FORM 2
THE PATENTS ACT, 1970 (39 OF 1970)
PATENTS RULES, 2006
COMPLETE SPECIFICATION (SECTION 10; RULE 13)
TITLE:
FABRICATION OF PIEZORESISTIVE PRESSURE SENSORS
Applicant : IITB MONASH RESEARCH ACADEMY Nationality: INDIAN
Address : IIT Bombay
Powai, Mumbai 400 076, India
THE FOLLOWING SPECIFICATION DESCRIBES THE INVENTION

TITLE: FABRICATION OF PlEZORESISTIVE PRESSURE SENSORS
FIELD OF INVENTION
[0001] The present invention relates to pressure sensors used in
diverse applications. In particular, the present invention relates to pressure sensors fabricated by using ink of graphene and multi-wailed nanotubes (MWNTs). More particularly, the present invention relates to flexible piezoresistive pressure sensors fabricated by synthesis of MWNTs doped reduced graphene oxide (rGO) ink in water.
[0002] The present invention also relates to a technique with fewer
steps for fabricating flexible and versatile pressure sensors by using MWNTs doped rGO ink for human-machine interfacing applications, e.g. biomedical
BACKGROUND OF INVENTION
[0003] Traditionally, transistors based sensors, especially silicon based
metal-oxide semiconductor field effect transistor reflects high sensitivity, but
being rigid, these sensors are incompatible to be used in flexible devices.
[0004] Currently, several flexible pressure sensors include
microstructured rubber as a dielectric layer capacitor, fractured microstructure, organic field effect transistors (OTFT), nanowire active array field effect transistors, reversible interlocking nano fibers piezoresistors, and ZnO-rGO based piezocapacitive sensor. Since the fabrication processes for such flexible pressure sensors requires a detailed study of the nanostructure configuration, which involves various complex processing, it affects the product's final manufacturing cost.
PRIOR ART
[0005] Typically, piezoresistive sensors devices have been extensively
employed due to their appealing advantages such as low-cost, feasible processing and convenient signal collection. These work on a very simple

principle in which the imposed pressure is transduced to electrical signals i.e.
resistance signal.
[0006] Conventionally, attempts have been made to use conductive
polymer films or conductive carbon black particles into the matrix of elastomeric rubbers as the probe material for the piezoresistive sensors. However, these probe materials employed for pressure sensing have certain limitations such as materials instability, insensitive and reproducibility in low-pressure regime (<10kPa), limiting their applications domain like artificial skin.
[0007] Recently, rational assemblies of micro as well as
nanoconductive materials have generated within the research community to fabricate sensitive pressure sensors by assembling polyaniline nanofibers and Au-coated polydimethylsiloxane (PDMS) micropillars and sandwiching ultrathin gold nanowire-impregnated tissue paper between PDMS sheets hierarchically compounding polyurethane (PU) fiber, nylon fiber, Ag nanowires, and piezoresistive rubber, etc. Alternative strategy for developing high-sensitivity sensors is also demonstrated by using mierostructure configurations like hollow-sphere microstructure, micro-pyramid array, porous structure, fractured microstructure and interlocking mierostructure etc.
DISADVANTAGES OF THE PRIOR ART
[0008] To date, a variety of pressure sensing mechanisms have been
suggested/developed which include piezoelectric sensing, piezoresistive sensing, triboelectric sensing, transistor sensing and capaeitive sensing mechanism respectively. However, the cost of any such technology implementation is quite high and involves complex processes.
[0009] Therefore, there is an existing need for developing an alternative
low-cost route which uses a fabrication technique with fewer steps for developing flexible pressure-sensitive materials with a sufficient sensitivity in a broad pressure range of 10s-1000s Pa (tens - thousands Pa).

[0010] The forthcoming tech-savvy generations would be well-
motivated to implement flexible and low-cost sensors in various prospective
research applications like electronic skin, speech recognition, sports motion
monitoring, portable healthcare monitoring, tactile sensing, robot prosthesis,
etc.
TECHNICAL SOLUTION
[0011] The devices based on porous foams or sponges offer an
alternative technique for fabricating pressure sensors, because their internal pore structure allows to behave elastically, enabling them to retain their original structure under external perturbations without undergoing any permanent deformation. Moreover, sponges or foams made of polyurethane (PU), polystyrene (PS) and polyvinyl chloride (PVC) are light-weight and flexible having varying densities from a low value of 1.6 kg/m3 to a high value of 960 kg/m3,
[0012] Further, the production methods of these sponges or foams are
also scalable, thus making them an ideal material for use in devices requiring mechanical transduction. These foams have been employed for the
fabrication of piezocapacitive and piezoresistive devices and exhibited good performances with regard to their sensitivity, pressure range, Signal-to-Nois© (SNR) ratio, fast response time, good linearity between the applied force (pressure) and the resistance cycling behavior (numbers) etc.
[0013] Conductive porous foams or sponges are another alternative for
fabricating piezoresistive pressure sensors carrying a combination of demanding properties, i.e. mechanical flexibility and electronic conductivity. Intrinsically, PU foams are light-weight with high flexibility due to their available pore structure which facilitates the fabrication of piezoresistive as
well as piezocapacitive based sensors.
[0014] Recently, various conductive sponges with high electrical and
mechanical properties have been demonstrated, which include conductive carbon nanotubes prepared via chemical vapor deposition, cellulose nanofiber

