Smart thermal patch for adaptive thermotherapy

A smart thermal patch for adaptive thermotherapy is provided. In an embodiment, the patch can be a stretchable, non-polymeric, conductive thin film flexible and non-invasive body integrated mobile thermal heater with wireless control capabilities that can be used to provide adaptive thermotherapy. The patch can be geometrically and spatially tunable on various pain locations. Adaptability allows the amount of heating to be tuned based on the temperature of the treated portion.

BACKGROUND

In the United States, 50 million individuals, including 300,000 children, suffer from arthritis. As treatment, they commonly use thermotherapy. Commercially available chemical-based pain relief patches come in different sizes based on the pain locations, have limited heating ranges, are non-reusable and effective for limited hours with a shorter overall lifetime, are prone to side-effects (skin irritation, allergy), and are not suitable for children. Laser heating can also be used, however in the impoverished parts of the world both of these are expensive and mostly unavailable.

SUMMARY

Web integrated flexible and stretchable electronics for thermotherapy can offer affordable advanced healthcare, for example for Arthritis, pain-strain-sprain, cancer cell destruction and many more. We provide a smart thermal patch therefor. In an embodiment, the patch can be a stretchable, non-polymeric, conductive thin film flexible and non-invasive body integrated patch. It can include conductive material for thermal heating. It can be a skin contour integrated spatially tunable mobile thermal patch. It can include wireless controllability, adaptability (for example that tunes the amount of heat based on the temperature of the body location), reusability, and/or affordability due to low-cost complementary metal oxide semiconductor (CMOS) compatible integration. A lithographically patterned mechanical design can be included to absorb the deformation strain in the conductive thin film while retaining high conductivity. It can be stretched and contracted back to its original form, allowing its usage as a geometrically and spatially tunable thermal patch on various pain locations. Web and battery integration can be included to make it a completely autonomous-mobile low-cost (for example, $1-2) smart electronic system, with precise temperature control using smartphones or mobile gadgets.

In an embodiment a thermal patch is provided, comprising: an array of heating pads; and a plurality of stretchable conductors interconnecting each of the array of heating pads with adjacent heating pads. In any one or more aspects the array of heating pads can be interconnected between a plurality of contact pads. The plurality of contact pads can be connected to adjacent heating pads by stretchable conductors. The thermal patch can include or be connected to a battery. The thermal patch can include or be connected to a flexible microcontroller. The thermal patch can include or be connected to a wireless transceiver configured to communicate with a mobile computing device. The wireless transceiver can be a Bluetooth transceiver. The mobile computing device can be a smart phone.

In an embodiment, a method is provided, comprising: a) forming a mask on a polymer layer, the mask defining a thermal patch; b) etching the polymer layer; c) depositing a conductive material to form stretchable conductors of the thermal patch; and d) vapor phase etching to release the thermal patch. In any one or more aspects, the mask can be an aluminum mask. The polymer layer can be a polyimide (PI) layer. The conductive material can be a metal, preferably copper, nickel, chromium, tin, silver, platinum or a metal alloy. The method can include wet etching to remove the mask prior to depositing the conductive material. The method can include depositing a seed layer for depositing the conductive material. The vapor phase etching can be XeF2vapor phase etching. In any one or more of the various embodiments the stretchable conductor(s) can have a lateral spring design. The design of the conductors can make them behave hyperelastically allowing the conductor(s) to stretch under applied strain and return to their generally unstretched shape when the strain is released.

DETAILED DESCRIPTION

Disclosed herein are various examples related to a smart thermal patch for adaptive thermotherapy. Reference will now be made in detail to the description of the embodiments as illustrated in the drawings, wherein like reference numbers indicate like parts throughout the several views.

As an effective alternative to chemical-based pain relief patches and laser heating, a wirelessly controllable heater can be used for the application of heat on specific points on the skin or thermotherapy. The use of thermotherapy has been proven useful for treating various serious diseases like arthritis, cancer, etc. The use of thin film-based thermal heaters on the human body has been restricted due to their natural rigidity and limited stretchability. Most material systems in use in electronics are not inherently stretchable. In particular, copper lines are commonly used as interconnects in state-of-the-art electronics. Since copper has a yield strain of 20-25%, the use of copper interconnects in stretchable electronics is restricted. Compatibility with large deformations can be provided by web integrated flexible and stretchable electronic devices that retain their electrical and thermal properties upon application of large strains (>100%).

