WO2014165908A1

WO2014165908A1 - Method and device for smart sensing

Info

Publication number: WO2014165908A1
Application number: PCT/AU2014/000381
Authority: WO
Inventors: Wenlong Cheng; Shu GONG; Yue TANG; Yi Chen
Original assignee: Monash University
Priority date: 2013-04-09
Filing date: 2014-04-09
Publication date: 2014-10-16

Abstract

A device for use in sensing a physical force, the device comprising a matrix of flexible material coated with an ultrathin layer and one or more electrodes wherein upon application of the physical force to the matrix brings at least some of the ultrathin layer into contact with the one or more electrodes, and remoyal of the physical force allows the matrix and ultrathin layer to recover an original shape, reducing the amount of ultrathin layer in contact with the one or more electrodes.

Description

METHOD AND DEVICE FOR SMART SENSING FIELD OF INVENTION

[0001 ] The present invention relates to the field of smart sensing.

[0002] In one form the invention relates to membranes for sensing physical parameters such as pressures and pressure changes.

[0003] In another form the invention relates to multifunctional nanoscale membranes.

[0004] In one particular aspect the present invention is suitable for use as a flexible membrane for measuring biological functions, such as pulse rate and blood pressure.

[0005] It will be convenient to hereinafter describe the invention in relation to measurement of blood parameters such as systolic or diastolic pressure and pulse rate, however it should be appreciated that the present invention is not limited to that use only and can be applied to a wide range of medical and non-medical uses, including industrial uses.

BACKGROUND ART

[0006] It is to be appreciated that any discussion of documents, devices, acts or knowledge in this specification is included 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 by the inventor. Moreover, any discussion of material such as documents, devices, acts or knowledge in this specification is included 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. [0007] Pressure is an important parameter for a wide range of industrial, non- industrial and biological processes and an enormous number of devices have been developed for pressure measurement. Pressure measuring instruments are usually divided up into hydrostatic instruments (such as piston! liquid column or McLeod gauges) or anaeroid (such as bourdon, diaphragm or bellows). Electronic pressure gauges typically work on the basis or thermal conductivity! or ionization. Pressure range, sensitivity, dynamic response and cost of pressure gauges varies widely according to the application.

[0008] In the field of human and veterinary medicine, sphygmomanometers are used to measure pulse and blood pressure. The sphygmomanometer was invented in 1881 to measure blood pressure and started to be routinely used in the early 20^lh century. In recent times electronic versions that measure both blood pressure and heart rate have become popular and affordable for both clinical and home use.

[0009] In their simplest form sphygmomanometers consist of an inflatable cuff to restrict blood flow and a mercury or mechanical manometer to measure the pressure. They also include means to determine at what pressure the blood flow starts when pressure in the cuff is released.

[0010] Commonly, blood pressure measurement is based on piezoelectric, capacitive response to external pressures.

[0011] Modern digital sphygmomanometers measure systolic and diastolic pressures by oscillometric detection, using a piezoelectric pressure sensor and electronic components including a microprocessor. They do not measure systolic and diastolic pressures directly, but calculate them from the mean pressure and empirical oscillometric parameters. Most instruments also display pulse rate.

[0012] Digital instruments use a cuff which may be placed, according to the instrument around the upper arm, wrist or finger, in all cases elevated to the same height as the heart. The correct size of cuff must be used or the reading will be inaccurately high (if too small a cuff is used) or inaccurately low (if too large a cuff is used). [0013] For animals, sites of placement depend on species and include the flipper or tail. However, blood pressure or pulse measurement in animals is notorious difficult unless they are anaesthetised to keep them quiet and immobile.

[0014] In recent times new types of pressure sensors based on nanotechnology have 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-coated polymeric nanofibres that are supported on thin potydimethylsiloxane (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 ef al, 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.

[0015] Conductive electrodes and circuits that can function when subjected to significant mechanical deformation are highly desirable for a wide range of applications including flexible displays, smart clothing and biological monitors. However, it is difficult to combine high conductivity and stretchability. Efforts have been made to overcome this problem by using one-dimensional nartostructures such as carbon nanotube and metal nanowires coated on stretchable fabric. Furthermore, highly stretchable electric circuits have been made from a composite material of silver nanoparticles and elastomeric fibres. A silver nanoparticle precursor is absorbed in electrospun polystyrene-block- butadiene-block-styrene rubber fibres, and then converted into silver nanoparticles in the fibre mat. (Park et al, Nature Nanotechnology (2012) 7, 803). Electric circuits have been designed directly on the electrospun fibre mat by printing on a solution of the nanoparticles precursor. These have been used to produce strain sensors, as well as highly stretchable antenna and highly stretchable light-emitting diode (LED).

[0016] Pressure sensitivity and mechanical self-healing are two vital functions of the human skin. Efforts have been made to combine these properties for electronic skin applications, useful for soft robotics and biomimetic prostheses. Specifically a composite material has been developed from suprarnolecular organic polymer having embedded nickel nanostructured microparticles. (Tee ef al, Nature Nanotechnology (2012) 7, 825). [0017] There is an ongoing need for development of flexible electronic devices for useful application such as measurement of biological parameters.

SUMMARY OF INVENTION

[0018] An object of the present invention is to provide a robust, flexible conductive device.

[0019] Another object of the present invention is to provide a flexible device for use in the measurement of one or more physical parameters.

[0020] Another object of the present invention is to provide a flexible pressure sensor.

[0021 ] Another object of the present invention is to provide a method of fabrication for a flexible pressure sensor.

[0022] Another object is to provide a pressure sensor that is more economic than existing pressure sensors.

[0023] A further object of the present invention is to alleviate at least one disadvantage associated with the related art.

[0024] 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.

[0025] In a first aspect of embodiments described herein there is provided a device for use in sensing a physical force such as pressure, the device comprising a matrix of flexible material coated with an uitrathin layer In a particularly preferred embodiment the present invention provides a device for use in sensing a physical force, the device comprising a matrix of flexible material coated with an uitrathin layer, preferably a metal layer, and one or more electrodes wherein upon application of physical force to the matrix brings at least some of the uitrathin layer into contact with the one or more electrodes, and removal of the physical force allows the matrix and uitrathin layer to recover an original shape, reducing the amount of ultrathin layer in contact with the one or more electrodes.

[0026] The device may include a thin film polymeric support on one or both faces.

