Wearable electronic devices are starting to save some battery power by harvesting an energy source that's right underneath them: human body heat. But while flexible devices are more comfortable to wear, they aren't quite as effective at soaking up that heat as rigid ones. Now a team from North Carolina State University has developed a system using liquid metal components, making it flexible, efficient and even self-healing.

Thermoelectric (heat-harvesting) devices come in all kinds of form factors, including smartwatches, wristbands, t-shirt patches, pockets and flashlights. Heftier, rigid devices generally perform better than the more flexible ones, but the latter have the advantage of being able to make contact with more skin, as well as being more comfortable to wear.

"We wanted to design a flexible thermoelectric harvester that does not compromise on the material quality of rigid devices yet provides similar or better efficiency," says Mehmet Ozturk, corresponding author of the study.

So the NC State team set about designing a flexible thermoelectric device with the efficiency of a rigid system, and using the same active materials. To connect sections of the thermoelectric elements, the team settled on a common, safe liquid metal mix called EGaln, made up of gallium and indium. This alloy has very low electric resistance, making it more efficient at generating power, and there's another benefit, too.

"Using liquid metal also adds a self-healing function: If a connection is broken, the liquid metal will reconnect to make the device work efficiently again," says Ozturk. "Rigid devices are not able to heal themselves."

Next, the team plans to further improve efficiency, by tweaking the materials and methods to eliminate parts that might be leeching energy. The team has a patent pending on the technology, and a paper describing the work was published in the journal Applied Energy.

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