Liquid Metal + x: A Review of Multiphase Composites Containing Liquid Metal and Other (x) Fillers

Multiphase mixtures containing both liquid metal and solid inclusions in a soft polymeric matrix can exhibit unique combinations of mechanical, electrical, magnetic, and thermal properties. Gallium‐based liquid metals have excellent electrical and thermal properties, and incorporating additional conductive, magnetic, or other solid fillers into liquid metal‐embedded elastomers can yield heightened electrical and thermal conductivities, enhanced elasticity due to lowered percolation thresholds, and positive piezoconductivity. This emerging class of liquid metal + x composites, where x denotes any solid filler type, has applications in stretchable electronics, wearables, soft robotics, and energy harvesting and storage. In this review, the recent literature is consolidated on liquid metal + x composites and their potential to offer uniquely amplified or multiplied bulk properties is highlighted. The literature related to the materials and processing of liquid metal + x composites is reviewed, through which it is found that the properties of the resulting multiphase composites are sensitive to the sequence in which the distinct liquid metal and solid inclusions are incorporated into the continuous phase. This review further includes a summary of relevant predictive modeling approaches, as well as identifies grand challenges and opportunities to advance liquid metal + x composites. This review consolidates the literature on multiphase mixtures containing both liquid metal and any solid (x) inclusions in a soft polymer, or liquid metal + x composites. The review aims to elucidate the potential to amplify or multiply bulk properties by adding liquid metal to existing filler‐matrix composite materials.