sponges, carbonization of hybrid (metal-polymer) nanocable sponge and dip-
coating of conductive nanomaterials on the backbone of commercial sponges to develop electrodes for fuel cells and supereapacitors. Although, the pressure sensors based on such proposed techniques help in developing highly conductive sponges, their sensitivity towards the low-resistance values under pressure has restricted their application in many human-machine interfacing applications, e.g. biomedical devices.
[0015] Accordingly, a simple and direct technique for fabricating flexible
piezoresistive pressure sensors is developed in accordance with the present invention, which involves the synthesis of MWNTs doped rGO ink in water (as a solvent), followed by embedding this doped rGO ink into commercially available PU foam.
[0016] Here, rGO ink (0.5 mg/mL) is synthesized from graphene oxide
(GO) solution by using hydrazine monohydrate (as a reducing agent) and water based noble additive (comprising benzisothiazolinone and methyisothiazolinone) which converts its intrinsic behavior from hydrophobic to hydrophilic.
[00173 On the other hand, the functionalized MWNTs are synthesized
by using dispersive agents (DISPERSKY 190) and additive (Anti-terra 250) with water as a solvent by using high shear forces. Thereafter, the optimized proportion of MWNTs are doped in rGO ink and directly exposed to PU foams,
since doped ink with a hydrophilic nature has a great affinity for PU backbone, quite similar to GO. The addition of MWNTs helps in enhancing electrical conductivity through interlinking of the flakes wrapped on the PU skeleton. The doped ink is infused into PU foam through repeated dipping, followed by thermal drying to achieve the PU skeleton wrapped with rGO flakes and to obtain the final product as a d-rGO@PU sponge piezoresistive pressure sensor.
[0018] A wide range of strain is recorded using from 10% to 70% strain
(corresponding to 2,22 kPa to 16.4 kPa) in real-time curve with a good

reproducibility of 5000 cycles. The versatile potential of d-rGO@PU sponge piezoresistive pressure sensor is also confirmed and demonstrated for observing the small-scale motion monitoring, such as blood pulse recognition through wrist as well as neck, voice recognition cheek blowing associated with health monitoring and large-scale monitoring such as finger moment inclined at different angles, OBJECTIVES OF THE INVENTION
[0019] Some of the objectives of the present invention - satisfied by at
least one embodiment of the present invention - are as follows:
[0020] An object of the present invention is to provide a flexible, low-
cost piezoresistive pressure sensor for diverse applications, such as those involving human-machine interfaces, e.g. in biomedical applications etc.
[0021] Another object of the present invention is to provide a flexible,
low-cost piezoresistive pressure sensor based on microcrack as well as macrocrack-configuration.
[0022] Still another object of the present invention is to provide a
piezoresistive pressure sensor by using the multi-walled nanotubes (MWNTs) doped rGO ink (with hydrophilic nature) in Polyurethane foam.
[0023] Yet another object of the present invention is to provide a
versatile piezoresistive pressure sensor having sufficient potential to detect both small-scale and large-scale motion monitoring.
[0024] A further object of the present invention is to provide a versatile
piezoresistive pressure sensor which is useful for application involving human-machine interfaces, e.g. biomedical applications.
[0025] A still further object of the present invention is to provide a
simple and direct technique a single step process for fabricating flexible piezoresistive pressure sensors suitable for various biomedical applications.

[0026] These and other objects and advantages of the present
invention will become more apparent from the following description when read
with the accompanying figures of drawing, which are, however, not intended to limit the scope of the present invention in any way.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
The present invention will be briefly described with reference to the
accompanying drawings, wherein:
[0027] Figures 1(a)-(b) represent the digital images of the PU foam
while embedding into doped rGO ink to obtain the d-rGO@PU sponge
piezoresistive pressure sensor fabricated in accordance with the present invention.
[0028] Figures 2(a)-(j) represent the schematics of the complete
procedure involved in the fabrication of d-r6Q@PU sponges piezoresistive pressure sensor of Figure 1(d).
[0029] Figure 2(k) shows the cross-sectional views of the PU foam
progressing through the fabrication of d-rGO@PU sponge piezoresistive pressure sensor by the process developed according to the present invention.
[0030] Figures 3(a)-(g) represent the HRTEM images of the rGO ink
and MWNTs doped rGO ink along with the SAED patterns thereof employed
for fabricating d-rGO@PU sponge piezoresistive pressure sensor of Figure 1(d).
[0031] Figures 4(a)-(d) represent the FEGSEM images with SOX
magnifications of purely cleaned PU foams and MWNTs doped rGO ink wrapped on PU backbone after first, second and third dips to fabricate d-
rGO@PU sponge piezoresistive pressure sensor of Figure 1(d).

[0032] Figures 5(a)-(d) graphically represent different sets of electrical
characteristics of d-rGO@PU sponge piezoresistive pressure sensor of Fig.1(d), e.g. conductivity v/s strain; pressure v/s strain; conductivity v/s
pressure under different applied strains; resistance (AR) V/S time at different applied strain and at defined intervals,
[0033] Figures 6(a)-(b) graphically represent the relative resistance
change (ΔR/RO) response curve of d-rGO@PU sponge piezoresistive pressure sensor of Figure 1(d) against a progressively increasing pressure (to calculate the sensitivity of as-prepared sensor) and the increase in applied
strain (to calculate the gauge factor of as-prepared sensor).
[0034] Figure 7 represents the reproducibility of d-rGO@PU sponge
piezoresistive pressure sensor of Figure 1(d) evaluated by using cyclic tests
under loading-unloading at 48% strain for 5000 cycles.
[0035] Figures 8(a)-(g) graphically represent the real-time response
(Current Vs Time) curves of the d-rGO@PU sponge piezoresistive pressure
sensor of Figure 1(d) for varieties of small-scale motion such as Blood pulse as well as neck pulse detection, cheek bulging, word recognition monitoring applications.
[0036] Figures 9{a)-(d) represent d-rGO@PU piezoresistive sensor
attached on the finger joints and finger is inclined at different angles such as (a) 15°, (b) 45°, (c) 60° and (d) 90° and including the response Current-Time (l-T) curves corresponding to these different angular (bending) positions.
DETAILED DESCRIPTION OF THE FIGURES/DRAWINGS
In the following, the present invention will be described in more details with
reference to the accompanying drawings without limiting the scope and ambit of the present invention in any way.
[0037] Description of method for fabrication of sensor