In an embodiment, the patch can be a stretchable, non-polymeric, conductive thin film flexible and non-invasive body integrated mobile thermal heater with wireless control capabilities that can be used to provide adaptive thermotherapy. The patch can be geometrically and spatially tunable on various pain locations. Adaptability allows the amount of heating to be tuned based on the temperature of the treated portion.

In one or more aspects, the conductive thin film can be a metallic thin film. As an example, low-cost complementary metal oxide semiconductor (CMOS) compatible integration can facilitate reusability and affordability of the device. Compared to prior demonstrations on stretchable electronics using mostly polymer or composite-based material systems involving 1D nanowires or 2D graphene for stretchable interconnects, electrodes, integrated circuits, light emitting diodes, super capacitors, artificial skins, and others, the smart thermal design enables the thin film's continued usage as a tunable-sized thermal heater by using design features to absorb the deformation strain in thin films with no impact on their low resistance. In an aspect the metallic thin film can be a copper (Cu) based thin film.

Based on the pain location and spatial requirements, the spatially tunable mobile thermal heater can be stretched to satisfy the user's needs and contracted back to its original form. The metallic nature of the film allows it to be used over a longer lifetime and in a reusable manner. Additionally, integration of web technology (such as advanced Bluetooth technology) and a battery can make it an autonomous mobile smart electronic system, with precise temperature control using a smartphone or other mobile interface device. The lithographically patterned mechanical design absorbs the deformation strain in the Cu (or other types of metallic) and conductive thin films while retaining their high conductivity, allowing the device to be stretched and contracted back to its original form. The geometrically and spatially tunable thermal patch can non-invasively conform to the skin contour at various pain locations. The availability of flexible and smart thermo-electronic systems with stretching capabilities enables their daily usage for thermotherapy by the global population, including patients, who are suffering from arthritis or more sophisticated cancerous tumor cell destruction, and individuals such as athletes or soldiers, who are experiencing body and/or muscle pains, strains, and sprains. As an example, the device can be used for thermotherapy (hyperthermia) for cancerous cell destruction. Moreover, we can use dissolvable conducting materials (examples include: tungsten, aluminum, molybdenum, etc.). After the treatment, the device can then be completely dissolved inside the body to eliminate any further surgery requirement to retract the device.

In one or more aspects, copper can be used as the conducting element since copper is used in state-of-the-art CMOS technology for metal interconnects, and is thus CMOS process compatible. Since copper is inherently non-stretchable, stretchability has been introduced by using a lateral spring design. In some implementations, other conductive materials, including conductive metals (e.g., nickel, chromium, tin, silver, platinum, or other metals or alloys) can also be used to form the metal interconnects.

Referring toFIG.1, shown is an example of a thermal patch design100. The design can be scaled using the scaling parameter λ to obtain devices of different dimensions. To demonstrate the scalability of the design, two versions of the design were fabricated by scaling the design parameter λ to 100 μm and 200 μm. As illustrated inFIG.1, thermal patch devices100aand100bwere fabricated and characterized with λ=100 μm and λ=200 μm, respectively. The thermal patch devices100can include one or more arrays or matrices of heating pads103interconnected between a plurality of contact pads106using stretchable conductors109.

The conductors109can be formed having a spring design, for example a lateral spring design. They can be coiled in a dimension that allows for stretching or flexing of the conductors. A non-limiting example is depicted inFIG.1wherein the conductor109is coiled, having a generally figure “8” configuration to provide a lateral spring design. One skilled in the art will recognize that other shapes can be used to provide a lateral spring design. For example the conductor109can be coiled as shown in either the upper or lower portion depicted inFIG.1in a generally circular or oval shape. A coiled design can allow the conductor(s)109to behave as a spring. The spring or coiled design can make the conductor(s)109behave hyperelastically, as described in more detail below.

