DESC:FIELD
The present disclosure relates to flexible heaters and a process for manufacturing thereof.
BACKGROUND
The background information herein below relates to the present disclosure but is not necessarily prior art.
Flexible heaters have attracted attention due to diverse application areas. One of the advantages of flexible electronics is that it complements the conventional electronics rather than replacing. Its flexibility gives the advantage that it can be used almost anywhere. Due to its special features, flexible electronics is being used in potential applications like flexible displays, flexible LEDs, flexible heaters, artificial skins, sensors like piezo sensors, biosensors and in sustainable energy. However, one of the disadvantages of the conventional flexible heaters is that the conventional flexible heaters are costly and require high technicality.
Further, portability of the conventional flexible heaters is a challenge as the traditional/conventional heaters made up of alloys having heavy weight, high rigidity, and low heating efficiency. Alloys of Fe and/or Cr are very rigid and low heating efficiency whereas the Indium-Tin-Oxide (ITO) based heater are costlier because of limited availability of Indium.
Recently, metal nanostructures are a potential component used in flexible heaters as they can be operated with low driving voltage due to better adhesion property and low sheet resistance. For fabricating flexible heater, different nanostructured materials have been used, however high flexibility and stability are still a matter of concern.
Still further, graphene and its oxides are used for making flexible heaters. However, they suffer from large sheet resistance problem and require high input voltage when high temperature is needed. Inorganic nanowires can also be used in fabrication of flexible heaters.
Therefore, there if felt a need for providing flexible heaters which mitigates the aforestated drawbacks.
OBJECTS
Some of the objects of the present disclosure, which at least one embodiment herein satisfies, are as follows:
It is an object of the present disclosure to ameliorate one or more problems of the prior art or to at least provide a useful alternative.
An object of the present disclosure is to provide flexible heaters.
Another object of the present disclosure is to provide a process for manufacturing flexible heaters.
Another object of the present disclosure is to provide flexible heaters that are economic.
Still another object of the present disclosure is to provide flexible heaters that are portable and can operate at low voltage.
Other objects and advantages of the present disclosure will be more apparent from the following description, which is not intended to limit the scope of the present disclosure.
SUMMARY
The present disclosure relates to flexible heaters. The flexible heater comprises at least one flexible substrate, at least two electrically conductive interdigitated electrodes screen printed on the flexible substrate, an electrically conductive thermally active layer screen printed on the interdigitated electrodes to form a grid structure by connecting interdigitated electrodes and thermally active layer, and a power source configured to facilitate flow of current through the electrodes and consequently through the thermally active layer to generate heat. The flexible heater of the present disclosure is adapted to provide heat at a temperature in the range of 35 oC to 40 oC at DC voltage in the range of 1V to 5V.
The present disclosure further relates to a process for manufacturing of flexible heaters. The process comprises obtaining a flexible substrate. The substrate is screen printed with at least two electrodes on the substrate by using silver nanoparticle ink to obtain a screen printed interdigitated electrodes on the substrate. The screen printed interdigitated electrodes on the substrate are electrically conductive. The screen printed interdigitated electrodes on the substrate are cured to obtain cured screen printed interdigitated electrodes on the substrate. An electrically conductive thermally active layer is screen printed on the cured screen printed interdigitated electrodes on the substrate, followed by curing to obtain a flexible heater. The thermally active layer comprises carbon ink. The carbon ink used in the present disclosure comprises a carbon paste and a first polymer solution.
BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWING
The flexible heater of the present disclosure will now be described with the help of the accompanying drawing, in which:
FIG. 1A illustrates a design of interdigitated electrodes of the flexible heater in accordance with the present disclosure;
FIG. 1B illustrates a design of carbon patch that can be placed over the interdigitated electrode of the flexible heater in accordance with the present disclosure;
FIG. 1C illustrates a design where interdigitated electrode is printed on the PET substrate of the flexible heater in accordance with the present disclosure;
FIG. 1D illustrates a design of the heater fabricated using screen printing on PET film in accordance with the present disclosure;
FIG. 1E and FIG. 1F illustrates a thermogram images of the flexible heater in accordance with the present disclosure;
FIG. 2 illustrates a graphical representation of the comparison of the resistance versus area for C60 and C80 ink;
FIG. 3 illustrates a graphical representation of the heater temperature profile with circuit module;
FIG. 4 illustrates a schematic diagram of a foldable low cost neonatal incubator containing the flexible heater in accordance with one embodiment of the present disclosure;
FIG. 5 illustrates a diagram of baby packs for rural and urban needs containing the flexible heater in accordance with one embodiment of the present disclosure; and
FIG. 6 illustrates a diagram of thermal jackets installed with flexible heater, health tracking sensors and control units in accordance with one embodiment of the present disclosure.

