Liquid Metal Core–Shell 3D Printing

Abstract

Alloys such as eutectic gallium indium (EGaIn) remain liquid at room temperature, enabling extensive, repeated deformations without structural damage, making them ideal electrical conductors for soft robotics and wearable devices. However, their liquid nature presents significant fabrication challenges, often mitigated by encasing them in flexible rubber sheaths. Extrusion-based 3D printing offers a rapid and integrated fabrication method, but significant rheological differences between the liquid metal (LM) core and rubber shell often lead to nonuniform shells and constrained core-to-shell ratios, which are crucial in optimizing functional properties like electrical and thermal conductivity. This study systematically investigates printhead design and process parameters to establish a generalizable framework for LM core–shell 3D printing. Key parameters, including nozzle core and shell diameters, flow rates, and shell material viscosity, are modulated to achieve uniform structures. Precise control of these process parameters enables core-to-total area ratios of up to 0.37, a nearly 50% increase over the current state of the art and comparable to commercial power and communication cables. The successfully printed core–shell features include overhangs, turns, and layers, demonstrating structural complexity akin to conventional material extrusion while maintaining high electrical conductivity.