[0027] In a particularly preferred embodiment, the device for use in sensing physical forces comprises:

• a first support layer, preferably a polymeric layer,

• a layer comprising a matrix of flexible material coated with an ultrathin layer,

• a current collector, preferably a conductive layer suitable for use as one or more electrodes, and

• a second support layer, preferably a polymeric layer.

[0028] The device of the present invention may include any other elements required for incorporation into an electric circuit or for making electrical contact with a circuit.

[0029] Matrix: In a preferred embodiment the matrix comprises an amorphous fibrous material such as tissue paper, cotton fibre, linen rag, abaca, mitsumata, silk or combinations thereof. Combinations of these fibrous materials with polymers may also be suitable. The matrix may be infused with polyvinyl alcohol, gelatin or other substances that provide extra strength.

[0030] However, other more structured materials may also be suitable including and man-made material such as porous forms of polyletrafluoroethylene and other fluoropolymer products, having a micro-structure characterised by nodes interconnected by fibrils.

[0031] Typically the matrix is sufficiently flexible that it can be readily worn or attached to a human or animal body as part of a wearable electronic device. [0032] Optimally the electronic device can feed information to a computer or diagnostic device providing continuous, real-time measurement. This could be very useful, for example, for both clinical measurement and remote or at-home monitoring of patients.

[0033] Nanometal Coating: In a preferred embodiment the metal coating is chosen from the group comprising noble metal (Au, Ag, Pd, Pt), carbon or Cu at nanoscale size, such as nanoparticles or nanowires. The noble metals, particularly Au, are preferred because of their superior resistance to oxidation.

[0034] Typically the metal coating is ultrathin, that is, less than 3 nanometers, more preferably less than 3 nanometers in thickness.

[0035] Typically the sensor measures resistance changes in response to external pressure - in contrast to equivalent sensors of the prior art which are based on piezoelectric, capacitive responses to external pressures.

[0036] In a particularly preferred embodiment, the device comprises ultrathin gold nanowires (AuNWs) or nanorods. The recently developed ultrathin AuNWs (~2nm in width, with an aspect ratio of >10,000) are mechanically flexible yet robust, enabling their uses in constructing novel superlattice nanomembranes and flexible transparent electrodes. Despite superior mechanical and electrical properties, ultrathin AuNWs have not yet been used in designing flexible sensors.

[0037] Current Collector: The current collector may be any metal or combination of metals capable as functioning as an electrode and may form one or more pads. For example, the conductive layer may comprise a Ti/Au thin film.

[0038] The current collector may be produced by any convenient means such as sputtering or electrodeposition.

[0039] The current collector is preferably configured as a pattern of electrodes, This can be achieved for example by using a laser-patterned shadow mask. [0040] In a particularly preferred embodiment, the device of the present invention comprises AuNWs-impregnated tissue paper sandwiched between a blank polydimethylsiloxane (PDMS) sheet and a patterned PDMS sheet with interdigitated electrode arrays. Thus, a superior wearable device for physical measurement, such as pressure sensing, can be fabricated.

[0041 ] Manufacture: The device of the present invention may be manufactured by any convenient means such as wet-chemical techniques.

[0042] In a second aspect of embodiments described herein there is provided a method of manufacturing a device of the present invention, the method comprising the steps of:

• depositing an ultrathin metal layer on the matrix by immersing a portion of matrix in an organic solution comprising the metal such that the matrix is at least partly coated with the metal, and

• drying the portion of metal coated matrix. j

[0043] The metal coated matrix may then be subjected to further processing so that it is suitable for the desired application. When used for pulse measurements, for example, it may be conformed as a strip so that it can be wrapped around the wrist of a patient.

[0044] The metal coated matrix may be subjected to further steps to form a useful electronic device. For example the coated matrix may be coated with a polymeric support on one or both sides. Thin film electrodes may be added, for example by electrodeposition. Current collector pads can be added so that the metal coated matrix may make electrical contact with a circuit.

[0045] In a further processing step:

• the metal coated matrix is sandwiched between a first support layer and a second support layer patterned with an electrode array. [0046] Preferably the support layers are polymer sheets, and may be the same or different polymers.

[0047] Without wishing to be bound by theory j it is believed that the sensing mechanism is due to pressing force-dependent contact between the nanoscale metal and electrode arrays. The quantity of nanometal bridging finger electrode pairs depended on the external forces applied. Upon applying an external pressure, a small compressive deformation of the matrix enables a greater proportion of nanometal to contact the electrodes, leading to more conductive pathways. This causes an increase in current when a fixed voltage is applied. Upon unloading, both the polymer support and matrix recovered to their original shapes, reducing¹ the amount of nanometal bridging the finger electrode pairs, therefore, leading to the decrease of the current.

[0048] In another aspect of embodiments described herein there is provided a method of measuring a change in physical force, such as pressure change, in real time, comprising the steps of:

• applying the device of the present invention to a locus associated with the change in physical force,

• passing a current across the device, and

• monitoring the current to detect changes in resistance associated with change in the physical force.

[0049] Typically the method is used for measuring a biological function in real time by applying a pressure sensor of the present invention to a human or animal subject. In a particularly preferred embodiment, the method of the present invention is used to measure biological functions that involve pressure change such as pulse or blood pressure. It is noted that change in pressure is not only important to the vascular system, but is important to a wide range of organs. For example, measurement of changes in intra-ocular pressure is important for monitoring progress of disorders such as glaucoma. [0050] Thus, according to the present invention AuNWs-impregnated tissue paper could be sandwiched between a blank PD S sheet and a patterned PDMS sheet with interdigitated electrode arrays, leading to a superior wearable pressure sensor Typically, a sensitivity of 1.14/kPa could be achieved, which is comparable to the record in organic transistor pressure sensors reported recently. (Schwartz G, Tee BC-K, Mei J, Appleton AL, Wang H, Bao Z. Nature communications 4, 1859 (2013)). Furthermore, the sensor is capable of responding to pressure changes within 0.05 second at a frequency up to 5.5Hz. Negligible loading-unloading signal changes are observed in over 50,000 cycles.

