DESC:Field of invention
The present invention relates to the field of sensors. More specifically, the invention is in relation to a sensor device, fabricated using crumpled sheets of reduced graphene oxide on a substrate. In particular, the invention relates to a sensor device based on crumpled sheets of reduced graphene oxide, for sensing wide range of pressure and strains with a gauge factor of over 4000 for a strain of the order 10-4 applied to it and provides for a cost-effective method of fabrication of the sensor device.
Background of invention:
Sensor technology is a rapidly growing field which has the potential to improve operation, utility and serviceability of various engineering systems. Sensors are basically devices which detect and respond, to any input from the physical environment, in the form of signals. The inputs could be light, heat, pressure or any other environmental feature. These sensors have versatile usages from monitoring functionality of various systems, health care, human machine interfacing, to prosthetics and various other safety measures.
Several different systems are available for sensing pressure and strain. A typical electronic sensor is based on the principle of force induced changes in capacitance, resistance, triboelectricity or piezoelectricity. Similarly, resistance based sensors are built on operational electronics with low power consumption. Use of percolation networks as sensors have also been explored wherein conducting nanomaterials are dispersed into/onto non-conducting polymer matrix. The document titled “Highly sensitive and stretchable multidimensional strain sensor with prestrained anisotropic metal nanowire percolation networks” Nano Lett., 2015, 15 (8), pp 5240– 5247 describes a multidimensional strain sensor composed of two layers of prestrained silver nanowire percolation network with a strain sensing capacity up to 35% maximum strain with a gauge factor of more than 20. Similarly, in article “Highly Stretchable and Ultrasensitive Strain Sensor Based on Reduced Graphene Oxide Microtubes–Elastomer Composite” ACS Appl. Mater. Interfaces, 2015,7 (49), pp 27432–27439 the authors report sensor that can be stretched more than 50% of its original length and which shows long-term durability and excellent selectivity to a specific strain under various disturbances. The sensitivity of this sensor is as high as 630 gauge factor under 21.3% applied strain.
Other strategies include reversible interlocking of nanofibers aligned carbon nanotube arrays or Au nanowire patch on polymers. Simple strategies like conductor dispersed rubber, Au nanowire impregnated tissue paper and laser scribed graphene have been used for detection of low pressure.
However, these systems have limited sensing capacity and the method of fabrication is expensive. Hence, there is a need for a sensor with a wide ranging sensing capacity, yet with a simple method of fabrication.
The present invention aims to provide a solution, wherein the invention provides for crumpled nanosheets which can form a loose packing of highly compressible matrix having a potential in designing of ultrasensitive strain pressure sensors.
Summary of invention
Accordingly, the present invention is in relation to crumpled nanosheets of reduced graphene oxide fabricated on a substrate for sensing wide range of pressure and strains. The sensor device wherein the nanosheets of reduced graphene oxide are dispersed in a solution of solvent, for example acetone and dropcasted on a substrate. The coated substrate is then taped to secure the nanosheets of reduced graphene oxide for experimental purposes. The taping of the substrate is done in two ways; a) wherein the coating on the substrate is fully confined, and b) wherein the coating is partially confined. The fully confined coated substrate is used in gauging high pressure while the partially confined coated substrate is used in gauging low pressure and strains. The sensing capacity of the sensor device is of more than 4000 gauge factor when strain of order 10-4 is applied on it with a cyclic stability of more than 7000 cycles coupled with a switching time of 20.4 ms. The invention provides for a cost effective means of fabrication of a sensing device wherein the device consumes low power.
Brief description of figures
The features of the present invention can be understood in detail with the aid of appended figures. It is to be noted however, that the appended figures illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope for the invention.
Figure 1: provides the schematic diagram of the sensor device (A)
Figure 2: Figure 2(a) depicts a schematic showing the fabrication process of taped rGOstrain/pressure sensor; Figure 2(b) and Figure 2(c) show SEM images at low/high magnification showing crumpled state of dropcasted rGO. Figure 2 (d) shows a photograph of the fabricated devices in two different geometries- Fully and Partially confined rGO.
Figure 3: Shows a schematic demonstrating the principle of detection of strain and pressure with taped crumpled rGO aggregate.
