Interest in wearable electronics for continuous, long-term health and performance monitoring is rapidly increasing. The reduction in power levels consumed by sensors and electronic circuits, accompanied by the advances in energy harvesting methods, allows for the realization of self-powered monitoring systems that do not have to rely on batteries.
For wearable electronics, thermoelectric generators (TEGs) offer the unique ability to continuously convert body heat into usable energy. For body harvesting, it is preferable to have TEGs that are thin, soft, and flexible. Unfortunately, the performance of flexible modules reported to date has been far behind that of their rigid counterparts. This is largely due to lower efficiencies of the thermoelectric materials, electrical or thermal parasitic losses, and limitations on leg dimensions posed by the synthesis techniques.
A flexible thermoelectric energy harvester was developed that has the potential to rival the effectiveness of existing power wearable electronic devices, using body heat as the only source of energy. The thermoelectric harvester provides the material quality of rigid devices, yet provides similar or better efficiency. Flexible electronics offer superior contact resistance — or skin contact — as well as the ergonomic and comfort considerations to the device wearer.
A key challenge to developing a flexible harvester is connecting thermoelectric elements in series using reliable, low-resistivity interconnects. A liquid metal of gallium and indium — a common, non-toxic alloy called EGaIn — was used 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.