Temporary tattoos are emerging as a preferred way to monitor health. Researchers at the University of California, San Diego have developed a sensor that monitors chemicals in the wearer’s sweat to gauge physical exertion. The temporary electrochemical tattoo relies on a carbon fiber backbone so that it bends with the skin during normal activity without loss of functionality, and a conductive polymer that is sensitive to pH-levels in sweat. The sensor is screen-printed onto the flexible adhesive. The tattoo is noninvasive and the results are immediate.