WO2017182929A2

WO2017182929A2 - A strain sensor and method thereof

Info

Publication number: WO2017182929A2
Application number: PCT/IB2017/052183
Authority: WO
Inventors: Giridhar Udapi Rao Kulkarni; Mallikharjuna Rao KUNALA DURGA; Kartikeya SRIVASTAVA; Ritu GUPTA
Original assignee: Centre For Nano And Soft Matter Sciences
Priority date: 2016-04-19
Filing date: 2017-04-17
Publication date: 2017-10-26
Also published as: WO2017182929A3

Abstract

The present invention provides a transparent strain sensor device having a Gauge factor (G.F) ranging from about 10⁶ to about 10⁹ and the method of fabricating the same.

Description

TITLE: A STRAIN SENSOR AND METHOD THEREOF

TECHNICAL FIELD

The present invention is in relation to the field of electronics. In particular to the strain sensors; designed to detect small strain of Gauge factor ranging from about 10⁶ to about 10⁹ in animals. The strain sensor is simple to fabricate and economically viable to be used for various applications.

BACKGROUND AND PRIOR ART The use of transparent strain sensors is becoming an integral part of various applications.

Stretchable and flexible sensors have attracted considerable attention for their potential applications in diagnostics, display gadgets and wearable electronics. Literature provides information on different types of strain sensors for various applications. Umpteen approaches and materials have been analysed to come up with strain sensors. However, traditional metallic and semiconductor based strain sensors are not suitable for stretchable applications, as they can withstand only limited strain (<5%).

Among the various typesanalysedin applications requiring stretchability and transparency, piezoresistive strain sensors are the most investigated ones.Zhou, J., et al reports about flexible piezotronic strain sensor(Nano Lett. 8, 3035 (2008)), Xioa li et al informs about stretchable and highly sensitive graphene-on -polymer strain sensors(Scientific reports 2 : 870); G. U. Kulkarni et al reports about semitransparent flexible resistive strain sensors fabricated by micromolding Pd alkanethiolate on polyimide substrates (ACS Appl. Mater. Interfaces 2011 , 3, 2173-2178); Alexandre Larmagnac et al (ACS Appl. Mater. Interfaces 2015, 7, 13467-13475) usesphotolithography processcompatible with soft and rigid substrates, enabling the fabrication of complex 3D interconnected patterns of silver

nanowire (AgNW) networks embedded in polydimethylsiloxane(PDMS)with gauge factors ranging from 0.01 to 100. Similarly Roh et al (Vol. 9 NO. 6 ' 6252-6261,2015) describes about a sensor with optical transparency of 62%, and gauge factor of 62.

It is explicit from the literature that an attempt to obtain transparent sensors with high Gauge factors hasbeen awarded with partial success. The devices are either opaque, or translucent and with low Gauge factor which are not suitable for sensing small strain. Further, usage of materials like graphene and CNT needs special skills, for the preparation of raw materials, especially one has to be very cautious while working with CNT's as it is considered as a health hazard.

The present invention is aimed to overcome the drawbacks and provide a simple device, which is easy to make and use; to measure strains in an animal. Accordingly the present invention provides a device to measure strain of gauge factor 10⁶ to about 10⁹ in an animal. The invention also provides a method of fabrication of the device in an economical and simple way. STATEMENT OF INVENTION:

Accordingly the present invention provides a device (A) to measure strain of Gauge factor ranging from about 10⁶ to about 10⁹ in an animal, wherein strain is measured by change in value of current output; wherein sensory material (1) comprising interconnected metal wire network of width ranging from about Ιμπι to 20μπι embedded on an elastomeric substance is connected to current measuring device (2) which is connected to a recorder and/or analyzer (3) to measure the strain; a sensory material (1) comprising interconnected metal wire network of width ranging from about^m to 20μπι embedded on a elastomeric substance; and a method of fabrication of sensory material (1), said method optionally comprising acts of Process A or Process B;

Process A:

A) coating a substance with a solution of colloidal dispersion to form crackled layer on the substance;

B) loading the substance with metal in the crackles and cleaning thereafter;

C) coating elastomericsubstrate solution on the substance lodged with metal;

D) curing the elastomeric substrate coated substance lodged with metal; and

E) peeling the elastomeric substrate from the substance to obtain the sensory material (1). Process B: a) coating elastomeric substrate with crackle precursor solution to obtain crackled layer on the substrate; b) lodging the metal in the crackled layer; and c) removing the crackle precursor on the substrate to obtain the sensory material (1). BRIEF DESCRIPTION OF FIGURES:

The present invention will be readily understood by the following detailed description in conjunction with the accompanying figures, wherein like reference numerals designate like structural elements, and in which:

Figure A: Schematic diagram of the device A.

