Scaleable asymmetric “Janus” patch with seamless elastomer-hydrogel integration for wet on-skin electronic interfaces

• A Janus patch was developed by seamlessly combining elastomer and hydrogel layers, each with unique functions. • The hydrogel layer achieves wet adhesion through the absorption of the interfacial hydration layer. • The elastomer layer forms a durable barrier that minimizes environmental interference. • The EHI can adhere to wet skin surfaces and monitor human motion and physiological signals. • The asymmetric hygroscopic structure of EHI enables it to achieve self-powered sensing via ion diffusion. On-skin electronics create comfortable, stable and durable interfaces with the skin through appropriate adhesion. However, external environmental disturbances and detachment between electronics and wet or sweaty skin often compromise signal quality in human–machine interfaces. Here, we address this challenge by developing a scalable asymmetric “Janus” patch, Elastomer-Hydrogel Integration (EHI), inspired by the paper-sizing process. A comprehensive series of analytical, adhesion, mechanical, and electrical tests demonstrated that the hydrogel layer of EHI exhibits excellent adhesion to wet tissues, high conductivity, low modulus, and seamless skin contact providing versatile stable on-skin electronic interfaces. Moreover, the tough elastomer layer, thermally welded to the hydrogel layer, forms a protective barrier to minimize environmental interactions. This seamless integration of the two layers offers an optimal balance between adhesion, elasticity, and resistance to external interferences, endowing the EHI with conformal long-term adhesion to wet-skin, robust electromechanical response, and a high signal-to-noise ratio for electromyography signals. Additionally, due to its asymmetric hygroscopic structural design, a moisture-electric generator assembled from EHI enables self-powered respiration and sweat monitoring via ionic diffusion. This “Janus” patch represents a promising solution for practical applications in on-skin electronic interfaces.