The heating pads103can be squares of size202. Copper lines112(or other types of metallic lines) on the heating pads103are placed so as to maximize the length of the conductor, and hence the resistance, of the conductor. A polyimide (PI) pad115has holes of 100 μm diameter, separated by 200 μm (center-to-center). Other soft polymers with characteristics similar to PI can also be used for pad115. The scale bar118is 1 mm. The contact pads106are 2 mm×20 mm in both cases. The total length (Lt) of the curved spring structure of the stretchable conductors109is 78.35λ, while the lateral length (Ll) of the spring is only 102. When the spring is stretched to its maximum capacity, the lateral length of the spring is approximately equal its total length. Hence, the stretchable lateral spring provides a maximum uniaxial stretchability of about 800% (Lt/Ll=7.835) for the individual stretchable conductors109. As shown inFIG.1, the stretchable conductors109are used to provide flexibility between heating pads103in two directions.

To demonstrate an application of the stretchable copper conductors109, stretchable thermal patches100are configured with stretchable conductors109connecting adjacent heating pads103to provide flexibility in two directions. The heating pads103do not contribute to the stretching, and have a constant lateral length of 202, along with interconnects of length52. This 52 length on both sides of the spring increases the lateral length before and after stretching, hence the stretchability at the device level is given by:

LtLl=(7⁢8.3⁢5+2*(2⁢02)+5+5)⁢λ(2*(2⁢02)+5+5+1⁢0)⁢λ=2.7⁢1
Thus, with the heating pads included, the total stretchability of the thermal patch system becomes about 270%.

Referring next toFIG.2, shown is an example of a CMOS compatible process to directly fabricate the thermal patch100using metal (e.g., copper) lines112on a flexible (e.g., a polymer such as polyimide) surface making it a transfer-less process. Beginning with a silicon (Si) wafer203, thermal oxidation is used form a silicon dioxide (SiO2) layer206. Plasma-enhanced chemical vapor deposition (PECVD) is then used to form a thin amorphous silicon (α-silicon) sacrificial layer209. A 4 μm thick polyimide (PI) layer212can then be spin-coated on the thermally oxidized wafer with the 1 μm thick amorphous silicon sacrificial layer209. In some implementations, other polymers having characteristics similar to PI can be used to form the layer212. The PI212is patterned into the lateral spring design using deposition and patterning of an aluminum hard mask215and O2plasma etching. Wet etching can be used to remove the aluminum mask215. A seed layer218for copper electroplating is deposited on the PI212, and 4 μm thick copper lines221are subsequently electroplated. The seed layer218is etched away using argon plasma and the thermal patch devices100are released using, e.g., XeF2based vapor phase etching of the amorphous silicon sacrificial layer209. The thicknesses of the copper lines221and the PI layer212can be engineered to be the same, so that the neutral axis during bending is at the copper/PI interface. The PI pad115includes holes of 100 μm diameter, separated by 200 μm (center-to-center), to reduce the time required for XeF2gas phase release. Hence, the interface of the two materials is under no stress even during flexing.

To fabricate working thermal patch devices100shown inFIG.3, the starting point was a thermally oxidized (300 nm), 4″ silicon (100) substrate203. A 1 μm thick layer of amorphous silicon was deposited on the substrate203using PECVD (e.g., SiH4, Ar plasma at 250° C. for 25 minutes) as a sacrificial layer209. The wafer was then spun with polyimide (e.g., HD MircoSystems PI2611) at 4000 rpm for 60 seconds to obtain a 4 μm thick coating212. The polyimide (PI) layer212was cured subsequently at 90° C. for 90 seconds, at 150° C. for 90 seconds, and at 350° C. for 30 minutes. A 200 nm aluminum layer was sputtered (25 sccm Ar plasma at 10 mTorr, 500 W DC power) on the wafer to act as hard mask215for the PI etching. The aluminum thin film was patterned using contact lithography and etched using Gravure Aluminum wet etchant (Technic France). The PI layer212was then etched using O2plasma at 60° C. and 800 mTorr, for 16 minutes. A Cr/Au (20/200 nm) bilayer was deposited on the wafer to act as a seed layer218for copper electroplating. A Cr/Cu bilayer can also be used as a seed layer218to reduce cost in batch fabrication. The wafer was spin-coated with photoresist, and the areas to be covered with copper were exposed by developing the photoresist. A 4 μm thick copper layer221was electroplated using CuSO4solution as the electrolyte and 0.698 Ampere current for 200 seconds. The photoresist was washed away using acetone and the seed layer218was etched using Ar (30 sccm) plasma for 3 minutes. The wafer was subjected to 60 cycles (30 seconds each) of XeF2etching (e.g., Xactix X3C) at 4 Torr pressure to release the thermal patch100.

