Three‐Stage Fluid‐Assisted Dual‐Mode Janus E‐Skin for Thermally Comfortable Multiplexed SERS Sweat Analysis and Strain Monitoring

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Wearable sweat sensors face persistent trade‐offs between multiplexed detection, signal stability, and wearing comfort. Here, we present a Janus electronic skin (e‐skin) with dual‐mode functionality, offering both thermal and moisture comfort. It integrates unidirectional sweat transport with multiplexed surface‐enhanced Raman spectroscopy (SERS) sweat analysis and strain sensing. This design is enabled by a three‐stage fluid‐assisted enhancement mechanism. The asymmetric structure establishes a wettability gradient and capillary pressure difference, driving directional fluid flow from the hydrophobic to hydrophilic side within 15.6 s. This directional transport enables a three‐stage fluid‐assisted enhancement mechanism: (i) efficient analyte delivery toward plasmonic hotspots; (ii) evaporative preconcentration increasing local analyte density; and (iii) dielectric environment optimization maintaining a refractive index of 1.33. This synergistic mechanism amplifies SERS signals by 22.79‐fold with an enhancement factor of 10 6 , validated by finite‐difference time‐domain (FDTD) simulations. Consequently, the platform enables simultaneous, label‐free component analysis of eight disease biomarkers, with principal component analysis confirming clear spectral discrimination among analytes, and real human sweat validation demonstrating practical feasibility. This markedly enhances multiplexing capability, overcoming the limited analyte range of conventional sensors. Beyond SERS biosensing, the same platform functions as a high‐sensitivity strain sensor (gauge factor = 445.27) with sweat‐resistant stability—a dual‐mode capability rarely integrated in existing Janus sweat sensors. Combined with high water vapor transmission (1141.04 g m −2 day −1 ) and evaporative cooling, this work resolves key trade‐offs between analytical performance, multiplexing breadth, and wearability for next‐generation personalized health monitoring.