Now, a team of researchers in Saudi Arabia says that it has developed a new type of wirelessly controlled thermopatch made from flexible copper-based thin films that could be a real alternative to such throwaway patches. As well as coming in handy for sports strains and everyday sprains, the films (which cost just $4 a piece) might also provide relief for patients suffering from arthritis.

Portable devices that can be worn by patients have come a long way in the last few years thanks to important advances in flexible electronics. For example, researchers have already developed temporary tattoo-like patches that can mould themselves to the soft structure of skin and be used to monitor its temperature. Other novel biocompatible applications such as electronic circuits that interface with internal organs like the heart and brain are also being developed.

A team led by Muhammad Hussain of the King Abdullah University of Science
and Technology (KAUST) in Thuwal, Saudi Arabia, has now made a stretchable thin-film, battery-operated, copper-based thermopatch that can be directly applied to the skin. The patch can be wrapped around a finger or wrist, for example, and can even be stretched to cover an elbow or knee. The temperature of the patch can be controlled wirelessly thanks to integrated circuits that can communicate remotely with mobile gadgets (via Bluetooth, for instance).

Patch is reusable

"You might be able to set the temperature of the patch using a dedicated application on your mobile phone and then fall asleep, allowing the device to do its job," explains Hussain. "During that time, the patch not only continues to provide heat but also monitors the temperature of the area being treated and calibrates the temperature as needed. The user can also set a timer to turn off the patch when required."

Since it is made of metal, the patch can be reused indefinitely. And, because copper is already widely used as the interconnect material in electronics circuits – thanks, in part, to its exceptional thermal conductivity (the reason why Hussain's team in fact used it in this work) – the patch can be made using CMOS-compatible deposition and patterning techniques. This means that it is cheap and easy to manufacture.

"Horseshoe" design

Being a metal, copper cannot be stretched, but Hussain and colleagues overcame this problem by arranging thin pieces of the element into spring-like structures that they then used to connect heat pads in the device (see figure). The researchers used electrochemically deposited copper on silicon oxide thermally grown on bulk silicon and patterned it in an array of heating unit islands interconnected in horseshoe patterns. Next they used a technique called reactive ion etching to release the silicon oxide/copper films from the underlying silicon and then transferred the released structure onto a polymeric material. The finished films can be stretched by around 800%, which is a record.

"Such extreme stretching in a metal was a critical and fundamental achievement in our experiments," Hussain told our sister site nanotechweb.org. "It is our horseshoe design (albeit not new) that absorbs deformation strains and allows the metal to become ultra-stretchable."

Copper is also prone to oxidation, but we fixed this drawback by encapsulating it in a transparent conductor, he adds.

The work, which was presented as one of the finalists in the Falling Walls competition held in Berlin last month, is published in Advanced Healthcare Materials doi: 10.1002/adhm.201400647.

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