Flexible wearable device harvests thermal energy

The team, from University of Washington, published its research in Advanced Energy Materials. The device is described as soft and stretchable, yet sturdy and efficient.

“It’s a 100 per cent gain if we harvest thermal energy that would otherwise be wasted to the surroundings,” said 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.”

According to the team, the device remains fully functional after more than 15,000 stretching cycles at 30 per cent strain — a highly desirable feature for wearable electronics and soft robotics. Researchers also found that it showed a 6.5 times increase in power density compared to previous stretchable thermoelectric generators.

To create the 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. The team said that the alloys address limitations in previous devices, including an inability to stretch, inefficient heat transfer and a complex fabrication process.

Hollow microspheres were also embedded 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, suggesting 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 characterisation," said Malakooti, who is also a researcher in 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.”

Youngshang Han, UW master’s student in mechanical engineering, was lead author on the paper. Leif-Erik Simonsen is an additional co-author. The research was funded by the US National Science Foundation.