[0038] Figure 1{a) represents a perspective view of the commercially
available polyurethane (PU) foams sliced into the rectangular form to be
subsequently soaked into the doped rGO ink for subsequently making d-
rGO@PU sponge piezoresistive pressure sensor fabricated according to the present invention,
[0039] Figure 1 (b) represents a perspective view of the pure PU foam was washed by using IPA (thrice) and dried in the vacuum at 60°C for 8 hrs.
Thereafter, foams were dipped in the prepared rGO ink and kept for degassing. The foam with the soaked ink was mildly pressed in order to remove the extra liquid content from it. It was then dried in vacuum oven at
70°C for 10 hrs and the process was repeated three times for subsequently fabricating d-rGO@PU sponge piezoresistive pressure sensor in accordance with the present invention,
[0040] Figure 1(c) represents the lateral view of the d~rGO@PU sponge
piezoresistive pressure sensor fabricated in accordance with the present invention by adhering two similar ITO coated PET sheets as top and bottom electrodes on the two circular faces of the PU foam doped in rGO ink (Figure 1(b)) by using a silver paste to eliminate the contact resistance and to achieve stable signals output.
[0041] Figure 1(d) represents the top view of the d-rGQ@PU sponge
piezoresistive pressure sensor of Figure 1(c) fabricated in accordance with
the present invention.
[0042] Figure 2(a) shows rectangular sliced piece of cleaned pur© PU foam of suitable size, e.g. 1.3 cm x 1 cm and 0.5 cm thick, for subsequent
soaking in rGO ink.
[0043] Figure 2(b) shows PU foam of Fig. 2(a) dipped in rGO ink for its first
soaking.

[0044] Figure 2(c) shows the first soaked PU foam in rGO ink being degassed/dried for embedding the rGO ink in its skeleton.
[0045] Figure 2(d) shows the degassed PU foam after first dipping in rGO
ink.
[0046] Figure 2(e) shows the step of drying the degassed PU foam of Fig.
2(d) in a vacuum oven. The steps of dipping (Fig. 2(b)), degassing (Fig. 2(c)) and drying (Fig. 2(c)) of PU foam was repeated three times.
[0047] Figure 2(f) shows the final dark black PU foam obtained after completion of dipping, degassing and drying the cleaned PU foam thrice.
[0048] Figure 2(g) shows the step of a respective ITO coated PET sheet is
laid on top and bottom surface of the dark black PU foam as an electrode and
adhered thereto by using a silver paste.
[0049] Figure 2(h) shows a respective wire is connected to each PU foam electrode to complete the fabrication of the d-r60@PU sponge piezoresistive
pressure sensor, e.g. sensor shown in Fig. 1(d).
[0050] Figure 2(i) shows the process of measurement under applied strain by using Mark 10 for measuring the electrical characteristics of the d-rGO@PU sponge piezoresistive pressure sensor of Fig. 2(h) by using KEYSIGHT Source/measurement unit connected to the laptop.
[0051] Figure 2(j) shows a magnified image of the d-rGO@PU sponge
piezoresistive pressure sensor under applied strain.
[0052] Figure 2(k) shows the respective cross-sectional views of the PU
foam progressing through the fabrication of d-rGO@PU sponge piezoresistive pressure sensor by the process developed according to the present invention. These include the cross-sections of: (i) pure PU foam, (ii) highly dispers rGO ink, (iii) PU foam after first dip, (iv) PU foam after third dip, and (v) ITO-coated PET sheet used as electrodes.

In order to confirm the formation of rGO flakes and to observe the distribution of the MWNTs on rGO flakes, high resolution transmission electron microscopy (HRTEM) was performed at various locations of the sample and these HRTEM images are discused in detail in the following;
[0053] Figure 3 (a) represents a HRTEM image (Scale 100 nm) of pure rGO ink used for fabricating piezoresistive pressure sensors and which is collected in dark field mode along with their selected area electron diffraction pattern (SAED), which includes wrinkles between the rGO flakes and folded •- edges.The average size of the rGO flake is estimated to be more than half a micrometer which is quite big to enable favorable percolation of charges,
[0054] Figure 3 (b) represents another HRTEM image (Scale 50 nm) of pure rGO ink used for fabricating piezoresistive pressure sensors and which is collected in dark field mode along with their selected area electron diffraction pattern (SAED), showing twelve bright spots (inset) appearing due to the coupling of two hexagon patterns, suggesting the presence of highly exfoliated few layer rGO.
[0055] Figure 3 (c) represents still another HRTEM image (Scale 20 nm) of
pure rGO ink used for fabricating piezoresistive pressure sensors and which is collected in dark field mode along with their selected area electron diffraction pattern (SAED), confirming the presence of single hexagon with six bright spots (inset). Such patterns were observed at several positions of the sample and confirm the agglomoration of the reduced films was restricted because of using a water based additive. These results also point out that the prepared rGO ink holds crystallinity in its chemical structure.
[0056] Fig 3(d) shows a HRTEM image (Scale 100 nm) or exfoliated MWNTs doped rGO ink, which clearly shows that MWNTs are decorated on the rGO flakes in random fashion connecting adjacents tubes as well as flakes. Few dark circular spots varying in diameters were also observed which