FIG.3includes images (a) through (k) of fabricated thermal patches100. All scale bars303are 2 cm. Optical images of the thermal patches100after release are as shown in images (a) and (b) ofFIG.3for A=100 μm and A=200 μm, respectively. The thermal patch100ashown in image (a) includes two arrays of heating pads103between three contact pads106. The application of the thermal patch100aof image (a) on human skin, with 200% lateral strain, is shown in image (c). The application of the thermal patch100bof image (b) on human skin in various locations is shown in images (d) through (h) ofFIG.3. In case of image (d), the thermal patch100bis under no strain, while in image (e) the thermal patch100bis under 150% uniaxial strain. Image (f) ofFIG.3shows the thermal patch100bunder biaxial strain, with both lateral and transverse strain being 150%.

The flexibility of the fabricated thermal patch100bwhen wrapped around various bodily features is illustrated in images (g) through (i) ofFIG.3, with a bending radius as low as 0.5 mm. Image (g) shows the thermal patch100bconformally bent around an elbow joint with a bending radius of 6.3 cm. Image (h) shows the thermal patch100bwrapped around two fingers with a bending radius 0.96 cm. Image (i) shows the thermal patch100bwrapped around a silicon wafer306with a bending radius about 0.5 mm. The van der Waals force enables conformal placement on skin micro-irregularities. Images (j) and (k) ofFIG.3compare an off-the-shelf medical patch309(WellPatch™ Capsaicin Pain Relief Patch) to the thermal patch100aof image (a). In image (k), the thermal patch100ais shown with a 200% strain.

The mechanical performance of the stretchable conductors109under uniaxial tensile strain is summarized inFIGS.4A-4B and5A-5Bfor A=200 μm and A=100 μm, respectively. As previously discussed, a maximum stretchability of approximately 800% is possible for the individual springs. This translates into an overall maximum stretchability of the device of approximately 300%. However, it was observed that this maximum point was not reversible. Rather, the elastic limit for the springs was determined to be about 600%.FIGS.4A and5Aare examples of plots of spring elongation versus applied force in the lateral direction (data set403for λ=200 μm and data set503for λ=100 μm), and resistance as a function of elongation for the first cycle (data set406for λ=200 μm and data set506for λ=100 μm), for a stretchable conductor109. The force versus elongation plots403and503obtained for the springs closely resembles hyperelastic, rubber-like materials. The yield point is marked with an “x”. The inset plots are the spring elongation versus applied force within the elastic limit. The stretchable conductors109returned back to their original state after 10 cycles of stretching up to 600%. Thus, the lateral spring design makes copper thin films behave hyperelastically.

For the stretchable conductors109, the resistance data sets406and506of the thermal patches100were almost invariant with strain—variation of only 0.6% within the elastic limit. The consistency in the resistance of the springs with the applied strain may be attributed to the design of the lateral spring system. The applied strain was absorbed in the deformation of the spring design, hence the copper interconnect was, at no point, under strain. Hence, the resistance of the metal lines (and the complete thermal patch100) remained unchanged throughout the experiment. Further, the slight variation in resistance shown inFIGS.4A and5Awas only for the first stretching cycle. The resistance became constant after the first cycle of elongation and remained constant after several cycles of stretching. In particular, after 10 cycles, there was no change in the resistance of the springs with strain. This may be attributed to, with the first few cycles, the springs undergoing a slight reorientation of their metal grains to accommodate for the twisting in the lateral spring. Once the metal grains “settle”, the resistance of the springs and the thermal patch100becomes invariant with respect to strain.

