Measuring the Salt Content of Sweat inside a Sweat-Absorbing Skin Adhesive

Biofluids contain a wealth of different biomarkers, and their concentrations are indicative of the state of the body. As one of those biofluids, sweat is easily accessible, and its composition can, for example, be related to particular diseases or sports performance. Due to the relatively low sweat flow rates, however, adequate sampling is paramount. Here, we aim to explore the potential use of sweat-absorbing skin adhesives as a sweat sampling system for wearable sensors with a simple construction. Upon absorption of sweat, the electrochemical properties of the skin adhesive are determined by the composition of sweat and the amount of sweat within the skin adhesive (i.e., hydration). Through the incorporation of two polarizable electrodes within the skin adhesive, its electrical properties can be monitored using impedance spectroscopy. Here, the double layer capacitance is used as an indicator of hydration, while the conductance depends on both the ion concentration and hydration (the mobility of ions). By evaluating the conductance as a function of hydration, the ion concentration within an electrolyte solution can be estimated. We demonstrate the concept based on a simple model sensor patch, which is exposed to electrolyte solutions containing various concentrations of NaCl and an artificial sweat solution. Finally, we show that ion concentrations in human sweat can be estimated when the model sensor patch is worn during exercise.