Starfish tube feet inspired hydrogel electrode for durable underwater sEMG acquisition

•An sEMG electrode uses a starfish tube-feet inspired sucker and a micro-swelling hydrogel to enhance skin contact underwater.•The micro-swelling hydrogel exhibits skin-like modulus and low skin contact impedance to guarantee high sEMG signal quality.•The waterproof electrode has advantages in durable underwater sEMG recording, promising for more practical scenarios. Underwater surface electromyography (UW-sEMG) acquisition with long-term high quality is crucial to underwater activities, including rehabilitation, fatigue measurement, and sports monitoring. However, most UW-sEMG electrodes suffer significant performance decline, especially for hydrogel electrodes, due to the swelling-induced mechanical, electrical, and adhesive degradation. Maintaining high performance underwater over extended periods is challenging. The controlled underwater adhesion-detachment behavior of starfish is inspirative. Here, a starfish tube feet-inspired hydrogel (STFH) electrode is developed for UW-sEMG recording, including a micro-swelling hydrogel pillar, a biomimetic silicone sucker, and an Ag/AgCl disc electrode. By soaking freeze-dried hydrophilic hydrogel in tannic acid (TA) solution, the hydrogel exhibits skin-like modulus of 22.4 kPa, low skin impedance of 18.3 kΩ (10 Hz) and low equilibrium swelling ratio. Unlike traditional strategies for improving anti-swelling properties, this work leverages the interaction between the sucker and the swollen hydrogel to promote conformal contact. Consequently, the sEMG signal exhibits SNR of 26.7 dB and baseline noise of 12.8 μV even after 60 min of underwater working. The STFH electrode also shows anti-noise capability to resist skin vibration and wave oscillation. This STFH electrode enables durable UW-sEMG acquisition, which could further extend from the arm to the wrist to adapt wearing habits.