Hydrogel-based sweat chloride sensor with high sensitivity and low hysteresis

Monitoring chloride ions (Cl⁻) in sweat is critical for assessing hydration, diagnosing cystic fibrosis (CF), and evaluating other health conditions. Existing wearable sweat chloride sensors either exhibit low sensitivity based on potentiometric sensing or irreversible readings based on colorimetric sensing. To address these challenges, we report the design of a hydrogel-based wearable sweat sensor that allows for monitoring of Cl⁻ based on an electrolyte concentration gradient. This reported sensor features a coplanar design with a cation-selective hydrogel (CH), a high-salinity hydrogel (HH), and a sweat chamber. The gradient between the HH and the sweat chamber drives ion diffusion through the CH, generating an open-circuit voltage (VOC) that corresponds to the Cl⁻ concentration in the sweat chamber. A stable device performance is achieved by further integrating a superhydrophilic poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) thin film that prevents ion exchange and hydrogel swelling. The resulting sensor exhibits an ultrahigh sensitivity of 1.7 mV/mM and a fast response time of 7.1 s, with excellent linearity and reversibility in the range from 10 to 100 mM. Integrating the sensor with a microfluidic module along with temperature calibration provides continuous and calibrated, accurate measurements during physical activities. The design concepts for real-time sweat Cl⁻ detection can also be applied to monitor the other biomarkers for personalized diagnostics and health monitoring.