Rewritable, Printable Conducting Liquid Metal Hydrogel

The development of high-performance printable electrical circuits, particularly based on liquid metals, is fundamental for device interconnection in flexible electronics, motivating numerous attempts to develop a variety of alloys and their composites. Despite their great potential, rewritable and printable electronic circuits based on liquid metals are still manufactured on demand. In this study, we demonstrate liquid metal-based hydrogels suitable for rewritable, printable electrical circuits. Our liquid metal hydrogels are based on sedimentation-induced composites of eutectic gallium-indium (EGaIn) particles in poly(ethylene glycol) diacrylate (PEGDA). The EGaIn particles are vertically phase-segregated in the PEGDA. When a composite surface with high EGaIn content is gently scratched, the surface covering PEGDA is removed, followed by the rupture of the native oxide layers of the particles, and the exposed EGaIn becomes conductive. The subsequent water-driven swelling of PEGDA on the scratched surface completely erases the conductive circuit, causing the system to reset. Our friction-responsive liquid metal hydrogel exhibits writing-erasing endurance for 20 cycles, with a dramatic change in the electrical resistance from metal (∼1 Ω) to insulator (∼107 Ω). By employing surface friction pen printing, we demonstrate mechanically flexible, rewritable, printable electrical conductors suitable for displays.