Figure 1: (a) Schematic illustration of fabrication of Gold wire network on PDMS substrate through solution processed crackle lithography, (b) Optical micrograph of Gold wire network after transferring to PDMS substrate, (c) SEM image of Gold wire network partially embedded in PDMS and inset is magnified view, (d) Transmittance spectrum of Gold wire network on PDMS in the wavelength range 300-900 nm. Inset shows the photograph of the Gold wire network on PDMS derived strain sensor on screw gauge. Figure 2: Optical micrographs of (a) crackle template on Si substrate, (b) crackle template after electroless Gold deposition, (c) Gold wire network on Si substrate following lift-off and (d) transferred Gold wire network on PDMS substrate. Scale bar, 50μπι.

Figure 3: (a) A plot between resistances of Gold wire network on PDMS versus applied strain as a function of time, (b) The resistance of Gold wire network on PDMS for an applied strain during stretching and relaxing, (c) The resistance of the Gold wire network on PDMS for 6 six stretching cycles at 0.2, 0.5, 1 and 1.5% strains, respectively and (d) the change in the resistance for a 120 stretching cycles between 0 and 1.2% strain.

Figure 4: A plot between gauge factor and applied strain of Gold wire network on PDMS strain sensor.

Figure 5: Change in the current of Gold wire network on PDMS strain sensor with respect to applied strain along (a) longitudinal and (b) transverse directions, respectively.

Figure 6: (a) I-V characteristics of Gold wire network/PDMS derived strain sensor at relaxed (0%, strain) and strained (2.5%, strain) positions. SEM images of (b) Gold wire network after applying 2.5% strain, and insets show the magnified view of a single Gold wires, (c) break junction of a single Gold wire before applying strain and after releasing strain.

Figure 7: (a) A plot between resistances of Gold wire network on PDMS as a function of time for various strains; (b) the resistance of Gold wire network on PDMS for applied strain during stretching and relaxing; and (c) a plot between gauge factor and applied strain of Gold wire network on PDMS strain sensor.

Figure 8: (a) Gauge factor histograms of various devices fabricated through vacuum deposition and solution processed crackle lithography.(b) SEM image of vacuum deposited Gold wire after 30 stretching cycles. The marked region shows a stripped of metal flake from the junction area.

Figure 9: Change in the resistance of vacuum deposited Gold wire network during stretching cycles between 0 and 1.4% strain.

Figure 10: Strain sensor made using Silver wire networks on PDMS, using Vacuum deposition, (a) a plot between resistances of Silver wire network on PDMS versus applied strain as a function of time, (b) the resistance of Silver wire network on PDMS for an applied strain during stretching and relaxing, (c) a plot between gauge factor and applied strain of Silver wire network on PDMS strain sensor.

Figure 11: Optical Micrograph of Silver wire network on PDMS.

Figure 12: Human nerve gesture recognition-Relative change in current of Gold wire network on PDMS, (a) while volunteer performs three gestures of hand and corresponding photographs of the sensor struck to the hand, (b) while volunteer blinks his eye with different frequencies, inset shows photography of the sensor struck to lateral canthusof eye, (c) Relative change in current of Gold wire network on PDMS, while the volunteer eats potato chips, chocolate, ground nut and chewing gum, respectively, where red colour indicates prehension of the food and black colour curves with pink lines represents mastication of the food.

Figure 13: Decrease in the current with respect to applied strain of two Gold wire network on PDMS strain sensors used for (a) eye blink and (b) eating experiments, respectively. Figure 14: Schematic illustration of the strain sensor at relaxed and strained positions.

Figure 15: (a) Image showing the bending of scale that generates tension and compression in the strain sensor, (b) schematic showing the bending experiment, (c) a plot between current versus time showing the oscillations of the scale, (d) asection has been enlarged to show the responsiveness of the sensor w.r.t the oscillations, (e) Comparison of Gauge factor versus strain for different values of PDMS thickness - 2mm, 180 um, 80 um. Slope for curve fits 1,2 and 3 are 14, 100 and 422 respectively.

DETAILED DESCRIPTION OF INVENTION:

The foregoing description of the embodiments of the invention has been presented for the purpose of illustration. It is not intended to be exhaustive or to limit the invention to the precise form disclosed as many modifications and variations are possible in light of this disclosure for a person skilled in the art in view of the Figures, description and claims.

The present invention is in relation to a device used for measuring the strain in animals including human beings. The various embodiments of the present invention however has been studied inhuman beings for exemplary and description purpose.

