Liquid Metals for Soft and Stretchable Electronics

Abstract

Liquid metals are the softest and most deformable class of electrical conductors. They are intrinsically stretchable and can be embedded in elastomeric or gel matrices without altering the mechanical properties of the resulting composite. These composites can maintain metallic electrical conductivity at extreme strains and can form soft, conformal contacts with surfaces. Gallium and several of its alloys, which are liquid metals at or near room temperature, offer a low toxicity alternative to mercury. These metals have negligible vapor pressure (so they do not evaporate) and low viscosity. The surface of these metals reacts rapidly with air to form a thin surface oxide ‘skin’ that allows these liquids to be patterned despite their large surface tension. For example, liquid metal can be 3D printed, molded, or injected into microchannels. This chapter summarizes the properties, patterning methods, and applications of these remarkable materials to form devices with extremely soft mechanical properties. Liquid metals may be used, for example, as conductors for hyper-elastic wires, stretchable antennas, optical structures, conformal electrodes, deformable interconnects, self-healing wires, components in microsystems, reconfigurable circuit elements, and soft circuit boards. They can also be integrated as functional components in circuits composed entirely of soft materials such as sensors, capacitors, memory devices, and diodes. Research is just beginning to explore ways to utilize these ‘softer than skin’ materials for biolectronic applications. This chapter summarizes the properties, patterning methods, and applications of liquid metals and concludes with an outlook and future challenges of these materials within this context.