25.4 Liquid Metal - Polymer Composites for Stretchable Circuits, Soft Machines, and Thermal Management.

Metals that are liquid at room temperature, typically referred to as liquid metals (LMs), have a long history of use in sensors and as electrical contacts. For decades, room temperature liquid metals like mercury and LM alloys like gallium-indium-tin have been used in strain gauges, thermometers, tilt sensors, and other use cases that require a combination of high electrical conductivity and fluidic rheology [1]. In recent years, eutectic gallium-indium (EGaln) has become especially popular as a liquid metal due to its negligible vapor pressure and low toxicity [2]. Because of their low viscosity ($\sim 2 \mathrm{cP}$, i.e. approximately twice the viscosity of water), EGaln have the potential to be used for highly stretchable circuits that can be embedded as microfluidic channels within a soft elastomer film. As the surrounding elastomers stretches, the fluidic EGaln channels can elongate and maintain electrical connectivity. Moreover, because of their high electrical conductivity, the channels can support digital circuit functionality and potentially replace the rigid metallic interconnects that are used in current circuit boards.