Look out, Spider-Man! Gecko inspires new glue

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.”

Miniature gecko-bots
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.

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