[0051] Notably, the key sensing elements, AuNWs-impregnated tissue paper could be easily fabricated in large amount at low-cost using scalable wet chemistry processing steps, This enabled facile large-area fabrication and patterning for spatial pressure mapping. In addition, AuNWs-based pressure sensors according to the present invention could be used to detect pressing, bending and torsional forces with high sensitivity, enabling their uses as wrist sensors and vibration sensors with low power consumption (<30|JW for an operating voltage of 1.5V). These performances are comparable to the recently reported pressure sensing devices ((Schwartz G, Tee BC-K, Mei J, Appleton AL, Wang H, Bao Z. Nature communications 4, 1859 (2013); Kaltenbrunner M, et al. Nature 499, 458-463 (2013); Wang C, et al. Nature materials 12, 899-904 (2013); Jeong JW, et al. DOI: 10.1002/adma.201301921. (2013); Yang Y, et al. ACS nano, DOI: 10.1021/nn403838y. (2013)). Furthermore, they offer advantages of low-cost and simplicity in device fabrication as well as versatility of detecting various force signals.

[0052] 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.

[0053] In essence, embodiments of the present invention stem from the realization that resistance changes can be used to measure changes in pressure - contrary to prior art wisdom which is based on piezoelectric, capacitive responses to external pressures. Furthermore, it has been realised that a structure can be provided in which nanoparticles change conformation and resistance in response to pressure changes. [0054] The present invention can be applied to a wide range of biological, medical, and veterinary functions including wearable, mobile sensors for measurement of various aspects of physiological parameters including:

• vascular parameters including pulse and blood pressure;

• parameters relating to other physiological fluids such as flow of lymph in lymph vessels;

• pressure in and around organs such as the eye, the ear, the bladder, and brain;

[0055] The present invention can be applied to a wide range of non-physiological functions, including industrial functions such as measurement of:

• pressure relating to audio elements such as speaker diaphragms,

• pressure relating to visual elements such as with touch-screens,

• pressure in miniaturised industrial processes.

[0056] The present invention can be applied to a wide range of new technological applications such as: soft electronic devices, such as soft keyboards, soft touch-screen displays - all potentially wearable; elastic switches; cyber skins,

• soft robotics,

• wireless e-skin sensors. [0057] Advantages provided by the present invention comprise the following:

• may be manufactured in wearable form,

• economic to manufactured in a very robust form,

• the device may be sufficiently flexible to wrap around a pulse point such as the wrist,

• more sensitive to pressure changes,

• greater sensitivity in lower pressure regimes,

• fast response to changes in pressure,

• can be integrated into existing systems,

[0058] 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, while 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

[0059] 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: Figure 1 illustrates a shadow mask design for a pressure sensor according to the present invention. The width (1), length and interval (3) of the interdigitate fingers were 300 micron, 10 mm and 100 micron respectively;

Figure 2 illustrates assembly of a flexible thin film pressure sensor according to the present invention and based on gold nanowires (AuNWs) coated tissue resistor. The illustration shows PDMS support (5), tissue paper (7) and electrodes

(9);

Figure 3 is a plot of the physical force over time of a human pulse under normal conditions (1 1 ) and with no load (13), measured using a pressure sensor of the present invention;

Figure 4 is a plot showing the detection of gentle finger touch with each time- dependent signal pattern of resistance ratio as a function of time;

Figure 5 presents scanning electron microscopy (SEM) images illustrating the bare tissue fibre mat (Fig.5a, 5b) and the tissue fibre mat after AuNW deposition (Fig.5c,5d). Fig.5e, 5f are cross-sectional SEM images of AuNW/tissue paper composite fibre; and

Figure 6 presents transmission electron microscope (TEM) images of AuNW coated fibres deposited on a carbon grid.

Figure 7 is a further depiction of assembly of a flexible thin film pressure sensor according to the present invention and based on gold nanowires (AuNWs) coated tissue resistor. Specifically, (a) is a schematic of the fabrication of the flexible sensor showing the components (tissue paper (15), blank PDMS sheet (17), AuNWs coating (19) and interdigitated electrodes on PDMS sheet (21 )) in a sandwich assembly (23) and connected with electrical wiring (25); (b) is a photograph showing the bendability of the sensor, (c) is a scanning electron microscopy image of the morphology of gold nanowires coated tissue fibers (scale bar, 100pm), (d) is a schematic illustration of the sensing mechanism to which pressure (27) is applied, changing it from an unloaded (29) state to a loaded state (31 ) and (e) shows current changes in responses to loading (33) and unloading (35)(loff: unloading, Ion: loading).

Figure 8(a) and (b) are TEM images of ultrathin gold nanowires under different magnifications.

Figure 9 illustrates the sheet resistance of gold nanowires coated tissue paper (8x8mm2) and corresponding off resistance of the pressure sensor as a function of dip coating cycles - circles indicating off resistance, squares indicating sheet resistance.

Figure 10(a) and (b) are the optical images of tissue paper before and after 10 dip-coating cycles in nanowire solutions, respectively, (c) and (d) are the cross- sectional SEM image of nanowire-impregnated tissue fibres. The dark area inside is tissue fibre (41) and the bright surface indicated the presence of gold nanowires (43).

Figure 1 1 illustrates the CAD design for the shadow mask used for fabricating interdigitated electrodes.

Figure 12 is an optical image showing the cross-sectional view of a gold nanowire based pressure sensor. In this view can be seen the PDMS supports (45), the AuNW coated tissue (47) and the interdigitated electrodes (49).

Figure 13 illustrates evaluation of sensing performances: (a) is a schematic illustration of the experimental setup showing the computer interface (51), stepping motor (53), force sensor (55), measurement device (57) and an AUNWs pressure sensor (59) according to the present invention; (b) illustrates sensitivity after repeated loading-unloading cycles at a frequency of 3Hz (square - 1^st, circle - 10'", triangle (upwards) - 100^th, triangle (downwards) - 1000^th, oval - 10,000^th); (c) shows current response to various pressures. (The dot line is a linear regression giving a sensitivity of ~1.14kPa); (d) illustrates electrical resistance changes under various strains. (Gauge factor, GFN could be derived by linear fitting); (e) shows the durability test under a pressure of 2,500 Pa at a frequency at 2 Hz. (The current change curves were recorded after each 10,000 cycles and 200 cycles of data was presented in each recording); and (f) is an enlarged view of the part of the l-t curve in c after 10,000 loading-unloading cycles.

Figure 14 is a detailed l-V curve for mechanical loads at various pressures.

Figure 15 is a graph illustrating the changes of the sensitivity of pressure sensor as a function of dip coating cycles. It indicates that the sensitivity increases with the increasing of dip coating cycles (squares - 15 cycles; dots 10 cycles; diamonds 6 cycles; triangle (downwards) 3 cycles; triangle (upwards) 1 cycle).

