Janus electrode with stable asymmetric wettability for robust biosignal monitoring on sweaty skin

Realizing an electrode that can stably monitor biosignals after multiple exposures to sweat is challenging. Utilizing a Janus electrode, which is composed of a stack of ultrathin hydrophobic microporous Au membrane and water-durable hydrophilic nanofiber layers, asymmetric wettability can be realized and maintained for 7 days. Thus, it can create spontaneous unidirectional sweat transport from the skin surface, ensuring that the skin-electrode interface remains dry, especially during sweating. The ultrathin hydrophobic membrane facilitates self-adhesion with an adhesion energy of 59.2 µJ cm−2, which creates a high conformal contact to the skin and self-adhering to the hydrophilic nanofibers. The hydrophilic nanofibers exhibit excellent durability even after continuous immersion in water for up to 1 month. Additionally, a thin translucent polyvinyl alcohol nanofibers frame assists the Janus electrode in achieving highly conformable attachment to both dry and sweaty skin. The Janus electrode also exhibits excellent breathability and high mechanical stability owing to its porous structure and fine thickness. With these properties, the Janus electrode can monitor an electrocardiogram signal after dynamic activities, including physical exercise, waking up, desk work, meals, and going out for 6 days, while maintaining a stable signal-to-noise ratio of ∼ 18.8 dB.