquid/fibres weight ratio around 200 %, and underwent the plate- inducing treatment at least 5 times to obtain a stable resistance of approximately 0.20 MQ-crrf¹. The amount of ionic liquid required per cm² of fibre network is relatively small at about 8.7 mg. Typical cost for the sensor is 2 cents cm² based on the typical cost of commercial ionic liquid being AUD$3.14 per gram, which makes this liquid-to-soiid type sensor economic to fabricate.

[0093] The gauge factor (GF) of a deformable sensor prepared as descri ed above has been tested (FIG. 8). The GF is defined as:

R - R,

R

OF .= - (Equation 3)

where R is the resistance when stretching force upon the sensor, and RQ is the resistance of sensor stay without stretching; ε is the strain of this fibre-based sensor.

[0094] The dependence of resistance change on diverse sensor strain (from 0 to 600 %) has been investigated. The separated fitting of these points showed good linear behaviour (R* = 0.995} with a GF value of 1.11 , indicating the device can serve as a reliable deformable sensor This linear dependence became more obvious in the small strain range (0 to 4 %_:r see the inset image in FIG. 8).

[0095] The responses of sensors of th present invention to both static and dynamic mechanical stretching have been measured. The sensor exhibited a steady response to static stretching and the resistance under each strain was constant (FIG. 15). To investigate the detected strain range of this sensor towards dynamic forces, a computer- controlled stretching machine with minimum displacement of 1pm was applied to the sensors,

[0086] As shown in FIG. 9, the noise-free, stable and continuous responses could be observed at the strain ranged from 1% to 200%. Actually, a strain of 0.05% could be detected (FIG. 18), which indicates a 5pm length increase for a 1 ^χ1 cm² sample. The eyeing stability of as-prepared sensor was tested under a 10 % strain at a frequency of 1 Hz (FIG, 10), The consistent resistance change with strain applied on the sensor could be maintained after 10,000 loading-unloading cycles, implying long working life and reliability of this liquid-to-solid type sensor. Besides this continuous working-life-time test, our sensors can keep their electrical responding ability for more than two months.

[0097] The sample was stored at atmosphere, and showed stable electronic performance as well as mechanical robustness. It should be note that two months is not the !ife-tirne limitation of this liquid-to-solid type sensor, since the ionic iiquid could keep its liquid state at atmosphere for several years. The response of this sensor towards different frequencies has aiso been observed (FIG. 11), It can be clearly found that the output electrical signals remained stable without obvious change in amplitude at typical frequencies of 1.4, 4.0 and 9.0 Hz. Furthermore, the as-prepared sensor could also respond other diverse frequencies from 0.1 to 8.5 Hz (FIG. 17) with stable resistance change, indicating a wide frequency adaption fo this sensor.

[0098] ionic liquid based piezoresistive sensors according to the present invention can be attached to skin or integrated in cloth, enabling comprehensive body motion and health monitoring. By attaching POE fibre mat-based sensor patches directl onto the skin or sewing them into clothes, stretching of skin and muscles associated with routine human motions could be monitored in real-time and in-situ. Even the minor movements associated with artery wrist pulses can be monitored using sensors according to the present invention.

[0099] To demonstrate the potential of this liquid-fo-solid type sensor in wearable devices, a series of proof-to-principle deformable human motion detectors were created from the same substrate sample. The resultant sensor was directly sewed onto the cloth by commercial conductive threads, providing a simple yet effective manner to integrate wearable fabric. Cloth fabrics are used daily to protect body temperature and adhere close to the body. As a result of the deformable device architecture, the skin, cloth and sensor behave as a single cohesive deformable object, so the deformation of the human skins, or muscles, can be monitored directly and precisely using this sensor (FIG. 12). [0100] After fixing the sensor sample to the human shoulder, gentle touching could be monitored by the upward and downward slopes of the relative resistance associated with finger clicking or scanning (FIG. 12). Under the external pressures, this fabric sensor would be physically sunken, indicating the elongation of ionic liquid covered fibres. Accordingly, the resistance of sensor increased. On unloading, these fibres recovered to its original shape, leading to the decrease of the resistance to the primary value. Larger pressure as well as contact area could yield an increased resistance change. Thus, touching by the hand generated larger electrical response than that b one/two fingers.

[0101] Besides passive responding, active monitoring of human actions, such as swiping (sensor on shoulder), swing arm (sensor on elbow), diverse leg movements (sensor on knee), shaking (sensor on trapezius) or nodding head , could also be surveyed by this sensor that has been directly sewed upon different parts of the cloth.

[0102] Diverse human activities cause different degrees of shape deformations to an attachable fibre sensor.

