DESC:TECHNICAL FIELD
The present disclosure describes a graphene based capacitive sensor comprising an substrate, graphene interdigitized electrode and electrolyte and polymer. Particularly, the present disclosure relates to graphene electric double layer capacitance capacitive sensor. The disclosure further describes a process of preparing the said graphene capacitive sensor.

BACKGROUND OF THE DISCLOSURE
Capacitance based sensors are being used in various fields like pressure and level sensing, accelerometers, proximity sensing etc. Capacitance based macro-scale differential pressure cells, popularly known as DP cells, are used in pressure and level sensing. Micro scale Micro-Electro-Mechanical Systems (MEMS), such as accelerometers and gyroscopes use differential capacitance measurement principle. These devices prove superior to other types of sensors in some respects like sensitivity but fall short in areas like noise immunity. Lead capacitance, stray capacitance and effect of temperature, humidity and pressure on the dielectric properties often affect the capacitive sensor performance. Though these limitations are addressed with corrective measures, such measures cause loss of energy and resources.

The performance of a capacitive sensor is determined by the degree of accuracy in measuring the change in capacitance of the sensor. The inherent shortcoming in most capacitive sensors is its low base capacitance, which is of the order of few pico farads. This low magnitude of the measurand signal makes the output highly prone to noise.

The most recent developments in capacitive sensing are focused towards reducing noise by means of electronic design and shielding technology. However, only limited improvement in noise elimination is obtained but with higher expenditure. The description of present disclosure intends to overcome the limitations observed in the art vis-à-vis capacitive sensors.

SUMMARY OF THE DISCLOSURE
Accordingly, the object of the present disclosure is to provide graphene based capacitive sensor comprising substrate, interdigitized electrode (IDE), electrolyte and polymer, which has high base capacitance, high noise immunity, cost effective and simple to fabricate and has good flexibility.

The present disclosure further relates to a process of preparing the graphene based capacitive sensor.
BRIEF DESCRIPTION OF ACCOMPANYING FIGURES
In order that the disclosure may be readily understood and put into practical effect, reference will now be made to exemplary embodiments as illustrated with reference to the accompanying figures. The figures together with detailed description below, are incorporated in and form part of the specification, and serve to further illustrate the embodiments and explain various principles and advantages, in accordance with the present disclosure where:

FIGURE 1 illustrates the schematic representation of the mask used for screen printing the reduced exfoliated graphene.

FIGURE 2 illustrates fabricated Capacitive sensor a) with only rEG IDE pattern and electrical leads b) with the rEG IDE, contact wires, PVA-H2SO4 gel and PDMS layer.

FIGURE 3 illustrates exploded schematic of the graphene based Capacitive sensor comprising polyimide substrate.

FIGURE 4 illustrates x-ray diffraction (XRD) scan of EG and rEG showing graphene (001) and (002) peaks.

FIGURE 5 illustrates Field emission scanning electron microscopy (FESEM) image showing rEG nano particles on the electrode of the graphene capacitive sensor of the present disclosure.

FIGURE 6 illustrates Raman spectra for EG and rEG showing D, G, and 2D peaks.

FIGURE 7 illustrates experimental setup of strain measurement using the graphene based capacitive sensor of the present disclosure.

FIGURE 8 illustrates cantilever test setup.

FIGURE 9 illustrates measured capacitance of the graphene based capacitive sensor of the present disclosure with respect to deflection.

FIGURE 10 illustrates measured capacitance of the graphene based capacitive sensor of the present disclosure with respect to percentage strain with calculated hysteresis.

FIGURE 11 illustrates the repeatability of the capacitance of the graphene based capacitive sensor comprising polyimide substrate. Multiple reading showed considerable adherence to standard error of mean.

FIGURE 12 illustrates linearity of response in 0-2000 µe input range by the graphene based capacitive sensor comprising polyimide substrate of the present disclosure.

FIGURE 13 illustrates graphene based capacitive sensor comprising poly(methyl methacrylate) (PMMA) substrate, wherein (a) illustrated exploded view of the sensor and (b) illustrates the actual photo of the sensor.

