Researchers in the US are creating a skin patch that can test sweat droplets while people exercise and send the results to their smartphones, in a new way to track health and fitness. The experimental gadget, the size of a small coin, goes well beyond activity monitors such as the Fitbit. “Sweat has biochemical components that tell us a lot about physiological health,” says JohnRogers, who led the research and directs Northwestern University’s Centre for Bio-Integrated Electronics in Illinois. Today’s wearable technology helps people track their calories, activity and heart rate. A wearable biosensor would be “radically different”, Rogers says. For simple fitness purposes, it could give an early warning that it’s time to replenish electrolytes before someone starts to feel dehydrated. But eventually with additional research, Rogers envisions more sophisticated use of such devices, such as real-time monitoring of how the body adjusts during military training, or to screen people for diseases such as diabetes or cystic fibrosis. Rogers, who did much of the research while at the University of Illinois at Urbana-Champaign, has worked to develop electronic devices that can stretch and twist with the body. The skin-like sweat patch adds a capability called microfluidics, capturing and analysing tiny amounts of body fluid. Tiny channels collect perspiration and route it to different compartments where it interacts with chemicals that change colour to reflect sweat loss, acidity level, and concentrations of chloride, glucose and lactate. Together, those measurements can indicate such things as hydration levels or electrolyte loss. Hold a smartphone over the patch, and an app takes a picture of the colours and interprets the meaning. In two studies reported last week, Rogers’ team stuck patches to the arms and backs of 21 healthy volunteers. Nine rode exercise bikes in a gym to compare the sweat patches’ performance with the decidedly lower-tech method of sweat testing used today – taping on absorbent pads and carting the resulting wet samples to a laboratory. For a more real-world test, the other 12 bicyclists wore the patches while competing in a long-distance outdoor race in Tucson, Arizona. The patches stayed in place and worked during the challenging outdoor race, and the biochemical test results agreed with the indoor bikers’ conventional sweat tests, the researchers reported in the journal Science Translational Medicine. “It seems really practical,” says Stanford University chemical engineering professor Zhenan Bao, who also researches novel biomedical materials but wasn’t involved with the sweat patch. By simply looking at a colour change, “such a patch allows people to have an opportunity to understand their health and how it changes depending on activities”. It’s a growing field: other research groups around the country, including some of Bao’s colleagues, are pursuing wearable biosensors. Rogers’ sweat patches are designed for one-time use over a few hours. While the studies used an early version that analysed sweat just once during the exercise, he is now testing a design capable of multiple measurements over time.