A sweat-sensing skin patch that connects wirelessly with a smartphone may soon be able to track health during exercise and warn against dehydration. In a new study of 21 healthy volunteers, the soft and stretchable device successfully collected sweat and captured key markers, ranging from sweat rate to glucose and pH. Wearable technologies offer a new way of monitoring health and fitness. Sweat is an untapped, yet rich source of biomarkers - including electrolytes, small molecules, and proteins - that could potentially be used to diagnose diseases like cystic fibrosis. Ahyeon Koh and colleagues have designed a soft and thin microfluidic device that sticks to the skin to catch and analyze sweat. The sweat is routed to different channels and reservoirs that can measure sweat volume, pH, lactate, chloride, and glucose concentrations using colorimetric readouts. Using image processing software and wireless data transmission, the researchers analyzed the readouts by taking a picture of the sweat patch with a smartphone. The devices were tested in nine healthy volunteers during indoor cycling in a controlled lab setting as well as in 12 participants during a long-distance outdoor bicycling race - mimicking a "real-world" setting. Readouts of biomarkers from the patch reliably matched those from conventional laboratory analysis of sweat. The devices also stayed in place without causing discomfort or irritation. The researchers say that their technology could potentially extend to other fluids, such as tears and saliva, and may even be adapted for detecting illicit drug use.