Figure 4: Figure 4 (a) illustrates an experimental setup to quantify the response of the sensor to pressure. Figure 4(b) demonstrates the repeatability of pc-rGOsensor at low pressure. Figure 4(c) shows the current response of the pc-rGO sensor to different applied pressures. Figure 4(d) shows variation of sensitivity of the same sensor with pressure. Figure 4(e) shows that at higher pressures, the base resistance changes significantly after each cycle because of permanent rearrangement of the rGO particles which is due to leaking of the particles from the sides. Figure 4(f) shows stable response of a fc-rGO device till high pressures. Figure 4(g) shows repeatability of fc-rGO sensor even at high pressure. Figure 4(h) shows anomaly in response at low pressure for a fc-rGO sensor which is not present in a pc-rGO sensor. Figure 4 (i) shows the higher sensivity of the pc-rGO sensor as compared to that of the fc-rGO device.
Figure 5: Figure 5(a) describes an easy device fabrication strategy that enables the fabrication of the device onto adhesive labels. Inset shows such a sensor pasted on the floor for robustness test. Figure 5(b) shows that the base current (hence base resistance) of the device remains same even after application of impulsive load over it with the ‘Impact Hammer’. Figure 5(c) depicts the effect of application of force through load cell demonstrating that the sensor can withstand forces as large as ~2.4 kN (~240 kg). Figure 5(d) shows that a maximum pressure of 80.2 MPa could be measured with adhesive label based devices.
Figure 6: Figure 6(a) illustrates a photograph showing the sensor pasted on a scale dipped in a beaker full of water. Figure 6(b) shows the response of the fc-rGO sensor to its placement in varying depths of water.
Figure 7: Figure 7(a) provides a schematic of the setup for testing the response of the sensor to variation in air pressure. Figure 7 (b) explains detection of fall in air pressure (shown in red), as the chamber is pumped-out till a pressure of 679 mbar as recorded using a reference Pirani gauge (shown in blue). Figure 7(c), and Figure 7(d) show plots of relative changes in resistance of the sensor as pressure inside the measurement chamber drops below and rises above atmospheric pressure respectively. Figure 7(e) and figure 7(f) shows the corresponding variation in sensitivity of the sensor with pressure.
Figure 8: 8(a) shows air pressure sensing below atmospheric pressure (expansion of the tape) and 8(b) compressive pressure (from the piston) sensing. Pressure sensing in compressive and tensile regime of the conducting nanomaterials show that rGO has the best sensitivity in both regimes compared to graphite and GO.
Figure 9: Figure9(a) shows the relative change in resistance of a typical sensor in response to strain in a cantilever. Figure 9(b) shows the corresponding gauge factor as a function of strain. Figure 9(c) demonstrates the detection of deformation in glass (before breaking) as a result of application of force at the centre of it. As can be seen, the applied force and the output signal from the rGO sensor match extremely well.
Figure 10: Figure 10(a) shows a schematic showing the set-up for dynamic straining tests of the strain sensor. Figure 10(b) shows variation of resistance of the sensor with strain that is varying at a frequency of 28 Hz. Figure 10(c) shows the relative change in resistance with strain. Figure 10(d) is a plot of the Gauge factor as a function of strain. Figure 10(e) shows the cyclic stability of the sensor for > 7000 cycles measured at 33 Hz. Figure 10(f) demonstrates the ultrafast switching time of the sensor (~20.4 ms) corresponding to a strain frequency of 49 Hz. Figure 10(g) describes response of the sensor to acoustic vibrations.
Figure 11: provides evaluation of the performance of the sensor at a lower bias voltage of 1 mV. The output current of the sensor is a few microamperes (base current 1.6 µA) indicating that the sensor can work at power as low as a few nanowatts (1.6 nW) with good signal to noise ratio.
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. It may further be noted that as used herein and in the appended claims, the singular “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.
The present invention is in relation to a device(A) for measuring pressure and strain; said device comprising crumpled rGO(1) deposited between metal contacts (2) on a substrate (3).
In an embodiment of the present invention, the size of crumpled rGO is ranging from 5 µm to 10µm.
In another embodiment of the present invention, the substrate is selected from a group comprising rigid, preferably glass and flexible substrates, preferably kapton.
In another embodiment of the present invention, the metal is selected from a group comprising gold, silver, aluminium, tin, copper, indium, titanium, chromium and alloys.
In another embodiment of the present invention, the pressure is ranging from 5kPa to 80MPa.
