Nanofiber Based Janus Electrode with Stable Asymmetric Wettability for Long-Term Biosignal Monitoring

To monitor the bio-signals with high accuracy for a long time, it is important to keep the skin and electrodes dry. Sweat can cause noise in the readings due to interactions with the electrode material. The Janus electrode, which is a type of dry electrode made from nanofibers, has a special property that lets it absorb sweat and move it to the outside, keeping the skin and electrode interface dry. This process is known as the wicking phenomenon. The Janus electrode can reduce noise from sweat and maintain stable contact with the skin. Janus electrodes are made from layers of hydrophilic (water-attracting) and hydrophobic (water-repelling) membranes, with the conductive part on the hydrophobic side, which touches the skin. The wicking effect happens due to the combined action of capillary and hydrostatic forces from the hydrophilic layer. However, it is challenging to make Janus electrodes that stick to the skin for days despite sweat. Common wet adhesives used for good skin contact can dry out over time and become unstable when exposed to water or sweat, reducing electrode adhesion and affecting bio-signal quality. Additionally, hydrophilic layers can dissolve in water, breaking their polymer chains.<br/>Here, utilizing a Janus electrode, a stacked layer of ultrathin hydrophobic microporous Au-membrane, and water-durable hydrophilic nanofibers, asymmetric wettability can be realized and maintained for 7 days. Thus, it can create a spontaneous unidirectional sweat transport out from the skin surface, ensuring the skin-electrode interface keeps dry, especially during sweating. The ultrathin hydrophobic membrane facilitates self-adhesion with an adhesion energy of 50.7 µJ cm^-2, which creates a high conformal contact with the skin and self-adheres to the hydrophilic nanofibers. The hydrophilic nanofibers show excellent durability to continuous water immersion for up to 1 month. Additionally, a thin translucent polyvinyl alcohol (PVA) nanofibers frame assists the Janus electrode to achieve a highly conformable attachment on both dry and sweaty skin. By the overall porous structure as well as the fine thickness scale, the Janus electrode also exhibits excellent breathability and high mechanical stability. With all these properties, the Janus electrode can monitor a long-term electrocardiogram (ECG) signal after dynamic activities, including physical exercising, after awaking, after desk work, after meals, and after going out for 6 days while maintaining a stable signal-to-noise ratio (SNR) of ~18.8 dB.