Pine cones -- the woody structures that litter the forest floor and disperse seeds from their parent trees -- recently inspired the development of tiny robots. While many studies have focused on animals when creating bio-inspired robots, researchers from the Seoul National University in Korea have taken a different approach and harnessed the natural movement in plants, which is powered by changes in humidity.

Compared to animals' use of complex muscles, the movement of plants is much simpler. However, plant motion is much more difficult to study since it is incredibly slow and can't be detected by the naked eye, according to a news release. Essentially, pine cones and some seeds, including those from mimosa leaves and Venus flytraps, respond to changes in humidity with motion, meaning as humidity increases, a plant's structure bends in response to swelling. This suggests that changes in environmental humidity can be converted into mechanical work.

"Some seeds consist of a head that contains all its genetic information, along with a long appendage called an 'awn' that is responsible for locomotion -- just like an animal's sperm," Ho-Young Kim, one of the study researchers and a professor in the Department of Mechanical and Aerospace Engineering at Seoul National University, explained in the release. "Awns are composed of two tissue layers: one that swells with humidity (active), and another that's insensitive to humidity change (inactive)."

For their study, researchers recreated the plant structures in a laboratory setting to generate motions in response to humidity changes. Periodic changes cause plants to bend and unbend repeatedly. Pine cones are considered an organ of conifer trees that contain the reproductive structures. The familiar woody cones with individual scales are female and produce seeds.

"Plants move slowly -- one cycle of bending and unbending can take an entire day. To increase the response speed of the bilayer, we had to develop a novel way to fabricate the active layer. Its response speed increases with the surface-area-to-volume ratio of the layer because humidity can be absorbed more rapidly, so we deposited active nanoscale fibers onto an inactive layer," Kim added in the release. (Scroll to read more...)

Researchers then had to covert this cyclic motion into directional motion, in order to create a robot that moves. To do this they attached the robot's legs, or "ratchets," to an actuator, which allows one-directional locomotion. This means that no electricity is needed to power the tiny robots.

"Making a bilayer for the robots isn't difficult, but making a fast one requires technical expertise," Kim said, adding that the group envisions a bright future for humidity-change-powered micro-robots simply because humidity changes are all around us. "Generally, it tends to be drier during the day and more humid at night -- the periodic humidity change cycle that enables seeds to bury themselves in the ground. Humidity changes occur even when we breathe, because humid air is exhaled."

Next researchers plan to explore the potential of human skin, which is more humid than the atmosphere. In this case Kim explained in the release, "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. Then the cycle begins again, and the robot continues to move based on changes in the skin's humidity."

Their findings will be presented at the 68th annual meeting of the American Physical Society's Division of Fluid Dynamics.

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