In another embodiment of the present invention, the pressure measured is air pressure below and above atmospheric pressure or water pressure.
In another embodiment of the present invention, the strain GuageFactor is about 4000 for a strain of 0.02%.
In another embodiment of the present invention, the rGO is optionally secured on all four sides or two sides.
In another embodiment of the present invention, the device is connected to an apparatus (5) for recording change in resistance of crumpled rGO.
The invention is also in relation to a method of fabrication of device (A)for measuring pressure and strain; said method comprising acts of
a) preparing crumpled rGO (1) of size ranging from 5 µm to 10µm;
b) coating the crumpled rGO between metal contacts (2) on a substrate (3); and
c) optionally securing the rGO on 4 sides or 2 sides to obtain the device (A).
In another embodiment of the present invention, the coating of rGO is carried out by drop casting rGO solution.
In another embodiment of the present invention, the rGO solution is of a solvent selected from a group comprising acetone, hexane and water
The present invention is also in relation to a method of measuring pressure or strain, said method comprising using the device (A) and measuring the change in resistance of rGO.
In still another embodiment of the present invention, the apparatus is for measuring change in resistance.
The present invention is in relation to a sensor device( Figure 1), devised for sensing pressure and strains over a wide range, comprising crumpled layers. Size of rGO particles is from 5µm to 20µm of reduced graphene oxide fabricated on a substrate, wherein the crumpled layers of reduced graphene oxide is partially or fully confined on the substrate using an adhesive tape. The said sensor device senses pressure or strain based on the principle of percolation network.
Flexible layers of rGO when drop casted on any substrate from its dispersion in a solvent; for example acetone, hexane and water crumples as the solvent dries due to the capillary forces by the solvent while drying. The crumpled rGO layers form a loosely packed matrix with poor contact with each other. For securing the rGO aggregates, an adhesive tape is used which confines the rGO either partially or fully. Application of pressure on the rGO patch improves the connectivity between the rGO grains leading to a reduced resistance of the rGO network (Figure 3).
It is found that the size of the crumpled rGO particle typically ranges from 5 µm to 20 µm in size.
Figure 2a shows the schematic description of the fabrication process of the pressure/strain sensor. Typically, the rGO powder synthesized wet chemically, is dispersed in acetone by ultrasonication(at 53 kHz) to form a thick ink (typically ~2 mg of rGO in 50 µL of acetone) and subsequently drop casted in between sputtered Aucontacts (separated by 2 mm) on glass plate/kapton sheet/paper based adhesive label (typically ~2.5 cm X 1 cm) to form a uniform layer. The contact material is not limited to sputtered Au. Any good contact material like silver, aluminium, tin, copper, indium, titanium, chromium or their conducting alloys can be used. The contacts can be prepared by sputtering, thermal evaporation or any other physical or chemical material deposition technique. The excess rGO powder is removed to define a square area (A) of 0.5 X 0.5 cm2. A typical resistance < 10 kO is achieved after drying of the rGO powder. Figure 2b and figure 2c show the SEM images of the crumpled rGO sheets obtained after drying of the solvent. The dried rGO powder is taped with adhesive tape (scotch tape, 3M; Young’s modulus 1.1 GPa, thickness 60 µm). Taping of the rGO patch leads to lowering of the resistance of the device due to partial compression of the rGO aggregate. The rGO patch is secured from all four sides, and the device is labelled as-fully confined rGO (fc-rGO) device. This type of device is fabricated on glass (rigid), kapton (flexible) or paper (adhesive label) based substrates. Alternately, the sides of the device can be cut so as to have the rGO secured from two sides only (dimensions ~2.5 cm X 0.5 cm). This is labelled as- partially confined rGO (pc-rGO) device. This kind of device is fabricated on Kapton based substrates only. The photographs of the two kinds of device are shown in figure 2d.
The sensor device thus fabricated is used in the detection of a wide range of pressures and strains as detailed below.The sensor has a patch of crumpled rGO which can detect any type of compression and expansion by changing its resistance. Such simple principle has been used to detect static and dynamic strain with high sensitivity of >4000 at a strain as low as 0.02%. It could successfully detect sinusoidal varying strain at a frequency of 49 Hz. Such capabilities find application of the sensor in monitoring structural health wherein very small strain needs to be detected. A power consumption as low as 1.6 nWis shown (while in operation) indicating it to be energy efficient. Detection of high frequency strain has application in detection of acoustic waves which could have implications in designing hearing aids and microphones.