Figure 16 is a graph illustrating the sensitivity of gold nanowire-based pressure sensor as a function of top layer PDMS thickness (squares - 1000 micron thickness; circles 500 micron; triangle (upwards) 250 micron; triangle (downwards) 120 micron). The bottom layer PDMS sheet was kept at 500pm during the test.

Figure 17 illustrates stable pressure sensing performances after repeated bending cycles (circle - the sensing responses without bending; square - the sensing responses after 1 ,000th bending cycles with 30 mm radius of curvature; triangle - the sensing responses after 5,000th bending cycles with 30 mm radius of curvature. Note that the device shows no appreciable changes in sensitivity even after 5,000 bending cycles.

Figure 18(a) and (b) illustrate hysteresis of output electric signal for an input pressure of -600 Pa with 1 Hz force frequency; Figure 18 (c) shows a comparison of input pressure and output signal for an abrupt unloading (from 1 ,600Pa to OPa), showing a response lag time of 17ms. (square - applied pressure; triangle - output signal; circle - loading; triangle - unloading)

Figure 19 illustrates the dynamic pressure response at the low pressure range and high pressure range wherein (a) is a plot of current response as a function of time (pressure input frequency: 0.5 Hz) for three applied pressures (13 Pa, 23 Pa and 37 Pa);, (b) is a plot of current response of the sensor as a function of time (pressure input frequency: 1 Hz) for the applied pressures at the range of 75-2600 Pa

Figure 20 illustrates the detection of other types of mechanical forces wherein (a)- (c) are plots showing the current responses tb dynamic loading and unloading cycles: (a) pressing, (b) bending and (c) torsion.

Figure 21 is a detailed l-V curve for bending at various radii of curvature for no bending, 100mm, 60mm, 30mm and 10mm.

Figure 22 illustrates monitoring in real-time and in-situ artery wrist pulses and acoustic vibrations wherein (a) is a photograph showing the skin-attachable sensor directly above the artery of the wrist. Scale bar, 3cm; (b) and (c) show measurement of the physical force of a heartbeat under normal (66 beats min-1 ) and exercise conditions (88 beats min-1 ); (d) j is a schematic illustration of the setup for acoustic vibration sensing including a stage (71 ), sponge (73), sensor (75), speaker (77) and measurement device (79) such as a Parstat 2273; (e) is a plot of the current responses to the acoustic vibrations from a piece of music (music on - 81 ; music off -83); (f) is the current responses to the acoustic vibrations from regularly clicking mouse, Note that three different voice volumes (upper box - 20%; middle box - 50%, and lower box -100%) were tested.

Figure 23 illustrates large-area integration and patterning wherein (a) is a photograph of a 5*5cm2 gold nanowires-impregnated tissue paper (scale bar, 2cm); (b) is a photograph of interdigitated Ti-Au electrode arrays on PDMS substrate (scale bar, 5 mm); (c) is an optical microscope image of one pixel electrode (scale bar, 1 mm); (d) is a photograph of a large-area pressure sensor with 5x5 pixels (scale bar, 2 cm); (e) is a top iew of a key (6.6 g) lays on the surface of the 5x5 pixel- pressure sensor (scale bar, 2 cm); and (e) is a map of pressure distributions.

Figure 24 is a plot showing the current changes at various applied pressure of AuNWs, MWCNTs and AuNRs pressure senor. DETAILED DESCRIPTION

[0060] The following describes the preparation of one particular embodiment of the present invention, specifically, a highly flexible and sensitive real-time pressure sensor based on tissue paper coated with ultrathin, high-aspect ratio (AR) Au nanowires.

[0061] The pressure sensor produced has been used to demonstrate monitoring of the human pulse in normal condition in real-time. The pressure sensor has also been used to demonstrate measurement of the pressure imparted by gentle finger touch at a frequency of 0.75 Hz, illustrating great sensitivity in low-pressure regimes (-100 Pa to ~10K Pa).

[0062] The following also illustrates wet chemical methods for the fabrication of device according to the present invention.

Experimental: Fabrication Method 1

[0063] 1.5 ml oleylamine (OA) and 44 mg HAuCU were added to 40 ml hexane to form a yellow solution. Triisopropylsilane (TIPS) 2.1 ml was added to the solution, which was kept standing at room temperature for 3 days.

[0064] Change of solution colour to dark-red, indicated the formation of AuNWs. The residual chemicals were removed by repeated centrifuging and thorough washing with ethanol/hexane (1/3 v/v) and finally concentrated into 2 ml dispersed in chloroform. Nanowire (NW) stock solution 80μΙ was drop-casted on 1 x cm2 tissue papers (Kimberly-Clark Worldwide, Inc.) and allowed to dry. After repeating the drop-casting and drying process for about 10 cycles, AuNW coated tissue paper with a square resistance of ~1 MO was obtained.

[0065] The effective sensing area was 10 mmx10 mm, with the thickness of the PDMS thin film at 500 pm. The total sensor size was 30 mm*27 mmxl mm (length width x thickness). Ti/Au thin film (2 nm/30 nm) was patterned as electrodes on the top surface of the PDMS membrane using a laser-patterned shadow mask. An inter- digitated geometry was chosen to maximize the changes of the AuNW coated tissue paper resistor when a pressure is applied. The width, length and interval of the Ti/Au interdigitate fingers were 300 μηι, 10 mm and 100 pm, respectively. Two 10 mmx10 mm contact pads were deposited at the two ends of the PDMS to establish external contacts.

[0066] Figure 1 is a schematic diagram of a shadow mask. The Ti/Au thin film (2 nm/30 nm) was patterned as electrodes on the top ; surface of the PDMS membrane using the laser-patterned shadow mask.

[0067] Figure 2 comprises schematic illustrations of the assembly and operation of a flexible, sandwiched device in which the AuNW coated tissue paper resistor was placed on interdigitate fingers of the bottom PDMS supports. Then, thin oxygen-plasma-treated PDMS supports were permanently sealed together with tissue paper resistor to ensure stable flexibility. When a pressure is applied on top of the upper PDMS support, more conductive tissue fibres on the intervals of Ti/Au electrodes are able to connect to electrode thus leading to a sudden increase of conductivity. This is the mechanism of the pressure sensor.

[0068] Fig. 2a is a schematic of the assembly and operation of the flexible sensor layer sandwiched between thin PDMS supports (500μιη thickness each). Fig. 2b is a photograph showing the AuNWs coated tissue paper resistor. Fig. 2c is a photograph showing the skin-attachable sensor directly above an artery of a patient wrist (size: 30 mmx27 mm). Fig. 2d is an SEM image of the structure of AuNWs coated tissue fibres. Fig. 2e illustrates operation of a flexible sensor layer by means of recording of resistance change.

