Researchers at the University of Washington say they have developed a first-of-its-kind flexible, wearable thermoelectric device that converts body heat to electricity, which offers a promising way to continuously power wearable electronics. The device, say the researchers, is soft and stretchable, yet sturdy and efficient — properties that can be challenging to combine.

“It’s a 100% gain if we harvest thermal energy that would otherwise be wasted to the surroundings,” says Mohammad Malakooti, a UW assistant professor of mechanical engineering. “Because we want to use that energy for self-powered electronics, a higher power density is needed. We leverage additive manufacturing to fabricate stretchable electronics, increase their efficiency and enable their seamless integration into wearables while answering fundamental research questions.”

Even after more than 15,000 stretching cycles at 30% strain, the prototype device remains fully functional, say the researchers – a highly desirable feature for wearable electronics and soft robotics. The device also shows a 6.5 times increase in power density compared to previous stretchable thermoelectric generators, reports Smart2Zero.

To create flexible devices, the researchers 3D-printed composites with engineered functional and structural properties at each layer. The filler material contained liquid metal alloys, which provide high electrical and thermal conductivity. These alloys address limitations in previous devices, including an inability to stretch, inefficient heat transfer, and a complex fabrication process.

The researchers also embedded hollow microspheres to direct the heat to the semiconductors at the core layer and reduce the weight of the device. The researchers showed that they could print these devices on stretchable textile fabrics and curved surfaces, which suggests that future devices could be applied to clothing and other objects.

“One unique aspect of our research is that it covers the whole spectrum, all the way from material synthesis to device fabrication and characterization,” says Malakooti, who is also a researcher at the UW’s Institute for Nano-Engineered Systems. “This gives us the freedom to design new materials, engineer every step in the process, and be creative.”