may be due to the agglomeration of rGO/MWNT or due to insufficient drying
of the sample.
[0057] Fig 3(e) shows another HRTEM image (Scale 50 nm) of exfoliated MWNTs doped rGO ink, which clearly shows few dark circular spots varying in
diameters, which may be due to agglomeration of rGO/MWNT or due to insufficient drying of the sample. SAED pattern demonstrates concentric rings (inset) instead of bright spots, which suggests that the crystalinity has disappered on doping MWNTs in rGO ink. This can happen due to the interconnection of flakes and nanotubcs, as well as by unzipping of MWNTs.
[0058] Fig 3(f) shows still another HRTEM image (Scale 10 nm) of
exfoliated MWNTs doped rGO ink, which confirms the disappearance of the crystalinity on doping MWNTs in rGO ink due to the interconnection offtakes and nanotubes, as well as by unzipping of MWNTs. Here, the exfoliation and unzipping of MWNTs is clearly visible (inset) by a circle, which leads to an increased number of layers.
Subsequently, in order to confirm the deposition of rGO flakes on the skeleton of PU foams, several Field Emission Gun Scanning Electron Microscopy (FEGSEM) images (Scale 100 urn) of the PU foam were collected
after performing the sequence of dipping, degassing and drying for once, twice and thrice, which are discused in detail in the following:
[0053] Figure 4(a) represents a FEGSEM image with 50x magnification of purely cleaned PU foam for fabricating the d-rGO@PU sponge piezoresistive
sensor of Fig. 1(d).
[0060] Figure 4(b) represents another FEGSEM image with 50x magnification of MWNTs doped rGO ink wrapped on PU backbone after first stage of dipping, degassing and drying for fabricating the d-rGO@PU sponge piezoresistive sensor of Fig, 1(d). On dipping the sponge in the ink first time,
the flakes and MWNTs having significant affinity for the PU foam to get
. . . .. . - -•■. ■ •. k ■ ■ ■ ' '''. .-■ i
attached to its backbone. Here, the neatly washed PU sponge is visible along

with a slight impression of the carbon tape (at the right top-corner) and
distinguishable by using color contrast. The thickness of the skeleton forming
the foam is estimated to be approximately 100pm.
[0061] Figure 4(c) represents still another FEGSEM image with 50x magnification of MWNTs doped rGO ink wrapped on PU backbone after second stage of dipping, degassing and drying for fabricating the d-rGO@PU sponge piezoresistive sensor of Fig. 1(d). On repeatly dipping PU foams in the ink, the rGO flakes try to wrap the uncovered region of PU skeleton and the flakes already deposited. This wrapping also leads to increasing the thickness of the skeleeton of PU foams with each consecutive dipping,
[0062] Figure 4(d) represents yet another FEGSEM image with 50x magnification of MWNTs doped rGO ink wrapped on PU backbone after the
third stage of dipping, degassing and drying for fabricating the d-rGO@PU sponge piezoresistive sensor of Fig. 1(d). Here, the flakes and MWNTs are seen attached to backbone of PU foam due to their significant affinity for PU
foam.
[0063] Figure 5(a) graphically represents a curve drawn for the
conductivity versus strain measured for the d-rGQ@PU sponge piezoresistive sensor of Fig. 1(d). Here, the conductivity behavior of the sensor under
different magnitudes of the compressive strain clearly demonstrates an increase in the conductivity by increasing the strain applied, although for the initial range of the applied strain (from 0 to 40%), the conductivity of the sensor drops down very slightly. This could be due to the disconnections of the mechanical micro as well as macro crack junctions in the doped rGO flakes wrapped on PU backbones. A very small increment was observed in the conductivity up to 70% of strain which could be due to the contact between d-rGO@PU conductive backbones while a transition phase was observed from 70% to 82% strain values. Further, a rapid shoot-up in the conductivity was measured on higher values of applied strain (82% to 100%) because of an increase in the contact area between the d-rGO@PU backbones; a higher conductivity increase was measured.

[0064] Figure 5(b) graphically represents a curve drawn for the pressure
under different values of the applied strain measured for the d-rGO@PU
sponge piezoresistive sensor of Fig. 1(d). Here, the pressure increases
smoothly up to 82% of strain associated with a pressure of 9.23 kPa, white a sudden pressure shoot-up is apparent as the applied strain surpasses 82%. it is quite obvious that the reaction force applied by PU foam at the initial stage was quite less, since most of the micro pores volume was occupied by air. Whereas, by increasing the load values further, PU foams wrapped with rGO flakes gets compacted, and leads to a sudden pressure shoot-up of 48.84kPa.
[0065] Figure 5(c) graphically represents a curve drawn for the conductivity
versus pressure measured for the d-rGO@PU sponge piezoresistive sensor of Fig. 1(d). Here also, the contact area between the backbones of d-rGO@PU is increased and therefore, a huge increment in the conductivity was measured. Again, the applied pressure i.e. up to 11 kPa (equivalent to 82% strain) shows a very slight increment in the conductivity which could also be confirmed in the inset graph, whereas at higher pressure, the conductivity represents a linear increase with more than four times increment in its magnitude, i.e. from 0.008 S/m to 0.035 S/m).
[0066] Figure 5(d) graphically represents curves drawn for the resistance (AR) versus time at applied strain values of 20% (1 mm), 40% (2 mm), 60% (3 mm), 80% (4 mm) and 100% (5mm) respectively measured at an interval of 10% (0.5 mm) for the d-rGO@PU sponge piezoresistive sensor of Fig. 1(d).
In order to study the response behavior (resistance v/s time) of d-rGO@PU sponge piezoresistive sensor of Fig. 1(d), a constant biased voltage of 5V was applied across d-rGO@PU sponge piezoresistive sensor and the resistance was measured with variation in the pressure, which was implemented in the normal direction for studying and recording the compressive strain under different magnitudes thereof, which is discussed in detail in the following:
[0067] Figure 6(a) represents the relative resistance change (ΔR/Ro)
response curve of d-rGO@PU sponge piezoresistive sensor against

