Gecko-inspired adhesives allow people to climb walls

In the 2011 movie Mission: Impossible – Ghost Protocol, Tom Cruise climbs the exterior of the world’s tallest building, Dubai’s Burj Khalifa, using nothing more than a pair of gloves. Now, scientists have invented the real deal: hand-sized, gecko-inspired adhesives that can lift a human up glass walls—and that one day may even catch space junk.

“This is one of the most exciting things I’ve seen in years,” says biomechanist Kellar Autumn of Lewis & Clark College in Portland, Oregon, who was not involved with the study. “This has been a real dream of mine.”

Geckos run up walls and scurry across ceilings with the help of tiny rows of hairs on their feet. The hairs, known as setae, generate a multitude of weak attractions between molecules on the two surfaces that add up to a secure foothold. Moreover, making and breaking the bonds that hold individual setae to a surface is easy. So, unlike glue or tape, a gecko’s sticky feet attach and detach effortlessly, a trait envied by mechanical engineers.

Scientists have recreated geckolike adhesion using silicones, plastics, carbon nanotubes, and other materials—but they’ve run into a scaling problem: The stickiness diminishes when the size of the adhesive exceeds a few square centimeters, severely limiting its practical applications. Even the gecko hasn’t solved this problem, Autumn says. In theory, each gecko hair is so sticky that the animal, which has about 6.5 million setae, should be able to hold up a 130-kilogram linebacker. In reality, a gecko can lift only 2 kilograms with its front feet.

The scaling problem stems in part from the fact that loads don't distribute uniformly across large areas of adhesives, report researchers from Stanford University in Palo Alto, California, in a study published online today in the Journal of the Royal Society Interface. In gecko toes, for example, only a fraction of the setae are in close contact with the surface, and those that are don’t share the load equally among themselves. This could be because the gecko skin, like a rubber band, becomes stiffer to stretch as more force is applied to it, the team suggests. When a gecko runs up a wall, the setae are stretched unevenly such that some have more force applied to them than others. So some hairs max out their stickiness, whereas others are underutilized or not attached to the surface at all. So a gecko patch’s stickiness should increase if researchers can find a way to distribute forces more evenly.

To create their artificial gecko adhesive, the Stanford team started by making silicone microwedges, which imitated gecko hair. They assembled these into 24 stamp-sized tiles, each of which contained hundreds of thousands of microwedges. The team then connected the tiles to springs with tendonlike strings and attached them on an octagonal-shaped plate. Unlike gecko skin, the springs apply the same force to the tiles after they are stretched beyond a certain threshold, thus distributing loads evenly among the tiles. This allowed the assembled patch to offer similar adhesive strength for sizes from a square millimeter to a human hand. Even if an individual tile peels off, the weight it carries is transferred to tiles with the lightest loads, where the springs haven't stretched past the threshold, so that the overall system remains sticky.

Mechanical engineer Elliot Hawkes of Stanford tested the new adhesive by strapping his feet to footholds connected to the patches through a rod. He found that he could easily peel off the patches one at a time and stick them to a wall with his hands—which was how the 70-kilogram Ph.D. student ascended a glass wall behind his lab on a crisp December afternoon (see video). At the end of the climb, he even let go of both his hands in a thumbs-up while the patches stuck securely onto the wall, providing him with two secure footholds.

Though he climbed only 3.6 meters, Hawke says the patches could work just fine carrying him up to the top of the Burj Khalifa—if he’s not too tired of walking up 830 meters, that is.

The Dubai sandstorm that nearly foiled Cruise in the movie would pose a problem, though, as the device works only on clean, smooth surfaces, Autumn says. But therein lies the beauty of the system: It could easily adapt to different situations, just by replacing the adhesive tiles with other types of geckolike adhesives—for example, adhesives that self-clean like real gecko hairs.

The team is now working with NASA’s Jet Propulsion Laboratory to create adhesive-equipped robots that can catch space junk such as defunct satellites, Hawkes says. In a recent experiment in a zero-g environment, a bot equipped with a small adhesive patch gripped the solar panel of another 400-kilogram robot, slowed it down, and gently pulled it in another direction.

“This is the future of bio-inspired design,” Autumn says. “It’s not just about copying nature, but understanding it deeply enough to go beyond it.”

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