It could make you want to climb the walls.
Inspired by the gravity-defying, rapid attach-and-release movement of geckos scampering up vertical surfaces, scientists have invented a non-sticky adhesive that mimics the reptiles’ padded feet and actually gets stronger with use.
The strange material could become the basis for legions of high-speed search and rescue gecko-bots, Band-Aids that finally leave your leg hairs alone, and yes, perhaps even oversized climbing gloves to help you live out your Spiderman fantasy.
The adhesive is fashioned after the foot pad of a diminutive reptile that can go seemingly anywhere.
“For a gecko to move around in the world, it can’t just have Scotch tape on its feet,” said Ron Fearing, director of the Biomimetic Millisystems Lab at the University of California at Berkeley and the co-author of two new studies describing how it all works. “Simultaneously, it’s got to be easy to attach, easy to release, as well as providing enough force to keep the animal from slipping off the surface,” he said.
Meeting all of those requirements without leaving any residue behind is “a completely revolutionary approach to sticking things together,” said Kellar Autumn, a biology professor at Lewis & Clark College in Portland, Ore., and a longtime Fearing collaborator who led the effort to reveal the animal’s unique ability. “We could have the next Mars rover crawling around with gecko feet.”
Inspired by nature
Other groups have found similar inspiration from geckos. Last year, a research team at Northwestern University announced that it had produced a gecko-mussel hybrid adhesive, dubbed “geckel” and deemed suitable for underwater use.
The secret to the gecko’s success is the collective power of tiny forces exerted by millions of odd bristles that extend out from each footpad and then branch into a forest of forked tips. “This is such a bizarre solution that I don’t think engineers would have thought of it if geckos hadn’t evolved it,” said Autumn.
At a microscopic scale, nearly everything in nature is attracted to everything else, and the branching bristles are equipped to take full advantage. The van der Waals forces of attraction, as they’re known, may seem unremarkable to casual observers. “If you put your hand down on a flat surface, there are van der Waals forces holding your hand to that surface,” Autumn said, “but they’re not strong enough to help you climb the walls like Spiderman.”
Ah, but that dynamic would change considerably if your hand was covered in tiny gecko bristles, each brushing up against the wall and helping to build up a considerable attraction.
“Think of Velcro or tape that doesn’t stick until it’s pulled parallel to a surface. But if you relax it, it just lets go,” Autumn said. “That works really, really differently from anything you can find in the store right now.”
No sticky residue
Conventional adhesives like Scotch tape are sticky because their gooey films can make intimate contact with other surfaces — sometimes too intimate, removing paint, varnish or other material when the tape is peeled away. By contrast, gecko bristles, or setae, are composed of the same material as our hair and fingernails.
For their gecko-like adhesive patch, Fearing and his colleagues started instead with a thick cluster of fibers made from polypropylene, the same plastic used to make gallon milk jugs. The plastic isn’t sticky at all, nor is it particularly soft.
“Think of it like a toothbrush. If I tap it lightly against a surface, a few of the bristles will make contact,” Fearing said. “But if I rub it across the surface, the bristles bend over and make much, much more contact.”
Unlike a toothbrush’s bristles, the tiny plastic fibers are so small that 40 million can fit within a square centimeter, or roughly the size of the fingernail on your pinkie. Dragging that patch across a surface brings all those bending fibers into particularly intimate contact with the surface, sticking the patch securely into place.
“It’s a very, very unintuitive type of contact,” Fearing said.
But it is strong. A fiber-filled patch about the size of a small postage stamp, in fact, can support nearly a pound of weight, even if only a fraction of the fibers are engaged.
“We’ve had weights hanging for several days and nothing happens to them,” he said. (Fearing’s linked studies, funded through the National Science Foundation, appeared last month in the Journal of the Royal Society Interface.)
Frequent use makes it stronger
The researchers also discovered that repeated use seems to make the adhesive stronger, a phenomenon Fearing likens to the difference between tensed and relaxed fingers.
“If I make my fingers very stiff and jab them into the desk, I can do some damage to them,” he said. “But if I curve them a little bit, they’re more compliant and they bend over.”
Once primed by repeated sliding in the same direction, faux gecko bristles could remain ever-so-slightly bent and become similarly compliant, providing more surface area for a stronger attraction the next time around.
If a real gecko can stick to walls even better than its mimic, how does it release its feet so quickly? High-speed videos, after all, show that the animal can step up to 20 times per second.
“So the gecko has this really funky peeling movement of its toes, a hyperextension,” Fearing said, comparing it to a person whose fingers bend back the wrong way. That movement quickly releases the grip of the bristles, allowing the gecko to take its next step. Science hasn’t yet replicated that feat, though the reduced strength of the artificial patch means it releases easily when lifted straight off a surface.
Unlike the nimble lizards, the artificial fibers don’t yet cling well to rough or dirty surfaces either. “There’s still a lot of things that geckos can do that the synthetic material can’t,” Autumn said. “But this is a first step.”
Northwestern’s Phillip Messersmith, a professor of biomedical engineering who led the team that invented geckel, said his group also began with multiple fibers arranged like a gecko’s, but then coated each with a thin polymer derived from mussels to yield the wet-dry combination.
Although Messersmith said his method couldn’t be directly compared to Fearing’s because the goals were different, he nonetheless praised the Berkeley project as coming closest to capturing a gecko’s full capabilities. “It really kind of demonstrates new strategies to mimic the gecko’s adhesion,” he said.
As for future applications, the new adhesive could be a boon for designing more body hair-friendly Band-Aids or cell phones that can be assembled easily but don’t break when dropped.
Fearing also plans to use the adhesive in an ongoing project to design miniature gecko-bots the size of a quarter. A battalion of such bristle-footed bots could crawl into collapsed buildings, five-alarm fires or other disaster sites where finding survivors quickly is a critical priority. Sending camera or microphone-equipped robots instead of dogs might make more sense in some situations, he said, especially if the cheaply made and disposable machines could be sent en masse.
The adhesive should easily support such small robots, the basic body plan of which could be ready within a few weeks.
Fearing has one caveat for would-be wall climbers, however: humans would require rather large gecko-inspired gloves to provide a similar safety margin, especially for preventing slips on dirty or oily spots. As Fearing pointed out, “A gecko weighs as much as a small coin, and it can better tolerate a fall from 10 body lengths than a person can.”
On second thought, maybe you should hold off on buying those Spiderman tights.