The various embodiments of the device of the present invention along with the method of fabrication is described below with reference to the figures.

It may further be noted that as used herein and in the appended claims, the singular forms "a", "an", and "the" include plural reference unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by person skilled in the art. Definition: Crackle is a crack down to the substance on which it is formed.

The present invention is in relation to a device (A) to measure strain of Gauge factor ranging from about 10⁶ to about 10⁹ in an animal; wherein strain is measured by change in value of current output; wherein sensory material (1) comprising interconnected metal wire network of width ranging from about Ιμπι to 20μπι embedded on a elastomeric substance is connected to current measuring device (2) which is connected to a recorder and/or analyzer (3) to measure the strain.

In an embodiment of the present invention, the device is of response time ranging from about 30ms to about 45ms.

In an embodiment of the present invention, the device sense strain ranging from about 0.08% to about 6.5%.

In an embodiment of the present invention, the metal is selected from a group comprising gold, silver, copper and mixture thereof.

In an embodiment of the present invention, the substrate is selected from a group comprising polydimethylsiloxane, polybutyrateadipate terephthalate, polyamide; preferably polydimethylsiloxane.

The present invention is also in relation to a sensory material (1) comprising interconnected metal wire network of width ranging from about Ιμπι to 20μπι embedded on a elastomeric substance. In another embodiment of the present invention, the sensory material (1) is of transparency ranging from about 80% to about 95 %. The present invention is also in relation to a method of fabrication of sensory material (1), said method optionally comprising acts of Process A or Process B;

Process A:

B) cleaning and lodging the substance with metal in the crackles;

Qcoating elastomeric substrate solution on the substance lodged with metal;

D) curing the elastomeric substrate coated substance lodged with metal; and

E) peeling the elastomeric substrate from the substanceto obtain the sensory material (1). Process B:

a) coating elastomeric substrate with crackle precursor solution to obtain crackled layer on the substrate;

b) lodging the metal in the crackled layer; and

c) removing the crackle precursor on the substrate to obtain the sensory material (1). In an embodiment of the present invention, the metal is lodged by a process selected from a group comprising electroless deposition process, and thermal vaporisation process.

In another embodiment of the present invention, the curing is carried out at a temperature ranging from about 75 °C to about 95 °C and for a period ranging from about 30min to about 60min. The present invention provides a device (A) capable of sensing small strain of Gauge factor ranging from about 10⁶ to about 10⁹. The device (A) thus can identify and detect the small strain of finger and hand movement; eye blinking; jaw movement while eating different type of food and the like for various applications including diagnostics. The stability of the device has been observed, by continuously stretching and releasing the strain, through 120 cycles.

The device (A) relays on electrical resistance, whenever a strain is applied there is a corresponding change in the resistance, which is read through the change in its current. The device (A) comprises sensory material (1) comprising interconnected metal wire network of width ranging from about Ιμπιιο about 20μπι embedded on a elastomeric substrate is connected to current measuring device (2) which is connected to a recorder and/or analyser(3) to measure the strain. The invention uses sensory material (1) fabricated with polymethyldisiloxane (PDMS) as the elastomeric substrate, having interconnected Gold wire networks on it, however other elastomeric materials like polybutyrateadipate terephthalate (Ecoflex), polyamide wherein the interconnected metal wire network is eligible to be transferred can be used. Elastomeric substrates like PDMS gives strechablity and transparency to the device, moreover it is also bio compatible, which further makes it suitable for present application. Any metal like Gold, Silver, Copper and the like including alloys of said metals are suitable for the present invention. The Gold and Silver metals have been used to exemplify the invention.

Since the principle underlying the invention is the detection of change in resistance of the interconnected metal wire network, the stretchability of the metal wire network is examined. For exemplary purpose, Gold wire network is used in the device. The device is glued to a screw gauge (inset of figure 1(d)) and the silver epoxy contacts are taken from outside of the stretching area. The device resistance is monitored for each 50 μπι stretching along its length using the screw gauge (least count, 10 μπι). The strain is calculated using the following formula

Strain = ε = ALVL x 100% (1) WhereAL is change in length and L is original length of the device's active area. As shown in figure 3(a), the device resistance is monitored as a function of time while stretching 50 μηιίη each step. With increase in the strain, changes in resistance are observed which are quite discrete with respect to the applied strain. As shown in figure 3(a), the resistance increase from the initial800 Ω to 4 kQ at 1.7% strain. Beyond 1.7% strain device suddenly jumps to 2 ΘΩ, the device continues to respond to the applied strain and a maximum resistance of 5 ΘΩ is observed at 2.6% strain. Most strikingly, when the strain is gradually withdrawn, the resistance of the device retraced and reached its original value (Figure 3(a)). The change in the resistance is shown in figure3(b) as a function of applied strain. The plot clearly depicts that the increase and decrease in the resistance is quite monotonous, without any observable hysteresis. In the above case the strain is applied along the longitudinal direction; however the device works even in transverse direction as in Figure 5(b).

