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The beautifully engineered gecko foot: What makes it so sticky?

Animal Research
Scientists get to the bottom of the gecko's supremely sticky feet
A closer look at what makes the bottom of a gecko foot so sticky

What gives geckos the remarkable ability to run upside down across ceilings and stop short on smooth vertical surfaces? You could call it "the hierarchy of hairs."

In the last 15 years, it has been determined that geckos make use of a relatively weak intermolecular force that exists between atoms known as van der Waals force. 

As you see in the pictures above, the sole of a gecko foot is covered in tiny folds of skin. Those folds are in turn covered in tiny hairs that branch, and then branch again until the tips of the hairs, called seta, are just a few nanometers across.

The beauty of this structure is that it provides millions of tiny locations where that weak molecular force can be activated so that, amplified millions of times over, it becomes strong enough to allow a gecko to hang from a ceiling indefinitely.

All of that is amazing, but perhaps even more amazing is that despite their hyper-sticky feet, geckos move fast, running at speeds of up to 20 body lengths per second. That means the bottom of a gecko's foot has to be able to unstick to surfaces just as efficiently as it sticks to them. 

In a paper published Tuesday in the Journal of Applied Physics, researchers at Oregon State University shed some insight on how the structure of the gecko's foot allows it to unstick from a surface with the same ease that it sticks to it.

It turns out that the seta on gecko feet are not just plentiful and minuscule, they are also flexible, curved and come shooting out at a 60-degree angle. 

The researchers found that for a gecko to stick on a ceiling or a wall, it pulls the hairs on the sole of its foot sideways just a bit, and because of the angle of the hairs (and how many of them there are), it provides enough force to support the weight of the gecko. But, when the gecko wants to move its foot, all it does is lift straight up, and it pulls away immediately with little extra energy expended. 

"The angle allows it to turn the stickiness on and off," said Alex Greaney, an assistant professor at the Oregon State University College of Engineering and the lead author of the paper.

The curviness of the seta and their flexibility help the gecko quickly change direction and catch itself. "It's a very delicate balance between the angle and the flexibility and the curviness," he said.

Insects and spiders have independently developed a similar system of foot stickiness, but it is unlikely that we humans will ever move with the freedom of the gecko and the fly, unfortunately. 

"Geckos are the upper limit of the size of animal that can use this technology," said Greaney. "There is a definite scaling problem."

Greaney is interested in working on synthetic adhesives that mimic the gecko's foot technology more accurately than in the past. He is also interested in studying more about the gecko physiology.

"The system is hierarchical -- there are four legs, then toes, then flaps of skin, then hairs that branch and branch again -- all this must be balanced with other stages of hierarchy and the muscular physiology of the gecko," he said. "We want to know even more, how he does it."

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