DETAILED DESCRIPTION
Embodiments, of the present disclosure, will now be described with reference to the accompanying drawing.
Embodiments are provided so as to thoroughly and fully convey the scope of the present disclosure to the person skilled in the art. Numerous details are set forth, relating to specific components, and methods, to provide a complete understanding of embodiments of the present disclosure. It will be apparent to the person skilled in the art that the details provided in the embodiments should not be construed to limit the scope of the present disclosure. In some embodiments, well-known processes, well-known apparatus structures, and well-known techniques are not described in detail.
The terminology used, in the present disclosure, is only for the purpose of explaining a particular embodiment and such terminology shall not be considered to limit the scope of the present disclosure. As used in the present disclosure, the forms "a,” "an," and "the" may be intended to include the plural forms as well, unless the context clearly suggests otherwise. The terms "comprises," "comprising," “including,” and “having,” are open ended transitional phrases and therefore specify the presence of stated features, integers, steps, operations, elements, modules, units and/or components, but do not forbid the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The particular order of steps disclosed in the method and process of the present disclosure is not to be construed as necessarily requiring their performance as described or illustrated. It is also to be understood that additional or alternative steps may be employed.
The terms first, second, third, etc., should not be construed to limit the scope of the present disclosure as the aforementioned terms may be only used to distinguish one element, component, region, layer or section from another component, region, layer or section. Terms such as first, second, third etc., when used herein do not imply a specific sequence or order unless clearly suggested by the present disclosure.
It is known in the art that the efficiency of the electro-thermal heater is mainly determined by the convective heat loss. Convective heat transfer/loss involves the combined processes of unknown conduction (heat diffusion) and advection (heat transfer by bulk fluid flow).
The present disclosure, therefore, envisages a flexible heater which is fabricated on the substrate by using a thermally active layer (ink composition) using screen printing techniques.
In an aspect, the present disclosure relates to flexible heaters. The flexible heater comprises at least one flexible substrate, at least two electrically conductive interdigitated electrodes screen printed on the flexible substrate, an electrically conductive thermally active layer screen printed on the interdigitated electrodes to form a grid structure by connecting interdigitated electrodes and thermally active layer, and a power source configured to facilitate flow of current through the electrodes and consequently through the thermally active layer to generate heat. The flexible heater of the present disclosure is adapted to provide heat at a temperature in the range of 35 oC to 40 oC at DC voltage in the range of 1V to 5V.
In accordance with the present disclosure, the substrate is selected from the group consisting of polyethylene terephthalate (PET), polypropylene copolymer (PPCP), polypropylene homopolymer (PPHP), Nylon, and fabric with high weave density. In one embodiment, the substrate is polyethylene terephthalate (PET) film.
The thickness of the substrate can be in the range of 100 microns to 150 microns. In one embodiment, the thickness of the substrate is 125 microns. The substrate used in the flexible heaters provides a mechanical base for the testing of the thermally active layer and later used for the fabrication of the flexible heater.
The interdigitated electrodes are obtained by screen printing silver nanoparticle ink on the substrate. The average particle size of the silver nanoparticles used in the ink can be in the range of 3 to 4 microns.
In accordance with the present disclosure, the thermally active layer comprises a carbon ink. The carbon ink comprises a carbon paste and a solution of a first polymer. Typically the solution of the first polymer is transparent. The first polymer is selected from the group consisting of polyvinyl pyrrolidone (PVP), polyvinyl butyral (PVB) and polyvinyl acetate (PVAc).
The ratio of the carbon paste to the first polymer can be in the range of 1:1 to 5:1. Typically, the ratio of the carbon paste to the first polymer is 1:2.
