Working up a good sweat can be its own reward. But instead of letting all that bodily moisture go to waste, why not let it power your smartphone — via a smart tattoo?

Researchers at the University of California in San Diego, led by Joseph Wang, are wringing more out of sweat — specifically, enough electrical current to potentially power personal electronics while you’re still on the treadmill.

See also: 50 Tattoos That Prove Nerds Are Badass

They’ve developed a temporary tattoo sensor that responds to lactate — a byproduct of physical activity, found in sweat and in the bloodstream.

The sensor strips electrons from the lactate, enough to generate an electrical current. That enables doctors to measure lactate levels — but the team also built a bio-battery into the sensor, storing some of the electricity.

At the moment, each sensor collects a few microwatts of power. A recent study found a suspended smartphone pulls close to 70 mW in 2 minutes — a few orders of magnitude more than the sweat tattoo can handle just yet.

So you probably won’t be powering your iPhone via your skin any time soon — even Usain Bolt covered head-to-toe in tattooed-on batteries would be hard-pressed to get a charge. But the researchers say they’re working on more efficient energy storage tech.

In the meantime, they’re presenting their findings at this week’s meeting of the American Chemical Society.