FIGURE 14 illustrates 3-point bending experimental setup, wherein (a) schematic diagram of the setup (b) UTM-3 point bending setup; and (c) in-house 3-point bending setup.

FIGURE 15 illustrates linearity of response of graphene based capacitive sensor comprising PMMA substrate.

FIGURE 16 illustrates repeatability shown by the graphene based capacitive sensor comprising PMMA substrate.

FIGURE 17 illustrates hysteresis curve of graphene based capacitive sensor comprising PMMA substrate.

FIGURE 18 illustrates transient response of graphene based capacitive sensor comprising PMMA substrate.

FIGURE 19 illustrates metal capacitive sensor (MCS).

FIGURE 20 illustrates loading and unloading curve of MCS.

FIGURE 21 illustrates the effect of lead capacitance on MCS and graphene based capacitive sensor of the present invention.

DETAILED DESCRIPTION
The present disclosure describes a graphene based capacitive sensor.

In an embodiment of the present disclosure, the graphene based capacitive sensor comprises-
substrate;
graphene interdigitized electrode;
electrolyte; and
polymer
wherein the substrate houses the graphene interdigitized electode, layered with electrolyte and polymer.

In an embodiment of the present disclosure, the graphene based capacitive sensor is graphene electric double layer capacitor capacitive sensor.

In an embodiment of the present disclosure, the graphene in the Interdigitized electrode is selected from a group comprising reduced exfoliated graphene (rEG), exfoliated graphene (EG), reduced graphene (rGO), carbon nanotubes and activated carbon.

In an embodiment of the present disclosure, the substrate is selected from a group comprising Polyimide substrate, Poly (methylmethacrylate) (PMMA) substrate, Mylar and Parylene C coated metal.

In an embodiment of the present disclosure, the graphene interdigitized electrode is connected to the metal leads in such a way that single capacitance is realized by smaller capacitances in parallel. Conductive polymer is layered on metal leads to establish sufficient contact between the metal leads and graphene interdigitized electrode.

In an embodiment of the present disclosure, the electrolyte is layered on the graphene interdigitized electrode and thereafter layered with polymer.

In an embodiment of the present disclosure, electrolyte is selected from a group comprising polyvinyl alcohol and free-ion contributing acids, bases or salts like (PVA)-H2SO4, (PVA)-H3PO4, (PVA)-NaOH and (PVA)-KCl.

In an embodiment of the present disclosure, the polymer is selected from a group comprising polydimethylsiloxane, Poly(methyl methacrylate) (PMMA), Paralyene-C and polyvinyl chloride (PVC).

In an embodiment of the present disclosure, the graphene based capacitive sensor comprising Polyimide substrate has a length and breadth dimension of 35mm x 28mm.

In an embodiment of the present disclosure, the graphene based capacitive sensor comprising polyimide substrate has a thickness ranging from about 300 µm to 350µm

In an embodiment of the present disclosure, the graphene based capacitive sensor comprising polyimide substrate weighs in the range of about 0.95g to 1.0g.

In an embodiment of the present disclosure, the graphene based capacitive sensor comprising polyimide substrate has sensitivity of about 14.2pF/µe.

In an embodiment of the present disclosure, the graphene based capacitive sensor comprising polyimide substrate has gauge factor of about 97.37.

In an embodiment of the present disclosure, the graphene based capacitive sensor comprising PMMA substrate has a length and breadth dimension of 60mm x 30mmx2.42mm.

In an embodiment of the present disclosure, the graphene based capacitive sensor comprising PMMA substrate has a thickness of about 2.42mm

In an embodiment of the present disclosure, the graphene based capacitive sensor comprising PMMA substrate weighs of about 10g.

In an embodiment of the present disclosure, the graphene based capacitive sensor comprising PMMA substrate has sensitivity of about 1.3pF/µe.

In an embodiment of the present disclosure, the graphene based capacitive sensor comprising PMMA substrate has gauge factor of about 7.2.

In an embodiment of the present disclosure, the graphene based capacitive sensor exhibits a base capacitance in nano farads range. There is about 1000-fold increase in the base capacitance demonstrated by the capacitive sensor of the present disclosure when compared to the capacitive sensors known in the art.

