Stick this sensor-embedded tattoo on your upper arm the next time you work out, and it will tell you how fit you are—and perhaps even power your smart phone someday. Scientists have developed a tattoo biobattery that converts lactate, a chemical in sweat, into electricity, researchers report today at a meeting of the American Chemical Society. During intense physical activity, the body activates a process called glycolysis to break down blood glucose and produce more energy, generating lactate in blood and sweat at the same time. Professional athletes monitor lactate levels to evaluate their training programs, and doctors look out for abnormally high lactate levels in patients as a sign of heart or lung disease. Until now, monitoring lactate levels has been a tedious, intrusive procedure that involves collecting blood at different times during exercise for analysis. So the researchers built a flexible sensor containing an enzyme that strips electrons from lactate to generate a weak electrical current. The sensor, which acts as the anode for the biobattery, is imprinted onto temporary tattoo paper with the cathode, according to a paper in Angewandte Chemie. As yet, electricity generated by the tattoo biobattery is too weak to power a watch, but researchers hope to enhance the design so it could eventually power small electronic devices.