Researchers at the University of California San Diego reported on their biobatteries at a recent meeting of the American Chemical Society. “Usually with wearable devices, you require a big battery,” Joseph Wang, distinguished professor of nanoengineering and head of the research team said. But these tiny temporary tattoos present an alternative.

The device runs on lactic acid, which is produced by the body during strenuous physical activity. In fact, the tattoos were initially designed as a diagnostic tool: Certain medical conditions can cause unusually high levels of lactic acid in sweat, and previous testing required doctors to collect blood samples throughout an exercise session. The sensors on the new device contain an enzyme that pulls electrons from lactic acid, producing a weak electrical charge that doctors can measure.

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Unfortunately, harvesting this power isn’t the most consistent way to power a device. In fact, test subjects who were less fit produced more lactic acid than their athletic counterparts, and therefore made more electrical power as well. And even on the sweatiest participants, who produced 70 microWatts of power per square centimeter of skin, the tiny electrodes could only store around 4 microWatts. It would take 10 microWatts to power a basic watch. But Wang doesn’t think that storing enough electricity to power cellphones and other “smart” devices is an inconceivable goal.