Stretchable ionic-electronic bilayer hydrogel electronics enabled in situ detection of solid-state epidermal biomarkers

Continuous and in situ detection of biomarkers in biofluids (e.g., sweat) can provide critical health data but is limited by biofluids accessibility. Here, we report a sensor design that enables in situ detection of solid-state biomarkers ubiquitously present on human skin. We deploy an ionic-electronic bilayer hydrogel to facilitate the sequential dissolution, diffusion and electrochemical reaction of solid-state analytes. We demonstrate continuous monitoring of water-soluble analytes (i.e., solid lactate) and water-insoluble analytes (i.e., solid cholesterol) with ultralow detection limits of 0.51 nmol cm^-2 and 0.26 nmol cm^-2, respectively. Additionally, the bilayer hydrogel electrochemical interface reduces motion artifacts by three times compared to conventional liquid-sensing electrochemical interfaces. In a clinical study, solid-state epidermal biomarkers measured by our stretchable wearable sensors showed a high correlation with biomarkers in human blood and dynamically correlated to physiological activities. These results present routes to universal platforms for biomarker monitoring without the need for biofluid acquisition.