[0069] Figure 3 illustrates a normal human pulse test in real time after attaching the device of the present invention directly above an artery of a patient wrist. The sensor was sufficiently flexible to make a conformal seal with: the aid of medical tape. Human pulse under normal condition (~64 beats min- with an average intensity of ~100 Pa) were monitored with time, leading to ~0.3% of resistance radio, indicating that the device of the present invention is capable of high sensitivity.

[0070] From Figure 3 it is clear that the device of the present invention is suitable for measurement of the physical force of a human pulse under normal conditions (-64 beats min-1 with an average intensity of ~100 Pa).

[0071] As a further example, the physical force of a gentle finger touch was tested (~10K Pa) and the results shown in Figure 4.

[0072] Figure 4 comprises plots that illustrate the detection of the gentle finger touch with each time-dependent signal pattern of resistance ratio expressed as a function of time. The resistance ratio was recorded as a function of time with a frequency of -0.75 Hz. The maximum resistance ratio of the device of the present invention is more than 50%, and is suitable for use in a testing range from -100 Pa to -10K Pa.

[0073] Figure 5 presents scanning electron microscopy (SEM) images of the bare tissue fibres (Fig. 5a,b) and the tissue fibres after AuNWs deposition (Fig. 5c-f ). Figs. 5a, b also show the laid and stacked fibres that make up the tissue paper, which have width and length of ~20 pm and several hundred micrometers, respectively. After deposition, the surfaces of the fibres are covered by AuNWs membranes, which merge with one another to form a continuous coating or shell (Fig. 5c,d). This AuNWs deposition around each tissue fibre providing the conductivity of tissue paper. A cross- sectional SEM image of AuNW/tissue paper composite fibres shows that the AuNWs deposition are densely coated on the fibres (Fig. 5e,f),

[0074] Figure 6 presents transmission electron microscope (TEM) images of AuNW coated fibres deposited on a carbon grid. AuNW membranes cannot be observed in most area of the fibres, because the fibre is too thick to be transmitted by TEM. However, AuNWs membrane still can be seen at the edges of fibres. These high aspect ratio AuNW membranes, having a width of -2.5 nm and length of several tens of micrometers give nanowires high mechanical robustness and flexibility, leading to hairy morphology.

[0075] Materials: Gold (III) chloride trihydrate (HAuCI4-3H20, = 99.9%), Triisopropylsilane (99%) and Oleylamine (OA) were purchased from Sigma Aldrich. Hexane and chloroform were obtained from Merck KGaA. All chemicals were used as received unless otherwise indicated. All aqueous solutions were made using demineralized water, which was further purified with a Milli-Q system (Millipore). All glassware used in the following procedures was cleaned in a bath of freshly prepared aqua regia and rinsed thoroughly in H20 prior to use.

[0076] The pre-polymer gel and the cross linker (Sylgard 184 by Dow Corning) were mixed at a 10:1 (w/w) ratio. The mixture was poured on a 6" flat-plate petro dish using 0.5 mm-height shims as spacers and cured at 65°C for 4 hours in an oven. After curing, the PDMS sheet (500 pm) was cut into 30 mm*27 mm strips for use.

[0077] The shadow masks were designed and fabricated by MasterCut Techniques with thickness of 0.1 mm and made by stainless steels. The Ti/Au thin film electrodes deposition was carried by an Intlvac Nanochrome II electron beam evaporation at a 10KV power supply. Morphology characterization was carried out using a Philips CM20 TEM at a 200 kV accelerating voltage and JEOL JSM-7001 F FEG SEM. Thin oxygen- plasma-treated PDMS was carried by a Harrick Plasma Cleaner PDC-001 .

Experimental: Fabrication Method 2

[0078] A new wearable and highly sensitive pressure sensor has been fabricated using ultrathin gold nanowires (AuNWs) via a simple, low-cost bottom-up approach. Ultrathin AuNWs (~2nm in width, with an aspect ratio of >10,000) have been described in various publications including Lu X, Yavuz MS, Tuan H-Y, Korgel BA, Xia Y. Journal of the American Chemical Society 130, 8900^8901 (2008); Huo Z, Tsung C-k, Huang W, Zhang X, Yang P. Nano letters 8, 2041 -2044 (2008); Feng H, et al. Chemical Communications, 1984-1986 (2009). They are mechanically flexible yet robust, enabling their uses in constructing novel superlattice nanomembranes (Chen Y, Ouyang Z, Gu M, Cheng W. Advanced Materials 25, 80-85 (2013)) and flexible transparent electrodes (Sanchez-lglesias A, et al. Nano letters 12, 6066-6070 (2012)), Despite superior mechanical and electrical properties, ultrathin AuNWs have not previously been used in designing flexible sensors.

[0079] The presently described method demonstrates that an AuNWs-impregnated tissue paper can be sandwiched between a blank PDMS sheet and a patterned PDMS sheet with interdigitated electrode arrays, leading to a superior wearable pressure sensor. Typically, a sensitivity of 1.14/kPa could be achieved. Furthermore, the sensor of the present invention can respond to pressure changes within 0.05 second at a frequency up to 5.5Hz. Negligible loading-unloading signal changes were observed over 50,000 cycles. Notably, the key sensing elements, AuNW-impregnated tissue paper could be easily fabricated in large amount at low-cost using scalable wet chemistry processing steps. This enables facile large-area fabrication and patterning for spatial pressure mapping. In addition, the AuNWs-based pressure sensors of the present invention can be used to detect pressing, bending and torsional forces with high sensitivity, enabling their uses as wrist sensors and vibration sensors with low power consumption (<30pW for an operating voltage of 1.5V). These performances are comparable to the recently reported pressure sensing devices simultaneously offering advantages of low-cost and simplicity in device fabrication as well as versatility of detecting various force signals.

[0080] Device fabrication: Figure 7 illustrates another method of fabrication of the AuNWs based pressure sensor of the present invention.

[0081] Firstly, high-quality AuNWs were synthesized and concentrated to a stock solution of -11mg/ml. Note that AuNWs were ultrathin (only about 2nm in width, comparable to typical polymer chain width), yet tens of micrometers long, corresponding to an aspect ratio of >10,000. In addition, AuNWs were mechanically robust yet flexible, with curved structures without breaking (see Figure 8).

