Electromechanical Characterization of a Highly Stretchable Liquid Metal Derived Conductor for Wearable Electronics

Highly stretchable and soft electronics as emerging technologies exhibit promising physical and electromechanical characteristics. One of the key efforts for expanding these emerging technologies includes developing intrinsically stretchable conductors as stretchable interconnects, which can withstand high tensile strain amplitude and repeated cyclic deformation while maintaining electrical conductivity and mechanical integrity. In this study, a highly conductive fluid phase conductor developed by Liquid Wire Inc. is stencil printed on a thermoplastic polyurethane (TPU) substrate followed by lamination of TPU as an encapsulant layer on top of the trace. The conductor is an alloy composed of a eutectic Gallium-Indium-Tin alloy stabilized by an oxide microstructure and additives. Test coupon includes 4 probe structure including stretch zone and non-stretch zone. Non-stretch zone is formed by assembling woven fabrics on TPU substrate. Electrical resistance is measured via 4-point probe in-situ monitoring of electrical resistance during fatigue cycling. Microstructure and morphology of the conductor are investigated by Confocal Laser Scanning Microscopy and Scanning Electron Microscopy (SEM). Electromechanical properties of the printed conductor are investigated by subjecting the trace to continuous and discrete tensile cyclic loading at strain amplitudes of 30% and 50% and distinct extension rates of 5, 10, 20 and 30 mm/s. The results of electromechanical analysis reveal that the conductor's resistance exhibits a linear response to changes in the length and exhibits no hysteresis. Extension rate and relaxation of TPU substrate does not show significant impact on rate of change in electrical resistance. The conductor shows fatigue free performance during continuous and discrete fatigue cycling at strain amplitude of 30% and 50% for 8000 cycles of fatigue cycling.