The sensor can also be used to detect pressure ranging from 5 kPa to over 80 MPa, from a piston.This aspect can be useful in designing pressure sensitive switches, smart mats and weighing devices. The sensor can even operate under water to detect water pressure and/or depth. Detection of water pressure ranging from 0.3 kPa to 2.7 kPa has been successfully demonstrated. The sensor can detect air pressure both below and above atmospheric pressure. It can detect air pressure ranging from 100 mbar (below atmospheric) to 18 psi (above atmospheric).
Detection of low pressure: Partially confined rGO device (pc-rGO) is used for detection of low pressure. The rGO forms a loose packing in this device configuration and hence it is more sensitive than fc-rGO device making it more suitable for measurement of low pressure.
Detection of high pressure: Fully confined rGO device (fc-rGO) is used for the detection of high pressure. This configuration prevents leaking of rGO powder from the sides and hence has better repeatability at high pressures.
Detection of water pressure: In fully constrained condition of the rGO aggregate the device (e.g. fc-rGO device on glass substrate) can be operated under water for the detection of fluid (e.g.-water) pressure by the same principle.
Detection of air pressure: Taping of the rGO aggregate from all sides (e.g. fc-rGO device on glass substrate used) leads to trapping of air along with the rGO aggregates which is used for detection of air pressure both above and below atmospheric pressure. As the pressure is increased above atmospheric pressure the tape deflates due to higher pressure outside the tape than inside it, which leads to lowering of resistance of the rGO aggregate. As the pressure is reduced below atmospheric pressure the tape inflates due to lower pressure outside the tape than inside it, which leads to increase in resistance of the sensor.
Measurement of pressure using the fully confined and partially confined sensor device is as follows:
Figure 4a show the schematic description of the pressure sensing setup. The sensor is placed under the universal testing machine (UTM) attached to a load cell (Max. force- 500 N and resolution ~83 mN) for the application of pressure. The force (and the pressure calculated from it) and displacement are simultaneously recorded as a function of time (approach rate -1 mm/s). A Keithley source-meter is used to bias the pressure sensor with a fixed voltage (1 V) and the current through it is measured. Figure 4b and figure 4c show the response of the pc-rGO sensor fabricated on kapton substrate to the pressure. Figure 4d shows the variation of sensivity (defined as- S = (?I/I0)/P) with pressure. The pc-RGO device has a higher sensitivity than that of the fc-rGO device (Figure 4i) and hence is better suited for the detection of low pressure reproducibly. However, at high pressure the pc-rGO device shows drift in the base current (Figure 4e) which is due to leaking of the rGO. Figure 4f and 4g show the pressure response of the fc-rGO pressure sensor fabricated on glass substrate. At high pressures, the fc-rGO pressure sensors show good repeatability. But at low pressure, the response is ambiguous (Figure 4h) and hence fc-rGO sensors are not suitable over this pressure range. Figure 5a shows the photograph of adhesive label based fc-rGO pressure sensor. Tests with ‘Impact Hammer’ indicates that this form of the device is structurally robust (Figure 5b). Formal measurement using UTM shows that the device can withstand large forces (Figure 5c) and that measurements of very high pressure is possible (Figure 5d). Figure 6a shows the schematic of the water pressure measurement setup. A fc-rGO pressure sensor integrated onto glass substrate is pasted at one end of a conventional plastic scale. The sensor is biased (1V) with a Keithley source-meter and the sensor current is measured. The sensor is dipped inside water to different depths to demonstrate the fluid pressure sensing ability of the sensor. The response is shown in figure 6b. Alternating dipping cycles over depths of 3 cm (0.3 kPa) to 28 cm (2.7 kPa) cause significant changes in the sensor current. However, random movement of the sensor horizontally maintaining the same depth does not cause any significant change in current. This implies that the sensor is extremely sensitive to depth and its response to turbulence is negligibly small. Figure 7a shows the schematic description of the air pressure measurement experiment. The fc-rGO sensor integrated onto glass substrate is placed inside a steel jacket and air is pumped in/out using a compressor/pump. A reference Pirani gauge is attached to calibrate the current response. The sensor is biased with a fixed voltage and the corresponding current is recorded. Figure 7b shows a typical response of the sensor to decrease in air pressure. The drop in current is more than two orders of magnitude for a pressure pulse of ~679 mbar indicating that it is highly sensitive. Figure 7c and 7d show the relative changes in resistance with pressures lesser than and higher than the atmospheric pressure respectively. The sensitivity in resistance of the air pressure sensor is defined by the formula, SR=(?R/R0)/?P. Figures 7e and 7f show the corresponding variation of sensitivity with pressure. The sensor is found to be more sensitive at pressures lower than atmospheric pressure.