progressively increasing pressure. The curves follow a systematic pattern which remains unaffected under different values of pressure imposed, while the difference in the resistance change increases. For measuring the sensitivity [(ΔR/Ro)/P] of d-rGO@PU sponge piezoresistive sensor, all cumulative measurement results associated with d-rGQ@PU sponge piezoresistive sensor are shown in Figure 6(a), which indicate that the pressure responsive behaviors of d-rGO@PU sponge piezoresistive sensor could be distributed in three regions: positive I region, negative II and HI regions. In positive I region, ΔR/Ro reflects a linear increment with progressively increase in pressure (up to 2.4 kPa), exhibiting a positive sensitivity slope of magnitude 0.022 kPa-1 which is slightly lower than the reported sensors. In this process, the resistance was induced due to micro as well as macro crack within doped rGO flakes and which played a decisive role in controlling the sensor conductivity. However, once the applied pressure exceeds 2.4 kPa, the sensor conductivity is driven by contacts between d-rGO@PU conductive backbones and it respectively results in a negative sensitivity slope of 0.088 kPa-1 in II regime and 0.034 kPa-1 in III region.
[0068] Figure 6(b) represents the relative resistance change (AR/Ro)
response curve of d-rGO@PU sponge piezoresistive sensor against
progressively increasing strain. It represents the response curve under different applied strain that varies from 20% to 100%. The resistance change against different values of applied strain was measured and shown here for achieving more insight on the effects of the progressively increasing strain. With up to 48% (2.4 KPa) of applied strain, a very slight increment, i.e. about 3.63% occurs in the magnitude of relative resistance change [AR/RJ, which drops down linearly until 80% of strain and finally has a smooth fall until it reaches 100% of strain value.
The response curves of d-rGO@PU sponge piezoresistive sensor in Figures 6(a) and 6(b) can be used for observing tiny motions such as Slood pulse, voice recognition, air blowing and so on. Similarly, the signal intensity at high strain values is very large with noise-free signal output. This allows it's use for large-scale human movement, such as joint bending.

[0069] Figure 7 represents the reproducibility of d-r60@PU sponge
piezoresistive sensor evaluated by using cyclic tests under loading-unloading at 48% strain for 5000 cycles. The response signal output reflects a slight fall
(0.2 to 0.3 kΩ) in the resistance signal prior to the saturation level due to a slight decrease in the conductivity. However, no further drop i/ resistance signal was observed after reaching the saturation level. The excellent reproducibility is attributed to the compression-resilience properties of the d-
rGO@PU sponge piezoresistive sensor
[0070] The d-rGO@PU sponge piezoresistive sensor has been used for different applications, which are broadly classified as small-scale motion monitoring e.g. blood pulse response sensor through the wrist and neck pulses, throat response while speaking different words, cheek response with
blowing etc. and also as large-scale motion monitoring sensors; e.g. finger response under different bending condition.
Accordingly, the capabilities of this flexible d-rGO@PU sponge piezoresistive pressure sensor were evaluated for monitoring some of the small-scale
human activities.
[0071] Figure 8(a) represents the real-time response (Current v/s Time) of
d-rGO@PU sponge piezoresistive pressure sensor for the small-scale motion monitoring of blood pulse detection through the wrist affixed via PDMS band.
[0072] Figure 8(b) represents the real-time response (Current v/s Time) of
d-rGO@PU sponge piezoresistive pressure sensor for the small-scale motion monitoring of the cheek at normal state as well as while blowing with air.
[0073] The current patterns recorded in Figures 8(a) and 8(b) are almost the same as those produced during the measurement. The reason fcr the difference in the current scale could be interpreted as the device was attached using bandage during the neck pulse response (Fig. 8(b)), while a PDMS band was used for pulse measurement through wrist (Fig. 8(a)). Therefore,

slight tightening of the wrist band may lead to the increase in current, but the overall response of the pulse in both the cases is quite similar. The insets in Fig. 8 (a) and 8(b) represent almost similar number of peaks (7 to 8 peaks)
associated with the time-dependent current signals within a time window of 6
secs (20 to 26 sec). This implies that the wrist as well as neck pulses are measuring the similar number of pulses, i.e. 80 beats per minute, which is obvious as both these places reflect the similar pulse response. These results
confirm that d-rGO@PU sponge piezoresistive pressure sensor has the capability to detect the difference in the blood pulses (BP), exhibiting it as a potential candidate for diagnostic applications.
[0074] Figure 8(c) represents the real-time response (Current v/s Time) of
d-rGO@PU piezoresistive sensor for the small-scale motion monitoring of the neck attached by a bandage. Here, d-rGQ@PU sponge piezoresistive pressure sensor was attached to the neck using bandage to observe the muscle motion near the throat. Initially, the base-run study was made by receiving the muscle response without uttering any word, which has some response features indicating the response to very small muscular motions, For demonstrating the capability of d-rGO@PU sponge piezoresistive pressure sensor for monitoring respiration through the lungs via air blowing of the cheeks, the fabricated d-rGO@PU sponge piezoresistive pressure sensor was attached to the cheek using bandage and was allowed to undergo periodic blowing of air.
The current signal response shows a significant drop in current (from 0.95 to 0.81 mA) with the unique signature repeating throughout the time domain of 40 sees for a normal person. In case, the person is suffering from the respiratory issues, there will be change in the features of the curve that may
assist in the diagnosis at the initial stage.
[0075] Figure 8(d) represents the real-time response (Current v/s Time) of
d-rGO@PU sponge piezoresistive pressure sensor for the small-scale motion monitoring for voice detection through the throat while pronouncing the word "Stop", in which the real-time curve shows a sharp peak repeatedly.