In an embodiment of the present disclosure, the increase in the base capacitance demonstrated by the capacitive sensor of the present disclosure decreases the effect of unwanted noise and stray capacitance. Thus, increases the accuracy of the sensor.

In an embodiment of the present disclosure, the graphene based capacitive sensor reduces the effect of lead capacitance and stray capacitance, thereby increases the accuracy of detection when employed in the sensor device, provides noise immunity and reduces electronic overheads.

In an embodiment of the present disclosure, the graphene based capacitive sensor provides an increased base capacitance ranging from about 100nF to 190nF as shown in examples, compared to the capacitive sensors available in the art which provide capacitance in the range of few Pico farads (~4pF to 200pF)

In an alternate embodiment of the present disclosure, the graphene based capacitive sensor comprising Polyimide substrate provides an increased base capacitance ranging from about 110nF to 125nF when compared to the capacitive sensors available in the art, which provide a capacitance in the range of few Pico farads (~4pF to 200pF).

In an alternate embodiment of the present disclosure, the graphene based capacitive sensor comprising PMMA substrate provides an increased base capacitance ranging from about 184nF to 190nF when compared to the capacitive sensors available in the art, which provide a capacitance in the range of few Pico farads (~4pF to 200pF).In an embodiment of the present disclosure, the graphene based capacitive sensor has a capacitance variation ranging from about 1.3 pF/µe to 14.2pF/µe based on the type of substrate when subjected to strain input.

In an embodiment of the present disclosure, the graphene based capacitive sensor on Polyimide substrate has a gauge factor of 97, and the graphene based capacitive sensor on PMMA substrate has a gauge factor of 7.2, wherein the gauge factor, GF = ((?C/C))/e.

In an embodiment of the present disclosure, the graphene based capacitive sensor facilitates high base capacitance, thereby facilitating easy and accurate measurement of the data from the capacitive sensor.

In an embodiment of the present disclosure, the graphene based capacitive sensor provides high noise immunity, thereby there is low requirement of noise shielding in the said capacitive sensor.

In an embodiment of the present disclosure, the graphene based capacitive sensor demonstrates good flexibility.

In an embodiment of the present disclosure, the graphene based capacitive sensor is economical.

In an embodiment of the present disclosure, the advantages achieved by the graphene based capacitive sensor are-
High base capacitance facilitating easy measurement;
High noise immunity, thus low requirement of noise shielding measures;
Cost effective and very simple to fabricate the sensor; and
Good flexibility.

In an embodiment of the present disclosure, the graphene based capacitive sensor demonstrates multi-platform applicability, such as ability to measure different parameters like strain, force, displacement, with minimum dependency on external parameters.

The present disclosure further describes a process of preparing the graphene capacitive sensor.

In an embodiment of the present disclosure, the process of preparing the graphene capacitive sensor comprises-
preparing solution of reduced exfoliated graphene (rEG);
molding the solution of rEG on a substrate, followed by annealing to obtain graphene interdigitized electrode (graphene IDE);
coating electrolyte on the molded graphene interdigitized electrode, followed by coating polymer solution to obtain the graphene based capacitive sensor.

In an exemplary embodiment of the present disclosure, the process of preparing the graphene capacitive sensor comprises-
preparing solution of reduced exfoliated graphene (rEG) and solvent;
molding the said solution of rEG on a substrate to obtain graphene interdigitized electrode;
annealing the substrate to remove the solvent;
coating the electrolyte on the molded graphene interdigitized electrode, followed by coating with polymer solution to obtain the graphene capacitive sensor.

In an embodiment of the present disclosure, synthesis of reduced exfoliated graphene was carried out by electrochemical exfoliation process followed by hydrazine hydrate reduction to obtained reduced exfoliated graphene.

In an embodiment of the present disclosure, the solution of reduced exfoliated graphene is prepared by mixing reduced exfoliated graphene and solvent in 1:1 ratio by weight, followed by homogenizing by ultrasonication for 30 minutes to obtain a uniform solution, wherein the solvent is selected from a group comprising N-methyl-pyrrolidone and deionized water.

