Development of a Microfluidic Sweat Rate Sensor for Continuous Monitoring in a Wearable Skin Patch

Abstract

Wearable technologies enable non-invasive and continuous monitoring of physiological parameters offering cost-efficient alternatives to clinical assessments. Among them, sweat sensors expand the available biometrics by providing access to biochemical information. Analyte concentrations in sweat vary substantially with inter- and intra individual differences in sweat rate (SR). Therefore, sweat sensors need to measure SR to deliver reliable data. This work presents the design and development of a microfluidic SR sensor for continuous monitoring in a skin patch. The device uses electrodes patterned on polyimide and embedded within polydimethylsiloxane (PDMS) microfluidic channels to quantify SR by tracking fluid progression through impedance measurements. Two electrode materials, thin-film gold electrodes and laser-induced graphene (LIG) electrodes, were investigated for this application. Gold electrodes showed a more stable SR sensing performance demonstrating high sensitivity across physiologically relevant SRs, ranging from values typical at rest (0.2 µL/min) to physical exercise (5 µL/min). Additionally, the sensor maintained a stable performance under mechanical deformation as well as during on-body testing. This supports the sensor’s suitability for wearable applications. The sensor response was also evaluated across electrolyte concentrations (10-100 mM) and temperature variations (20-40 ºC), highlighting the need to include calibration strategies that can compensate for these effects. The proposed sensor architecture was designed to be compatible alongside ion-selective electrodes, for sodium and chloride sensing, to support future incorporation into a complete wearable sweat analysis platform.