A team of researchers at the University of California, San Diego, has designed a sensor applied as a temporary tattoo that can not only monitor a person’s progress during exercise but produce power generated by their perspiration that may be used to energize small electronic devices.

They imprinted a flexible lactate sensor onto temporary tattoo paper and applied the tattoo to the upper arms of 10 healthy volunteers. The sensor contained an enzyme that strips electrons from lactate, generating a weak electrical current. Then, they measured the electrical current produced as the volunteers exercised at increasing resistance levels on a stationary bicycle for 30 minutes, which allowed them to continuously monitor sweat lactate levels over time and with changes in exercise intensity.

But measuring the lactate was just the first step. The team then built upon its findings to create a sweat-powered biobattery. They also discovered that the exercisers who were less fit produced more power by forming more lactate than those who were moderately fit.

The team explained that biobatteries offer certain advantages over conventional batteries: They recharge more quickly, use renewable energy sources (in this case, sweat), and are safer because they do not explode or leak toxic chemicals.