With the rise of soft robotics and wearable electronics, there rises the secondary, but equally limiting issue, of dissipating the heat produced by the electronics while maintaining flexibility. Bartlett et al.1 recently demonstrated that soft composites, consisting of liquid metal micro-droplets dispersed in a silicone matrix, can be used to attach a high-power LED to a user as well as prevent it from overheating. The integration of liquid metal micro-droplets allows for a material with a maximum thermal conductivity of 10 Wm-1K-1, when stretched, while being able to sustain a minimum flexibility of 200% of its original length. However, as in the case of traditional electronics, the power density of wearables is bound to increase with time, ultimately necessitating use of active liquid cooling. In fact, a liquid cooled viscoelastic actuator has been recently used in a high power compliant robotic leg prototype. However, as in thermoregulatory liquid cool garments, the tubing material used in this design has a very low thermal conductivity, of around 0.2 Wm-1K-1; a value far too low to effectively deal with the amount of heat that will inevitably be generated by wearable electronics.
To address this issue, in this work we introduce a high performance stretchable heat exchanger made out of the liquid metal-silicone composites. Since stretching violates most assumptions used in design of conventional heat exchangers (e.g. constant areas and cross sections, heat transfer coefficients, and flow rates), we developed a new theoretical frame for design of stretchable thermal management devices. Specifically, we will discuss time scaling and the quasi-static shape models for single stream heat exchanger undergoing axial stretching and compression. To validate this thermofluidic design framework, a prototype stretchable heat exchanger design has been built and its performance characterized. Using this data, an optimized heat exchanger design has been proposed that minimizes the required pumping pressure and maximizes heat dissipation while maintaining the essential flexibility of wearable electronics.
1. Bartlett et al. PNAS, 114, 2017.