The carbon ink used in the thermally active layer of the present disclosure is prepared by mixing the carbon paste and transparent solution of the first polymer at a temperature in the range of 50- 80 oC for a time period in the range of 45 to 75 min. Typically, the mixing is carried out at 60 oC for 60 mins.
In accordance with the present disclosure, the carbon paste comprises carbon flakes, at least one second polymer, at least one solvent, and at least one surfactant. In the present disclosure, the formation of conductive pathways inside the polymer matrix is due to the formation of infinite clusters. In the carbon paste, carbon flakes can be used as filler and the second polymer can be used as a binder. The conductivity of carbon-polymer paste varies with respect to the amount of the filler.
The ratio of the second polymer to the solvent can be in the range of 1:2 to 1:6. Typically, the ratio of the second polymer to the solvent is 1:4.
Carbon flakes which are the main constituent of the carbon paste are commercially available and have been purchased from Sigma Aldrich. The particle size of the carbon flakes can be in the range of 0.5-10 µm. Typically, the particle size of the carbon flakes can be in the range of 1-4 µm.
The second polymer used in the preparation of the carbon paste is selected from the group consisting of polyvinyl pyrrolidone (PVP), polyvinyl butyral (PVB), and polyvinyl acetate (PVAc).
The solvent can be selected from the group consisting methanol, ethanol, propanol, and ethers. Typically, the solvent is ethanol.
The surfactant used in the preparation of carbon paste can be selected from sodium dodecyl sulphate, and hydrophilic surfactant. In an embodiment, the hydrophilic surfactant can be as phospholipid polyethylene glycol amine, 4-(5-Dodecyl) benzene sulphonate. In an exemplary embodiment, surfactant is sodium dodecyl sulphate used in the carbon paste for better dispersion of the carbon flakes.
In an embodiment, the carbon paste is prepared by mixing the carbon flakes, at least one second polymer, at least one solvent and at least one surfactant at a temperature in the range of 50-100 oC for a time period in the range of 60 minutes to 150 minutes to obtain the carbon paste.
The amount of carbon flakes used in the preparation of the carbon paste can be in the range of 60-75 wt%. Typically, the amount of carbon flakes is 65 wt%.
The amount of the second polymer used in the preparation of the carbon paste can be in the range of 5-30 wt%. Typically, the amount of the second polymer is 25 wt%.
The amount of surfactant used in the preparation of carbon paste can be in the range of 5-15 wt%. Typically, the amount of surfactant is 10 wt%.
In accordance with an embodiment of the present disclosure, the thermally active layer having desired properties can be prepared by mixing the carbon paste and PVAc/ethanol solution at different weight ratios under magnetic stirring for 60 min at 60 oC, to obtain ink compositions, such as C80, C60, C40, and the like.
In an exemplary embodiment, C80 ink composition comprises 80% of carbon paste and 20% of PVAc/ethanol solution, C60 ink composition comprises 60% of carbon paste and 40% of PVAc/ethanol solution, and C40 ink composition comprises 40% of carbon paste and 40% of PVAc/ethanol solution.
The effect of patch area can be demonstrated by printing the thermally active layer of various sizes and studying the temperature profile. In an embodiment, patches of C80 ink of different area are printed and its temperature profile is measured for 20V supply. It can be clearly seen from Fig 2 that the patch of area-4 (0.21cm2) has most suitable temperature profiles for heating applications in a range. Because of less connecting paths for current to flow in small areas compared to large area, high amount of current flow through them resulting in higher overall temperature output.
After studying the C80, C60 and C40 ink compositions, C60 and C40 have less amount of carbon paste (conducting material), hence less connected paths for current to flow resulting in less temperature output and high resistance.
C80 ink composition has a comparatively large amount of carbon paste present therein (having more conduction material), C80 ink composition is used for designing the flexible heaters.
The thermally active layer of the present disclosure is carbon polymer based inks that are prepared using different carbon concentration. Viscosity measurements are performed on these inks and it is observed that the ink composition/thermally active layer of the present disclosure is compatible for screen printing. The ink composition with various carbon contents is characterized by Raman spectroscopy and SEM imaging.
In another aspect of the present disclosure, there is provided a process for manufacturing flexible heaters comprising conductive interdigitated electrode and carbon patches. The process is explained in detail herein below.
