A Californian professor has designed a “tattoo” that can turn sweat into electricity. But there’s still a lot of work before it becomes a practical form of charger.

Dr Wenzhao Jia of the University of California, San Diego, was originally working on a device to monitor lactate levels, designed for use by athletes in training. The BBC notes that she used temporary tattoo paper as a way to collect sweat samples to analyze for lactate levels, avoiding the disruption of collecting blood samples.

That inspired the idea of using an enzyme to isolate lactate electrons to turn into a current. It works as a proof of concept, but in the current form only generates around six microwatts of power.

Jia notes that in terms of the technology, this is a “substantial” amount to harvest, but it’s not enough for any practical purpose. Possible ways round this could be to use multiple biofuel cells or to explore making the device more sensitive to lactate.

The team also face the challenge of harnessing the power in a usable way, either to hook it up directly as a charge for mobile devices, or to add a capacitor for storage.

And for those of you thinking you aren’t fit enough to take advantage, it’s actually the unhealthiest athletes who generate most power — not just because they sweat more quickly, but because they get tired and produce high lactate levels sooner.