[01033 Due to the long-range sensitive feature (strain from 0.05% to 600%, see FIG, 9 and FIG. 16) of this iiquid-to-solid type sensor, a small muscle traction generated by shaking head (resistance change up to 1 %, see FIG. 12), or kicking the leg by a large margin (resistance change from -10% to 10%, see FIG. 12), could all be detected. Such sensors might be useful to monitor athletes for the early detection of over-exercise induced muscle bruise, alerting coaches to any potential problems. Besides wearable property upon cloth, this sensor could also be directly attached onto the human skin to monitor th muscie movement (Note: this ionic liquid as well as the poiymer is non-toxic to human body).

[0104] Besides integration to the cloth, this ionic liquid sensing patch could also be directly attached onto th human skin to monitor the muscie movement (Both ionic liquids and FOE polymers are non-toxic to human skins). FIG. 12 shows the dependence of resistance on the grip fist behaviour by real-time monitoring of muscle tension/stretching associated with making a fist. About 5% resistance change appeared following rapid systolic diastoiic cycles of the hand muscle. Briefly, our ionic liquid infused sensor can be used to monitor diverse human physical behaviours. At least two advantages existed in our approach: the first is its facility since the fibre sensor can be directfy sewed onto the cloth, indicating an easy way to integrate the sensor in the cloth fabrication process in future. The other one is our liquid-to-sotid strategy can solve the connect problem between sensor and throughout wires that commonly causes the poor electrical performance in mostly existing wearable sensors. The ionic liquid used in our sensors can stably connect the sensor with the conductive threads due to its fluidity even at a large strain.

[0105] Sensors according to the present invention have been shown to be sensitive enough to monitor a human radial artery pulse in real-time. The wrist pulses could be read out accurately with two dearly distinguishable peaks (P1 and P3) and a late systolic augmentation shoulder (P2). This line shape is commonly caused by constitution of the blood pressure from the left ventricle contracts and reflective wave from the lower body. The radial augmentation index Air = P2/P1 is a characteristic value for arterial stiffness, which is closely related to the age of people. From a series of waveforms measured with a sensor according to the present invention, an average Air of 0.43 was estimated, agreeing quite well with the literature data for a health 25-year-old female. This result demonstrates the potential of the sensors to serve as wearable diagnostic device to monitor human health in real-time. It likewise could be incorporated on to prosthetics to help deliver real "feel" to the prosthetic wearer.

[0106] No fabricated deformab!e sensor reported until now possesses ail the unique characteristics of ionic-liquid/fabric one. A fast yet simple preparation, universe adaption to any elastic fabric, long-range sensitivity, long life-times for both thousands of testing or stored at atmosphere for more than two months, and an easy attachable way to the cloth towards wearable sensors. Considering the fluidity of ionic liquid to contaminate the human cloth or skin, we have sealed the sensor by sandwiching it between two layers of commercial Ecof!ex elastomer. The electrical performance of this sealed sample (a GF value of 1.63) was even better compared with that of the unsealed one (a GF value of 1.11 , see FIG. 8), Meih o<is

[0107] Materials. Poiyolefln elastomer (Engage 7457) was purchased from Dow while Ecofiex (00-30) was purchased from Smooth-On, inc. Rhodamine B ( 95%) was purchased from Sigma-Aidrich. Chloroform (~99.9%) and ethano! (~99.0%) were purchased from Merck KGaA. Porous polyurethane sponges, cotton T-shirt and sport clothes are common commercial products that can be purchased at the supermarket. The stainless thin conduciive threads for electrical connecting were purchased from Adafruit industries,

[0108] Preparation of fabric-based deformabie sensors: For fibre-based samples, 6g of potyolefin elastomer pellets were dissolved in 60 ml chloroform and stirred for 24 h. 3 mi ethanoi was added into the solution and stirred For another 24 h. The well dissolved polyme solution has been forcespun on the Force Spinner Cyclone L-1000M (Fibreio) at 5000 rpm to generate the fibre mats, which have been dried at room temperature. Then, the fibre mat was cut into -1 * 2 cm² fibre sheets and drop-casted by ionic liquids (the weight ratio of liquid/solid is 2/1). In order to generate a uniform liquid conductive layer along the fibre surfaces, a plastic plate has been utilized to induce the spreading of sonic liquid for at least 5 times, yielding stable deformabie sensors.