A process for manufacturing flexible heaters comprises obtaining a flexible substrate. At least two electrodes are screen printed on the substrate by using silver nanoparticle ink to obtain a screen printed interdigitated electrodes on the substrate. The screen printed interdigitated electrodes on the substrate are cured to obtain a cured screen printed interdigitated electrodes on the substrate. A thermally active layer is screen printed on the cured screen printed interdigitated electrodes on the substrate followed by curing to obtain flexible heaters. The thermally active layer comprises carbon ink.

Patches of different area can be screen printed and change of resistance with change in area is measured. The output temperature at constant voltage is found to be different for different area patches. Ink and patch size with highest temperature deliverance at low power consumption is selected for further development of the heater.
In one embodiment, the flexible heater consists of printing of two layers on the substrate, such as PET. Firstly, interdigitated electrodes are printed with nanoparticle ink (such as silver nanoparticle ink) following by curing; and secondly the so obtained nanoparticle ink cured substrate is printed with the carbon ink composition of the present disclosure, especially C80, ink composition connecting the interdigitated electrodes are as shown in Figure 1A. In one embodiment, the patches are prepared by printing two layers. Firstly, interdigitated electrodes are screen printed on PET film using conductive silver nanoparticle ink and secondly a thermally active layer is printed on the electrodes. The thermally active layer consists of carbon-polymer ink patch printed in an array between the interdigitated electrodes.
In one embodiment, silver nanoparticles ink is used in the manufacturing of interdigitate electrodes and carbon ink in the manufacturing of the functional part of the heater.
In another embodiment, the patches made of screen of 40 mesh with 48 microns diameter are used so that both carbon and silver nanoparticles can easily pass through it. In an embodiment, for preparing conductive interdigitated electrode, ink of silver (Ag) nanoparticles with an average size of 3-4 microns can be used. Polyethylene terephthalate film of thickness of 125 microns was used as a substrate to provide mechanical base for the testing of ink and later used for the fabrication of heater.
FIG. 1B illustrates a design of carbon patch that can be placed over the interdigitated electrode of the flexible heater in accordance with the present disclosure; and FIG. 1C illustrates a design where interdigitated electrode printed on the PET substrate of the flexible heater in accordance with the present disclosure.
A grid like patches of C80 ink is printed as the functional layer connecting the electrodes as illustrated in FIG. 1D. The C80 ink patches are connected in parallel to each other which can be used to generate heat when DC voltage of 5-10 volts is applied. Grids like patches are used to achieve heat in the whole surface. The inter electrode distance can be maintained at 3 mm to 8 mm, each carbon patch is having a diameter of 7 mm and the distance between two patches of carbon is 12.50 mm. Size of the heater is 100 X 90 mm2. In an embodiment, the inter electrode distance can be 5 mm.
PET based substrate helps to allow the heat to be transferred in the surface area. The patches are cured at 140 oC for 6 minutes in hot air oven. FIG. 1E and FIG. 1F illustrate thermogram images of the grid patches fabricated using screen printing on PET film in accordance with the present disclosure.
In accordance with the present disclosure, the curing of the screen printed interdigitated electrodes carried out at a temperature in the range of 100 oC to 150 oC and the screen printed thermally active layer is also cured at a temperature in the range of 100 oC to 150 oC. In an embodiment the curing temperature is in the range of 120 oC to 130 oC. In an exemplary embodiment, the curing temperature of screen printed interdigitated electrodes is carried out at 130 oC and the curing of screen printed thermally active layer is carried out at 140 oC.
Temperature profiling was done by measuring temperature of the heater with respect to time at different voltages (Figure 3). Figure 3 illustrates a graphical representation of the heater temperature profile with circuit module. Temperature vs time measurement is performed at different supply voltages. It is evident from Figure 3 that change in the temperature is very high in the first 50 sec, and after that temperature/time is almost zero and reaches a steady state. It is also observed that when the supply power increases, steady state temperature also increases. Increase in steady state temperature of the heater with increase in applied voltage is observed. Bend test is performed by measuring temperature at various bend angles. Changes in temperature with various angles are found to be in window of 6-8 oC and decreases with time. This heater was able to provide 37 oC (human body temperature at DC voltage below 5V. Therefore, the heater of the present disclosure is an ideal component for health care applications, operating at low voltages which can be used in thermal jackets and in portable, controlled environment, such as baby-incubators.
