Flexible Skin Electrodes Based on Conductive Hydrogel for Hand Electromyography Sensing and Gesture Recognition: Exploring Conductivity and Mechanical Properties

Robots want to collaborate with humans or mimic human actions, needing various flexible sensors. Flexible skin electrodes are used as human–robot interfaces (HRIs), providing an opportunity for sensing interactions between robot and user. However, traditional electrodes with low skin adhesion prevent their applications in human-assisted robots, especially sweat on the skin surface, resulting in them slipping and even falling off. Thus, sodium casein (SC) is used for improving polyacrylamide (PAAM) hydrogel to fabricate PAAM-SC-conductive hydrogel with excellent conductivity and mechanical properties. The strong interaction of PAAM-SC improves its adhesion, which has an elongation at a break of roughly 3,000%. Additionally, it exhibits extremely low resistance (< 5kΩ), which can be used for electromyography (EMG) sensing. Herein, flexible skin electrodes with four-layer structures are designed. To fabricate flexible skin electrodes for hand gesture recognition, a 3D printer is employed to pattern high-performance Ag wires with structurally soft and stretchable filamentary serpentines to connect an external circuit and the PAAM-SC-conductive hydrogel. Based on the Transformer network model analysis, the classification results in cross validation are more than 85%. Notably, the PAAM-SC hydrogel as a strain sensor attached to a robotic finger can quickly respond to robotic finger movements. We might anticipate that our strategy will improve the harmonized integration between flexible electronics and human skin for smart HRIs, and when a corobot is equipped with the PAAM-SC-conductive hydrogel, the robots can interact and collaborate with humans seamlessly through active perception and matching.