[0109] To prevent the fabric-based sensors from mechanical damages, an Ecofiex capsuiated strategy has been used. The ionic liquid infused fibre sheet has been sewed with two conductive threads at two ends, and sandwiched between two layers of Ecofiex films with thickness of ca. δθθμι ι, A few Ecofiex liquid prepolymer (a mixture of component A with component B at weight ratio of 1/1) has been used to seal these two layers by heating at 80^C'C. For the generation of cotton fabric or sponge type deformabi sensors, ~1 * 2 cm² sample pieces have been cut from commercial cotton T-shirt and polyurethane washing sponges, respectively. The following processes were similar to that of fibre based sensors.

[0110] Electrical performance of deformafole sensors: The electrical characteristics for ail the deformabie sensors were recorded by the Pars tat 2273 Electronchemical System (Princeton Applied Research) with the assistance of a translation stage (Model LTS150/M, Thorlabs). For proof-to-principle demonstrations to 2β

monitor human behaviors, ail the fibre based sensors have been cut into suitable sizes that densely attached to the human body. To connect the electronic system as well as physical integration to doth, commercial conductive threads were directly sewed upon the two ends of sensors as the source-drain electrodes.

[011 1] Characterizations: Confocal images were obtained using a Nikon A1 Rsi laser scanning confocal microscope with the excitation wavelength at 561.3 nm. Optical images and demonstration videos were taKen by a Nikon Digital Sight DS-Fil camera. SEM images were carried out using a FEI Nova FEG-SEM operated at 5kV beam voltage. The contact angle was measured on a contact angle goniometer (OCA 15EC).

[0112] While this invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modificat!on(s). This application is intended to cover any variations uses or adaptations of the invention following in general, the principles of the invention and including such departures from the present disclosure as come within known or customary practice within the art to which the inventio pertains and as may be applied to the essential features hereinbefore set forth.

[0113] As the present invention may be embodied in several forms without departing from the spirit of the essential characteristics of the invention, it should be understood that the above described embodiments are not to limit the present invention unless otherwise specified, but rather should be construed broadly within the spirit and scope of the invention as defined in the appended claims. The described embodiments are to be considered in all respects as illustrative only and not restrictive.

[0114] Various modifications and equivalent arrangements are intended to be included within the spirit and scope of the invention and appended claims. Therefore, the specific embodiments are to be understood to be illustrative of the many ways in which the principles of the present invention may be practiced. In the following claims, means-plus-function clauses are intended to cover structures as performing the defined function and not only structural equivalents, but also equivalent structures.

[0115] [0118] "Comprises/comprising'⁵ and "includes/including" when used in this specification is taken to specify the presence of stated features, integers, steps or components but does not preclude the presence or addition of one or more other features, integers, steps, components or groups thereof. Thus, unless the context clearly requires otherwise,, throughout the description and the claims, the words 'comprise', 'comprising', '^'includes'. Including' and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the sense of "including, but not limited to".

Claims

1. A deformabie sensor comprising a deformabie substrate and a conducting !iquid.

2. A deformabie sensor according to claim 1 wherein the conducting liquid is hydrophilic or hydrophobic ionic liquid.

3. A deformabie sensor according to claim 1 wherein the conducting liquid is an ionic liquid impregnated into the e!astomeric structure.

4. A deformabie sensor according to claim 1 wherein the conducting liquid is an ionic liquid deposited onto the elastomeric structure.

5. A deformabie sensor according to claim 1 wherein the conducting liquid has high viscosity , low surface tension of between 15 and 35 mN-rri¹ and a melting temperature or eutectic tem erature of less than ambient tem eratu e.

8. A deformabie sensor according to claim 1 wherein the conducting liquid is an ionic liquid chosen from the group comprising tetrabutylphosphonium meihanesuiphonate, 1- mefhyl-3-octylimida2olium chloride, l-butyl-S-methylimidazoiium bromide, 1-(3- cyanopropyl)-c-methylimida2oSium bis(trifiuorQmethylsulphonyl) amide, and 1-buty!-2,3- dimefhyl-imidazolium tetrafluoroborate.

7. A deformabie sensor according to claim 1 having a lower operating limit of about 0.05%.

8. A deformabie sensor according to claim 1 having an upper operating limit of about 800%.

9. A deformabie sensor according to claim 1 having negligible ioadtng-unloading signal changes over at least 10,000 cycles.

10. A method of fabricating a deformabie senso according to claim 1, the method including the step of wetting the substrate with a conductive liquid.

11. A method according to claim 10 wherein the conductive liquid is an ionic liquid.

12. A method according to claim 11 wherein the substrate is natural or synthetic, and chosen from the group comprising fibres, fabrics, sponges and sheet material.

13. A deformabie sensor according to claim 1 when incorporated into a wearable garment.

14. A deformabie sensor according to claim 1 when attached to skin or hide of a human or animai for sensing of physical parameters associated with physiological phenomena.