In an embodiment of the present disclosure, the solution of reduced exfoliated graphene is molded on a substrate selected from a group comprising polyimide substrate, Mylar, PMMA and Parylene C coated metal.

In an embodiment of the present disclosure, the solution of reduced exfoliated graphene is molded on to a substrate by screen printing technique.

In an embodiment of the present disclosure, the molding of solution of reduced exfoliated graphene is carried out on a substrate having the shape i.e., Interdigitized Electrode (IDE) as illustrated in figure 1.

In an embodiment of the present disclosure, the process of preparing the capacitive sensor comprises a step of attaching metal lead/wire to the graphene IDE and contacting the metal lead and graphene IDE with conductive polymer / ink such as silver epoxy.

In an embodiment of the present disclosure, the metal lead/wire includes but it is not limited to copper, silver, gold, nickel.

In an embodiment of the present disclosure, the graphene capacitive sensor obtained by the process has an electrode height of about 150µm, line width of about 0.5mm and gap between the lines of about 0.5mm.

In an embodiment of the present disclosure, the electrolyte is selected from a group comprising polyvinyl alcohol and free-ion contributing acids, bases or salts like (PVA)-H2SO4, (PVA)-H3PO4, (PVA)-NaOH and (PVA)-KCl.

In an embodiment of the present disclosure, the poly vinyl alcohol (PVA) with average molecular weight of about 1,12000 is mixed with solution of deionized water and H2SO4 to make an electrolytic gel. The said electrolytic gel is maintained at a temperature of about 85ºC to retain viscosity.

In an embodiment of the present disclosure, the electrolyte is drop casted on the molded reduced exfoliated graphene.

In an embodiment of the present disclosure, the polymer solution is selected from a group comprising polydimethylsiloxane (silicone elastomer), PMMA, Parylene-C and PVC.

In an embodiment of the present disclosure, the polymer solution is polydimethylsiloxane (PDMS) which is prepared by mixing silicon elastomer (Sylgard 184 Silicone Elastomer Base) and curing agent (Sylgard 184 Silicone Elastomer curing Agent) in a ratio of about 1:10.

In an embodiment of the present disclosure, the polymer solution was applied using spin coater in the form of a film. The thickness of the polymer film was controlled by the spin speed.

In an exemplary embodiment, the process of preparing the graphene based capacitive sensor on Polyimide substrate comprises-
mixing reduced exfoliated graphene (rEG) and N-methyl-2-pyrrolidone (NMP) to obtain a uniform solution;
metal mask was fabricated on flexible polyimide substrate, thereafter the solution was molded to realize rEG interdigitized electrode, using screen printing technique;
the substrate was annealed to remove NMP solvent;
attaching two leads to the rEG interdigitized electrode in a way such that single capacitance is realized by smaller capacitance in parallel;
layering conductive silver epoxy on the electrode to provide proper contacts between the lead wire and rEG interdigitized;
drop casting PVA-H2SO4 electrolyte gel on the rEG interdigitized electrode; and
coating the substrate with polymer solution such as Polydimethylsiloxane solution by spin coater.

In another exemplary embodiment, the process of preparing the graphene capacitive sensor on PMMA substrate comprises-
mixing reduced exfoliated graphene (rEG) and De-Ionised (DI) Water to obtain a uniform solution;
metal mask was affixed on flexible Poly (methyl methacrylate) (PMMA) substrate to deposit Chromium-gold metal layer, thereafter the solution was molded to realize rEG interdigitized electrode, using screen printing technique;
the substrate was annealed to remove DI-water;
attaching two leads from the gold pads using copper wire to the rEG interdigitized electrode in a way such that single capacitance is realized by smaller capacitance in parallel;
layering conductive silver epoxy on the electrode to provide proper contacts between the lead wire and rEG interdigitized;
drop casting PVA-H2SO4 electrolyte gel on the rEG interdigitized electrode; and
coating the substrate with polymer solution such as Polydimethylsiloxane solution by spin coater.

In an embodiment of the present disclosure, the graphene based capacitive sensor is applicable to tactile sensor, accelerometer, displacement sensor, shear sensor, flow sensor, speed sensor, acoustic sensor, dynamic pressure sensor, static pressure sensor, impact sensor, force sensor, heart beat sensor, breath sensor or gas sensor.

