Biphasic Liquid Metal Composites for Sinter‐Free Printed Stretchable Electronics

Abstract This work introduces and presents a comprehensive study on a series of biphasic liquid metal (LM) composites that benefit from high conductivity, excellent stretchability, a low gauge‐factor, excellent adhesion to a wide range of substrates, for sinter‐free writing complex stretchable circuits. These trinary material systems are composed of a block‐co‐polymer binder, EGaIn liquid metal, and a microparticle (μP) filler (Ag flakes, Ag‐coated‐Ni, Ag‐coated‐Fe, Ni, Ferrite, or TiC). They combine the fluidic behavior, resilience, and self‐healing properties of LMs, and the printability, adhesion, and elastic integrity of elastomers. Unlike the previous efforts with LM‐polymer composites and printed EGaIn nanodroplets, these composites are intrinsically conductive and do not require any thermal/optical/mechanical sintering. The binary combinations (LM‐SIS, LM‐Ag, Ag‐SIS) are first synthesized and characterized, and then the trinary LM‐μP‐SIS composites are evaluated. This includes analysis of microstructure, surface roughness, conductivity, electromechanical coupling, and LM smearing/leakage during mechanical loading, as well as the examination of the influence of filler particle size and composition. It is found that a binary combination of Ag‐EGaIn or EGaIn‐SIS does not result in the desired properties, and only trinary combination with conductive μP, preferably Ag, results in a printable, stretchable and sinter‐free composite. As an application, a digitally‐printed epidermal sticker for respiration monitoring is demonstrated.