Direct Wiring of Eutectic Gallium-Indium to a Metal Electrode for Soft Sensor Systems

Abstract

For wider applications of the liquid metal-based stretchable electronics, an electrical interface has remained as a crucial issue, due to its fragile electromechanical stability and complex fabrication steps. In this study, a direct writing-based technique is introduced to form the writing paths of conductive liquid metal (eutectic Gallium-Indium, eGaIn) and electrical connections to off-the-shelf metal electrodes in a single process. Specifically, by extending eGaIn wires written on a silicone substrate, the eGaIn wires were physically connected to the five different metal electrodes, of which stability as an electrical connection was investigated. Among the five different surface materials, the metal electrode finished by electroless nickel immersion gold (ENIG) had reproducible and low contact resistance without time-dependent variation. In our experiments, it was verified that the electrode part made by an ENIG-finished flexible flat cable (FFC) were mechanically (strain≤100 %, pressure≤600 kPa) and thermally (temperature≤180 Celsius) durable. By modifying trajectories of eGaIn wires, soft sensor systems were fabricated and tested to measure finger joint angles and ground reaction forces, composed with 10 sensing units, respectively. The proposed method enables the eGaIn-based soft sensors or circuits to be connected to the typical electronic components through a FFC or weldable surfaces, using only off-the-shelf materials without additional mechanical or chemical treatments.