Gauge factor is an important parameter to assess the performance of strain sensor. In this study, the gauge factors are calculated using the equation (2).

Gauge factor = GF = (AR/R)/s (2)

WhereAR is change in resistance, R is original resistance and ε is applied strain. For R = 850Ω,

AR = 5.2x 10 9 and ε = 0.0275. The estimated gauge factor for device is 2.2x 108°. Change in the gauge factor with respect to applied strain is plotted in figure 4.

The fabricated strain sensor is taken through a cycled strain at various strain values as shown in figure 3(c). In a single strain cycle, the strain is applied and then released to its rest position; likewise 6 cycles are performed on the device at various strains. As the strain increases, the change in the resistance also increases which is evident in figure 3(c). Further, the same device is stretched for 120 cycles by switching strain in between 1.2% and 0% i.e. relaxed state (Figure 3(d)). The device performance is consistent within 5% indicating reliability over large number of stretching cycles. To facilitate the understanding of strain sensing mechanism, a detailed and in-situ microscopy and electrical characterization is carried out. In the relaxed state (rest position, no strain) of the device, I-V characteristics are linear as shown in figure 6(a)indicating, that the conduction through the percolative wire network is ohmic but diffusive in nature. In this case, all Gold wires are connected as shown in Figure 1(b). However, with 2.7% applied strain, the I-V characteristics became non-linear as shown in Figure 6(a). SEM images during in-situ application of 2.7% strain clearly demonstrate the opening of Nano gaps in the strain direction of Gold wire network. A magnified view of Nano gaps in figure 6(b) reveals that the gaps are around few nanometres to -100 nm. Surprisingly, the Gold wires in the direction perpendicular to the strain direction are intact as shown in figure 6(b). The electrical characterization is performed along the applied strain direction. The non-linear I-V characteristics observed are attributable to tunnelling or filamentary conduction in the Gold wire network under high strain and release of the strain, results in linear I-V characteristics. The SEM of a break junction before applying strain and after releasing the strain as shown in figure 6(c) reveals that the gaps are closed without giving any room for tunnelling current. Thus, all the nano gaps and openings are closed as soon as the strain is released in accordance with the principle mechanism for strain sensing.

In order to examine reproducibility and reliability of the fabrication process, various devices are fabricated using vacuum deposition and solution processed deposition of the metal, while keeping rest of the process unchanged. In case of vacuum deposition, the crackle template, metal deposition and lift-off are carried out directly on cured PDMS surface. The gauge factors of all devices fabricated in the study is shown in the form of a histogram (Figure 8(a)). From the histogram, it is clear that there is more variation in the gauge factors of sensors obtained from vacuum deposition (Red bars in Figure 8(a), whereas solution processed devices exhibited consistent and high gauge factors (Blue bars in Figure 8(a)). More importantly, vacuum deposited strain sensors are stable only up to 10 stretching cycles,(Figure 9) whereas solution processed strain sensors are stable beyond 120 stretching cycles as shown in figure 3(d). The inconsistency in vacuum deposited strain sensors may be attributed to permanent breakage of the interlock junctions, as depicted SEM image of Figure8(b). Both detachment and stripping of metal flakes from the wire network is clearly seen in figure8(b). The vacuum deposited Gold wires tend to peel off after few stretching cycles, unlike solution processed Gold wire networks. This may be attributed to embedding of Gold wire network in PDMS matrix. The embedding of Gold wire network in PDMS matrix not only furnishes a tight hold but also provides a back bone support during the stretching and compression. The back bone PDMS matrix ensures the connectivity of Gold wire network break junctions over large number of stretching cycles. A Gauge factor value of 7.43 x 10⁹is achieved at a strain value of 6.5%, (Figure 7(c))which is by far the highest value reported amongst various values in the literature (Table 1). These values are obtained for R = 11.25Ω, AK = 5.44 x 10⁹and ε = 0.065.The change in the resistance with respect to time and strain is shown in Figure 7(a) and 7(b) respectively. Thus, the solution processed crackle lithography is a promising method to fabricate consistent and reliable strain sensors.

