A 3D Printed Liquid Metal Emulsion for Low-Stress-Activated Stretchable Electronics

Integrating liquid metal (LM) into 3D stretchable devices remains a manufacturing challenge due to its intrinsically low viscosity and high surface tension. Current manufacturing methods require the use of complicated lithography and molding techniques which increase fabrication complexity and limit widespread adoption. Modifying the LM rheology by creating LM emulsions dispersed in a carrier liquid can create a stable 3D printed emulsion, which can be integrated with other printed materials in a single fabrication process, reducing manufacturing complexity. This work presents a 3D printable strain-induced electrically conductive liquid metal emulsion for the programmable assembly of soft conductive composites. This liquid metal emulsion exhibits shear yielding and shear thinning rheology that is compatible with direct ink writing (DIW). Examples of complex self-supported 3D printed structures with spanning features are presented to demonstrate the 3D printability of this emulsion. Stretchable liquid metal composites are fabricated by integrating this emulsion into a multi-material printing process along with a 3D printable elastomer, combined with manual pick and place of off-the-shelf electronics. The as-printed composites exhibit a low electrical conductivity but can be transformed into highly conductive composites by a single axial strain at stresses that are an order of magnitude smaller than previous stress-activated emulsions. The effects of axial strain and cyclic loading on the electrical conductivities and sensitivities of these composites are characterized. The electrical conductivity increases with activation strain, with a maximum observed conductivity of 2.32x10⁵Sm^-1at strains greater than or equal to 200%. The electrical conductivity of these composites reaches a steady state for each strain after one cycle and remains stable with low variation (< 7%) over 1,000 cycles. The strain sensitivities of these composites are constant and significantly lower than that of a bulk conductor, making them suitable as stretchable conductors. The utility of these composites is shown by employing them as wiring into a single fabrication process for a stretchable array of LEDs.