Electronic skins based on liquid metals

The implementation and exploration of liquid metals for soft electronics, especially electronic skins (e-skins), are fast increasing. The growing field has received special attention since research regarding gallium-based alloys has intensified as these alloys are much safer in comparison to their more hazardous counterpart, mercury. Liquid metal alloys of gallium provide unique physical and chemical properties for e-skin. These properties originate from their high thermal and electrical conductivities and the fact that the liquid metal is an electronic melt in contrast to the ionic liquid. The formation of 2-D oxides on the surface of liquid gallium alloys, with large van der Waals forces, also adds to their uniqueness. Liquid metals, whether in bulk form or particulated morphologies, provide stretchabilities that surpass any other systems, allowing for the formation of super malleable e-skins. As such, they present certain opportunities for developing elements with extraordinary softness, malleability, and skin compatibility: stretchable wires and electrodes, memories, electronic components such as resistors, coils, diodes, and transistors, soft sensors, energy harvesting/storage elements, and self-healing systems. Presence of the 2-D metal compound skin also helps in accessing the non-Newtonian characteristics of gallium-based alloys that permit the formation of microparticles/nanoparticles and specific fluidics and grant printability in three dimensions. Liquid alloys of gallium, their properties, and applications for e-skins are discussed in this review, and the wealth of opportunities for future applications within soft and stretchable electronics is explored.