A comparative analysis of the sensing principle with different carbon based compressible materials like graphite, Graphene Oxide and crumpled rGO of present invention is carried out. The pressure sensing tests are conducted for both tensile and compressive regime of the aggregate of the different materials. For the tensile regime, air pressure detecting capabilities below atmospheric pressure (wherein the aggregates loose contact with each other) is conducted. For the compressive regime, UTM based compression of the aggregate patch (wherein the aggregates establish better contact with each other) is conducted. The tests demonstrate that the device fabrication principle is general and may work for any compressible material. However, rGO has the highest sensitivity as compared to graphite and Graphene Oxide in both the regimes.Figure8 with results calibrated graphically indicate the result of the tests.
Detection of static/dynamic strain:
The pc-rGO device on Kapton substrates is used for strain sensing. As the device is flexed the adhesive tape either compresses or relaxes the rGO aggregate leading to either increase or decrease of the current at a fixed bias voltage (Figure 3).
Measurement of strain using the partially confined sensor device:
The pc-rGO device integrated onto kapton substrate is used for sensing strain (d). The sensor is pasted at the fixed end of a cantilever. A known displacement is given to the free end of the cantilever (using UTM) to strain it. The strain is calculated from the standard cantilever bending theory. Measurement is carried out in a probe station, by biasing the sensor with a fixed voltage and measuring the current at high sampling rates. Figure 9a show the response of the sensor to strain due to bending of the cantilever. The figure of merit of a strain sensor is given by the gauge factor (G.F.) expressed as (?R/R0)/d. Figure 9b shows the variation of G.F. of the sensor with strain. A strain of the order of 10-4 can be detected with high G.F. Such capability is useful in structural health monitoring. To demonstrate this, such a sensor is pasted on a glass slide (~1 mm thick) which is pivoted from two ends as shown in the inset of figure 9c. Strain in (~10-4) of the glass could be successfully detected before it reached the breaking point. The same sensor can also detect dynamic strain in a vibrating cantilever. Figure 10a shows a schematic description of the setup. For setting the cantilever into vibration, it is placed on a speaker which is excited through a function generator (sine wave, Vpp-1 V). Laser Doppler Vibrometry (LDV) is used to monitor the oscillation of the cantilever to obtain the temporal strain. Figure 10b shows the temporal response of the sensor at cycling strain at a frequency of 28 Hz. The relative change in resistance versus strain at dynamic straining conditions is shown in figure 10c. The corresponding variation in G.F. is shown in Figure 10d. A G.F. over 4000 is observed at a strain of the order of 10-4. A cyclic stability over 7000 cycles is observed (Figure 10e). Strain at a frequency as high as 49 Hz is read out which (Figure 10f) indicates that the sensor has a fast switching time of <20.4 ms even at low strains. The said response is exploited in detection of acoustic waves. In a typical experiment, the sensor is pasted on a tissue paper clipped over the mouth of a beaker as shown in the inset of figure 10g. Music played from at a distance of 10 cm (80-85 dB) can be detected with high fidelity using the sensor.
The power consumption of the sensor device is shown in Figure 11. Evaluation of the performance of the device at low bias voltage show that its power consumption is extremely low. Figure 11 also shows that pressure sensing with the device at a bias voltage of 1 mV consumes power as low as few nanowatts.
Comparative analysis of various sensors with crumpled rGO based strain sensor.
Further the Guage Factor (GF) of the strain sensor is above 4000 at a strain as low as 0.02% as compared to the GF of commercially available strain sensors ( Table 1) and those reported in literature (Table 2).

