Engineering the Skin-Device Interface for Efficient Skin-Mounted Thermoelectric Generators

The recent advancements in skin electronics have led to complex devices capable of sensing, actuating, computing, and communication. Such an increase in functionality comes with a higher demand for power. Batteries alone cannot always fulfill this demand because frequent replacement might be unfeasible in many scenarios, or because the bulkiness of batteries might hinder wearability. Moreover, batteries often include toxic liquids that pose safety challenges for integration with the human body. In this context, energy harvesters hold promise to complement or even replace batteries. In particular, the body is a rich source of waste heat that could be harvested by thermoelectric (TE) devices to power wearable devices. An important challenge of TEs is the high thermal contact resistance between the device and its surroundings at its hot and cold sides. Such (parasitic) contact resistance leads to a large temperature drop at the surroundings-device interfaces that cannot be then utilized for electrical power generation. This challenge is accentuated for wearable TEs, where the dynamic nature of the skin makes it hard to optimize the thermal interface at the hot side, i.e., between the skin and the device. Moreover, wearability is not compatible with the use of bulky heat sinks, which results in a further increase of the thermal contact resistance at the cold side of the device, i.e., the device-air interface. Consequently, wearable TE generators utilize only a very small fraction of the thermal gradient available to them, resulting in a disappointing value of skin heat-generated power.<br/>In this work, I will present the last progress in my group on skin-mounted thermoelectrics with an emphasis on optimizing the skin-device thermal interface. First, I will emphasize the importance of using low thermal conductivity TE materials when high thermal contact resistances are present, like in wearables. To address this point, we are developing TE aerogels that are electrically conducting but present extremely low thermal conductivity. Those aerogels are to be integrated on soft substrates designed to minimize the thermal contact resistance with the skin. This device-skin thermal contact resistance minimization is achieved by promoting the substate tight mechanical compliance with the skin, and by enhancing the substrate thermal conductivity. Furthermore, the substrate will be engineered to channel skin lateral heat flows toward the TE generators to further improve power generation. This research will improve the efficiency of skin-mounted TE generators by tackling, from a material point of view, aspects related to their practical implementation. Our holistic approach includes not only developing innovative TE and substrate materials but also tuning their interface.