In an embodiment of the present disclosure, the substrate material gives an added design feature to obtain desirable parameters of the capacitive sensor. High gauge factor can be achieved by using flexible substrate like polyimide, whereas high linearity can be obtained by using stronger and thicker substrate, such as PMMA.

In an embodiment of the present disclosure, the figure 1 illustrates the schematic representation of the mask used for screen printing the reduced exfoliated graphene. The mask is fabricated using copper metal by standard chemical milling technique.

In an embodiment of the present disclosure, the figure 2a illustrates the capacitive sensor with rEG lines screen printed on the Polyimide substrate and leads taken using copper wire and silver epoxy to realize an Interdigitized Electrode (IDE) pattern. Figure 2b illustrates the capacitive sensor on Polyimide substrate with PVA-H2SO4 layer followed by PDMS layer coated on the IDE pattern.

In an embodiment of the present disclosure, the figure 3 illustrates the exploded schematic view of the capacitive sensor on Polyimide substrate. Each component is false color marked for identification.

In an embodiment of the present disclosure, the figure 4 illustrates the XRD scan image of exfoliated graphene (EG) and reduced exfoliated graphene (rEG), which clearly shows graphene oxide peak at 2? equals to 13.38º and pure graphitic peak at 2? equals to 26.35º which corresponds to (001) and (002) planes of graphene. The graphene (100) crystal plane peak at 2? approximately equals to 43º can be observed. Broadening of the peak may be attributed to presence of functional groups oxides and hydroxyl groups attached to the basal graphene plane bringing non-uniformity in the structure.

In an embodiment of the present disclosure, the figure 5 illustrates the FESEM image of rEG indicating multi-layered structure similar to graphene flakes.

In an embodiment of the present disclosure, the figure 6 illustrates Raman spectroscopy to determine the sp2 hybridization of graphene. The said figure illustrates the Raman spectroscopy at temperature of about 20ºC to 40ºC for rEG and EG. The spectrum shows the characteristic D and G graphite peaks at 1353cm-1 and 1591cm-1, respectively. This G peak gives the degree of ordered graphitized carbon and the D peak denotes presence of disorder in the carbon layers.

In an embodiment of the present disclosure, the figure 7 illustrates the Lab experimental setup to test capacitive sensor on Polyimide substrate. The capacitive sensor was fixed at one end using a rigid support and the other end was deflected using a digital height gauge (Mitutoyo). The capacitance of the capacitive sensor was measured using an LCR meter (Gwinstek LCR-819) at 1kHz frequency and 1V bias voltage.

In an embodiment of the present disclosure, the figure 8 illustrates cantilever test setup, demonstrating the testing of the capacitive sensor of the present disclosure, wherein the sensor acts as a fixed cantilever. One end of the sensor is fixed using rigid support and the other end is deflected. The deflection applied induces a known strain which is calculated using the equation e=3yhx/(2Z^3 )
where, y is the deflection of the free end, h is the thickness of the substrate, x is the distance from center of the sensor to the free end, and Z is the total active substrate length.

In an embodiment of the present disclosure, the figure 9 illustrates the sensor response in terms of capacitance to the applied known deflection. It is observed that the capacitance of the capacitive sensor of the present disclosure varied from 125nF to 92nF as deflection is varied.

In an embodiment of the present disclosure, the figure 10 illustrates measured capacitance of the capacitive sensor with respect to percentage strain. The hysteresis seen is about 2% which is acceptable for a prototype device.

In an embodiment of the present disclosure, the figure 11 illustrates the repeatability of the sensor of 95% as seen from the indicated error bar. Multiple reading showed considerable adherence to standard error of mean.

In an embodiment of the present disclosure, the figure 12 illustrates linearity of response in 0 µe to2000 µe input range. The linearity was not satisfactory outside this range of strain.

In an embodiment of the present disclosure, the figure 13a illustrates the exploded schematic view of the capacitive sensor on PMMA substrate. Each component is false color marked for identification. Figure 13b illustrates the capacitive sensor on PMMA substrate.