The present disclosure provides the development of positive temperature coefficient (PTC) ink made up of carbon paste with thermoplastic polymer. The ink is used for the fabrication of low cost, thin sheet flexible heater on a PET substrate for wearable electronics.
In one embodiment, the flexible heater of the present disclosure is used in making a foldable low cost neonatal incubator which is light weight and portable. The body of the foldable neonatal incubator mainly consists of plastic integrated with printed circuitry for hybrid electronics and other components at the rigid ends such as UV light inlets for sterilizing incoming air, signal transmitter for remote controlling and monitoring system and battery is attached to main foldable part consisting of flexible heater as illustrated in Figure 4.
Figure 5 illustrates baby packs for rural and urban needs and Figure 6 illustrates thermal jackets installed with heaters, health tracking sensors and control units in accordance with the present disclosure.
The foregoing description of the embodiments has been provided for purposes of illustration and not intended to limit the scope of the present disclosure. Individual components of a particular embodiment are generally not limited to that particular embodiment, but, are interchangeable. Such variations are not to be regarded as a departure from the present disclosure, and all such modifications are considered to be within the scope of the present disclosure.
The present disclosure is further described in light of the following experiments which are set forth for illustration purpose only and not to be construed for limiting the scope of the disclosure. The following experiments can be scaled up to industrial/commercial scale and the results obtained can be extrapolated to industrial scale.
EXPERIMENTAL DETAILS
Experiment-I: Preparation of carbon ink in accordance with the present disclosure:
Carbon ink was prepared by using a carbon paste developed in stock.
The carbon paste was prepared by blending 6.5 gm of carbon flakes having particle size 2-5 micron (which was purchased from Sigma Aldrich) in 0.5 gm of polyvinyl butyral solution and 2 gm of polyvinyl pyrrolidone solution with 1gm of sodium dodecyl sulphate. The so obtained mixture was heated to 80 oC under stirring for 2 hours to obtain a carbon paste.
Polyvinyl butyral solution was prepared by dissolving 2 gm of polyvinyl butyral in 8 gm of ethanol.
Polyvinyl pyrrolidone solution was prepared by dissolving 2 gm of polyvinyl pyrrolidone in 8 gm of ethanol.
Polyvinyl acetate solution was prepared by dissolving 2 gm of polyvinyl acetate in 8 gm of ethanol.
Carbon paste and transparent polyvinyl acetate solution were stirred for 1 hour at heating of 60 oC under stirring to obtain carbon ink of predetermined ratios as given below i.e. C80 and C60.
C80 ink was prepared by blending 80 wt% of carbon paste and 20 wt% of PVAc/Ethanol solution.
C60 ink was prepared by blending 60 wt% of carbon paste and 40 wt% of PVAc/Ethanol solution.
Experiment 2: preparation of Flexible heaters in accordance with the present disclosure:
Polyethylene terephthalate film of thickness of 125 microns was used as a substrate to provide mechanical base for screen printing of carbon ink and later used for the fabrication of heater.
For conductive interdigitated electrode, ink of silver nanoparticle having an average size of 3-4 microns were screen printed on the PET film substate to obtain screen printed interdigitated electrode. FIG. 1C illustrates a design where interdigitated electrode printed on the PET substrate of the flexible heater in accordance with the present disclosure.
FIG. 1B illustrates a design of carbon patch that can be placed over the interdigitated electrode of the flexible heater in accordance with the present disclosure. A grid like patches of C80 ink was printed as the functional layer connecting the electrodes as illustrated in FIG. 1D. Each carbon patch having a diameter of 7 mm and the distance between two patches of carbon is 12.50 mm was printed on the PET film substrate. The inter electrode distance was 5 mm. The C80 ink patches were connected in parallel to each other and generated heat when DC voltage of `6.5 volts was applied.
The patches were cured at 140 oC for 6 minutes in hot air oven. FIG. 1E and FIG. 1F illustrate thermogram images of the grid patches fabricated using screen printing on PET film in accordance with the present disclosure.
