To improve performance and lifetime of wearable electronic devices, numerous wearable energy harvesting approaches have been explored in recent years. However, existing wearable energy harvesting devices do not convert energy at a high enough rate to enable continued operation of wearable sensors. One promising approach is to use thermoelectric devices (TED) that convert body heat to usable electrical energy. TEDs operate on two main principles: the Seebeck effect, in which temperature differentials across oppositely doped semiconductors drive a potential difference; and the Peltier effect, in which current driven across the depletion regions of oppositely doped semiconductors actively heat and cool opposing sides. Traditionally, TEDs are made of rigid materials that do not interface efficiently or comfortably with the human body. In this talk, we will discuss our results making flexible TED that convert body heat into voltage potential. We use a materials-first design approach, utilizing flexible and stretchable materials throughout the design process to enable conformal body contact to improve performance. In contrast to the rigid ceramic layers typically used as the interface between the device and the human wearer, we utilize soft, thermally conductive liquid-metal embedded elastomers (LMEE) comprising liquid metal microdroplets inside an elastomer matrix, paired with a fully-stretchable 3D-printed TED substrate. Additionally, we utilized electrically conductive LM based composites as our interconnect material to maintain reliability over many bending cycles.
In this talk, we begin by describing our fabrication process, including stencil lithography and 3D printing steps. This is followed by Seebeck characterization, including voltage and power output for several candidate designs and configurations, to optimize performance and understand the relevant tradeoffs inherent in the introduced design space. Next, we characterize the flexible TED’s Peltier cooling performance, finding that the LMEE interface increased the achievable temperature reduction while improving the cooling stability. This is followed by characterization of electro-mechanical performance for cyclical buckling and strain testing. Next, wearable demonstrations, including the analysis of the influence of biomechanical movement and exertion on harvesting performance, are introduced. Lastly, integrated device demonstrations and future directions are explored, including our efforts to design a wearable biosensor entirely powered by body heat.