In an embodiment of the present disclosure, the figure 14a illustrates the schematic of Lab experimental setup to test capacitive sensor on PMMA substrate. Figure 14b illustrates the 3-point bending test in Universal Testing Machine (UTM). Figure 14c illustrates an in-house 3-point bending setup. The capacitive sensor was rigidly supported at the two ends and force was applied in between these two points. The capacitance of the capacitive sensor was measured using an LCR meter (Gwinstek LCR-819) at 1kHz frequency and 1V bias voltage.

In an embodiment of the present disclosure, the figure 15 illustrates the linearity response of graphene based capacitive sensor comprising PMMA substrate, wherein the non-linearity of about 0.23% was observed.

In an embodiment of the present disclosure, the figure 16 illustrates the repeatability graphene based capacitive sensor comprising PMMA substrate, wherein the repeatability of about 99.4% was observed across 10cyles.

In an embodiment of the present disclosure, the figure 17 illustrates the hysteresis curve of graphene based capacitive sensor comprising PMMA substrate, wherein the hysterisis of about 0.36% was observed.

Additional embodiments and features of the present disclosure will be apparent to one of ordinary skill in art based upon the description provided. The embodiments provide various features and advantageous details thereof in the description. Description of well-known/conventional methods and techniques are omitted so as to not unnecessarily obscure the embodiments. The examples provided herein are intended merely to facilitate an understanding of ways in which the embodiments provided may be practiced and to further enable those of skilled in the art to practice the embodiments provided. Accordingly, the following examples should not be construed as limiting the scope of the embodiments.

EXAMPLES

EXAMPLE 1: Synthesis of Graphene
Two graphite rods were used as electrode material. An aqueous solution of about 0.1M ammonium sulphate was used as electrolyte. DC voltage of about 15V was applied, the graphite on anode side was exfoliated and sheets of graphene or graphite drops into the solution. The exfoliated graphite particles contain monolayer to few layer graphene flakes.

EXAMPLE 2: Synthesis of graphene based capacitive sensor employing Polyimide substrate
About 50mg rEG and about 50ml N-methyl-2-pyrrolidone (NMP) was mixed and homogenized by ultrasonication to obtain a uniform solution.
Flexible polyimide substrate was fabricated with a metal mask (illustrated in figure 1) using screen printing, wherein the uniform solution was molded to realize rEG interdigitized electrode (IDE). Figure 2 illustrates the graphene electrode of height 150µm, 0.5mm line and 0.5mm gap in between the lines. The pattern was annealed at 60ºC for about 4hours to remove NMP solvent. Two leads were taken out from the IDE in a way such that single capacitance was realized by smaller capacitance in parallel. Conductive silver epoxy was layered on the electrode to make proper contact with lead wire and the electrode. PVA-H2SO4 was drop casted on the electrode and kept until the gel settles. Further, Polydimethylsiloxane (PDMS) solution was spin coated. The thickness of the PDMS film was controlled using the spin speed. PDMS was left to dry in ambient environment to avoid bubbling of the entrapped electrolyte on annealing to obtain graphene based capacitive sensor comprising Polyimide substrate.
The details of the graphene based capacitive sensor comprising Polyimide substrate is provided in table 1 below-
Parameter Specification
Type of Sensor Graphene EDL Capacitive sensing film (GECS) on Polyimide substrate
Dimensions 35 x 28 x 0.175 mm
Active Area 18 x 28 mm
Thickness ~ 325 µm
Weight ~ less than one gram
Tested Strain Range 0 – 2050 µe
Hysteresis 2.15%
Repeatability 95%
Non-Linearity 2.06%
Gauge Factor 97.37
Sensitivity 14.2pF/µe
Table 1:

EXAMPLE 3: Analysis of Graphene based capacitive sensor comprising polyimide substrate
The performance of graphene based capacitive sensor on polyimide substrate was studied using the cantilever bending technique. The capacitance of the fabricated device was measured using an LCR meter (Gwinstek LCR-819) at 1kHz frequency and 1V bias voltage. The device was fixed at one end using a rigid support and the other end was deflected using a height gauge (Mitutoyo). Therefore, a known strain was applied to the device and corresponding capacitances variations were recorded. Figure 7 shows the experimental setup and mechanism for subjecting the cantilever to strain.
The sensor acts as a fixed cantilever. One end of the sensor is fixed using rigid support and the other end is deflected. The deflection applied induces a known strain which is calculated by the following equation-
e=3yhx/(2Z^3 )
Where, y is the deflection of the free end, h is the thickness of the substrate, x is the distance from center of the sensor to the free end, and Z is the total active substrate length (illustrated in figure 8).
Deflection applied at the free end was in the range of about 0mm to 5mm and induced a strain of about 0 to 2050 µe in the sensor. The capacitance of the sensor varied from about125nF to 93nF. Capacitance variation of about 14.2pF/ µe was observed which can be considered as good response.
The response was observed to be more non-linear of about 2000 µe. The non-linearity was about 2.06% (illustrated in figure 12). A maximum hysteresis of about 2.15% was observed (illustrated in figure 10) from the loading and unloading cycles. The sensor displayed a repeatability of about 95% (illustrated in figure 11) across 6 cycles. The root mean square (RMS) error and root of sum of squares (RSS) error were calculated and found to be about 1.49 and about 4.47, respectively.

EXAMPLE 4: Synthesis of graphene based capacitive sensor employing poly(methyl methacrylate) (PMMA) substrate
About 50mg rEG and about 50ml DI Water was mixed and homogenized by ultrasonication to obtain a uniform solution.
Flexible poly(methyl methacrylate) (PMMA) substrate affixed with metal mask (illustrated in figure 1) was deposited with chromium-gold metal layer having thickness of about 20nm and 100nm respectively, using RF Sputtering System. The uniform solution was screen printed to realize rEG interdigitized electrode (IDE) and annealed at 60ºC for about 4hours to dry the pattern. Two leads were taken out from the IDE in a way such that single capacitance was realized by smaller capacitance in parallel. Conductive silver epoxy was layered on the electrode to make proper contact with lead wire and the electrode. PVA-H2SO4 was drop casted on the electrode and kept until the gel settles. Further, Polydimethylsiloxane (PDMS) solution was spin coated. The thickness of the PDMS film was controlled using the spin speed. PDMS was left to dry in ambient environment to avoid bubbling of the entrapped electrolyte on annealing to obtain graphene based capacitive sensor comprising poly(methyl methacrylate) (PMMA) substrate.
The details of the graphene based capacitive sensor comprising PMMA substrate is provided in table 2 below-
Parameter Specification
Type of Sensor Graphene EDL Capacitive sensing film (GECS) on PMMA substrate
Dimensions 60x30x2.42 mm
Active Area 18 x 28 mm
Thickness ~ 2.42 mm
Weight ~ 10 grams
Tested Strain Range 0 – 4502 µe
Hysteresis 0.36%
Repeatability 99.4%
Non-Linearity 0.3%
Gauge Factor 7.2
Sensitivity 1.3pF/µe
Table 2:

EXAMPLE 5: Analysis of graphene based capacitive sensor comprising Poly(methyl methacrylate) (PMMA) substrate
3-Point bending technique was employed to characterize the device performance. In this set-up, the capacitive sensor was fixed at two ends and known load is applied at the center (illustrated in figure 14). The substrate bends, thus undergoing strain, which further is experienced by the IDE capacitor on the substrate. This strain leads to change in capacitance of the senor which is measured using the LCR meter (LCR-819). Initial trials were carried out on a universal testing machine (UTM) in compressive loading mode. A total strain of 4500µe was applied.
The strain was calculated using the following equation-
e=(3PL_0)/(2E_f wh^2 )
where, P is the applied load,
L0 is the original device length,
Ef is the modulus of elasticity,
w and h are width and thickness of the device.

The Au-metal film underneath the graphene material acts as a capacitor electrode. This film ensures that all the rEG flakes form part of the capacitor by contact through the metal film. Even if the rEG line is discontinuous, rEG flakes, after the discontinuity point, would be in contact with metal film and hence minor breaks in rEG line would not be of much concern.
The PMMA substrate of thickness about 2.4mm in the sensor performed consistently even after multiple loading trails.

