Stick-on patch monitors sweat compounds to determine exercise efficiency

When you're sweating away in the gym or pounding the streets on your morning run, it's important to think that all the misery is going to do some good in the end. But physiology is a more complex thing than most of us realise, and if you aren't managing your nutrients and hydration properly, it is possible that not only is your exercise regime not going to make you more healthy, it might even pose a risk. One thing that can help determine what is going on in your body as you exercise is, ironically enough, your sweat. Researchers at Northwestern University in Illinois have now developed a small, flexible stick-on patch that can analyse sweat and tell wearers or physicians monitoring them whether they are putting themselves in danger, or are on the road to better fitness.

Prof John Rogers, a specialist in biomedical engineering, materials science and neurosurgery at Northwestern's medical school, has for many years worked with Yonggang Huang on stretchy electronics that can move with the skin, and they have now for the first time incorporated these into a device that can analyse biofluids. "Sweat is a rich, chemical broth containing a lot of important compounds with physiological health information," Rogers said. "By expanding our previously developed 'epidermal' electronics platform to include a complex network of microfluidic channels and storage reservoirs, we can now perform biochemical analysis of this important biofluid."

About the size of a 10p piece, the device is soft and flexible and designed to be stuck on the forearm or back for a couple of hours. When sweat forms freely on the skin during exercise, the liquid is drawn into microfluidic channels in the device by capillary action, and carried through to four circular reservoirs. The inside of each of these is coated with a reagent that changes colour when exposed to specific chemical species: hydrogen ions (indicating pH), glucose, chloride and lactate.

The device also contains sensors that detect the proximity of a smartphone. When the user or monitor brings a phone close to the device, it automatically triggers an app which takes a photo of the device and analyses the colours of the four reservoirs, which indicates the concentrations of the four species. "We chose these four biomarkers because they provide a characteristic profile that’s relevant to health status information," Rogers explained. "The device can also determine sweat rate and loss, and it can store samples for subsequent laboratory analysis, if necessary.”

Huang noted that even with the group's previous experience, some more innovation was still necessary. "We already knew how to place electrons on the skin in a natural manner – here the challenge was dealing with fluid flow and the collection, storage and analysis of sweat in a thin, soft and flexible device," he said. "The sweat analysis platform we developed will allow people to monitor their health on the spot without the need for a blood sampling and with integrated electronics that do not require a battery but still enable wireless connection to a smartphone."

The team, which included collaborators from the University of Arizona and dermatology experts L'Oreal, tested their device on cyclists both indoors and participating in El Tour de Tucson, a long-distance endurance race in arid conditions. Comparing the results from the skin patch with those obtained by conventional methods of off-site analysis of absorbent swabs, they found the two sets were in agreement; moreover, the patches did not leak, stayed stuck on even in the desert, and provided high-quality data. Rogers’ team explains the research in a paper in the journal Science Translational Medicine, also mentioning that the device can detect markers for cystic fibrosis and may well have applications in diagnostic medicine as well as fitness.