Direct 3D Printing of Stretchable Circuits via Liquid Metal Co-Extrusion Within Thermoplastic Filaments

Liquid-metal alloys are now extensively used for stretchable electronic applications due to their superior electrical conductivity, non-toxicity, and mechanical stability in micro-channels. Needle-injection and direct-writing are the most popular techniques for patterning micro-structured liquid metal alloys. However, embedded channels often require a very high pressure to inject liquid-metal, and direct writing by dispensing is relatively complex due to the low viscosities and high surface tension of the metal which cause liquid to normally dispense in droplets rather than a stream. In this work, a technique to co-axially extrude liquid-metal alloy within an encapsulating cover fluid has been presented to obtain a continuous stable stream of liquid-metal. Fused deposition modeling (FDM) 3D printing has been adapted to co-extrude a liquid-metal core with a shell made from a thermoplastic elastomer. A custom extruder system is used to directly produce conductive micro-wires (diameter: approximate to 25 mu m) of liquid-metal having an insulating shell of styrene-ethylene-butylene-styrene which can be stretched up to four times the original length without any noticeable mechanical and electrical loss. The system is capable of printing in-plane conductive pathways as well as out-of-plane functional devices with direct-stable encapsulation of liquid-metal wires. This technology has been successfully used to print 2D-pressure and 3D-strain sensors.