Table -1 : List of Gauge factors of with different sensors. I. Gauge Strain %T Hyste Resp. Type Material usedo factor values Resis Time

(%)

[1] 2- 14 - 70 — High - Resistive AgNW's with PDMS

[2] 10 ^J 2 - 6 — - - Resistive Graphene with PDMS

10 ^b > 7 Resistive

[3] 11.5 < -2 — Moderate 100 ms Resistive Pt coated, polymeric

Pressure nanofibres, on PDMS

0.75 < -4 Resistive

Shear

8.53 < -5 Resistive

Torsion

[4] 20 - 35 — Very low - Resistive Ag NW with PDMS

[5] 2 x 10⁴ - 100 -- - - Resistive CNT and graphene with

PDMS

[6] 10^J 2-6 — - - Resistive GWFs on PDMS

10^b >7 Resistive

35 0.2 Resistive

[7] 0.7 - 50 — Very low 40ms Capacitive AgNW and PDMS

[8] 62 -100 62 - - Resistive SWNT + PU

[9] 12.4 -100 75.3 Low - Resistive AgNW

[10] 27.8 0 - 40 — moderate - Resistive Piezoresistive

1084 40 - 90 Resistive interlocked microdome

9617 90 - Resistive arrays

120

[11] 0.56 0- 200 — - 12ms Resistive CNT fibre

[12] 30 0.2 -- - - Resistive Piezoresistive doped

Nano crystalline Si

[13] 20 3 — - - Resistive Si nanowire

[14] 1250 1.2 — - 10ms Resistive ZnO nanowire

[15] 6.7 x10" 1.4 — - < 100ms Resistive ZnOnanorod

[16] 10 1.5 -- - - Resistive Piezoresistive carbon

Filament

[17] 75 2 — Moderate - Resistive MWCNT/epoxy composite

[18] 210 0.08 — - - Resistive SWNT

[19] 269 0.24 — - - Resistive SWNT

[20] 290 0.22 — - - Resistive Pd μ -strips

[21] 35 6 — - - Resistive Graphene- rubber composite

[22] 0.99 100 -- High - Resistive CNT based percolation n/w

[23] 3740 0.32 — - - Resistive ZnSn0₃ nanowires/microwires

[24] ~ 1 300 90 Very < 100ms Resistive CNT

Low

[25] 0.82 0 - 40 — - 14ms Resistive SWNT

0.06 60 - Resistive

200

[26] 14 5-10 Resistive Nanocarbon

Nanocom po si tes

[27] 2.35 0- 17 — - - Resistive Cu NW ink

13.17 18- 30 — Resistive

54.38 40 - 90 — Resistive

[28] 100 50 6 Moderate - Resistive Ag NW on PDMS

[29] 7.38 0-14 — Low < 17ms Resistive Au NW PDMS

1.82 14-25 — Resistive

[30] 60.6 - 0.22 — - 110ms Resistive Graphite

150.5 0.22 - -- Resistive

0.32

536.6 0.32- -- Resistive

0.62

[31] 150 0.08 - -- Moderate - Resistive V0₂ nano beam

0.2

[32] 2.05 20 — Low 1000ms Resistive Ag NP on PDMS

[33] 1.5 1.75 — Low - Resistive CNT

[34] 233.5 2 -- Moderate - Resistive Graphene PDMS composite

[35] 2 0.15 — - - Resistive Ni wire (commercial)

[36] 7.43xlO^y 6.5 80 Low 40ms Resistive

30-180 0.2-2.5 80 Resistive Present work

(Au NW on PDMS)

0.1-11.2 0.08- 80 Resistive

0.8 (oscillatory

strain)

Lastly, silver epoxy is used to take contacts from the PDMS substrate, and it is ready to be use. Also, PDMS gives an additional advantage of transparency. From the point of aesthetics, our device is almost invisible. Various sets of experiments are performed to show the versatility of the strains, which can be detected by the device, for example hand movement, eye blinking, and jaw movements. The minimum response time obtained is 28 ms, which indicates the reliability and accuracy of the device.

Transparency of the strain sensor is an additional attribute though it does not contribute functionally. However in the context of therapeutics, it may add a great value, particularly in situations where the sensor is to be concealed while being attached to the outer parts of the body of an individual such as face or hand, not affecting the aesthetic appearance. In such situations, transparency of the sensor plays important role if the device should enable a physician to see through it for monitoring the underlying skin/body part for possible swelling, injuries or for rashes. Here, the utility of transparent strain sensing device is examined in three contexts. In each context, the strain is estimated using calibration plots as shown in Figure 13. In the first context, the sensory material (1) is placed on opisthenar ofa volunteer as shown in the photographs of figurel2(a). The current passing through the sensory substance (1) is monitored as a function of hand gestures. The photographs show three hand gestures, where the hand is held momentarily at those positions during stretching and releasing of the hand grip. The device showed high current in gesture T i.e completely relaxed position and in '2' gesture i.e., partially stretched position, the current decreased to 50% of the original value, whereas in '3' gesture i.e., completely stretched position, the device exhibited minimal current(~2 nA) indicating near complete breakage of the wire network. The strain developed due to hand gesture is estimated as -3%. In the data shown 8 such cycles have been performed with reproducibly as shown in figure 12(a).

