With the increasing attention to underwater scenes, real-time detection and analysis of underwater EMG signals is of great significance for underwater rehabilitation, underwater training, and diver safety monitoring. However, traditional EMG electrodes are prone to desorption due to the presence of a hydration layer between the electrode and the skin, and their performance in underwater applications is limited. Conductive hydrogels have attracted much attention due to their high water content, low mechanical modulus, good chemical and biocompatibility, and high tunability. However, most hydrogels have serious swelling behavior underwater, resulting in the failure of their three-dimensional network structure, the loss of conductive components, and the challenges of reduced mechanical strength and toughness, poor electrical conductivity, and low skin adhesion, which cannot meet the needs of underwater EMG signal acquisition. In addition, the existing underwater EMG electrodes mainly record EMG signals through the arm, while wrist acquisition is more in line with wearing habits and can reveal richer fine motor information of the hand.
In the ocean, starfish are able to use their tubular feet to walk on rocks underwater, i.e., to continuously adhere and detach. Among them, the tubular feet of starfish have a flattened sucker-like structure, and underwater adhesion is achieved by secreting adhesion proteins. Therefore, in this work, an STFH electrode inspired by the structure of starfish tubular feet was designed, including an external silicone suction cup, an internal micro-swollen hydrogel, and an Ag/AgCl electrode base. The preparation of DAT for microly-swollen hydrogels includes three steps: synthesis, freeze-drying, and soaking modification of DMAEA-co-AA hydrogels. The waterproof silicone suction cup partially isolates the external water environment and the hydrogel, and enhances the underwater contact between the electrode and the skin through negative pressure and the interaction between the micro-swelling of the hydrogel and the suction cup, so as to meet the needs of continuous high-quality underwater collection of EMG signals.
Figure 1. The schematic diagram of the design of underwater electromuscular electrodes imitating starfish tubes was first explored and characterized from four aspects: mechanical properties, adhesion properties, conductivity and anti-swelling properties. DAT-200 (i.e., 200 mg/ml TA solution) has the best comprehensive performance, TA is deposited on the polymer skeleton and forms a hydrogen bond network with the original polymer network, which gives DAT hydrogel anti-swelling performance and provides a basis for long-term underwater work. Among them, DAT-200 has a skin-like Young's modulus (22.4 kPa) and a low skin contact impedance (18.3 kΩ) at 10 Hz.
Figure 2. Electrical properties and anti-swelling properties of DAT hydrogels. a) Ionic conductivity of DAT hydrogels, b) skin contact impedance of DAT hydrogels, c) Contact impedances of DAT hydrogels and commercial gels at 10 Hz and 100 Hz, d) Nyquist plots of contact impedance of DAT hydrogels and commercial gels, e) Swelling rates of DAT hydrogels vs. original hydrogels, f) Swelling of DAT hydrogels vs. original hydrogels, g) Shear strength of DAT hydrogels before and after immersion for one hour, h) Ionic conductivity of DAT hydrogels before and after immersion for one hour, i) Compressive modulus of DAT hydrogels before and after immersion for one hour. Secondly, mechanical simulation was used to verify the effect of the interaction between the micro-swelling gel and the external suction cup on skin contact. Through finite element analysis and related experiments, the pressure and mechanical behavior of the suction cup and gel on the skin under dry and swollen conditions were studied, and the simulation results were in good agreement with the experimental data. When the hydrogel swells, it completely touches the inner wall of the suction cup, and the overall top stress increases, and the stress concentration decreases. At the same time, the stress distribution on the skin is more uniform, and the contact area between the hydrogel and the skin is increased, which is conducive to the recording of EMG signals.
Figure 3. Simulation of the mechanical mechanism of the electrode pressing process. a) Simulation model in the dry state, b) Von Mises stress distribution in the dry state and photos of the electrodes, c) Stress distribution in the skin contact area during compression and photos of the pigskin, d) Simulation model in the swollen state, e) Von Mises stress distribution in the swelling state and photos of the electrodes, f) Stress distribution in the skin contact area during swelling and photos of the pigskin, g) Node selection along the labeled path on the hydrogel in the dry state, h) The swelling state selects nodes along the labeled path on the hydrogel, i) hydrogel pressure-position map. Finally, the STFH electrode was connected to the OpenBci acquisition system for underwater EMG signal acquisition test, and the STFH electrode was compared with the pure DAT-200 gel electrode and the commercial 3M electrode. After 1 hour of continuous underwater recording, the electrode is able to maintain a high-quality EMG signal because it is able to continuously increase contact with the skin as the gel swells, which has the advantage of a high signal-to-noise ratio and low baseline noise compared to other electrodes. In addition, the STFH electrode is resistant to wave noise and maintains high signal quality in the working environment of the underwater motor. Therefore, STFH electrodes can achieve stable underwater EMG signal acquisition, and are expected to be integrated with wireless, miniaturized, portable, and waterproof EMG acquisition systems in the future to meet the needs of more practical underwater scenarios such as underwater rehabilitation and underwater training.
Figure 4. Underwater sEMG signal acquisition and acquisition system integration. a) Experimental setup for underwater sEMG signal acquisition, b) sEMG signal recorded by commercial electrode, DAT-200 hydrogel electrode and STFH electrode within 1 hour, c) Time-frequency graph of sEMG signal recorded by STFH electrode, d) SNR and e) baseline noise of different electrodes at different operating hours, f) Experimental setup for underwater motor noise interference, g) sEMG signal recorded by three electrodes and SNR under h) interference, i) Mixing STFH electrode with Velcro, Bolton wireless EMG acquisition system integration, j) integration with self-developed wireless EMG acquisition wristband. The above results were recently published in the journal Chemical Engineering Journal under the title of "Starfish tube feet inspired hydrogel electrode for durable underwater sEMG acquisition". The first author is Ye Yuanming, an undergraduate student of Northwestern Polytechnical University; The corresponding authors are Associate Professor Wang Tengjiao and Associate Professor Ji Bowen of Northwestern Polytechnical University.
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Paper Links:
https://doi.org/10.1016/j.cej.2024.153882 Source: Frontiers of Polymer Science