FIGS.4B and5Bshow images of stretchable conductors109with λ=200 μm and λ=100 μm, respectively, during elongation. The top images409and509show the spring before the beginning of elongation cycles. Images412and512are scanning electron micrographs (SEM) for the released spring. The middle images415and515show the spring completely stretched with about 800% spring elongation. Images418and518are SEMs of portions for the springs under tension with a strain of 200%. It can be seen that the lateral spring twists at certain points to absorb the strain energy. The bottom images421and521show the spring after 10 elongations cycles of 600% elongation within the elastic limit of the spring. ForFIG.4B, scale bar424ais 2 cm and scale bars424bare 50 μm. ForFIG.5B, scale bar524ais 4 mm and scale bars524bare 50 μm.

Heating capability and effective operational time can be limited for commercially available thermotherapy.FIGS.6A-6C and7A-7Cillustrate the thermal performance assessment of the thermal patch100designs for λ=100 μm and λ=200 μm, respectively. As shown inFIG.1, the heating pads103of the thermal patches100were contacted by soldering copper wires of the stretchable conductors109to the 2 mm×20 mm contact pads106. The total parasitic resistance introduced into the thermal patch device100due to the contact pads106was measured to be 0.05 Ohm (or about 0.6% of the total device resistance). During the evaluation, the thermal patch100was energized using a constant voltage source (e.g., an Agilent E3631A power supply) and the temperature of the thermal patch device100was measured using an Optotherm Mirco thermal imaging system. To measure the average temperature of the glass substrate for a given applied voltage, a square area was defined with an area equal to four times the size of the heating pad103as a unit. The mean temperature of this unit area was plotted against voltage to obtain the thermal characteristics of the heating pad under thermal load (glass substrate).

FIGS.6A and7Ashow examples of the temperature of the thermal patch100versus applied voltage (data sets603for λ=100 μm and data sets703for λ=200 μm) and the power consumed for the applied voltage (data set606for λ=100 μm and data set706for λ=200 μm). The maximum temperature for the thermal patch in air and on a glass substrate (with load) was plotted (data sets603a/603band703a/703b, respectively) for an applied voltage. Image (i) ofFIG.3shows a thermal patch100b(FIG.1) wrapped around a silicon wafer306. The mean temperature data603cand703ccorresponds to the mean of the temperature readings of the unit area defined as a square four times the heating pad103area.

As can be seen inFIGS.6A and7A, the thermal patches100achieved higher temperatures while in ambient air, as compared to the glass substrate load, for the same applied voltage. This is expected as air offers only convective cooling of the heating pad103(FIG.1), while glass substrate offers convection through air (top portion) as well as conduction through the glass substrate, and has a higher thermal capacity. In the case ofFIG.6λ=100 μm), a maximum temperature of about 80° C. was measured for an applied voltage of 1.6 V with power consumption of 1.5 W. The temperature range with the glass substrate as thermal load is obtained as 25-53° C. In the case ofFIG.7λ=200 μm), a maximum temperature of 102° C. is recorded for 3.8 V applied voltage with a 1.4 W power consumption. The temperature range in case with glass substrate as thermal load is obtained as 25-66° C.

Referring toFIGS.6B and7B, shown are plots of the temporal response of the heater temperature for a given applied voltage. The glass substrate was also seen to heat up gradually to a certain temperature for a specific applied voltage. The power was switched on after the indicated “Power On” time. InFIG.6B, the applied voltage for the thermal patch100awith λ=100 μm was varied from about 1 V to about 1.6 V in steps of about 0.1 V (as indicated by the arrow). InFIG.7B, the applied voltage for the thermal patch100bwith λ=200 μm was varied from about 2.6 V to about 4 V in steps of about 0.2 V (as indicated by the arrow).FIGS.6C and7Care plots of temperature of the thermal patches100for various applied voltages for thermal patch designs100with =100 μm and λ=200 μm, respectively. Voltages of 0.5 V (top left), 1.0 V (bottom left), 1.5 V (top right) and 2.0 V (bottom right) were applied inFIG.6Cand 1 V (top left), 2 V (bottom left), 3 V (top right) and 4 V (bottom right) were applied inFIG.7C. The scale bars603and703are 2 mm.