In second context, the device is stuck over volunteer's lateral canthus of eye to monitor the strain developed during eye blinking. As shown in figure 12(b) each blinking cycle involves a decrease in the current and recovery. Importantly, the change in the current in this particular case is two orders less as compared to hand gesture and corresponding strain developed is just 0.02 %. The eye blinking test is performed with different frequencies 0.15, 0.4 and 0.9 Hz, respectively and same is reflected in relative change in current of sensor as depicted in figure 12(b). Thus, the strain sensor is able to detect such small strains with fast response.

In third context, the sensor is placed on a volunteer's right temple of head over temporal branch of the facial nerve as shown in the photograph in figure 12(c). The main focus of this experiment is to monitor the differential strain developed in facial nerve while eating different food articles, as the concerned serve is directly linked to jaw movements. In the relaxed state of the facial nerve, the device is in rest position and while eating the food articles, the facial nerve gets strained which is translated to the device as shown in the schematic in figure 14. Thus, the strain, relaxation or compression in the facial nerve could be directly transferred to the strain sensor. Four food articles with different properties, potato chips (crispy), chocolate (soft), ground nut (hard, powdery) and chewing gum (soft and elastic) are consumed. In first example, a potato chip is consumed while monitoring the device current. The prehension of potato chip led to increase in the current sharply marked with red colour curve in figure 12(c) attributed to compression in the nerve. During mastication, the current showed spike-like features with minimum current even below the initial (rest) value, indicating that the sensor could distinguish both signs of strains i.e tension and compression. A single mastication cycle consists of a compression -tension cycle in the facial nerve, which is clearly reflected in the current (Figure 12(c)). At the end of mastication, the oscillations died down in amplitude corresponding to the swallowing stage. In case of chocolate mastication, the increase in the oscillation amplitude is due to the relatively big size of the food article (Figure 12(c)). Conversely, during mastication of a ground nut, the oscillation amplitude decreased rapidly due to its powdery nature and came to relaxed state after 8-10 mastication cycles as shown in figure 12(c). On the other hand, the oscillation frequency in the case of the chewing gum is relatively high and the decrease in the amplitude is rather slow. The device current did not come back to the relaxed state at all and mastication is forcefully stopped (Figure 12(c)). This is attributed to the soft and elastic nature of the chewing gum. Hence, the strain sensor fabrication in this method is able to sense eating of various food articles with well differentiated signals. Such a device can be easily integrated on any part of the body and highly local body movements can be monitored. This may be used in the automatic monitoring of the hand gestures or other parts of body as a patient recovers from an injury or fracture of bones.

In order to find out the response time, a scale bending experiment is performed. Here, the sensor is pasted over a flexible structure surface (stainless steel ruler, having a thickness of 0.5 mm,Figure 15(a)). Then, it is pushed downwards manually and left on its own to oscillate till it stopped, as can be seen from the figure 15(c). Also, in its corresponding Figure 15(d), there aretwo regions of operation i.e. tension and compression. When the scale bends in the downward direction, the sensor feels the mechanical stretching (Tension) due to which resistance increases and the current value goes down. Similarly, when scale bends in the upward direction, which is to complete the one oscillation cycle, the strain senor feels compression, which reduces resistance, and eventually increases current value. Here, a response time is 30 millisecond. Gauge factor values have been plotted versus strain (Figurel5(e)), w.r.t different PDMS thickness 2 mm, 180 um, 80 um. As PDMS thickness decreases, sensitivity of the sensor increases up to a large extent. For a strain value of 0.10 %, GF values for strain sensor having a PDMS thickness of 2mm, 180 um and 80 um are 0.27, 8.39 and 36.0 respectively. Since, the active material for the sensor are, for example the gold networks; actual strain transferred to the gold networks increases, when the PDMS thickness goes down. The strain sensor with PDMS thickness of 80 urn, detects low strains of 0.04 %, with a G.F of 0.46.

