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Electronic Materials

Self-healing device could use body heat to power wearable electronics

Novel design gives a super-stretchy, powerful thermoelectric generator

by Prachi Patel, special to C&EN
February 18, 2021 | A version of this story appeared in Volume 99, Issue 6

A photo of a hand wearing a ring that looks like a yellow band with reddish strips that look like a rolodex on it.
Credit: Science Advances
A new stretchable thermoelectric generator can wrap around the finger to convert body heat into electricity. The device consists of several soft chips printed with thermoelectric alloys and inserted vertically like fins into an elastic, self-repairing polyimine substrate.

Recharging the batteries in activity trackers, wearable heart monitors, and smart watches can be annoying. Thermoelectric generators (TEGs), which convert heat into electricity, could be a way to power such wearable devices continuously by using the person’s own body heat. With a clever combination of materials and design, researchers have now made a highly stretchable thermoelectric device that produces enough power for wearables. It could be worn as a ring or armband (Science Advances 2021, DOI: 10.1126/sciadv.abe0586). And unlike previously reported generators, this one can heal itself if damaged and is easy to recycle.

TEGs based on bismuth telluride alloys and lead telluride are already found in spacecraft and are being developed to harvest waste heat from engines and industrial pipes. In these devices, a temperature difference across two different thermoelectric materials creates an electric current—a larger temperature difference produces more power. For scavenging body heat, researchers have made flexible devices using thermoelectric films and nanowires.

But to conform well to the body, TEGs need to be very stretchable, says Jianliang Xiao, a mechanical engineer at the University of Colorado Boulder. So he and his colleagues came up with an architecture in which vertical thermoelectric chips are arranged like fins on a substrate made of polyimine, a stretchable polymer known to be self-healing. The vertical design allows for a large power output by creating a large temperature difference between the tops of the chips, which are exposed to air, and bottoms, which touch the skin.

The chips are soft polyimide pieces patterned with bismuth telluride alloy thin films. The researchers insert the chips into laser-cut slots in the polyimine substrate, which they first print with electrodes of gallium-indium alloys to connect the chips. These highly conductive alloys behave like liquids at room temperature, so they can stretch and flex. The researchers apply a small amount of polyimine solution to encapsulate the liquid metal wires.

The team reports a 1 cm2 device that can be worn on the finger like a ring. Its power output is comparable to previous flexible TEGs, but unlike its predecessors, it can stretch to over twice its size without a change in output. A device the size of a sports wristband should be able to power a simple fitness tracker or electrocardiogram monitor from the heat generated while the wearer is walking, says Xiao. It should cost around $10.

The TEG can be remade easily. Soaking it for a few hours in an amine-based recycling solution dissolves the polyimine substrate, which can be used to remake a polyimine film. The thermoelectric modules and metal electrodes can be recovered intact from the solution and reused. A TEG made from recycled components worked just as well as the original.

To demonstrate self-repair ability, the team cut the polyimine substrate and liquid metal wiring, and then simply pressed the cut ends back together. The liquid metal immediately fused together, repairing the circuit. In 1.5 h the device was back to its original mechanical strength and stretchiness. You can also combine two devices into one with this property, Xiao says.

This ability makes the devices easy to configure for different sizes and shapes, says Renkun Chen, a mechanical and aerospace engineer at the University of California, San Diego. Plus, he adds, “if the power output is enhanced, which is entirely possible for this novel device architecture, it may also be used to power smart watches.”

Chong-An Di, a chemist at the Chinese Academy of Sciences calls the device innovative with “impressive output [and] outstanding mechanical properties.” To make it more comfortable to wear and easier to integrate with electronic devices, though, the researchers might have to reduce the vertical height of the device and study the possibility of liquid metal leakage, he says.


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