In an effort to make the batteries that power wearables and medical devices safer, scientists have developed new flexible batteries that can run on saltwater — and maybe one day blood, sweat, or tears.

The lightweight batteries, described in a paper published this week in the journal Chem, can withstand being folded in half a hundred times. And they’re safer: instead of running on toxic chemicals, they’re powered by relatively harmless liquids like saltwater and IV rehydration solutions. That’s key if you have a battery powering a device that’s worn on or inside your body. That way, if the battery leaks, the stuff inside won’t hurt you.

Batteries work by storing electrical energy as chemical energy. They have three main components: two metal electrodes that conduct electricity — one that’s positively charged, one that’s negatively charged — and a soup or paste of electrolytes in between. When the battery is powering a device, ions shed their electrons and then drift through the electrolyte soup from one electrode to the other.

In a lot of the flexible batteries out there, these electrolyte solutions are made out of strong acids or toxic chemicals, the study says. That stuff is corrosive, flammable, or toxic, and you definitely don’t want it dribbling onto or into your body. That’s why scientists at Fudan University in China came up with a way to replace these toxic electrolyte solutions with something much less harmful.

Their batteries come in two different forms. One looks like a strip of tape, made of two flattened electrodes that sandwich the electrolyte in between. The other battery is composed of two tiny threads made out of carbon nanotubes. One thread incorporates specks of a positively charged electrode, while the other weaves in negatively charged electrodes. The two threads are then packed together into a tiny, hollow tube filled with the electrolyte solution. The idea is that these thread-like batteries could one day be woven into wearables or smart clothing.

The researchers experimented with a few different types of electrolyte solutions. The one that worked best was sodium sulfate, which is sometimes used as a laxative. But saline solutions, which are literally diluted salt water, also worked well. Eventually, bodily fluids like blood, sweat, or tears might take over the roll of the electrolyte solution to power medical implants, the study says.

You know what that means, right? The future of power might be in your pee.