The strain sensor fabrication in general comprises following steps; namely template formation using crackle lithography, metal deposition by electroless method and transferring the metal wire mesh on to PDMS, adding the electrodes and connecting to recorder and/or analyser. Crackle dispersion is spin coated on Silicon substrate and after the formation of crackle template; the Silicon substrate is dipped in Gold electroless plating solution as shown in Figure 1(a). Following the lift-off, a continuous Gold wire network is formed on Si substrate. Polydimethylsiloxane (PDMS) is poured on the Gold wire networked Si substrate and cured as shown in Figure 1(a). In the final step, the PDMS film is peeled off from the Si substrate. The Gold wire network gets completely transferred onto PDMS without any breakage or discontinuity. The optical micrograph in Figure 1(b) reveals the presence of Gold wire network on PDMS, quite similar to vacuum deposited Gold wire network. The optical micrographs for each step is depicted in Figure 2. The Gold wire network is seen interconnected without any breakage after transferring to PDMS substrate. As expected the Gold wire networks are partially embedded in PDMS as shown in SEM image in figure 1(c). The magnified view of a single wire shows that at least 50% of the wire width(~ 1 um) is embedded in PDMS. The wire surface is rough and textured because of the electroless deposition. The transmittance spectrum shown in the Figure 1(d) clearly demonstrates that the transmittance is around ~ 85%, which is also evident from the photograph in the inset. Importantly, the sheet resistance is only 100 Ω/square.

Alternately, the metal can also be deposited on an elastomeric substrate to form the sensory material by thermal evaporation process as described in example 4 (figures 10 and 11) and used in the sensory device (A). Experimental Details

The fabrication of the device involves the following steps- a) General outline of preparation of sensory layer (1):

A mixture of diluter i.e., chloroform and crackle precursor, i.e., colloidal dispersion of an acrylic resin available commercially as crackle nail polish (Ming Ni Cosmetics Co., Guangzhou, China) at concentration of 0.55 g/ml.80 uL of said solution is spin coated at RPM 1000 for about 100 sec on a silicon substrate of area(3 cm xl.5 cm) After the development of interconnected crackles, it is dipped in gold solution for a period of 2-4 min to deposit the gold in the interconnected crackle pattern. Then the substrate is cleaned with chloroform to remove the sacrificial layer. Then PDMS is coated on the gold deposited silicon substrate. The substrate is then heated for 45min at 80°C and the PDMS layer is peeled to obtain the sensory material, i,e., PDMS embedded with the gold wire network. Copper wires along with the silver epoxy are applied on to the PDMS layer to form electrodes and dried in hot air oven at 90 °C for 45 min.

B) Preparation of PDMS for the invention

The commercially available PDMS is sonicated for 8 min at a power of 50w and a frequency of 37 Hz. The mixture is degassed in a desiccator for 20 min.

C) Preparation of gold electroless solution of concentration 10 mM

A solution of 17 mg of Chloroauric acid (HAuC14) and 2 ml of water is prepared. Similarly a solution of 1.9 ml of water and add 1.1 ml of Hydrofluoric acid(HF) is prepared and both the solutions are mixed to be used as Gold solution having a concentration of lOmM. Similarly, a solution having 20 mM concentration, is prepared by using 34 mg of Chloroauric acid(HAuC14). Experiment-2

To make Gold solution having a concentration of 10 mM, 17 mg of Chloroauric acid (HAuC14) and 2 ml of water is mixed with 1.9 ml water and 1.1 ml of Hydrofluoric acid (HF). A total of 5 ml of solution is prepared. Then, the silicon substrate (3 cm x 1.5 cm) having crackle patterns is dipped in the gold solution for 2 min. Later, it is cleaned using chloroform andpurge nitrogen gas to make it dry. Then, 5 ml of uncured PDMS is poured, and it is kept for 2-4 min, so that PDMS can settle down on the surface, and fill in between the gold networks properly. After this, sample is kept in hot air oven for 45 min at 80 °C. After 45 min, PDMS gets fully cured and is peeled from the silicon substrate, and cut into a rectangular size of 2 cm x 1.5 cm. Then copper wires are placed on the edges of the PDMS and silver epoxy is applied on top of the copper wires to make electrodes. The sample is kept back in the oven for 45-50 min at 90° C; so that silver epoxy gets dry and sticks to the substrate firmly. Finally, a digital multimeter (MS8218, Mastech) is used to measure the resistance of the sensory device, which comes out to be 250 Ω. Transmission measurements are done using a Perkin Elmer Lambda 900 UV/visible/near-IR spectrophotometer, using a blank PDMS as a reference; i.e. 80 %.

