The cucumber mimicking experiment is the first demonstration of plant-like tropism in an actuator, and it’s part of a move toward “soft” robotics, which uses actuators made of liquid materials such as cloth, paper, fibers and polymers , instead of rigid metal joints, to prioritize versatile movements. Softness would improve robots in situations where flexibility and unobtrusive design are important, such as during operations. And an autonomous soft robot could work in places where there is no power supply – and no people.
“For our work, the success is in proving that the artificial materials can also behave like natural creatures — plants, in this case,” says Aziz. “So we’ve given artificial materials a certain amount of natural intelligence.”
Yarn of course cannot move independently. It needs to be infused with an additional material that makes it responsive.
Aziz ran his yarns through three different solutions. One, an alginate hydrogel, would allow the device to absorb water. Another, a hydrogel made from polyurethane, made it less brittle. The last layer was a heat-sensitive coating. He then wrapped the yarn around a metal bar to make it coil like cucumber vines. The finished product looks like a long, dark magenta feather. The smooth curls overshadow the many layers of fibrous twists, but they’re all there.
His team tested the yarn muscle’s abilities with a series of experiments. First, they attached a paper clip to the lower end of the coil. Then they watered the coil a few times. The hydrogel swelled and absorbed the water. The spool contracted, flinched, and pulled up the paperclip.
But why did the swelling of the hydrogel make the coil? contract instead of expanding? It’s because of that helical microstructure: The swollen hydrogen pushed the helix to expand radially into wider coils, and the yarn muscle contracted lengthwise to compensate.
Next, the researchers applied air heated by a hot plate. This had the opposite effect: the coil relaxed and lowered the paperclip. That’s because hot air helps release water molecules from the hydrogel, which allows the muscle to expand. (Cool air allows those molecules to reabsorb, causing the muscle to contract again.)
Then they asked, Can this thing close a window? (That may seem like an odd challenge, but they wanted a demo to prove that the little muscle could accomplish a useful task on its own — no power source, no air hoses or wires needed.) Of course, a yarn is too thin to move a glass window of full size no matter how many twists you make in it. So Aziz’s team made their own palm-sized plastic version. The window had two panes that could come together to close like shutters. They wove the little magenta muscle through both panes. With a jet of water, the yarn contracted, bringing the shutters together until the window was completely closed.
For Aziz, the beauty of this microstructure is that these kinds of shape changes are reversible. Other artificial muscle materials, such as shape memory materials, often deform irreversibly, limiting their repeated use. But in this case, the coil can contract or relax indefinitely, responding to atmospheric conditions. “When the rain comes, the window can close,” he says. “And when it starts to rain, the window opens again.”
