Rheological Modification of Liquid Metal for Additive Manufacturing of Stretchable Electronics

One of the challenges to rapidly manufacturing flexible electronics is the complexity involved in printing circuitry from stretchable conductors. Eutectic gallium alloys are typically used as the conductive material because they have unique high conductivity, self-healing, and stretchable properties. However, limited 3D printing has been demonstrated by leveraging the structural stabilization provided by the thin gallium oxide film. Vertical structures are difficult to print with a liquid metal (LM) due to the low viscosity and high surface tension of the gallium alloy, which easily leads to coalescence. A method is presented to alter the physical structure of the liquid metal through the incorporation of a conductive nano- or micronickel fillers. The resulting rheological modification of the liquid metal to a paste drastically increases the fluidic elastic modulus and yield stress, rendering it 3D printable. Further, the modification retains the high electrical conductivity (3.9 x 10(6) +/- 9.5 x 10(5) S m(-1)) and stretchability (over 350% strain) of pure liquid metal. The ability to print 3D standing structures using this highly conductive metal paste opens up new opportunities to manufacture more complex stretchable electronics.