An improved performance was seen with respect to non-linearity, repeatability and hysteresis. Non-linearity was observed to be about 0.23% (illustrate in figure 15). Repeatability of about 99.4% was seen across 10 cycles (illustrated in figure 16) and hysteresis offset of about 0.36% was observed (illustrated in figure 17).

Further, the sensor was tested for transient response performance. The sensor was subjected to instantaneous loading and unloading using a fixed weight (about 0.5kg). the capacitance drops instantaneously on application of load and comes back when the load is removed (illustrated in figure 18).

EXAMPLE 6: Comparison of the Graphene based capacitive sensor (GECS) of the present disclosure and conventional Metal capacitive sensor.
Conventional Metal capacitive sensor (MCS) was fabricated with the same dimensions as that of the graphene based capacitive sensor of the present disclosure (illustrated in figure 19). The MCS made up of chromium-gold thin film deposition, showed approximately 7.5pF base capacitance and a loading of about 4500µe (0.5mm displacement) in the 3-point bending test, resulted in 11fF change in capacitance. The obtained signal was largely affected by noise and smaller loading conditions were difficult to identify (illustrated in figure 20).
It was observed that capacitance of extra lead length themselves had capacitance in the range of pF and hence greatly affected by MCS capacitance. On the other hand, the capacitance of GECS was hardly affected by change in the lead length, as illustrated in figure 21 (about 1.6% change at 25cm extra lead length). This shows that increasing the base capacitance has indeed helped in minimizing the effect of noisy component in the sensor.
Experiments were carried out to study the effect of stray capacitance by alternatively measuring the MCS and GECS with and without any ground shield. It was observed that MCS showed about 4.8% rise in capacitance without any ground shielding around the sensor, whereas GECS showed only 0.19% rise. This shows that increase base capacitance has reduced the effect of stray capacitance caused by the surrounding components and EMI effects.
,CLAIMS:1. A graphene based capacitive sensor comprising-
substrate;
graphene interdigitized electrode;
electrolyte; and
polymer
wherein the substrate houses the graphene interdigitized electrode, layered with the electrolyte and the polymer

2. The graphene based capacitive sensor as claimed in claim 1, wherein the substrate is selected from a group comprising Polyimide substrate, Poly (methylmethacrylate) PMMA substrate, Mylar and Parylene C coated metal.

3. The graphene based capacitive sensor as claimed in claim 1, wherein the electrolyte is selected from a group comprising polyvinyl alcohol and free-ion contributing acids, bases or salts like (PVA)-H2SO4, (PVA)-H3PO4, (PVA)-NaOH and (PVA)-KCl.

4. The graphene based capacitive sensor as claimed in claim 1, wherein the polymer is selected from a group comprising polydimethylsiloxane, Poly(methyl methacrylate) (PMMA), Paralyene-C and polyvinyl chloride (PVC).

5. The graphene based capacitive sensor as claimed in claim 1 has base capacitance ranging from about 100nF to 190nF.

6. The graphene based capacitive sensor as claimed in claim 1 has capacitance variation ranging from about 1.3 pF/µe to 14.2pF/µe.

7. A process of preparing the graphene based capacitive sensor as claimed in claim 1, comprises-
preparing solution of reduced exfoliated graphene (rEG);
molding the solution of rEG on a substrate, followed by annealing to obtain graphene interdigitized electrode (graphene IDE);
coating electrolyte on the molded graphene interdigitized electrode, followed by coating polymer solution to obtain the graphene based capacitive sensor.

8. The process as claimed in claim 7, further comprises attaching metal lead or wire to the graphene intergitized electrode, wherein the metal is selected from a group comprising chromium, copper, silver, gold and nickel.

9. The process as claimed in claim 7, wherein the solution of reduced exfoliated graphene is prepared by mixing the reduced exfoliated graphene and solvent in 1:1 ratio by weight, followed by homogenizing by ultrasonication for about 30 minutes to obtain a uniform solution, wherein the solvent is selected from a group comprising N-methyl-pyrrolidone and deionized water.

10. The process as claimed in claim 7, wherein the polymer solution is prepared by mixing the polymer and curing agent in a ratio of about 1:10.