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April 7, 2013

Super Sticky Stuff

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Imagine a pair of gloves that lets you climb walls like Spider Man. Now imagine a pair of gloves like that which also work under water. Scientists say they can make adhesives that do that, but as this ScienCentral News video explains, they needed the help of a reptile and a shellfish to figure out how.

Amazing Feet

While growing up in India, Ali Dhinojwala recalls geckos as a household nuisance. "As a child I grew up watching geckos climbing walls, but at that time it was more of a pest for us rather than any kind of scientific curiosity. We tried to avoid them and they would come and climb on the walls and we tried to get rid of them."

But his perception changed after he attended a biology conference in 2000, where other researchers presented evidence showing how the hairs on geckos' feet act as a super strong adhesive without using any chemical substance or suction.

"You'd never expect that the feet of these lizards were actually so beautiful and so intricate in design," says Dhinojwala, now a polymer physicist at the University of Akron. "And that is when actually the curiosity started, of understanding what these hairs are for. And by understanding the mechanism, can you come up with something that can be a synthetic analog of those hairs?"

This microscopic image shows the spatulae that split off from the setae on the bottom of a gecko's foot.
image: Kellar Autumn & Ed Florance
"Dhinojwala and his team are among a handful of others currently developing the next generation of adhesives, using the gecko as their main inspiration. Geckos have millions of tiny hairs, called setae, on the bottoms of their feet. And each seta is the split into up to 1,000 more endings. All told the bottom of a gecko foot has billions of tiny endings that work to create van der Waals forces -- natural but weak attractive forces between things. But since there are so many endings, the forces combine to be very strong. And since they are natural atomic forces between two things, there is no suction involved, and no chemical used to cause stickiness.

This means gecko feet are strong, reusable, reversible, they don't get dirty, and don't leave behind any kind of sticky residue. In other words, they're nature's answer to the near-perfect adhesive.

Why only near-perfect? Turns out geckos aren't nearly as good at sticking underwater. For this reason, other researchers have looked to a different sticky creature: the mussel. A mussel's stickiness isn't designed to be reversible like a gecko's, but combing features from the two may bring forth the best of both worlds.

Mimicking Nature

Dhinojwala and his team tried to mimic the gecko foot using nanotechnology.

"We have very, very tiny carbon nanotubes which are very tiny hairs--almost 100 times smaller than a human hair--and bundled them up together in the size of a human hair," he says.

They created a small piece of tape-- about the size of a dime-- and put it to the test against an actual gecko.

The geckos are tested on a vertical plate with a harness. Researchers record how much weight can it support on a surface before it starts slipping.
"The geckos are actually tested on a vertical plate," he explains. "We put a little harness on this and we pull at it until it starts sliding. And we record how much weight can it support on that glass piece, or any other substrate we want to test it on. And that number typically, which has been measured by previous researchers in this field, is about 10 newtons per centimeter square for a Tokay gecko."

But Dhinojwala's synthetic gecko tape has more nano-hairs per square millimeter than a real gecko's foot. For this reason, he expected the tape to actually be stronger than a gecko foot. As he wrote in Proceedings of the National Academy of Sciences, he was right.

"These synthetic tapes can support four times the weight the natural gecko feet can support on these surfaces," he says. "And it actually also sticks really well to Teflon, which the natural gecko has a hard time walking on it."

The small dime-sized patch of carbon nanotube tape was able to support almost nine pounds, and the tape kept its grip a significant amount of time.

"Like if you can imagine using a scotch tape or an adhesive tape in your house, those tapes cannot hold the weight for a long period of time," he explains. "This synthetic gecko tape, which is based on van der Waals forces, can hold this weight for much longer. We have tested these synthetic tapes supporting weights for about 10-20 hours and it still holds there. And in fact some of the experiments we have done in the lab the weight has been held for weeks."

Mussel Power

But Teflon isn't the only thing geckos have a hard time sticking to. They also suffer a drop off in performance underwater, according to Northwestern University biochemical engineer Phil Messersmith. So Messersmith decided to take the best features of both creatures, and create what he calls a "geckel" adhesive.

"The geckos use a temporary but low strength adhesion which is necessary for them to crawl up surfaces and on inverted surfaces, trees, rocks, etcetera, in nature. So that's a temporary adhesive much like a sticky note in your office," he explains. "Mussels on the other hand use a permanent wet adhesive strategy whereby they, when they secrete their adhesive, its intended to be a permanent and very strong adhesion. And so we're combining features of the temporary gecko adhesion, with the wet properties of the mussel, into this one adhesive that now exhibits both temporary and reversible adhesion, but can work underwater."

image: Lewis and Clark College
Messersmith also used nanotechnology to create the tape surface – except rather than nanotubes he used a silicone tape made from a nano-mold. (His nano-hairs are significantly larger than Dhinojwala's, and therefore not designed to be as strong overall.) Messersmith's team then created a liquid polymer derived from the protein which allows mussels to stick underwater, and dip-coated their gecko tape with this mussel polymer. They then tested the gripping strength of a non-coated piece of tape to the polymer-coated tape underwater.

"That adhesive functions about 15 times better than just the pure gecko adhesive does when you have the adhesive working under a liquid environment," he says.

As he and his team wrote in the journal Nature, the tape also worked over and over-- up to 1,100 repetitions, with only a 20 percent drop in adhesion near the end. They also looked "extensively" at the testing surface for signs of residue coming off the polymer coating, and found none.

Scaling Up

The applications for these new adhesives are far ranging. A reversible waterproof adhesive could greatly improve dentures and bandages (simple band-aids or even a drug delivery patch that is allowed to get wet). Super sticky reversible tapes could allow people to hang picture frames in their homes without putting holes in the wall, and without leaving any gooey residue on their paint job. And both researchers mentioned the vast interest it could have for athletes: special grips on golf clubs or tennis rackets, special gloves and shoes for rock climbers, maybe even a special coating on car tires. Since they would work in a vacuum, gecko adhesives could be used in outer space. And since they can be very small and leave no residue, they would be perfect for microelectronics.

PNAS logo

This material is made possible by the Proceedings of the National Academy of Sciences and the National Academies.

NAS logo

Messersmith co-founded a company, Nerites Corporation, which owns the license to his geckel technology and will look to bring it to market. Dhinojwala is in preliminary talks about doing something similar for his adhesive. But both researchers agree a product is not about to hit the shelves. They say scaling up the work to manufacture large pieces of their materials is a significant undertaking that could take several years.

Publications: Dhinojwala - Proceedings of the National Academy of Sciences, June 18-22, 2007. Messersmith - Nature, July 19, 2007.

Research funded by: Dhinojwala - National Science Foundation. Messersmith - National Institutes of Health and NASA.

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