Plants don’t always need sunshine to get the energy necessary to move; sometimes humidity supplied by the air is all they need. Now, researchers from Seoul National University have co-opted this technique into tiny robots, allowing them to move without a battery, the sun, or any other electrical methods—meaning these robots could work almost perpetually, and in areas without electrical supply.
In certain plants and seeds, this technique of motion is fairly simple.
“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 co-author Ho-Young Kim, a professor in the Department of Mechanical and Aerospace Engineering at Seoul National University, in a statement.
“Awns are composed of two tissue layers: one that swells with humidity (active), and another that’s insensitive to humidity change (inactive).”
So, as humidity increases, the active layer swells, causing the two tissue layers (otherwise known as the bilayer) to bend together. When the humidity drops, the bilayer unbends, and over time, the seeds end up with a slow wriggling motion that helps propel them into the soil.
The structure of the researchers’ robots, which was presented at the American Physical Society’s 68th Annual Meeting of the Division of Fluid Dynamics in Boston on the 22nd, derives directly from plants.
“We mimicked the bilayer structure to make an actuator that can generate motions by using environmental humidity change,” said Kim.
So as the active layer of the robot swells, its bilayer bends, propelling it forward.
“This cyclic motion must be converted into directional motion to create a robot that moves,” said 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.”
Robots extra receptive to humidity
Of course, the motion of seeds is quite slow; the amount of change in humidity necessary for flexing often requires the sunrise or sunset. But the researchers designed their robots to be extra receptive to humidity.
“Plants move slowly — one cycle of bending and unbending can take an entire day,” Kim said. “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.”
Thanks to this technology, the robots are sensitive enough to be able to walk using the humidity gradient surrounding human skin.
“The concept is that by bending, some part of the robot will move away from the [humid] skin to encounter dry atmospheric air. When it dries, the robot will return to an upright position near the skin,” he said.
Once returned, the robot will swell with humidity again, ratcheting the legs forward, before shrinking in the drier air away from the skin. And the cycle continues again and again.
Which means these robots could have countless uses, including medical ones, “like disinfecting wounds, removing skin wrinkles, and nourishing skin tissues,” Kim said.
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Feature Image: Thinkstock
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