Sweat does more than just cool down an overheating body. Measuring the chemical makeup of an individual's sweat - specifically the levels of chloride, a chemical component of salt - can serve as an early warning system to help inform the diagnosis of cystic fibrosis, a genetic disease that damages the lungs and digestive system.
A group of researchers at Penn State recently developed a wearable device capable of accurately tracking chloride ion levels in sweat, which is essential for evaluating hydration status and health conditions like cystic fibrosis and more. Their sensor allows for real-time tracking of an exercising person's sweat through a hydrogel-based design that allows the device to operate with enhanced sensitivity, accuracy and efficiency, all while being reusable. Their research, available online, is set to publish in the November issue of Biosensors and Bioelectronics.
"The traditional method of measuring chloride ion levels is to go to a hospital and have the measurements taken, which is time consuming and expensive," said Wanqing Zhang, a doctoral candidate in engineering science and mechanics and co-author of the paper. "The wearable sensors we developed process sweat and track chloride ion levels in real time, directly on a subject's body. This gives researchers a lot of information about an individual's health and, specifically for this study, can identify the high chloride ion levels that signify the presence of cystic fibrosis."
Wearable sensor technology is not new, with several other devices - including those that detect specific biomarkers in sweat - originating just from research at Penn State. However, Zhang explained how different existing designs face different major issues. Colorimetric based sweat sensors, which change color depending on the presence of a specific chemical or reaction, cannot produce reversible readings. If the sensor detects high chloride ion levels, it cannot revert to a neutral state and measure low levels, meaning that researchers can only take one accurate reading before needing to apply a new sensor. Another design, known as a potentiometric sweat sensor, operates by measuring the potential energy difference between two electrodes. While these sensors offer continuous monitoring, they typically have a limited sensitivity and rely on expensive ion-selective membranes to function.
According to Zhang, the research team's new sensor uses multiple types of hydrogel - a water-rich, gel-like material made of networks of connected molecules called polymers - to address these issues simultaneously.
The team's sensor contains a sweat chamber, a cation-selective hydrogel (CH) with mobile cations and a high salinity hydrogel (HH) with high salt content like sweat. When sweat enters the chamber, the difference in salt concentration between the sweat and the HH causes the mobile cations in the CH to move from the HH side to the sweat chamber side, generating open-circuit voltage (OCV) between the two points. By tracking this voltage - which indicates how many chloride ions are present in the sweat sample - they can track the levels of chloride ions.
"In other sensor designs, it is extremely difficult or impossible to effectively track small fluctuations in the chloride ion levels," said Huanyu "Larry" Cheng, the James L. Henderson, Jr. Memorial Associate Professor of Engineering Science and Mechanics and corresponding author on the paper. "By incorporating two different types of hydrogel into the design of our sensor, we can measure the change in OCV across the sensor in real time, meaning we can follow the fluctuation of chloride ion levels in our subject's sweat."