[0082] Secondly, AuNWs were deposited into Kimberly-Clark tissue paper by simple dip-coating and drying. The dip-coating process was repeated for 10 times, giving a dark tissue paper with a sheet resistance of 2.5+0.4ΜΩ sq-1 measured from 10 samples, and leading to an average off-resistance of 91.1±52kO after their integration into pressure sensors (Figure 9). Both optical microscopy and scanning electron microscopy demonstrated uniform deposition of AuNWs onto tissues papers (Figure 10). Finally, the AuNWs-impregnated tissue paper was sandwiched between a blank 500pm-thick PDMS sheet and a PDMS sheet patterned with interdigitated electrode arrays (Figure 11). PDMS surfaces were treated by plasma before sandwiching, enabling their conformal contact and firm bonding with tissue paper (Figure 12). Such-fabricated devices are wearable and bendable due to the flexible nature of both tissue paper and AuNWs (as shown in Figures 7 b, c).

[0083] The sensing mechanism is due to pressing force-dependent contact between AuNWs and interdigitated electrode arrays. Unlike a bulk rigid planar metal, soft tissue paper has porous and rough surfaces with hairy AuNWs. The number of AuNWs bridging finger electrode pairs depended on the external forces applied. Upon applying an external pressure, a small compressive deformation of tissue paper enabled more AuNWs in contact with finger electrodes, leading to more conductive pathways (Fig. 7d). This caused an increase in current when a fixed voltage of 1.5 V was applied (Fig. 7e). Upon unloading, both PDMS and tissue paper recovered to their original shapes, reducing the amount of AuNWs bridging the finger electrode pairs, therefore, leading to the decrease of the current.

[0084] Cycling tests and sensitivity: To measure the responses of the AuNWs- based sensors to both static and dynamic mechanical pressures, a home-made system containing a computer-controlled stopping motor and a force sensor were designed (Fig. 13a). Such a system can provide an external pressure of up to 50kPa (static pressure) and 3kPa (dynamic pressure) with electrical signals simultaneously recorded. The AuNWs-based pressure sensor showed a steady response to static pressure up to 50kPa and the resistance under each pressure was constant (Figure 14). For dynamic force measurement, multiple loading-unloading tests were performed under various pressures at a frequency of 3Hz. After approximately 10,000 cycles, the sensor performance showed little changes at various pressures ranging from 500Pa to 3000Pa (Fig. 13b).

[0085] The sensitivity of pressure sensors of the type described herein is defined as S = (AI/loff)/AP, where ΔΙ is the relative change in cur ent, loff is current of the sensor under no load and ΔΡ is the change in applied pressure. An approximately linear relationship between ΔΙ/loff and applied pressure P in the range of 0-5kPa, leading to a sensitivity value S = 1.14/kPa. This value is only next to the record value in organic transistor pressure sensors3 and higher than the typical sensitivity of 5x10-3-0.55/kPa reported from other pressure sensors which have a sensing area of 0.6x0.6mm2-8><8 mm22, 34-37. In addition, higher sensitivity could be achieved by further increasing the dip-coating cycles of AuNWs (Figure 15) and reducing the PDMS thickness (Figure 16).

[0086] To assess the ability of the device of the present invention to function as a strain-gauge sensor, the gauge factor (GF) was measured, which is usually defined as the ratio of relative change in electrical resistance (AR/Roff) to the mechanical strain, ε. For this case, the forces were applied at normal directions, causing the thickness changes, εΝ. Then, GF at the normal direction GFN= (AR/Roff)/ N. As shown in Figure 8d, the gauge factor below the strain of 14% showed a higher linear slope of 7.38. In the high strain range (14%-25%), the average gauge factor dropped to 1.82.

[0087] The high durability of the AuNWs pressure sensor under a pressure of 2,500 Pa at a frequency of 2Hz (Fig, 8e) was also tested. Note that the high signal-to-noise ratios were well maintained and the current amplitude exhibited negligible changes after 50,000 loading-unloading cycles. The high stability was also demonstrated in a bending test and the device sensitivity didn't show evident changes after repeated bending for over 5,000 cycles at 30mm radius of curvature (Figure 17).

[0088] Time-resolved responses: To examine the response time of sensors according to the present invention to external forces, the output current signals were compared with the dynamic pressure inputs at a frequency of 1 -5.5Hz. Note that the current waves were almost the same as the input pressure waves under a pressure of ~400Pa at the lower frequency (1 -2Hz). Furthermore, the applied pressure was increased to 600Pa at a frequency of 1 , 3 and 5.5Hz. The output electrical signals remained stable without evident change in amplitude up to 5.5Hz. A negligible hysteresis was observed under a pressure of 600Pa at Hz (Figure 18), which increased with the increasing frequency up to 0.05s at 5.5Hz. This hysteresis may be attributed to elastic deformation of tissue paper during loading-unloading process and the viscoelastic effects of the PDMS layers. However, the bandwidth and the line shapes kept almost unchanged as the force frequency increased. Similar quick response time was also observed in the abrupt unloading process from 1 ,600Pa to OPa (Fig. 18c), where a response time of 17ms was obtained. [0089] Pressure-resolved responses: To estimate the pressure range of the AuNWs-based sensors towards dynamic forces, a piezoelectric stepping positioner with minimum displacement of only 1 pm was applied to the sensors. As shown in Figure 19a, a pressure of 13Pa could be detected, which is equivalent to the weight of a water droplet (~13pL) on a surface of 10mm2. At the higher pressure range, the noise-free, stable continuous responses could be observed up to 2600Pa (Fig. 19b).

[0090] Detection of other types of mechanical forces In addition to pressing forces, pressure sensors according to the present invention can simultaneously be used to detect the bending and torsional forces. Remarkably, the response curves are characteristic for each kind of mechanical forces for the same device (Figure 20). High signal-to-noise ratios were observed in all three types of force measurements, further demonstrating the high sensitivity of AuNWs pressure sensors according to the present invention. The device also exhibited stable linear responses to various bending forces (Figure 21 and Table 1 ).

Table 1 :

[0091] Detection of wrist pulses: The AuNWs-based pressure sensors of the present invention are wearable and could be used to monitor the blood pressure of a human radial artery in real-time (Fig. 22a,). The wrist pulses could be read out accurately under both normal condition (~66 beats! per minute) and after physical exercise (~88 beats per minute) as shown in Figure 22b. Note that the amplitude and frequency of pulses under two conditions were clearly diflerent. Under normal condition, a typical radial artery pulse waveform was obtained with two clearly distinguishable peaks (P1 and P3) and a late systolic augmentation shoulder (P2) (Fig. 22c). The line shape is known to be caused by constitution of the blood pressure from the left ventricle contracts and reflective wave from the lower body.

