Liquid metal in the flexible thermoelectric device allows for self-healing. (Credit: Mehmet Ozturk, NC State University)

Engineers have designed a flexible thermoelectric energy harvester that has the potential to rival the effectiveness of existing power wearable electronic devices using body heat as the only source of energy.

Wearable devices used to monitor a variety of health 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.

The researchers wanted to utilize the best thermoelectric materials used in rigid devices in a flexible package, so that manufacturers wouldn’t need to develop new materials when creating flexible devices. A key challenge of a flexible harvester is to connect thermoelectric elements in series using reliable, low-resistivity interconnects. So, the researchers used a liquid metal of gallium and indium — a common, nontoxic 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.

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. Future work will focus on improving the efficiencies of these flexible devices, by using materials and techniques to further eliminate parasitic resistances.