[0076] Figure 8(e) represents the real-time response (Current v/s Time) of
d-rGO@PU sponge piezoresistive pressure sensor for the small-scale motion
monitonng for voice detection through the throat while pronouncing the word
"Sponge".
[0077] Figure 8(f) represents the real-time response (Current v/s Time) of d-rGO@PU sponge piezoresistive pressure sensor for the small-scale motion monitoring for voice detection through the throat white pronouncing the word "Morning",
[0078] Figure 8(g) represents the real-time response (Current v/s Time) of
d-rGO@PU sponge piezoresistive pressure sensor for the small-scale motion monitoring for voice detection through the throat while pronouncing the word
"Good Morning".
Further, the current patterns recorded in Figures 8(f) and 8(g) show a considerable change in the signature on pronouncing different words, since the vocal muscle undergo different kind of motions during pronouncing. For instance, i.e. while switching from speaking 'Morning' (Figure 8(f)) to 'Good Morning' (Figure 8(g)), an increment of the sharp kink is apparent in the peak feature i.e. from two to three keeping rest of the current signal characteristics unchanged. Moreover, the current-time response of the throat muscle in Figures 8(d) to 8(g) shows almost the similar kind of current patterns while pronouncing the same words repeatedly (morning, Good Morning, Stop), thereby indicating the stability of this fabricated d-rGO@PU sponge piezoresistive pressure sensor. Thus, the significant changes among these current response curves with different motions of throat muscles makes this sensor suitable to be used as a word recognition device. Besides the small-scale motion detection, d-rGO@PU sponge piezoresistive pressure sensor are qualified enough to identify large-scale movement monitoring, benefiting from its capability of high compressibility as well as its piezoresistive performance under high compressive strains. This d-rGO@PU sponge piezoresistive pressure sensor was mounted at the middle joint of the index

finger by using a bandage and the current singals were collected during the

alternating sequence of bending and releasing.
[0079] Figure 9 (a) represents d-rGO@PU sponge piezoresistive pressure
sensor attached on the finger joints and finger is inclined at an angle of 15°
This figure also includes the response Current-Time (l-T) curve corresponding to this angular position (bending),
[0080] Figure 9(b) represents d-rGO@PU sponge piezoresistive pressure
sensor attached on the finger joints and finger is inclined at an angle of 45°. The figure also includes the response Current-Time (l-T) curve corresponding
to this angular position (bending),
[0081] Figure 9(c) represents d-rGO@PU sponge piezoresistive pressure sensor attached on the finger joints and finger is inclined at an angle of 60° The figure also includes the response Current-Time (l-T) curve corresponding to this angular position (bending),
[0082] Figure 9(d) represents d-rGO@PU sponge piezoresistive pressure
sensor attached on the finger joints and finger is inclined at an angle of 90°.
The figure also includes the response Current-Time (l-T) curve corresponding to this angular position (bending).
Accordingly, the digital images of the finger inclined at different angular positions (bending), i.e. at 15°, 45°, 60° and 90° are shown in Figures 9(a) to 9(d) and their corresponding current signal responses at different orientations
are reported for the index finger. It can be seen from these curves that the d-
rGO@PU sponge piezoresistive pressure sensor generated different responsive signals associated to different finger moments.
On moving from lower angle (15°) to higher one (90°), the signature as well as the magnitude of the current showed significant changes, i.e. 0.04 mA (15°C), 0.2 mA (45°C), 0.67 mA (60°C) and 0.8 mA (90°C) respectively. Therefore,
larger the inclination range, the higher would be the peak intensity.

This could be interpreted as the small angular motion causing mild
compression led to the disconnection of microcrack, while the larger angular
moment resulted in larger area interaction of the flakes wrapped on the PU backbone.
Thus, it is experimentally confirmed that d-rGO@PU sponge piezoresistive pressure sensors can be applied to distinguish different orientation of the finger movements as well.
EXPERIMENTAL SECTION
A) Materials details
Graphite flakes were supplied by the Talga Resources Ltd, via Talga Advanced Materials GmbH and commercially available PU foams were purchased from Sigma Aldrich, whereas sodium nitrate, concentrated Sulfuric acid (98% pure), potassium permagnate, and hydrogen monohydrat© (35% aqueous) were purchased from Merck. The additive material was brought from BYK company (in Germany) and MWNTs from NANOGYL® NC7000™ series.
B) Synthesis of GO and Reduced Graphene Oxide (rGO)
Graphene oxide (GO) was initially synthesized according to the modified Hummers method. 20 ml of the prepared GO solution of the concentration 0.5 mg/mL was then kept for ultrasonication continuously for 45 min in order to obtain highly dispersed GO solution. Thereafter, hydrazine mononydrate and a noble additive composed of benzisothiazoline and methylisothiazolinone was added, wherein hydrazine serves as a reducing agent, while the additive helps in inhibiting the agglomeration of rGO graphene flakes. The prepared solution was then kept in vacuum oven at 90°C for 24 hrs for obtaining the highly dispersive reduced graphene oxide (rGO) ink. The conversion of light yellow color of GO solution to deep black

color after reduction confirms the formation of rGO. The prepared rGO ink
was then allowed to cool naturally and was further used for making sensors,

C) Synthesis of functionnalized MWCNTs and MWCNTs doped rGO ink
1.5 g of MWNTs was mixed with 1:10 ratio of surfactant and hydrolysed siloxane (30% solid contents), followed by the addition of 50 mL of DI water. Since, MWNTs exist in the form of agglomerates in this solution, the uniformly dispersed solution of MWNTs was obtained by applying high shear forces (using Ross Mixture model HSM 100 LSI) at 8000 rpm for 8 hrs. The high shear forces assist in overcoming the Van der Waals attraction between different CNTs and in unzipping MWCNTs providing electron rich sites at the edges to form a chemical bond with hydrolyzed siloxane. Thereafter, the mixture was ultra-sonicated for 1 hr, followed by centrifugation at 12,000 rpm for 30 minutes and the supernatant with well-dispersed MWCNTs was collected.
In order to obtain ink made of MWCNTs doped rGO solution, different volumes (20-140 μL) of highly exfoliated MWNTs were added to 1 mL of rGO (0,5 mg/mL) solution in order to increase the conductivity of rGO solution. The mixture was kept for 8hrs by stirring at room temperature, followed by 30 minutes of ultrasonication. The films were prepared by using this ink on glass substrates and their electrical conductivity was measured. The ink with 100 ut of MWNTs in rGO solution provided the highest electrical conductivity, and was thus employed for sensor fabrication.
D) Fabrication of piezoresistive pressure, sensor
The commercially available polyurethane (PU) foams was sliced into the rectangular form with dimensions of 1.3 cm x 1 cm and thickness of 0.5 cm. It was washed by using I PA (thrice) and dried in the vacuum at 60°C for 8 hrs. Thereafter, foams were dipped in the prepared ink and kept for degassing. The foam with the soaked ink was mildly pressed in order to remove the extra

