No other animal has mastered camouflage like the octopus. The mightiest of these morphing creatures, the mimic octopus, contorts its body into a thin ribbon and adopts the colors of a venomous sea serpent to scare predators away. Divers have seen mimic octopuses masquerade as lion fish, sea stars, shrimp and anemones. When peckish, the octopus takes the form of a lusty crab. Crustaceans fooled by the display end up eaten.
Materials scientists and engineers also have fallen under the octopuses’ spell. A team of Cornell University researchers, with the aid of octopus expert Roger Hanlon, successfully mimicked the mimic using sheets of rubber and mesh.
As they report in a study published Thursday in the journal Science, the researchers created a thin membrane that contorts into complex 3-D shapes – much like the shape-shifting skin of an octopus. The membranes can inflate in seconds to the shapes of everyday objects, such as potted plants or a cluster of stones.
Octopuses are covered in muscly bundles called papillae, Hanlon and his colleagues documented in the Journal of Morphology in 2014. The papillae go slack when an octopus wants to decrease the drag of water against skin, allowing it to speed away in a hurry. Contractions cause fleshy nubs to appear, and the skin bulges. Octopuses can match the texture of seaweed so closely they become almost invisible.
Itai Cohen, a materials expert at Cornell and an author of the new study, said he was awed by videos of the shape-shifters. (Such a scene, filmed by Hanlon and posted to YouTube, is described as the Woods Hole Marine Biological Laboratory scientist’s “most famous underwater video clip.”)
“You are staring at this coral reef. You have no idea [an octopus] is there.” And then: “It changes color. It changes texture. It appears out of nowhere.”
Cohen was impressed and inspired. “That is just awesome,” he thought. “How the heck do we do that?”
He and fellow Cornell researcher Robert Shepherd assigned James Pikul the grunt work of figuring it out. “For a few decades, scientists and engineers have been trying to control the shape of soft, stretchable materials,” said Pikul, a graduate student at the time of this research and now an assistant professor at the University of Pennsylvania. But cheaply fabricating these materials proved difficult.
Success brought together two concepts: the musculature of the octopus with the mechanics of blowing up a balloon. The trick was to cut concentric rings into a thin surface of silicone rubber and mesh. Inflating the rubber caused the membrane to contort following the shape of the cuts.
“The width of the concentric rings determine how much radial stretch there is in the membrane,” Pikul said. The wider the rings, the less stretch it had. “This stretch is directly related to the slope of the inflated shape, so if you know your final shape, you can calculate the slope and match the ring patterns to that slope.”
Programming the laser cuts in just the right way enabled the rubber to inflate not only outwardly, like a kickball, but as a 3-D structure with concave regions. It’s a bit like the sculptures made by twisting together sausage-shaped balloons. Except, in this case, it’s all one membrane.
To show their membrane in action, the researchers had it take the shape of river stones, collected in Ithaca. During demonstrations of their work, Shepherd begins with the inflated membrane tucked among real stones. “Many times people are surprised that they weren’t actually rocks,” he said.
The team also picked a succulent with spiral leaves – Graptoveriaamethorum – to show that the membrane can mimic lifelike shapes. “And succulents are pretty cool plants,” Pikul said.
Theirs is the latest invention in the field of soft materials to borrow ideas from the cephalopods. Researchers at Harvard University announced in August 2016 that they’d created the first autonomous soft robot, patterned after the eight-legged creatures.
“I have been thinking about octopuses for a while,” Shepherd said. In previous research, he and his colleagues created flexible, color-changing robots also inspired by octopus skin.
The Army Research Office provided funding for the recent work. “You could imagine shipping out camouflage sheets,” Cohen said. Once delivered to the field, an inflated sheet might obscure a location from the enemy.
“We didn’t go into this with an application in mind,” Shepherd said. But he envisions creating touch mechanisms used in concert with virtual reality. A person running a finger along a morphing membrane might feel something like gravel suddenly switch to a glassy surface – or a virtual-reality octopus that goes from nubby to smooth in the blink of an eye.
