In the future, tiny robots such as the hygrobot will be able to avoid the need for batteries and electricity to power them. Like a worm or a snake, moisture will power these tiny wriggly robots.

Hygrobots actually inch forward by absorbing humidity from their surrounding environment. Created by researchers at the Seoul National University, South Korea, these tiny robots can twist, wriggle forwards and back, and crawl just as snakes or worms do. The researchers envisage these hygrobots being useful for a variety of applications in the future, which could include delivering drugs within the human body.

According to the researchers, they received the inspiration for hygrobots from observing plants, and they have described their findings in the journal Science Robotics. Using hydroexpansion, plants change their shape and size when they absorb water from the air or ground. For instance, pinecones know when to close and when to open, depending on whether the air is wet or dry, and this helps them to disperse seeds more effectively. Even earlier, plants have provided inspiration for robots—researchers created robots in imitation of algae.

Although hygrobots are not made of plant cellulose, they mimic the mechanism the plants use. As moisture is available almost everywhere, using it as a source of power for operating robots makes sense. Unlike batteries, moisture is non-toxic, and does not have the tendency to explode. This is an important consideration, as microbots, for instance the spermbot, are usually required to operate within the human body.

One can visualize the motion of hygrobots by observing the Pelargonium carnosum seed bristle—a shrub-like plant found in Africa. The hygrobot mimics the motion of the bristles, as it has two layers made of nanofibers. While one layer absorbs moisture, the other does not.

Placing the bot on a wet surface causes the humidity-absorbing layer to swell up, making the bot move up and away from the wet surface. This allows the layer to lose moisture and dry up, and the bot comes back down—the cycle repeating itself—allowing the bot to move. The researchers demonstrated a hygrobot coated with antibodies crawling across a bacteria-filled culture plate. It could sterilize the entire plate without requiring any artificial power source.

This is how the researchers imagine the bots of the future will deliver drugs within the human body, propelling themselves using only the moisture of the skin. Other than responding only to water vapor, researchers say they could equip them with sensors that respond to other gases as well.

However, this is not the first instance of scientists working with tiny robots. Last year, researchers had created a hydrogel bot for biomedical applications that a magnet could activate. It was able to release localized chemo doses for treatment of tumors.

Not only medical, military, and industrial applications will also benefit from light and agile microbots that do not require additional power inputs to operate. Hygrobot, the biologically inspired bilayer structure harnessing energy from the environmental humidity uses ratchets to move forward. The hygroscopically responsive film quickly swells and shrinks lengthwise in response to a change in humidity.