Scientists have designed a flexible energy harvester that may pave the way for wearable devices which are powered using only body heat. Wearable devices used to monitor a variety of health and environmental measures are becoming increasingly popular. The performance and efficiency of flexible devices, however, pale in comparison to rigid devices, which have been superior in their ability to convert body heat into usable energy. “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,” said Mehmet Ozturk, a professor at North Carolina State University in the US.
Using rigid devices is not the best option when you consider a number of different factors. Superior contact resistance – or skin contact – with flexible devices, as well as the ergonomic and comfort considerations to the device wearer, researchers said. One of the key challenges of a flexible harvester is to connect thermoelectric elements in series using reliable, low-resistivity interconnects, they said. The team used a liquid metal of gallium and indium – a common, non-toxic alloy called EGaIn – to connect the thermoelectric ‘legs’. “The electric resistance of these connections is very low, which is critical since the generated power is inversely proportional to the resistance: Low resistance means more power,” researchers said.
“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. Rigid devices are not able to heal themselves,” Ozturk said. Future work will focus on improving the efficiencies of these flexible devices, by using materials and techniques to further eliminate parasitic resistances, researchers said. The study was published in the journal Applied Energy.