Electrophysiological signals (EPS), such as electromyograms (EMG), electrocardiograms (ECG), and electroencephalograms (EEG), are vital for understanding human physiology and have numerous applications in healthcare monitoring, prosthetic control, and human-machine interfaces. However, accurately monitoring these signals during strenuous exercise poses a significant challenge due to the dynamic movement and profuse sweating of the skin. Current methods utilize gel electrodes and dry electrodes. Gel electrodes, while providing good signal quality, suffer from dehydration, skin irritation, and discomfort with prolonged use. Dry electrodes offer a more comfortable alternative, but improvements are needed to handle the conditions of exercise and sweating. Previous research has focused on enhancing electrode-skin adhesion to mitigate motion artifacts during exercise using techniques like ultrathin electrodes, microstructured electrodes, and in-situ formed electrodes. Other studies have tackled the problem of sweating by incorporating porous materials such as porous films, leather, textiles, and electrospun mats to improve water vapor permeability. However, a single electrode system effectively addressing both rapid movement and profuse sweating during strenuous exercise remains elusive. This research aims to address this gap by developing a novel dry electrode that maintains a stable interface with the skin under dynamic movement and sweating conditions, ensuring reliable EPS acquisition during strenuous activity.

The existing literature demonstrates advancements in both motion artifact reduction and sweat management in dry electrodes. Researchers like Takao Someya's group have developed sub-300 nm dry thin-film electrodes that minimize motion artifacts through self-adhesive and conformable designs, enabling EP signal monitoring even with skin vibrations up to 15 µm. Seokwoo Jeon's group integrated gecko-inspired microstructures to enhance adhesion and achieve biosignal measurement during various movements. Cunjiang Yu's group has explored in-situ formed electrodes that accommodate stretching. For sweat management, several approaches have been explored, focusing on enhancing water vapor permeability through porous substrates. While these studies individually address motion artifacts or sweat, a comprehensive solution for both remains a challenge. This work builds upon these prior efforts, aiming to create a dry electrode that is both highly stretchable and highly water-permeable, resulting in superior performance during strenuous exercise.

The researchers fabricated a porous, nano-thick stretchable dry electrode system using a novel one-step method based on the Marangoni effect. This method involves dropping a polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) solution onto the surface of water. The evaporation of toluene from the SEBS solution leads to a decrease in surface tension and an increase in viscosity, resulting in the formation of pores in the ultrathin SEBS film due to the Marangoni effect. This process is quick (less than 30 seconds) and requires minimal equipment. A layer of gold was then sputtered onto the porous SEBS film to create the conductive electrode. The electrode's thickness, stretchability, and permeability were characterized. The researchers employed a tensile testing machine to quantify the force between the electrode and the skin. The connection between the electrode and the signal acquisition equipment was designed to ensure stability using flexible conductive medical dressing and anisotropic conductive films. The electrode's performance was evaluated under various conditions, including dynamic stretching, arm swinging with varying accelerations (up to 10g), running at different speeds (up to 6 km/h), and artificial sweating conditions. EMG, ECG, and EEG signals were recorded and analyzed to assess the electrode's performance. The water vapor transmission rate (WVTR) and sweating rate (SR) were measured using established methods (ASTM E96-95). The study included multiple participants, and all experiments were conducted with ethical approval.

The fabricated electrode exhibited exceptional characteristics: a thickness of 200 nm, a stretchability of 120%, and a water permeability of 25.3 g m⁻² h⁻¹. The electrode maintained a stable interface with the skin through van der Waals forces, even under strenuous exercise. High-quality EMG signals were successfully recorded during arm swinging with an acceleration of 10g and a sweating rate of 2.8 mg cm⁻² min⁻¹. Similar results were observed during running at 6 km/h. The electrode also demonstrated excellent performance in ECG and EEG recording under both resting and strenuous exercise conditions, with sweat rates reaching 3.2 mg cm⁻² min⁻¹ for ECG and 2.6 mg cm⁻² min⁻¹ for EEG. The high water vapor permeability of the electrode prevented sweat accumulation, maintaining skin comfort and preventing inflammation. The 200 nm thickness was crucial for maintaining stability during stretching, unlike thicker electrodes (1 µm and 5 µm) which showed motion artifacts. High-density EMG acquisition was successfully demonstrated using an 8-channel electrode array.

The findings demonstrate a significant advancement in stretchable dry electrode technology for continuous multi-channel electrophysiological monitoring during strenuous exercise. The combination of high stretchability and high water permeability addresses the long-standing limitations of existing dry electrodes in accurately monitoring biosignals under dynamic and sweating conditions. The success of EMG, ECG, and EEG recording under these challenging conditions highlights the potential of this technology for various biomedical applications. The novel fabrication method based on the Marangoni effect offers a simple, scalable, and cost-effective approach to produce high-performance electrodes. The results validate the feasibility of using this type of electrode in realistic scenarios involving vigorous physical activity. This technology could revolutionize wearable health monitoring, advanced prosthetic control, and human-machine interfaces.

This study successfully developed a porous nano-thick stretchable dry electrode system that excels in continuous multi-channel electrophysiological monitoring during strenuous exercise. Its superior stretchability and water permeability ensure stable signal acquisition even under challenging conditions. The findings demonstrate its potential to transform various biomedical fields, paving the way for more accurate and comfortable wearable sensor technologies. Future research could focus on further enhancing the electrode's mechanical robustness, improving the electrode-connector interface, and exploring its applications in various clinical settings.

While the electrode demonstrates excellent performance, certain limitations exist. The electrode's relatively low mechanical strength makes it susceptible to damage under high pressure. The difference in Young's modulus between the electrode and connector could lead to strain concentration at their interface, potentially affecting signal acquisition. Further research is necessary to address these issues and enhance the electrode's overall durability and performance under extreme conditions.