TECHNICAL ADVANCEMENTS
The present disclosure described herein above has several technical advantages including, but not limited to, the realization of flexible heaters that:
? are light weight and hence portable;
? are user friendly;
? are low cost;
? operate at low voltage; and
? are flexible.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the invention to achieve one or more of the desired objects or results. While certain embodiments of the inventions have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Variations or modifications to the formulation of this invention, within the scope of the invention, may occur to those skilled in the art upon reviewing the disclosure herein. Such variations or modifications are well within the spirit of this invention.
While considerable emphasis has been placed herein on the specific features of the preferred embodiment, it will be appreciated that many additional features can be added and that many changes can be made in the preferred embodiment without departing from the principles of the disclosure. These and other changes in the preferred embodiment of the disclosure will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the disclosure and not as a limitation.
,CLAIMS:WE CLAIM:
1. A flexible heater comprising:
i. at least one flexible substrate;
ii. at least two electrically conductive interdigitated electrodes screen printed on said flexible substrate;
iii. an electrically conductive thermally active layer screen printed on said interdigitated electrodes forming a grid structure, said thermally active layer being electrically connected to said interdigitated electrodes; and
iv. a power source configured to facilitate flow of current through said interdigitated electrodes and consequently through said thermally active layer to generate heat.
2. The flexible heater as claimed in claim 1, wherein said flexible substrate is selected from the group consisting of polyethylene terephthalate (PET), polypropylene copolymer (PPCP), polypropylene homopolymer (PPHP), Nylon and fabric with high weave density.
3. The flexible heater as claimed in claim 1, wherein said interdigitated electrodes are screen printed on said substrate by using silver nanoparticle ink, wherein the average particle size of the said silver nanoparticles is in the range of 3-4 microns.
4. The flexible heater as claimed in claim 1, wherein said electrically conductive thermally active layer is screen printed by using carbon ink, wherein said carbon ink comprises a carbon paste and a solution of a first polymer.
5. The flexible heater as claimed in claim 4, wherein the ratio of said carbon paste to said first polymer is in the range of 1:1 to 5:1.
6. The flexible heater as claimed in claim 4, wherein said carbon ink comprises carbon flakes, at least one second polymer, at least one solvent, and at least one surfactant.
7. The flexible heater as claimed in claim 6, wherein said first polymer and said second polymer is independently selected from polyvinyl pyrrolidone (PVP), polyvinyl butyral (PVB), and polyvinyl acetate (PVAc).
8. The flexible heater as claimed in claim 6, wherein the particle size of the said carbon flakes is in the range of 0.5µm to 10 µm.
9. The flexible heater as claimed in claim 6, wherein the ratio of said carbon flakes to said polymers is in the range of 10:90 to 90:10.
10. The flexible heater as claimed in claim 6, wherein said surfactant is selected from sodium dodecyl sulphate, phospholipid polyethylene glycol amine, and 4-(5-Dodecyl) benzene sulphonate.
11. The flexible heater as claimed in claim 6, wherein said solvent is selected from the group consisting of methanol, ethanol, propanol, and ethers.
12. The flexible heater as claimed in claim 1, wherein said flexible heater is adapted to provide heat at a temperature in the range of 35 oC to 40 oC at DC voltage in the range of 1V to 5V.

13. A process for manufacturing flexible heaters, said process comprising the following steps:
i. obtaining a flexible substrate;
ii. screen printing at least two electrodes on said substrate by using silver nanoparticle ink to obtain screen printed interdigitated electrodes on said substrate;
iii. curing said screen printed interdigitated electrodes on said substrate to obtain cured screen printed interdigitated electrodes on said substrate;
iv. screen printing thermally active layer on said cured screen printed interdigitated electrodes on said substrate to obtain screen printed thermally active layer on said cured screen printed interdigitated electrodes on said substrate; and
v. curing said thermally active layer screen printed on said cured screen printed interdigitated electrodes on said substrate to obtain flexible heaters, wherein said thermally active layer comprises carbon ink.
14. The process as claimed in claim 13, wherein said curing of said screen printed interdigitated electrodes in step (iii) and said curing of said screen printed thermally active layer in step (v) is carried out at a temperature in the range of 100 oC to 150 oC.

15. The process as claimed in claim 13, wherein said thermally active layer is a grid like patch connected to said electrodes.