Thermally Responsive Polymers for Wearable Calorimeters

Body core temperature (BCT) is a rich source of clinical information: it can flag hypothermia and heat stroke as well as inflammation and infection, and its continuous monitoring opens further possibilities for well-being applications such as estimating burnt calories, predicting the ovulation window in the female population, and assessing mental-health status. Integrating a BCT sensor into a wearable device is, however, very difficult because conventional methods cannot simultaneously achieve minimal invasiveness and high measurement accuracy. Dual heat flux (DHF) thermometry is a recent technique that permits an accurate estimate of BCT from a skin-temperature measurement. Nevertheless, the limited precision of currently available temperature sensors has prevented the widespread adoption of devices based on this architecture. This thesis presents the fabrication of a fully wearable DHF thermometer built by integrating new polymers that possess a remarkable temperature sensitivity. In these particular polymers, a temperature increase produces a change in ionic conductivity. The first part of the work concentrates on understanding the ion-transport mechanism in these materials and, specifically, on the nature of the interaction between the functional groups on the polymer backbone and the conducting species (i.e., metal cations and water molecules). It is shown that the coordination environment of the ion is the key to making these materials strongly temperature sensitive. The second part addresses the fabrication of a BCT sensor by integrating these temperature-responsive polymers into an ultrathin DHF thermometer. Building on the understanding of the temperature response, the polymer composition is optimized to obtain a thermal sensitivity that allows good precision in measuring BCT. Finally, the fabricated DHF thermometer is characterized under different conditions, assessing the sensor’s accuracy and response time.