Nature is full of expert climbers, but perhaps none is more impressive than the gecko.
Many of the small reptiles are known for sticky toes that allow them to crawl up sheer walls, hang from ceilings, and grip surfaces as smooth as glass, Spider-Man style.
The gecko’s superlative stickiness is so impressive that scientists have been hard at work for over a decade to replicate it for practical human uses, like tape and glue. (See “Watery Gecko Grip Could Lead to Stickier Tape.”)
Now, a new study published August 12 in the Journal of Applied Physics has unraveled some of the complexity of how geckos can turn their stickiness on and off.
Scientists used models to show that tokay geckos—a species native to Southeast Asia—can change the angles of their toe hairs to make them more or less sticky.
While other creatures use methods like secretions or claws to attach to surfaces, the gecko uses an intricate system called “dry adhesion.”
That’s due to a phenomenon known as the van der Waals force, which occurs when electrons in one atom create a magnetic field that stimulates and attracts the electrons in a neighboring atom.
“Van der Waals forces are the weakest sort of interatomic forces that we have,” said study co-author P. Alex Greany, a professor of mechanical engineering at Oregon State University in Corvallis. “It’s amazing that geckos are able to use this really weak force.”
They do so by taking advantage of the remarkable anatomy of their toes, which sport millions of microscopic hairs called setae that in turn branch out to form billions of tiny contact points called spatulae. (Watch a video of gregarious geckos.)
Spatulae allow the reptiles to maximize the amount of contact made with surfaces, spreading the load of their weight and exponentially increasing the attractive force between them and the surface, Greany said.
Ready, Setae, Go
But how does the gecko unstick from a surface when it wants to take a step? The key is their angled, microscopic toe hairs, Greany and his team discovered.
Using a mathematical model, the scientists calculated that a gecko can simply change the angle of its setae to easily detach from a surface.
What’s more, the setae are not just angled, but curved—allowing the gecko to store an immense amount of energy and change direction very quickly, Greany said.
In effect, the curved setae act like a “spring-loaded detachment mechanism,” said Kellar Autumn, a biomechanics professor at Lewis and Clark College in Portland, Oregon, who wasn’t involved in the study.
Autumn, whose team proved that geckos use van der Waals in 2002, called the study “a big step forward in terms of the theory and physics underlying” the adhesion system that geckos use.
“What’s amazing is just how finely balanced and finely tuned this whole system is,” Greany added. “We understand it at one level, and as we learn more and more about it, it turns out there’s a really subtle interplay of things going on.”
The oblique angle of the gecko’s setae combined with their flexibility is key to the process, said Greany.
The new research will help scientists working on reusable adhesives for robot grippers or feet—think robots that can climb walls or grab on to things, said Greany. (See “Gecko, Mussel Powers Combined in New Sticky Adhesive.”)
Autumn agreed that gecko-inspired technology has applications ranging from the futuristic—such as robots fitted for extreme environments and disaster areas—to the mundane, such as holding together the cell phones of the future.
And with the future of gecko tech looking bright, perhaps it’s time to give some credit where credit is due.
“Adhesive nanostructures are such a different way of sticking things together,” Autumn said, “that I don’t think engineers would have invented this if geckos hadn’t done it first.”
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