As strange as it sounds, the future of skin care could involve tiny robots powered by changes in humidity. This week, a group of Seoul National University researchers shared their research into creating such devices, inspired by the slow motions of plants.

Many plants have parts that swell and move in response to humidity, like mimosa leaves or seeds found in pine cones. Changes in humidity can cause these parts to bend and unbend repeatedly, something we've already seen inspire new materials and "smart fabrics," but the scientists from Seoul were more interested in the potential to harness that motion to produce forward movement.

"This cyclic motion must be converted into directional motion to create a robot that moves," explains Seoul National University's Ho-Young Kim. "So we attached legs to our actuator, which allows only one-directional locomotion. We call the legs 'ratchets' and combined them with an actuator to build our bio-inspired robot."

Because plants move incredibly slowly, with one cycle of bending and unbending often taking an entire day, the researchers used active nanoscale fibers to increase the surface area-to-volume ratio of the absorbent layer, allowing it to take in more humidity more rapidly.

The potential for robots powered by humidity changes may be more far-reaching than it sounds, when you consider that they would require no batteries, could work remotely and that humidity changes frequently – between day and night, for example.

This schematic shows the concept behind the team's plant-inspired microrobot(Credit: Ho-Young Kim)

"Humidity changes occur even when we breathe, because humid air is exhaled," Kim says.

But the group is most interested in placing the robots on another surface where humidity changes are easy to tap into – human skin.

"The concept is that by bending, some part of the robot will move away from the skin to encounter dry atmospheric air. When it dries, the robot will return to an upright position near the skin," he says. "Such a robot could do jobs like disinfecting wounds, removing skin wrinkles, and nourishing skin tissues."

One of the robots can be seen in action, in the video below.

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