Direct Writing-based Wiring of Liquid Metal to a Metal Electrode for Soft Sensor Systems

As demands for stretchable electronics have increased in the field of wearable devices, liquid metal, such as eutectic Gallium-Indium (eGaIn), has gained much attention due to its metallic conductivity with liquid reconfigurability. Although various applications have been suggested using eGaIn, an electrical connection has remained as a technique challenge. Wires have been directly inserted into the microfluidic channel filled with eGaIn, resulting in electromechanically unstable connection, bulky size, and time consuming fabrication steps for the electrode part. In this study, a novel solution for the electrode is proposed, connecting eGaIn wires directly to the metal electrode based on direct writing of eGaIn. The two electrode materials were considered as candidates, including electroless nickel immersion gold (ENIG) and immersion tin (Im-Sn) plated surfaces. Among them, only the ENIG-finished surface had stable electrical connection with eGaIn, allowing sufficiently low contact resistance 0.1Ω The suggested electrode part was mechanically durable under strain up to 100 %. As an application, a sensing skin embedding 10 sensing units was fabricated based on direct ink writing, using a flexible flat cable finished by ENIG plating.