The cucumber-mimicking experiment is the first demonstration of plant tropism in an actuator, and is part of a move towards “soft” robotics, which uses actuators constructed from fluid materials like fabric, paper, fibers and polymers, rather than rigid. metal joints, to favor versatile movements. Softness would improve robots in situations where flexibility and low-profile design are important, such as during surgery. And an autonomous soft robot could work in places where there is no power supply and no people.
“For our work, success is proving that man-made materials can also behave like natural creatures – plants, in this case,” says Aziz. “So we gave artificial materials a degree of natural intelligence.”
The wire, of course, cannot move on its own. It needs to be infused with some additional material that makes it reactive.
Aziz ran his yarn twists through three different solutions. One, an alginate hydrogel, would allow the device to absorb water. Another, a polyurethane-based hydrogel, made it less brittle. The final layer was a heat sensitive coating. He then wrapped the thread around a metal rod to make it coil like cucumber tendrils. The final product looks like a long dark magenta spring. Its smooth coils dwarf the many layers of fibrous twists, but they’re all there.
His team tested the capabilities of the wire’s “muscle” with a series of experiments. First, they attached a paper clip to the lower end of the coil. Then they gave the coil a few sprays of water. The hydrogel swelled, absorbing water. The coil contracted, shrinking and pulling the paperclip upwards.
But why did the swelling of the hydrogel make the coil contract rather than expanding? It’s because of this helical microstructure: the swollen hydrogen caused the helix to expand radially into wider coils, and the wire muscle contracted lengthwise to compensate.
Next, the researchers applied air heated by a hot plate. This had the opposite effect: the spool relaxed and lowered the trombone. This is because the hot air helps release the water molecules from the hydrogel, allowing the muscle to expand. (Cool air allows these molecules to reabsorb, contracting the muscle again.)
Then they asked: could this thing close a window? (It may sound like a weird challenge, but they wanted a demonstration to prove that the little muscle could do a useful job on its own — no power source, air tubes, or wires needed.) One wire is of course too flimsy to move a full size glass window no matter how many twists you give it. So Aziz’s team created their own palm-sized plastic version. The window had two panes that could join together to close like shutters. They wove the little magenta muscle through the two panes. With a stream of water, the thread contracted, bringing the shutters together until the window was completely closed.
For Aziz, the beauty of this microstructure is that this type of shape change is 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 expand indefinitely, responding to atmospheric conditions. “When the rain comes, it can close the window,” he says. “And when the rain falls, she will open the window again.”