Experiment - 3

To make Gold solution having a concentration of 20 mM, 34 mg of Chloroauric acid (HAuC14) and 2 ml of water is mixed with 1.9 ml water and 1.1 ml of Hydrofluoric acid (HF). A total of 5 ml of solution is prepared. Then, the silicon substrate (3 cm x 1.5 cm) having crack patterns is dipped in the gold solution for 1 min. After that, lift off is done using chloroform. Then nitrogen gas is purged to dry it. Then, 5 ml of uncured PDMS is poured, and it is kept for 2-4 min, so that PDMS can settle down on the surface, and fill in between the Gold networks properly. After this, sample is kept in hot air oven for 45 min at 80°C. After, 45 min, sample is taken out and PDMS is fully cured. PDMS is peeled from the silicon substrate, copper wires are kept at the edges and silver epoxy is applied to make electrodes. The sample is keptin the oven for 45-50 min at 90° C; so that silver epoxy gets dry and sticks to the substrate firmly. A digital multimeter (MS8218, Mastech) is used to measure the resistance of the device, which comes out to be 150 Ω. Transmission measurements are done using a Perkin Elmer Lambda 900 UV/visible/near-IR spectrophotometer. The transmittance observed is 82 %.

Using the aforementioned methods the details of the sensory materials prepared are provided in Table 2.

Table 2: Details of sensor materials prepared

Experiment 4: Strain sensor with silver wire network as

PDMS (with elastomer to curing agent ratio 10:1) is prepared,cut into pieces of dimension 3 cm x 1.5 cm and treated with UV radiation for 15-20 min. The crackle precursor is spin coated on UV treated PDMS pieces at a speed of 1000 rpm for 90 sec. The crackled layer of PDMS pieces is deposited with silver by thermal evaporation process in a Vacuum chamber unit for 4-5 mins, out of which actual deposition took 1-2 mins and the amount of silver used for evaporation is 250 - 280 mg. After deposition, crackle precursor is removed through mechanical exfoliation using scotch tape. Finally silver epoxy contacts are made, and used in the strain sensor.

Thus the present invention provides a potential solution to the typical problems related to the strain sensing of high Gauge factors. The device is cost effectively fabricated and can be easily used. The device exhibits excellent transmittance having a very high gauge factor. Importantly, the sensor is very sensitive to low strains.

It will be apparent that other variations and modifications apart from the above mentioned may also be made to the above described exemplary embodiments and functionality, with the attainment of some or all of their advantages. It is an object of the appended claims to cover all such variations and modifications as come within the true spirit and scope of the invention.

Claims

WE CLAIM:

1. A device (A) to measure strain of Gauge factor ranging from about 10⁶ to about 10⁹ in an animal; wherein strain is measured by change in value of current output; wherein sensory material (1) comprising interconnected metal wire network of width ranging from about Ιμπι to 20μπι embedded on a elastomeric substance is connected to current measuring device (2) which is connected to a recorder and/or analyser (3) to measure the strain.

2. The device (A) as claimed in claim 1, wherein the device is of response time ranging from about 30ms to about 45ms.

3. The device (A) as claimed in claim 1, wherein the device sense strain ranging from about 0.08% to about 6.5%.

4. The device (A) as claimed in claim 1, wherein the metal is selected from a group comprising gold, silver, copper and mixture thereof.

5. The device (A) as claimed in claim 1, wherein the substrate is selected from a group comprising polydimethylsiloxane, polybutyrateadipate terephthalate, polyamide; preferably polydimethylsiloxane.

6. A sensory material (1) comprising interconnected metal wire network of width ranging from about Ιμιη to 20μπι embedded on a elastomeric substance.

7. The sensory material (1) as claimed in claim 6, wherein the sensory material (1) is of transparency ranging from about 80% to about 95 %.

8. A method of fabrication of sensory material (1), said method optionally comprising acts of Process A or Process B ;

Process A: A) coating a substance with a solution of colloidal dispersion to form crackled layer on the substance;

B) lodging the substance with metal in the crackles and cleaning thereafter;

Qcoating elastomeric substrate solution on the substance lodged with metal;

D) curing the elastomeric substrate coated substance lodged with metal; and

E) peeling the elastomeric substrate from the substance to obtain the sensory material (1). Proces B:

b) lodging the metal in the crackled layer; and

c) removing the crackle precursor on the substrate to obtain the sensory material (1).

9. The method as claimed in claim 8, wherein the metal is lodged by a process selected from a group comprising electroless deposition process, and thermal vaporisation process.

10. The method as claimed in claim 8, wherein the curing is carried out at a temperature ranging from about 75 °C to about 95 °C and for a period ranging from about 30min to about 60min.