[0092] The radial augmentation index Alr=P2/P1 is a characteristic value for arterial stiffness, which highly related to the age of people. From a series of waveforms measured with the AuNWs-based pressure sensor of the present invention, an average Air of 0.7 was estimated, agreeing quite well with thb literature data for a healthy 37- year-old male. (Nichols WW.. American Journal of Hypertension 18, 3S-10S (2005)) After the physical exercise, the pulse wave shapes exhibited a change, in which the late systolic augmentation (P2) disappeared. This could be due to a few reasons such as altered heart rate/ventricular ejection characteristics, large artery stiffness/PWV, or change in tone of muscular arteries influencing pressure wave reflection as described in the literature. The above results demonstrate that the subtle differences in blood pulses could be identified with AuNWs-based pressure sensors according to the present invention, indicating its potential to serve as a wearable diagnostic device to monitor human's health in real-time under various conditions.

[0093] Detection of acoustic vibrations: AuNWs-based pressure sensors of the present invention could be also applied to detect acoustic vibrations. To prove such a capability, sensors according to the present invention were attached to a sponge and positioned them close to a speaker with a fixed spacing from a few millimeters to tens of millimeters (Fig. 22d). Interestingly, the tiny vibrational forces from both music and repeated sounds could be accurately resolved. Note that the reliable current signals accompanied to both amplitude and frequency of acoustic vibrations could be detected when the sensor was 30 mm away from the speaker. Even when the voice volume was decreased to 20%, the well-defined current waves could still be obtained (Fig.22f).

[0094] Large-scale Integration and Patterning Both synthesis of AuNWs and fabrication of AuNWs-impregnated tissue papers are essentially based on wet chemistry techniques and both are scalable, enabling facile large-scale integration and patterning. In a typical demonstration, an AuNWs-impregnated tissue paper sheet of 5><5cm2 (Fig 23a) was fabricated and cut into 25 pieces (4*5mm2 each) and then integrated into a 5*5 pixel arrays of sensing matrix fabricated by shadow mask lithography (Fig. 23b, 23c). Each sensing pixel element was extended to two additional contact pads and connected by copper wires. When an irregular object such as a key (6.6g) was positioned on the top of sensors according to the present invention, the output current intensity depended on specific locations (Fig. 23e). The colour contrast mapped local pressure distribution in consistency with the shape of the key. [0095] The fabrication approach is general, and could be extended, for example, to carbon nanotubes and gold nanorods (Figure 24) as well. (Each sensor was fabricated by a dip coating process. The mass fraction of AuNWs, MWCNT and AuNRs were kept at ~50%, respectively. (MWCNTs were purchased from Bayer Material Science. The diameter, length, number of walls and bulk density were 5-20nm, 1->10pm, 3-15 and 140-230kg/m3, respectively. Aqueous CNT solutions (1 mg/ml) with sodium dodecylbenzenesulfonate (SDBS) (1 : 10 in quality ratio) as a surfactant were prepared according to the reference40. AuNRs were synthesized in accordance with known techniques. The loads were applied by the application of various weights placed directly on the sensing area.)

[0096] However, AuNWs showed the best sensitivity, which is perhaps due to high mechanical flexibility in conjunction with high conductivity. These attribute origins from the unique structures of AuNWs, including the ultra-thinness in width (~2 nm) and the high aspect ratio (>10,000).

[0097] Thus the present invention provides a simple yet efficient, low-cost, bottom-up approach to fabricate a wearable and highly sensitive pressure sensor using ultrathin, high-aspect-ratio AuNWs. This new sensor featured a sensitivity of >1 ,14/kPa, a fast response time of <17 ms, high stability over >50,000 cycles and a low power consumption of <30pW when operated under a battery voltage of 1.5V. With this new sensor, it was possible to detect dynamic forces in a wide pressure range (13~50,000Pa) with the ability of resolving various complex forces including pressing, bending, torsion, and acoustic vibrations. These attributes enabled monitoring in real-time and in-situ the real-world force signals from radial artery blood pulses and acoustic vibrations. Notably, the entire device fabrication process is scalable without the need of complex and expensive equipment. The methodologies disclosed herein open a new route to low-cost pressure sensors with potential facile integration into future wearable electronics such as flexible touch-on displays, human-machine interfacing devices and prosthetic skins.

[0098] Methods: Synthesis of ultrathin AuNWs , Ultrathin gold nanowires were synthesized as described elsewhere [Park M, Im J, Park J, Jeong U. Micropatterned Stretchable Circuit and Strain Sensor Fabricated by Lithography on an Electrospun Nanofiber Mat. ACS applied materials & interfaces 5, 8766-8771 (2013): Maheshwari V, Saraf RF. High-resolution thin-film device to sense texture by touch. Science 312, 1501- 1504 (2006).]

[0099] In detail, 44mg HAuCU-3H₂0 was added into 40 ml hexane (40ml), followed by addition of 1.5ml Oleylamine (OA). Note that the gold salts weren't dissolved until the addition of OA which acted as a phase-transfer reagents. After completely dissolving the gold salts, 2.1 ml Triisopropylsilane (TIPS) was added into the above solution. The resulting solution was left to stand for 2 days without stirring at room temperature until the color turned from yellow into dark-red, indicating the formation of AuNWs. The residue chemicals were removed by repeated centrifugation and thorough washing by ethanol/hexane (2/1 , v/v) and finally concentrated to a 2ml stock solution in chloroform. For the entire process, OA molecules played a critical role in dissolving gold salts in Hexane, directing nanowire growth as well as stabilizing AuNWs.

[0100] Sensor fabrication: 8><8mm2 tissue papers were dipped into a chloroform solution of the AuNWs. After evaporating the chloroform, the color of tissue paper changed from white to dark red. Then the dip-coating and drying process were repeated for about 10 cycles until the electrical resistance of paper sheets reached to -2.5 Ω sq- 1. The Ti/Au interdigitated electrodes (Thickness at 3nm/30nm) were deposited onto PDMS substrates (30><27mm2) using a designed shadow mask by an electric beam evaporator (Intlvac Nanochrome II, 10kV). The spacing between the adjacent electrodes was typically 0,1 mm, with the width of interdigitated electrodes at 0.5mm. Two 10x10mm2 contact pads were deposited at the two ends of the interdigitated electrodes to establish external contacts, Then, the bottom layer electrode coated PDMS and upper layer blank PDMS supports were treated by thin oxygen-plasma (Harrick Plasma Cleaner PDC-001 ) and permanently sealed outside the AuNWs coated tissue paper to ensure conformal contact of tissue paper and interdigitated electrodes.

