Engineers have developed a device that can monitor an athlete's workout progress and also generate power from perspiration. Dr. Joseph Wang and his team at University of California San Diego recently demonstrated their technology at the national meeting of the American Chemical Society in San Francisco. It works by measuring a chemical called lactate that is naturally present in sweat - generally, the more intense the workout, the more lactate the body produces. The device is essentially a temporary tattoo with a lactate sensor imprinted on it. The sensor also strips electrons from lactate to generate a small electrical current. A biobattery in the sensor stores the electrical current; currently only about a few dozen microwatts of power can be harnessed. Wang hopes the device can eventually power a smart watch, heart rate monitor, or smartphone.