Wearable Organic Electrochemical Transistor Biosensor for Real-Time Measurement of Sweat Salinity and Saturation

Wearable sweat sensing offers a noninvasive strategy for continuous monitoring of hydration, electrolyte balance, and physiological stress. Here, we demonstrate a flexible textile-based organic electrochemical transistor (OECT) biosensor for real-time quantification of sweat salinity and saturation during physical activity. The device consists of cotton fibers functionalized with the conductive polymer PEDOT:PSS, configured as channel and gate electrodes within a biocompatible patch. The gate and drain currents measured by the device in response to sweat are decoded using a mathematical model that estimates both the total salt concentration and the sweat saturation level. Unlike conventional enzymatic or membrane-based sensors, the platform operates without selective coatings, improving robustness, reproducibility, and mechanical stability. In in vitro tests, the device exhibited a linear response in the 10–4 M–1 M concentration range, a limit of detection as low as 10-5 M, and a response time below 40 s. Moreover, the sensor device was validated in three participants during treadmill running sessions lasting up to 1 h. The saturation profiles derived from the sensor showed a very high correlation with facial skin temperature measured by infrared thermography in all participants (up to 0.90), and with heart rate variability (HRV) derived from ECG recordings in two of the three participants (up to 0.86). Cross-correlation analysis revealed that the time lag between the onset of the exercise-induced increase in body temperature and the onset of sweating ranges from 600 to 740 s, in line with values predicted by the literature. These results demonstrate accurate, sensitive, and continuous tracking of sweat dynamics in real time. The proposed wearable OECT platform establishes a robust foundation for biomedical applications including monitoring of hydration disorders, electrolyte imbalance, metabolic dysfunction, and autonomic dysregulation.