Dual-Mode Paper-Based Epidermal Microfluidics for Sweat Rate Normalized Chloride Sensing

In resource-limited clinical settings, sweat chloride testing frequently departs from standardized protocols, leading to unreliable measurements and elevated false-negative rates. A central contributor to this failure is the lack of a unified, passive device that enables efficient sweat collection, quantitative chloride measurement, and sweat-rate normalization. Here we report a dual-mode paper-based epidermal microfluidic sensor that enables sweat-rate–normalized chloride analysis by integrating real-time sweat-volume quantification with distance-based and colorimetric chloride sensing in a fully passive skin-conformal patch. The laser-engraved paper microfluidic channel quantifies sweat volume with a sensitivity of 6.541 mm.µL⁻1 and a detection limit of 0.98 µL for localized sweat-rate estimation. In parallel, distance-based chloride sensing based on reverse Mohr chemistry provides a linear response across 0–100 mmol.l⁻1 with a sensitivity of 24.768 mm.mM⁻1 and a limit of detection of 0.094 mmol.l⁻1, while companion colorimetric zones enable rapid visual classification under low-sweat conditions using smartphone imaging. The device shows high repeatability (CV = 1.84%), selectivity against common sweat interferents, and storage stability for up to 71 days (CV = 9.25%). Pilot on-body measurements capture dynamic sweat chloride changes during exercise and electrolyte modulation, highlighting the value of sweat-rate normalization for physiologically meaningful interpretation. With an estimated material cost of less than USD 0.02 per device, this platform provides a scalable approach to sweat chloride testing in resource-limited settings.