The thermal patches100were designed such that the widths of the copper lines112on the heating pads103(50 μm) were half and quarter of the copper lines on the stretchable conductors109for λ=100 μm and λ=200 μm, respectively. Also, the copper lines112on the heating pads103were designed so as to maximize their length. Hence, most of the resistance of the thermal patch devices100was concentrated across the heating pads103. This maximized the amount of power dissipated on the heating pad103, and hence maximized the heating being applied. With this design, the total resistance of the thermal patches100was measured to be 8.85 Ohm, including the contact metal resistance. The ratios of the resistance of the heating pad103and the stretchable conductors109were calculated to be 3.35 and 15.5 for λ=100 μm and λ=200 μm, respectively. The high difference in resistance ratios can be mainly attributed to the lower width of copper lines (λ/4 in case of λ=200 μm as compared to λ/2 in case of λ=100 μm), and because of the larger length of the heating lines (6.84 cm in case of =200 μm as compared to 14.8 cm in case of λ=100 μm). Hence, for a given current, the ratio of power dissipated on the heating pads to the total power supplied is calculated to be 0.92 and 0.69 for the λ=200 μm and λ=100 μm designs, respectively. Thus, in terms of heat dissipation on the heating pads, the design with λ=200 μm was found to be more efficient.

The thermal patch with λ=200 μm was also tested on a consenting, adult human subject (in compliance with Institutional Bioethics Policy). The thermal patch100bwas taped on the hand of the subject using double sided scotch tape. The thermal patch100bwas powered using a constant voltage DC power supply, and the temperatures of the pad and the skin were measured. Referring toFIG.8A, shown is a plot of the maximum and mean temperatures attained (curves803and806, respectively) versus applied voltage, at 60 seconds after application of the voltage. The mean temperature806was calculated for the entire area of the thermal patch100b, just after switching off the power supply. It was found that the thermal patch103beffectively heated the human skin up to several degrees over the normal temperature.FIG.8Bplots the temporal response of the skin temperature for given applied voltages of 1 V, 2 V, 2.5V and 2.75 V. The power was switched on after the indicated “Power On” time.

Further, the heating effect was not only under the heating pads103, but had also extended to the entire region of the thermal patch100b. This was observed by measuring the mean temperature of the skin just after the power to the thermal patch100bwas switched off.FIG.8Cshows examples of temperature of the skin for various heat application conditions. The initial temperature conditions for the skin and thermal device is show at the top left. The temperature changes caused by applying 1 V after 60 seconds is shown at the bottom left and caused by applying 2.75 V after 60 seconds is shown at the top right. The temperature just after switching the power off, having applied 2.75 V for 60 seconds, is shown at the bottom right. The scale bar809is 2 cm. Thus, it was observed that in real application conditions, and under a high strain of 150%, the thermal patch100bcan bring about uniform temperature increment on the human skin.

The stretchable and flexible thermal patches100have several applications in the biomedical industry. A thermal patch100can be stretched up to 3 times its original size and can be applied to any part of the human body, and may be reused thereafter, for thermotherapy. In many real life applications, a wired constant voltage power supply is not available for use, and can be impractical to carry around for thermotherapy. Hence, a practical thermal patch system can be wireless to be portable and easily usable. Also, as an additional function, the thermal patch100should be easily controllable using a readily available device such as, e.g., a smart phone or tablet. To this effect, a thermal patch100that is wirelessly controllable using Bluetooth enabled Android-based smart phones was examined. The wireless connectivity was achieved using an open source hardware module (Arduino Uno) along with a Seeedstudio Bluetooth shield. The voltage applied to the thermal patch100was controlled using a PWM output from one of the outputs of the Arduino system. The thermal patches100can also be used in other applications where heating in a defined area is desired. The flexibility of the thermal patches100allows them to be positioned on or around non-uniform surfaces and to be molded to fit the area. For example, a thermal patch100may be placed around a pipe to apply heating to correct or avoid freezing of fluid in the pipe. The heating temperature of the thermal patch100may be controlled to, e.g., avoid damage to the heated component, control heating variations over time, or maintain a constant temperature.