liquid content from it. It was. then dried in vacuum oven at 70°C for 10 hrs and
the process was repeated three times, To fabricate the piezoresistive sensor,
ITO coated PET sheets of similar dimensions were employed as electrodes (top and bottom), which were adhered to the two circular faces of the PU foam using silver paste to eliminate the contact resistance and to achieve stable
signals output. The complete fabrication process of sensor is schematically outlined in Figure 2.
E) Materials Characterizations
The high-resolution transmission Electron Microscopy (HRTEM) images of the ink along with their Selected Area Electron Diffraction (SAED) images were performed using JEOL JEM-2100 F. The Field Emission Gun-Scanning Electron Microscopy (FEG-SEM) images were obtained through JSM-7600F with a resolution of 1.0 nm (15 kV) using an accelerating voltage ranging from 0.1 to 30 kV using focused electron beam. Raman measurements were performed with a Lab RAM HR 800 micro-Raman spectroscopy using the 532nm line of an Argon (Ar+) laser as the excitation source. An integration time of 100 sec was used and the spectra were averaged over 2 accumulations. The incident laser beam exposed on the sample was focused on the sample surface through a 100x objective lens with a laser spot diameter of approximately 2 urn. The scanning range for all the samples for Raman spectra was 1000-3000 cm-1. Samples for X-ray photoelectron spectroscopic characterization were prepared on Si/SiO2 substrates from dispersions using the drop cast method. The samples were analysed using a ULVAC-PHl PHI5000 Versa Probell XPS system with an Al Ka X-ray source. The survey scan was acquired using a constant pass-energy of 187.5 eV, and the detailed scans were acquired using constant pass energy of 58.7 eV.
F) Sensor. Characterization
The electro-mechanical characterization of sensors was p. rformed by simultaneously using Universal testing machine (Mark 10 model ESM303)
and source measure unit (KEYSIGHT model B2902A). ESM303 is a highly

configurable single-column force tester for tension and compression
measurement applications up to 300 IbF [1,5 kN] and available in speed range
from 0.5 to 1100 mm/mins. Here, the device is placed on the Chuck attached
at the bottom end and pressure is applied in a normal direction through the top electrode. It was placed on a vibration-less platform in order to discard
any external effect.
G) Results and Analysis
In order to confirm the formation of rGO flakes and to observe the distribution of the MWNTs on rGO flakes, high resolution transmission electron microscopy (HRTEM) was performed at various locations of the sample. Figure 3 (a-c) represent HRTEM images of pure rGO ink collected in dark field mode along with their selected area electron diffraction pattern (SAED). It is observed that there were wrinkles between the rGO flakes and the edges were folded [Figure 3(a)].The average size of the rGO flake is estimated to be more than half a micrometer which is quite big to enable favorable percolation of charges. The diffraction pattern of rGO in Figure 3(b) shows twelve bright spots appearing due to coupling of two hexagon patterns, suggesting the presence of highly exfoliated few layer rGO. On the other hand, Figure 3(c) confirms the presence of single hexagon with six bright spots. Such patterns were observed at several positions of the sample which confirms that the agglomoration of the reduced films was restricted because of water based additive. The results also point out that the prepared rGO ink holds crystallinity in its chemical structure. Fig 3(d-f) shows the HRTEM images of exfoliated MWNTs doped rGO ink, which clearly shows that MWNTs are decorated on the rGO flakes in random fashion connecting adjacents tubes as well as flakes. Few dark circular spots varying in diameters were also observed which may be due to the agglomeration of rGO/MWNT or duo to the insufficient drying of the sample. SAED pattern demonstrates concentric rings instead of bright spots, which suggests that the crystalinity has disappered on doping MWNTs in rGO ink. This can happen due to the interconnection of flakes and nanotubes, as well as by unzipping of MWNTs. This is confirmed

through Figure 3(f) where one can see the exfoliation and unzipping Of
MWNTs (shown by a circle) leading to increase in the number of layers'.
In order to confirm the deposition of rGO flakes on the skeleton of PU foams, FEGSEM images of the PU foam in its pure form (Figure 4(a)) and dipping it once (Figure 4(b)), twice (Figure 4(c)) and thrice (Figure 4(d)) in the ink were collected at three different magnifications. In the first three images [Figure 4(a)], the neatly washed PU sponge can be observed along with slight impression of the carbon tape (at the right top-corner) which can be distinguished by using color contrast The thickness of the skeleton forming the foam is estimated to be approximately 100μm.
On dipping the sponge in the ink first time, the flakes and MWNTs having significant affinity for the PU foam, get attached to its backbone, which is as shown in Figure 4(d). On repeatly dipping PU foams in the ink, the rGO flakes try to wrap the uncovered region of PU skeleton and the already deposited flakes. The wrapping also leads to increasing the thickness of the skeleton of PU foams with each consecutive dipping, which can be observed from Fig 4(a).
In summary, a very simple, cheaper and one-step technique is developed according to the present invention for fabricating a flexible and versatile pressure sensor using MWNTs doped rGO ink for human-machine interfacing applications. The challenge of the agglomeration of rGO flakes was resolved via water-based additive to obtain a highly dispersive rGO ink that can be employed directly for the sensor fabrication. Incorporation of functionalized MWNTs assist in percolation of charges among rGO flakes with the improved conductivity. The sensing mechanism includes the combination of the macro and micro crack junctions as well as conductive backbones of PU foams wrapped with doped rGO flakes within d-rGO@PU sponge pie .oresistive pressure sensor with capabilities to monitor small-scale as well as large-scale motions. It has an excellent reproducibility over 5000 cycles. Numerous human activities such as pronouncing words, blood pulse, cheek blowing, joint

