Highly Stretchable and Strain-Insensitive Liquid-Metal Based Elastic Kirigami Electrodes (LM-eKE)

For stretchable electronics, there remains a critical need for electrically conductive materials that are highly deformable. Among these, gallium (Ga) based liquid metal (LM) alloys have become increasingly popular because of their high metallic conductivity and intrinsically deformable characteristics. Here, we demonstrate an LM-coated elastic kirigami electrode (LM-eKE) architecture that combines extreme mechanical deformability (up to 820% stretchable) with high electrical conductivity and relatively low electromechanical coupling. This stretchable electrode is based on a unique “Elastic Kirigami” design in which the paper-like foldable substrate typically used in kirigami is replaced with a soft and highly elastic thin film. Specifically, the LM-eKE is composed of a kirigami-patterned silicone membrane that is coated with a thin gold (Au) film and layer of eutectic gallium-indium (EGaIn) LM alloy. The biphasic Au-EGaIn film has negligible mechanical stiffness and allows the LM-eKE to maintain a low elastic modulus (kPa scale) and high degree of deformability. Moreover, the elasticity of the silicone substrate allows the LM-eKE to be stretched beyond the structural elongation of conventional kirigami since it combines folding deformation with high elastic strain. In addition, the coupling between this geometric and elastic deformation of the substrate allows the LM-eKE to maintain highly stable electrically conductive pathways along the biphasic Au-EGaIn surface. In this way, these electrodes exhibit low strain-sensitivity with only a 33% increase in electrical resistance when stretched to 820% strain and only a 1.3% ~ 1.7% increase when placed on a human knee and stretched during physical activities (walk, run, jump, bend). This low strain-sensitivity is an essential property of highly stretchable electrodes since it allows data to be transmitted without interruption from external mechanical stimuli. Moreover, as shown in this study, the high electromechanical stability of the LM-eKE electrodes allows for the avoidance of motion artifacts when collecting physiological data within wearable health monitoring applications.