A bi-phasic ternary Ag–In–Ga ink that demonstrates high electrical conductivity, extreme stretchability, and low electromechanical gauge factor (GF) is introduced. Unlike popular liquid metal alloys such as eutectic gallium–indium (EGaIn), this ink is easily printable and nonsmearing and bonds strongly to a variety of substrates. Using this ink and a simple extrusion printer, the ability to perform direct writing of ultrathin, multi-layer circuits that are highly stretchable (max. strain >600%), have excellent conductivity (7.02 × 105 S m–1), and exhibit only a modest GF (0.9) related to the ratio of percent increase in trace resistance with mechanical strain is demonstrated. The ink is synthesized by mixing optimized quantities of EGaIn, Ag microflakes, and styrene-isoprene block copolymers, which functions as a hyperelastic binder. When compared to the same composite without EGaIn, the Ag–In–Ga ink shows over 1 order of magnitude larger conductivity, up to ∼27× lower GF, and ∼5× greater maximum stretchability. No significant change over the resistance of the ink was observed after 1000 strain cycles. Microscopic analysis shows that mixing EGaIn and Ag microflakes promotes the formation of AgIn2 microparticles, resulting in a cohesive bi-phasic ink. The ink can be sintered at room temperature, making it compatible with many heat-sensitive substrates. Additionally, utilizing a simple commercial extrusion based printer, the ability to perform stencil-free, digital printing of multi-layer stretchable circuits over various substrates, including medical wound-dressing adhesives, is demonstrated for the first time.

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.

A novel technique that permits, for the first time, fabrication of stretchable traces with linewidths as low as 20 µm and line‐spacing of 30 µm, based on simple coating and printing techniques, performed entirely at ambient condition, is demonstrated. By relying on existing inkjet printing technique, the proposed sinter‐free method is a step toward scalable fabrication of high‐resolution stretchable circuits, with application in logic gates, transparent conductors, and solar panels. This is accomplished by coating a layer of poly(vinyl alcohol) (PVA) over an elastic substrate, inkjet printing a circuit with silver nanoparticle (AgNP) ink, and then coating the printed circuit with a thin film of eutectic gallium‐indium‐tin (Galinstan) alloy. The Galinstan coating selectively wets to the printed AgNPs, resulting in highly conductive (6.65 × 106 S m−1) circuits that can withstand over 100% of strain with a modest gauge factor of ≈2.7. The process does not need thermal sintering, thanks to the Galinstan fusion with AgNPs, thus being compatible with heat‐sensitive substrates. The PVA coating has a critical role as a hydrophilic surface that absorbs the water‐based ink but resists wetting of the Galinstan. This method is demonstrated over a variety of substrates, including ultrasoft polyurethanes, ultra‐stretchable styrene–ethylene/butylynestyrene, and polyimide.