Forget batteries and electricity — the tiny robots of the future could run on moisture.

Researchers at Seoul National University in South Korea, have created robots that can “inchworm” forward by absorbing humidity from their surrounding environment. The so-called hygrobots, described this week in the journal Science Robotics, can crawl, wriggle back and forth, and twist like a snake. And in the future, they could be used for a variety of applications, including delivering drugs to human skin.

The inspiration for the tiny bots came from plants, which can change shape and size by absorbing water from the ground or air (called hygroexpansion). For instance, pine cones close when it’s wet, and open when it’s dry so that the seeds inside them can disperse farther away. Plants have inspired robots before: last year, for instance, researchers made robots out of algae.

The robots in question aren’t made of plant material; they just mimic the mechanisms behind plants. Making a robot that runs on moisture is valuable because moisture is an all-natural source of power. It’s also not toxic, unlike batteries, which can and do explode. That’s key for microrobots that need to be used inside the human body (like the spermbot).

Mimicking the bristles of the Pelargonium carnosum seed, a shrubby plant from Africa, the hygrobot has two layers made out of nanofibers: one layer absorbs moisture and the other doesn’t. When the bot is placed on a wet surface, the humidity-sucking layer swells, shooting the bot up, away from the surface. Once the layer dries, the bot goes back down and the cycle repeats. That allows the bot to move.

To prove its potential, the researchers showed that a hygrobot soaked in antibiotics could inch across a culture plate filled with bacteria, leaving behind a sterilized trail, a bit like a slug would leave a trail of slime in its wake. In the future, these robots could deliver drugs to the human body, propelling themselves only by using skin moisture. They could also be equipped with sensors that respond to other gases, not just water vapor.

This is not the first tiny robot scientists are working on for biomedical applications. Last year, for instance, researchers created a hydrogel bot that’s activated by a magnet and could release localized doses of chemo to treat tumors.