It doesn’t sound appealing, but scientists at the University of California, Berkeley, are developing wearable skin sensors that can detect what’s in a person’s sweat. The researchers believe that by monitoring perspiration, they could bypass the need for more invasive procedures like drawing blood and provide real-time updates on health problems such as dehydration or fatigue.

The research, recently published in Science Advances, describes a new sensor design that can be rapidly manufactured using a roll-to-roll technique that essentially prints the sensors onto a sheet of plastic. The sensors are used to monitor the sweat rate, and the electrolytes and metabolites in sweat, from volunteers who were exercising, and others experiencing chemically induced perspiration.

“The goal of the project is not just to make the sensors, but start to do many subject studies and see what sweat tells us—I always say ‘decoding’ sweat composition,” said Ali Javey, a professor of electrical engineering and computer science at UC Berkeley and the paper’s senior author.

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“For that we need sensors that are reliable, reproducible, and that we can fabricate to scale so that we can put multiple sensors in different spots of the body and put them on many subjects,” added Javey, who also serves as a faculty scientist at Lawrence Berkeley National Laboratory.

The sensors contain a spiraling microscopic tube, or microfluidic, that wicks sweat from the skin. The sensors track how fast the sweat moves through the microfluidic to determine the person’s sweat rate. At the same time, chemical sensors in the microfluidics detect concentrations of electrolytes like potassium and sodium, and metabolites like glucose.

The researchers placed the sweat sensors on different spots on volunteers’ bodies—including the forehead, forearm, underarm and upper back—and measured their sweat rates and the sodium and potassium levels in their sweat while they rode on an exercise bike. They found that local sweat rate could indicate the body’s overall liquid loss during exercise. This information could be used to warn athletes if they are pushing themselves too hard.

The sensors were also used to compare sweat glucose levels and blood glucose levels in healthy and diabetic patients, finding that a single sweat glucose measurement cannot necessarily indicate a person’s blood glucose level.

“There’s been a lot of hope that non-invasive sweat tests could replace blood-based measurements for diagnosing and monitoring diabetes, but we’ve shown that there isn’t a simple, universal correlation between sweat and blood glucose levels,” said Mallika Bariya, a graduate student in materials science and engineering at UC Berkeley and the other lead author on the paper. “This is important for the community to know, so that going forward we focus on investigating individualized or multi-parameter correlations.”

To manufacture the sensors, Javey and his team worked with researchers at the VTT Technical Research Center of Finland to produce sensor patches in a roll-to-roll processing technique similar to screen printing.

“Roll-to-roll processing enables high-volume production of disposable patches at low cost,” said Jussi Hiltunen of VTT. “Academic groups gain significant benefit from roll-to-roll technology when the number of test devices is not limiting the research. Additionally, up-scaled fabrication demonstrates the potential to apply the sweat-sensing concept in practical applications.”