Image copyright J.Wang/ACS Image caption The biobattery tattoo is fuelled by lactate - naturally present in sweat after vigorous exercise

A sweaty gym workout is not only good for your health - it could also energise your phone.

A tattoo that produces power from perspiration has been unveiled at the American Chemical Society meeting.

The biobattery is fuelled by lactate - which is naturally present in sweat after vigorous exercise.

It could soon power heart monitors, digital watches and eventually even smartphones, say scientists in California.

The dream of "people power"- using the body to charge portable electronic gadgets - has inspired many innovative approaches.

Some harness movement - via piezoelectrics - while others use blood to power implanted biofuel cells.

"Our device is the first to use sweat. It's a proof of concept," said Dr Wenzhao Jia of at the University of California, San Diego, who gave details of her method in the journal Angewandte Chemie.

"At the moment the power is not that high - only four microwatts. But we are working on enhancing it so it can power small electronic devices."

Interestingly, her team did not set out to build a biobattery. Their aim was to make a wearable monitor for lactate.

Athletes in training measure their lactate levels to evaluate their work-rate and fitness.

But monitoring it can be inconvenient as it typically relies on taking blood samples.

To develop a faster, more comfortable test, Dr Jia printed a lactate sensor onto temporary tattoo paper.

"I've worn it myself - you don't even feel it. It really is like a tattoo," she told BBC News.

"It's not just for athletes. Most people who exercise want to know how they can improve their workout.

"We can measure our heart rate - but if you combine that physical feedback with chemical data you get a much more comprehensive view of your exercise status."

Her team then went a step further, turning the sensor into a sweat-powered biobattery.

They incorporated an enzyme that strips electrons from lactate, generating a weak electrical current.

When volunteers on an exercise bike wore the tattoo, they were able to generate up to 70 microwatts per sq cm of skin.

Interestingly, people who were less fit produced the most power. While those who exercise most (more than three times per week) produced the least.

"We think that's because less fit people become fatigued sooner, so they form more lactate," Dr Jia explained.

"A fit person is going to have to work out much harder to power the battery."

Her lab has partnered with a start-up company to develop the product.

Next steps include linking the tattoo to portable gadgets, and adding a way to store the generated current - by integrating a device such as capacitor.

But the main challenge is to ramp up the power. More than double the current value would be needed for a digital watch - 10 microwatts.

"It's a challenge because our electrodes are only very small - just 2x3mm," said Dr Jia.

One route is to make the device more sensitive to lactate.

Another is to incorporate several biofuel cells - connected in serial or parallel.

But why use the body at all? Why not simply miniaturise conventional batteries and make them wearable?

"Because biobatteries offer certain other advantages," explained Dr Jia.

"They recharge more quickly. They are safer as there is no risk they will explode or leak toxic chemicals.

"And they use a renewable energy source. You."

Follow James on Twitter.