Cap-Sweat: a capillary microfluidic platform for digitized sweat sampling and time-resolved biomarker analysis

Wearable biofluid sampling systems are often limited by poor temporal resolution, evaporative loss, and dependence on glandular pressure for fluid propulsion. We present Cap-Sweat, a first-of-its-kind, ventless capillary microfluidic device that passively digitizes sweat into sequential, time-resolved compartments without requiring absorbent pads, hydrogels, or external actuation. Adapted from our Cap-Drop droplet platform for 3D cell culture, Cap-Sweat incorporates pre-programmed capillary circuits, multilayer laser-fabricated architecture, and hydrophilic-hydrophobic patterning to autonomously guide, delay, and arrest flow at discrete microchambers. A centralized, sealed venting mechanism prevents sample evaporation while maintaining bubble-free, pump-free operation. In vivo studies during exercise demonstrated continuous cortisol chrono-sampling with sub-minute resolution, capturing a transient early surge followed by fluctuating oscillations-dynamics that are undetectable using conventional continuous-flow or absorbent-based patches. This high-fidelity, temporally resolved biomarker profiling reveals previously obscured features of acute stress physiology and supports applications in neuroendocrine research, psychological health monitoring, and performance tracking. Cap-Sweat's compatibility with ELISA and future biosensor integration, combined with its scalable, low-cost design, positions it as a versatile platform for point-of-care diagnostics and personalized health analytics.