Sweat can now generate electrical energy, thanks to a fuel cell that’s powered by perspiration (Angew. Chem. Int. Ed. 2013, DOI: 10.1002/anie.201302922). The thin-as-skin device is designed to use lactate, a common chemical in sweat, as a biofuel. Joseph Wang and colleagues at the University of California, San Diego, prepare the devices on temporary transfer tattoos by using electrode printing technology. At the cathode, they use platinum, which reduces oxygen to water. At the anode, they use the commercially available enzyme lactate oxidase, which oxidizes the lactate in sweat into pyruvate and releases electrons. Wang’s team tested the devices on volunteers with various levels of physical fitness. When asked to cycle while wearing biofuel-cell tattoos, the least fit volunteers produced the most lactate in their sweat and therefore generated the most power—55 to 70 μW per cm2. The researchers tell C&EN that’s not enough electricity to power an iPod, for example, but the energy generated could be stored in a wearable device and used to power electronics.