Directional permeation-driven microfiber composite hydrogel towards rapid sweat uptaking and hydration monitoring

Wearable sweat rate and electrolyte sensors offer real-time assessment of hydration status. Current epidermal microfluidic devices represent the widely adopted approach; however, their limitation for microliter-scale sweat collection often results in response latency and compromised detection accuracy. A rapid sweat-absorbing material (RSAM) filled in the collection chamber between the microfluidic device and the skin has been demonstrated as an effective solution. This work proposes a polyvinyl alcohol@polyurethane microfiber composite hydrogel (PVA@PU MH) with unidirectional sweat-transport capability in the inlet chamber of a microfluidic. The optimized PVA@PU MH exhibits a sweat collection efficiency that is 49.76 ± 6.75% higher than traditional methods. With anisotropic microchannels, PVA@PU MH leverages capillary action to confine sweat laterally and drive vertical transport directionally. Additionally, the integration of conductivity-sensing components within the microfluidic system enables the detection of both sweat rate and electrolyte concentration. A low-power unit was developed to process and wirelessly transmit real-time sweat data to mobile devices for continuous monitoring. The PVA@PU MH facilitated both faster sweat uptake and more physiologically representative analyte readings, as evidenced by a strong correlation with whole-body measurements. The proposed strategy rapidly acquires microliter sweat samples, substantially expanding wearable monitoring capabilities.