Referring toFIGS.9A and9B, shown are images of a thermal patch100being controlled wirelessly, using an Android smart phone. These images illustrate the control of temperature of the thermal patch100using the smart phone. The Android-based temperature control system was also tested with human subjects. Using the off-the-shelf Arduino board and its Bluetooth shield in the thermal patch system can make the thermal patch heavy and immobile, which may restrict the full potential of its use as a generic autonomous portable thermotherapy solution. However, using trench-protect-peel-release based transformational silicon electronics to make a flexible microcontroller similar to the one used in the Arduino board can overcome this limitation. Thus, a complete system level solution can be obtained.

Portability of the thermal patch100may also be limited by the supply of power from a constant voltage source. In the previous examples, the maximum power drawn by the thermal patch was about 1.5 W. Thus, the thermal patch100can be supported by a commercially available coin battery (e.g., a Panasonic CR2477 with capacity of 1000 mAh), for a period of 2 hours, at maximum operating temperature. The battery may also be flexible and stretchable and can be recharged to make the thermal patch100reusable. Control of the thermal patch100shown inFIGS.9A and9Butilizes an open loop control system, wherein the thermal patch100and the control software have been calibrated beforehand. In some cases, the control mechanism can lead to inaccuracies in the temperature control of the thermal patch100. To overcome this problem, the thermal patch100can use itself as a temperature sensor.

Since the thermal patch100employs copper lines for heating, and the resistance of copper increases with increase in temperature, the resistance of the thermal heating device100increases with raising temperature. The resistance can sensed based on the current consumed by the thermal patch100in the PWM mode of operation. For example, during testing the temperature response of the resistance of the thermal patch device100was tested using a thermal chuck probe station set-up (e.g., Cascade Microsystems M150). The thermal chuck was set at a particular temperature (for 5 minutes at every temperature to achieve steady state), and a small sensing current was applied to the thermal patch100to measure the resistance of the thermal patch device100without heating it more than the thermal chuck temperature.

FIG.9Cshows a plot of the variation of resistance of the thermal patch100with change in temperature. The error bars indicate the maximum and minimum values of the measured resistance (curve903) and the chuck temperature (curve906). As shown inFIG.9C, the thermal patch100can be used as a temperature sensor with a non-linearity of 1.49% in the temperature response of the resistance903. Further, the sensitivity of the temperature sensor is reported to be 0.0308 Ohm/° C. The temperature co-efficient of resistance (a) for copper was determined to be 0.00397° C.−1. Therefore, the current levels in the thermal patch100can be used to sense the temperature of the thermal patch100. This temperature feedback can be used to implement a closed loop control system such that the temperature control of the thermal patch100is accurate, which allows the whole system to be adaptable.

FIG.9Dis an image of an example of a thermal patch design100with a flexible silicon microcontroller for wireless temperature control and a coin battery as the power supply. The scale bar909is 2 cm. Cost calculations indicate that such an autonomous system can be within about $1-$2, which is a cost effective solution compared to many other status-quo solutions or products. A small flexible silicon piece can be used to house microprocessors and other communication devices, while a coin battery can provide power to the electronics as well as the thermal patch100for several hours of operation. Finally, integration of logic processors and memory can add functionality for constantly monitoring a patient, storing the data locally, communicating the in-situ processed data to another computing device or a cloud computing platform, which enabling big data analysis.

The current disclosure describes various examples of thermal patches100with wireless control capabilities. To overcome limited stretching capabilities in widely used low-cost metallic thin film copper (Cu), a lithographically patterned mechanical design was used to desorb the deformation strain allowing 800% stretchability while maintaining its high conductivity. A geometrically and spatially tunable, readily usable, affordable thermal patch100for thermotherapy was engineered using the flexible spring design. The resulting thermal patch100is usable at various locations on human body by providing conformal attachment to irregular skin contours and irregular sizes and shapes of inflamed areas. The thermal contact areas, which can be used as a temperature sensor, allows the patch to adapt to the inflamed area's condition by adjusting the therapy based on the measured temperature of the inflamed area. Wireless interface and battery integration make the system an autonomous, portable and adaptable unit with precise temperature control using smart phones or mobile device.