Tiny Robots Inspired by Pine cones

This schematic shows the concept behind the team’s plant-inspired microrobot. Credit: Image courtesy of American Physical Society’s Division of Fluid Dynamics

The future of bio-inspired engineering or robotics will greatly benefit from lessons learned from plants eg tiny robots powered exclusively by changes in humidity. To generate motion, plants and some seeds – such as mimosa leaves, Venus flytraps and pine cones – simply harness the supply or deprival of water from plant tissues.

“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,” explained Prof Ho-Young Kim. “Awns are composed of 2 tissue layers: one that swells with humidity (active), and another that’s insensitive to humidity change (inactive).” If environmental humidity increases, the bilayer bends from changes in length-wise swelling. Periodic humidity changes cause the bilayer to bend and unbend repeatedly – meaning that changes in environmental humidity can be converted to mechanical work.

“We mimicked the bilayer structure to make an actuator that can generate motions by using environmental humidity change,” Kim said. “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.”

While a key step in creating a robot, repeated bending and unbending produces no net locomotion. “This cyclic motion must be converted into directional motion to create a robot that moves,” he said. “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.”

The group’s work is significant because it opens the door for tiny robots capable of locomotion based solely upon changes in environmental humidity – no electrical power supplies are involved. The group has also developed a mathematical model to find the optimum design for the robot to achieve the fastest speed for any given robot size.

The team is exploring the possibility of placing a tiny robot directly on human skin – one that bends because it’s humid near skin. “…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 said. Then the cycle begins again, and the robot continues to move based on changes in the skin’s humidity. “Thanks to bio-inspiration from plants, “such a robot could do jobs like disinfecting wounds, removing skin wrinkles, and nourishing skin tissues,” Kim added.
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