bending could be efficiently monitored in real-time by using this versatile
Piezoresistive pressure sensor.
Thus, a simple, efficient and cheaper method is developed, whereby rGO ink is directly incorporated for fabricating piezoresistive sensors to be employed in various biomedical applications. TECHNICAL ADVANTAGES & ECONOMIC SIGNIFICANCE OF THE INVENTION
The d-rGO@PU sponge piezoresistive pressure sensor developed'' in accordance with the present invention has the following advantages:
• Flexible, low-cost sponge piezoresistive pressure sensor for diverse applications, such as those involving human-machine interfaces, e.g. in biomedical applications etc.
• Sensor is based on microcrack as well as macrocrack-configurations.
• Uses multi-walled nanotubes (MWNTs) doped rGO ink (with hydrophilic
nature) in Polyurethane foam.
• Versatile d-rGO@PU sponge piezoresistive pressure sensor can
detect both small-scale and large-scale motion monitoring.
• Sensor is useful for applications involving human-machine interfaces,
e.g. biomedical applications.
• Uses a simple and direct fabrication technique.
. Involves a single step process for fabricating flexible piezoresistive pressure sensors suitable for various biomedical applications.
Throughout this specification, the word "comprise", or variations such as "comprises" or "comprising", shall be understood to implies including a described element, integer or method step, or group of elements, integers or method steps, however, does not imply excluding any other element, integer or step, or group of elements, integers or method steps.

The exemaplary embodiments described in this specification are intended
merely to provide an understanding of various manners in which this
embodiment may be used and to further enable the skilled person in the relevant art to practice this invention. The description provided herein is purely
by way of example and illustration.
Although the embodiments presented in this disclosure have been described in terms of its preferred embodiments, the skilled person in the art would readily recognize that these embodiments can bo applied with modifications possible within the spirit and scope of the present invention as described in this specification by making innumerable changes, variations, modifications, alterations and/or integrations in terms of materials and method used to configure, manufacture and assemble various constituents, components, subassemblies and assemblies, in terms of their size, shapes, orientations and interrelationships without departing from the scope and sprit of the present invention,
While considerable emphasis has been placed on the specific features of the preferred embodiment described here, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiments without departing from the principles of the invention.
These and other changes in the preferred embodiment of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive mater is to bs interpreted merely as illustrative of the invention and not as a limitation.

WE CLAIM;
1. Aprocess for fabricationof piezoresistive pressure sensor for real time health-monitoring
applications.The sensor fabrication comprises of two steps (i) synthesis of hydrophilic d-rGO ink and (ii) employing the as-prepared ink for the fabrication of the sensor using dip and dry technique.
2. The materials required for synthesis of ink as mentioned in claim 1 are MWNTs and graphite flakes [0015], [0016], [0017].
3. The base material as mentioned in claim 1 is Polyurethane (PU) foam. Other foams [0011] are used based on desired properties (light: weight, high flexibility [0013], pore size, Young's modulus, elasticity, etc.). However, the sensing mechanism based on macro and microcracks [0063], [0064], contact area [0065]with the skeleton of ink coated foam are samesame in each case.
4. The sensor properties are enhanced by introducing dopants such as nanoparticles (Au, Ag, etc), nanowire (Ag), silver ink, and conductive polymeric ink.
5. The solvent used for the synthesis as in claim 1 for the rGO and MWNTs is hydrophilic solvents, preferably DI (Deionized) water. Other solvents can also be utilized.
6. As mentioned in claiml, the sensor comprises of PU foam (base material), d-MWNT-rGO ink (probe materials), ITO coated PET substrate (to make it flexible), Ag paste (for firm connection and reduce the damping losses due to contact issue), conducting wires (as an electrodes for collecting output signals)makes it economically viable.
7. The sensor [0042]can have any geometry but in the present case [0063], [0064], [0065] a rectangular geometry is chosen.
8. The present invention as a piezoresistive pressure sensor as mentioned in claim I can be converted in the form of a complete product by packaging it in the mould. This is prepared using elastomers (PDMS, ECOFLEX, etc.) to maintain its flexible and electrical properties.
9. The as-prepared piezoresistive sensor as claimed in 1 has been successfully demonstrated
as an application for the health-monitoring, both small-scale motione.g. blood pulse
response sensor through the wrist and neck pulses, throat response while speaking

different words cheek response with blowing etc. and also as large-scale motion monitoring sensors, including finger response under different bending condition [0070],
10. The sensor as mentioned in claim 1 can be employed for using detecting the health of the
mechanical motors such as gear-box, different pumps and any such application whereit is
possible to detect the difference in the vibrations or audio signals originating from the new mechanical machine with respect to old one,
11. As mentioned in claim 1, that the device is quite compact, sensor can be implemented to monitor the signals at any time and any place without the need of bulky equipments in order to do the real time monitoring.
12. The sensor as claimed in 1 has a detection mechanism of the d-rGO@PU foam is based on the piezoresistive pressure sensor which implies the relative change in the electrical response (Resistance vs Time) as a function of applied pressure. The method for recording health-monitoring signals are done in the following sequence:
The sensor is biased with a certain amount of voltage. The signal associated to this voltage is known as base signal or base run. "Now, the device is permitted to undergo some sort of perturbation which may small-scale or large-scale motion which leads to change in the thickness of the sensor (compression of foam) and thus, affect the electrical signals. In case, the subject perform certain process periodically such as blood-pulse response, breathing, etc, the output signals of the sensor will response accordingly and synchronizes with the subject. Thus, the received signal will closely inform the present status of the subject. Therefore, this simple principle along with the involvement of the macro and microcracks has been declared as a governing mechanism for monitoring different kind of motions.