[0101] Large-area fabrication and patterning A 5*5cm2 tissue paper was firstly dipped into the AuNWs stock solution and dried for 10 cycles, leading to a uniform black tissue paper (Fig. 23a). Then AuNWs-impregnated paper was cut by paper knife into 4x5mm² pieces. In the meantime, a patterned Ti/Au interdigitated electrode arrays on PDMS substrates (65><65mm2) was fabricated using shadow mask lithography mentioned earlier in this work (Figs. 23b, c). The spacing and width of electrodes were kept at 100pm and 200pm, with each interdigitated electrode pixel at 5x5mm². Finally, each AuNWs-impregnated paper piece was addressed to the specific electrode pixels and sandwiched between the plasma-treated PDMS sheets, leading to large-area, patterned pressure sensors.

[0102] Device characterization: SEM images were characterized using a JEOL JSM-7001 F FEG SEM operated at 5kV beam voltage. TEM images were carried out using a Philips CM20 TEM at a 200 kV accelerating voltage. Optical images were taken by a Nikon SMZ800 microscope with a Nikon Digital Sight DS-Fi1 camera. The sheet resistances of AuNWs coated tissue papers were carried out on a Jandel 4-point conductivity probe by using a linear arrayed four-point head. The strains were measured by an Instron (Micro Tester, 5848, Instron) using a 100N load cell and strain control mode with a strain rate of 100% per minute, The current differences and the l-V characteristics for the pressure sensor were recorded by the Parstat® 2273 electrochemical system (Princeton Applied Research). For the dynamic force measurement, a computer-based user interface and a micro force sensor (ATI Nano17 Force/Torque Sensor, 1/80N resolution without filtering) and a Maxon Brushless DC motor using a high resolution quadrature encoder (15pm of linear resolution) were used to apply an external pressure up to 2600Pa with frequency up to 5.5Hz. For the dynamic low pressure measurement, a piezoelectric stepping positioner (SLC-1730) was used by a custom LabView program and the force data was measured by an electrical balance (Mettler Toledo NewClassic MF, MS105DU).

[0103] Materials: Gold (III) chloride trihydrate (HAuCI4-3H20, >99.9%), Triisopropylsilane (99%) and Oleylamine were purchased from Sigma Aldrich. Ethanol, Hexane and chloroform were obtained from Merck KGaA, All chemicals were used as received unless otherwise indicated. All glassware used in the following procedures was cleaned in a bath of freshly prepared aqua regia (highly corrosive!) and rinsed thoroughly in H20 prior to use. Tissue papers were brought from Kimberly-Clark Worldwide Incorporation (Kleenex Brand optimum towel, 30.5x24cm2). PDMS substrates were made by the mixing of the pre-polymer gel (Sylgard® 184 Silicone Elastomer Base) and the cross linker (Sylgard® 184 Silicone Elastomer Curing Agent) at the weight ratio of 10:1. The mixture was poured on a 6" flat-plate petro dish using 0.5mm-height shims as spacers and cured at 65Λ for 2 hours in an oven. After curing, the PDMS sheet with a thickness of 500μιη was cut into 30x27mm2 strips for further treatment. The stainless shadow masks were purchased from MasterCut Techniques, Australia. Silver paste was from Dupont (Dupont 4929N, DuPont Corporation, Wilmington, DE, USA).

[0104] While this invention has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modification(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 invention pertains and as may be applied to the essential features hereinbefore set forth.

[0105] 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.

[0106] 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

[0107] "Comprises/comprising" 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

THE CLAIM DEFINING THE INVENTION ARE AS FOLLOWS

1. A device for use in sensing a physical force, the device comprising a matrix of flexible material coated with an ultrathin layer and one¹ or more electrodes wherein upon application of the physical force to the matrix brings at least some of the ultrathin layer into contact with the one or more electrodes, and removal of the physical force allows the matrix and ultrathin layer to recover an original shape, reducing the amount of ultrathin layer in contact with the one or more electrodes.

2. A device for use in sensing a physical force, the device comprising:

• a first support layer^'

• a layer comprising a matrix of flexible material coated with an ultrathin conductive layer,

• a current collector, and

• a second support layer.

3. A device according to claim 1 or claim 2 wherein the matrix comprises an amorphous fibrous material chosen from the group comprising tissue paper, cotton fibre, linen rag, abaca, mitsumata, silk or combinations thereof,

4. A device according to claim 1 or claim 2 wherein the ultrathin layer is chosen from the group comprising noble metals, copper or carbon.

5. A device according to claim 1 or claim 2 wherein the ultrathin later comprises nanoparticles, nanowire or nanorods or combinations thereof.

6. A device according to claim 2 comprising first and second support layers of polydimethylsiloxane polymer, tissue paper coated with an ultrathin conductive layer of gold nanowires and a Ti/Au thin film current layer.

7. A device according to claim 2 wherein the second support layer is patterned with an electrode array.

8. A device according to claim 1 or claim 2 which further includes one or more elements for incorporating the device into an electric circuit.

9. A method of manufacturing the device of claim 1 , the method including the steps of:

• immersing a portion of the matrix in an organic solution comprising a metal such that the matrix is at least partly coated with the metal, and

• drying the portion of metal coated matrix.

10. A method of manufacturing the device according to claim 9 which further includes the step of including the coated tissue between a first support and a second support.

1 1. A method of manufacturing the device according to claim 9 which further includes the step of applying a current collector to the matrix.

12. A method of measuring a change in a physical force in real time, comprising the steps of:

• applying the device of claim 1 or claim 2 to a locus associated with the change in physical force,

• passing a current across the device, and

13 A method of measuring change according to claim 12 wherein the physical force is physiological, chosen from the group comprising pulse, blood pressure, lymph flow, organ pressure or combinations thereof.

14. A method of measuring change according to claim 12 wherein the physical force is pressure associated with soft electronic devices.

15. A method of measuring change according to claim 12 wherein the physical force is pressure associated with audio or visual elements.