Making Sweat Measurable: Induction, Sampling, and Refreshment in Wearable Biofluid Sensing.

Sweat has emerged as a promising biofluid for wearable health monitoring due to its ability to provide noninvasive and continuous access to a wide range of biomarkers. While advances in sensing materials and device integration have accelerated the field, measurement reliability is fundamentally governed by how sweat is induced, intercepted, transported, and refreshed at the skin-device interface. Outside of exercise or thermal stress, sweat secretion is often low, transient, and spatially heterogeneous, making sweat availability-not sensing chemistry-the primary constraint for continuous monitoring. This Review reframes sweat sampling as an integrated engineering pipeline spanning physiological induction, on-skin capture, directional transport, flow regulation, storage, and outlet-driven refreshment. We summarize sweat gland physiology and compositional dynamics that define biomarker accessibility, compare whole-body, localized thermal, and cholinergic induction strategies, and analyze modern microfluidic architectures designed to sustain temporal resolution under low and variable sweat flux. Particular emphasis is placed on system-level coupling between programmable induction and controlled fluid handling to enable quantitative and long-duration monitoring. By shifting focus from sensing chemistry alone to induction-sampling integration, we outline design principles required to make sweat a reliable and comparable biofluid for real-world wearable applications.