Liquid metal composites for flexible thermoelectric energy harvesting

Thermoelectric generators (TEGs) have received immense attention in the area of wearable electronics because they offer self-sustainable power sources. Regardless of the past advances in wearable thermoelectric generators, the level of flexibility and durability of such devices has remained limited. Here, we present our recent work on wearable TEGs based on liquid metal (LM) composites that are utilized as thermal material interfaces. These multifunctional layers of LM composites provide conformity with skin and efficiently transfer body heat to thermoelectric modules while they increase heat dissipation on the upper side of the device. The LM composites consist of eutectic gallium indium (EGaIn) inclusions and a silicone-based elastomer matrix (polydimethylsiloxane, PDMS). The embedded EGaIn droplets significantly enhance the thermal conductivity of the silicone matrix which is suitable for heat absorption and dissipation. In the case of EGaIn microdroplets, electrically conductive pathways can be created to function as an integrated circuit board. This is achieved by mechanical sintering of the embedded EGaIn droplets. Moreover, the soft and stretchable nature of the LM composites allows for intimate contact between the hot/cold surfaces (i.e., human skin) and the thermoelectric device. The TEG devices with LM composite show enhanced power generation particularly in wearable devices. Different types of flexible TEGs are fabricated here, and their performance is characterized by thermoelectrical and electromechanical measurements. Lastly, their potential application in self-powered wearable devices is discussed.