Metallic gels for conductive 3D and 4D printing

This paper reports printable metallic gels (pendular suspensions) consisting of an aqueous suspension of copper particles connected by bridges of liquid eutectic gallium indium alloy (EGaIn). Pendular suspensions rely on capillary forces to form networks between solid particles with a composition-dependent rheology, but prior studies have focused on insulating suspensions. Here, the rheology of a conductive solid-liquid-liquid suspension is tuned for 3D printing by varying the composition and the pH; the latter promotes metallic wetting. The dry printed parts have metallic electrical conductivity (1.05×105 S/m) without requiring a sintering step. Drying at elevated temperatures can accelerate the removal of water while creating stress that drives shape change (i.e., 4D printing). As a demonstration, we print a conductive spider that lifts and assembles its own body from an initially flat shape. Such conductive inks are promising for printing metallic structures under ambient conditions.