Table 1: Comparative analysis of commercially available strain sensors
Material Gauge Factor
Metal foil strain gauge 2-5
Thin-film metal (e.g. constantan) 2
Single crystal silicon -125 to + 200
Polysilicon ±30
Thick-film resistors 100
p-type Ge 102

Table 2: Composition of Gauge Factor of the strain sensor with other sensor reported in literature showing ultra-high Gauge Factor at strains of the order 10-4.
Journal ,Year Principle G.F. Strain
Nat. Mater. 11 (9), 795 (2012) Reversible interlocking of fibers 11.45(w.r.t vertical strain) 0-2%
Nat. Nanotech. 6 (5), 296 (2011). Aligned CNT array 15 1%
ACS Nano 9 (6), 6252 (2015). Nanohybrid of CNT and conducting elastomer 39.4 >1.5%
ACS Nano 8 (5), 5154 (2014). Silver Nanowire Elastomer Nanocomposite 2-14 70% stretchable
Nano Lett. 15 (8), 5240 (2015). Metal nanowire percolation network >20 ~35%
Nano Lett. 12 (11), 5714 (2012). Overlapping 2D graphene sheets >150 0-2%
ACS Nano 9 (6), 5929 (2015). CNT in polymer 0.56 0-200%
Sci.Rep. 2, 870 (2012).
Graphene on polymer ~103 2~6%
Nano Lett. 12 (4), 1821 (2012). Percolating nanotube network (Capacitive) 0.99 Till 100%
Nat. Commun. 5, 3132 (2014). Au nanowire impregnated tissue paper 7.38(w.r.t.vertical strain<400) Till 14%
Nano Lett. 8 (9), 3035 (2008). Single ZnO piezoelectric nanowire <400 0.2%
Nat. Nanotechnol. 6 (12), 788 (2011). Transparent elastic films of CNT(Capacitive) 1250
0.004 1%
0-50%
Present invention Crumpled flexible nanosheets >4000 0.02%

Experimental:
A. Method of preparation of reduced Graphene Oxide(rGO).
Step 1: The synthesis of rGO starts with synthesis of GO by Hummer’s method {J. Am. Chem. Soc., 1958, 80, 1339-1339}. Concentrated H2SO4 (23 ml), taken in a beaker, is first cooled down to 0°C by placing the beaker in an ice bath, followed by addition of KMnO4 (3 g) and is stirred for 30 minutes. Then, deionized (d.i.) H2O (45 ml) is added to the solution, which increases the temperature of the solution. After 15 minutes of addition of H2O, the reaction is terminated by adding H2O (140 ml) followed by addition of H2O2 (10 ml of 30% v/v). The sample is then washed with 5% HCl solution multiple times. Then the sample is washed few times with acetone, followed by drying in air for overnight.
Step 2: rGO is then synthesized by reduction of GO in water at high temperature {Carbon, 2008, 46, 1994-1998}. Dry GO powder (100 mg) is dispersed in d.i. water (15 ml) by ultrasonication. The mixture is then heated via microwave irradiation in closed vessel condition at 200°C for an hour. The product is washed few times with water followed by final washing with acetone. The settled final product is left overnight for drying.
B. Method of fabrication of sensor with crumpled rGO
The rGO powder (~2 mg) synthesized wet chemically, is dispersed in acetone(50 µL) by ultrasonication(at 53 kHz) to form a thick ink (1) and subsequently drop casted in between sputtered Au contacts (separated by 2 mm) (2 ) on glass plate/kapton sheet/paper (3) based adhesive label (typically ~2.5 cm X 1 cm) to form a uniform layer (few hundreds of micron in thickness). The excess rGO powder is removed to define a square area (A) of 0.5 X 0.5 cm2. A typical resistance < 10 kO is achieved after drying (at room temperature, ~26°C) of the rGO powder. The dried rGO powder is taped with adhesive tape (scotch tape, 3M; Young’s modulus 1.1 GPa, thickness 60 µm). The rGO patch is secured by a sheet (4) (as described in schematic of Figure 2a) from all four sides, if fabricated on glass (rigid), kapton (flexible) or paper (adhesive label), and the sensor device (A) is labelled as-fully confined rGO (fc-rGO) device.
Alternately, the sides of the sensor device (A) can be cut (with scissors) so as to have the rGO secured by a sheet (4) from two sides only (dimensions ~2.5 cm X 0.5 cm) if fabricated on Kapton. This is labelled as- partially confined rGO (pc-rGO) device.
