Microfluidic valves for wearable sweat analysis: a review.

Wearable sweat analysis can extend health monitoring from clinics to daily life. However, sweat is produced slowly and at changing rates, which makes sweat handling difficult. During collection and transport, sweat may remain in the device, flow backward, mix with older sweat, or become contaminated. The liquid that reaches the sensor may therefore differ from freshly secreted sweat. Microfluidic valves can control when sweat moves, where it flows, and how it reaches the sensing region. This review compares five valve classes relevant to wearable sweat systems. They are capillary, Tesla, mechanical, stimuli-responsive, and electro-actuated valves. Each class is assessed in terms of its working principle, flow-control function, power requirement, reusability, suitable task, and practical limit. The comparison covers sweat retention, backflow control, time-separated collection, reagent isolation, and replacement of older sweat at the sensor. The reviewed studies are grouped by tested medium and setting, covering laboratory liquids, artificial sweat, collected human sweat, and direct on-body transport. The main unresolved problems are changing sweat rate, deformation during wear, limited reusability, and unstable materials or interfaces. Solving these problems is important for reliable valve operation during long-term wear.