One-Step Molded Free-Standing Liquid Metal Electrode and Its Application in Wearable Pressure Sensors

A flexible electrode is a critical element in enabling real-time, stable charge transport between the soft device’s internal functional material and the external measuring unit. Though the traditional methods of preparing flexible electrodes, which depend on coating conductive materials on insulating flexible substrates, can provide excellent electrical conductivity, the flexibility is still limited by the ductility of the conductive film and the substrate, as well as their compatibility. To address the challenge of improving flexibility, a liquid metal (LM)-based one-step molding process is developed in this work to fabricate the free-standing flexible electrode, which is demonstrated to be capable of improving the sensitivity and responding time of the pressure sensor as a proof of concept. By this method, the conductive and insulating layers may be formed simultaneously, enabled by LM particle sedimentation and APU solidification during the molding process. The as-prepared LM/APU composite thin films are elaborately characterized from micromorphology, chemistry, and flexible electronics viewpoints. Furthermore, in the proof-of-concept application, the high sensitivities in the ranges of 0–140 and 140–300 kPa, the rapid response (83 ms), and recovery times (105 ms) demonstrated that the proposed free-standing LM electrode was able to provide an unobstructed electrical channel between the pressure-sensitive layer (i.e., the carbon nanofiber-coated elastic sponge) in the tested pressure sensor and the measuring appliance. With the use of these free-standing LM-based electrode-assembled flexible pressure sensors, the mechanical stimuli generated by joint extension and contraction were successfully converted to electrical signals in real time. Conclusively, this one-step molded LM-based flexible electrode is demonstrated to afford unfailing, durable, and adaptable electronic contacts for flexible devices. We think it may pave a new way in developing wearable and soft electronic products.