C. Detection of air-pressure
The sensor device is fixed inside of a vacuum chamber, and the pressure inside the chamber is varied in a controlled manner, either using a vacuum pump (below atmospheric pressure), or by pressurizing by inserting gas from a gas-cylinder. As the pressure changes [in a pressure range of ~100 mbar (below atmospheric) to ~18 psi (above atmospheric), in a typical case], the resistance of the device changes by several orders of magnitude, which allows detection of air-pressure inside the chamber. For calibration, the air pressure inside the chamber is measured using a commercial gauge.
D. Detection of water pressure
Detection of water pressure ranging from 0.3 kPa to 2.7 kPa has been successfully demonstrated using the sensor. However, application of the device is not limited to this range. In a typical experiment, the sensor is fixed on a long-plate with scale, and inserted in a beaker containing water. The water pressure is calculated from the depth of the sensor (read from the scale) inside water. The resistance of the sensor device decreases with increase in depth, i.e. increase in water pressure.

E. Detection of mechanical Force
External mechanical pressure of known value is applied on the sensor using mechanical load (of known value), and the change in resistance is measured. The sensor demonstrates to be used to detect pressure ranging from 5 kPa to over 80 MPa.
F. Detection of bending and deformation in glass
Strain in (~10-4) of the glass can be successfully detected, using the sensor device mentioned in this invention, before it reached the breaking point. For a demonstration, the sensor is pasted on a glass slide (~1 mm thick) which is pivoted from two ends, and pressure (of known value) is applied in the centre of the slide. The change in the resistance of the sensor is measured as the force is increased, till the glass reached its breaking point.
G. Detection of strain
A strain of the order of 10-4 can be detected with high G.F (>4000). For a demonstration, the sensor is pasted at the fixed end of a metal cantilever, and a known displacement is given to the free end of the cantilever (using UTM) to strain it, while the change in the resistance of the device is measured.
H. Detection of acoustic wave
The sensor can be used in detection of acoustic waves. In a typical demonstration, the sensor is pasted on a tissue paper clipped over the mouth of a beaker, and the resistance of the sensor is monitored continuously. A music (80-85 dB) played from a distance can be detected with high fidelity using the sensor. Frequency components present in the sound can also be detected.
Thus the present invention provides for a sensor device with sensing capacity over a wide range of pressures and strains. The invention involves cost effective method of fabrication of the sensor device and also the method of detecting the pressure and strains.
,CLAIMS:1. A device(A) for measuring pressure and strain; said device comprising crumpled rGO(1) deposited between metal contacts (2) on a substrate (3).
2. The device (A) as claimed in claim 1, wherein the size of crumpled rGO is ranging from 5 µm to 10µm.
3. The device (A) as claimed in claim 1, wherein the substrate is selected from a group comprising rigid, preferably glass and flexible substrates, preferably kapton.
4. The device (A) as claimed in claim 1, wherein the metal is selected from a group comprising gold, silver, aluminium, tin, copper, indium, titanium, chromium and alloys.
5. The device (A) as claimed in claim 1, wherein the pressure is ranging from 5kPa to 80MPa.
6. The device (A) as claimed in claim 1, wherein the pressure measured is air pressure below and above atmospheric pressure or water pressure.
7. The device (A) as claimed in claim 1, wherein the strain Guage Factor is about 4000 for a strain of 0.02%.
8. The device (A) as claimed in claim 1, wherein the rGO is optionally secured on all four sides or two sides.
9. The device (A) as claimed in claim 1, wherein the device is connected to an apparatus (5) for recording change in resistance of crumpled rGO.
10. A method of fabrication of device (A)for measuring pressure and strain; said method comprising acts of
a) preparing crumpled rGO (1) of size ranging from 5 µm to 10µm;
b) coating the crumpled rGO between metal contacts (2) on a substrate (3); and
c) optionally securing the rGOon 4 sides or 2 sides to obtain the device (A).
11. The method as claimed in claim 7, wherein the coating of rGO is carried out by drop casting rGO solution.
12. The method as claimed in 10 and 11, wherein the rGO solution is of a solvent selected from a group comprising acetone, hexane and water.
13. A method of measuring pressure or strain, said method comprising using the device (A) and measuring the change in resistance of rGO.
14. The method as claimed in claim 13, wherein the apparatus is for measuring change in resistance.