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Tree Frog Inspires New Easy-Off Stickies

But can they out-stick wall-walking gecko robots?
tree frog



BARNES RESEARCH GROUP
Scotch tape, packing tape, Post-its—no man-made adhesive holds a candle to the sticky world of animal adhesives, where geckos scurry across ceilings and tree frogs leap from leaf to leaf on tacky toe pads without missing a step.

Unable to beat nature, researchers have joined it. The ripping sound of Velcro echoes the prickly, sweater-grabbing burdock seeds that inspired the childhood wonder material. Gecko feet bristle with millions of branching, self-cleaning fibers called setae, the inspiration for so-called "gecko tape." Even the "glue" that keeps zebra mussels anchored to rocks has been imitated.

Now, inspired by the clingy toes of tree frogs and insects, researchers have created a new adhesive that can become up to 30 times stickier on demand or peel off easily, allowing it to re-adhere. Although weaker than ordinary Scotch tape, researchers say the concept may lead to custom-strength, reusable adhesives that peel off without losing their gluey power.

The secret to the film's ability to grip and release, they say, lies in narrow, oil-filled channels just below the surface, reminiscent of the honeycomb grooves on tree frog toe pads and the feet of bush crickets and other insects. Unlike hairy gecko feet, tree frogs and insects secrete a viscous fluid onto their flat, adhesive pads.

Like speed bumps, the grooves slow the growth of cracks that form when adhesives stretch to their breaking point. Adding fluid diffuses the rupture-causing forces on the grooves through the so-called capillary effect, says materials chemist Animangsu Ghatak of the Indian Institute of Technology Kanpur.

Inspired by the concept, Ghatak and his colleagues carved micro-scale tubes in a layer of the polymer PDMS (polydimethyl siloxane), which sticks very weakly on its own, then filled the channels with silicone oil. They sandwiched the film between two plates and pulled the top plate up to test the stickiness. The best results—a 3,000 percent gluey boost—came from tubes 710 microns wide (nearly ten times thicker than a human) filled with moderately viscous oil, the researchers report in Science.

To switch the adhesive on and off, they added a second layer of tubes to the film, filling one set with oil and the other with air. When the oil was on the bottom, the adhesive held fast, but when it was flipped to the top, the film gave way without leaving a residue, the group reports. Ghatak says the adhesive can re-stick up to 25 times without weakening.

"The results are really exciting," says Phillip Messersmith, associate professor of biomedical engineering at Northwestern University. "To my knowledge, this is the first study describing the use of subsurface features—the air- or silicone-filled channels—to enhance adhesion."

The findings also help solve the mystery of why tree frog and insect pads consist of a stiff outer layer over a fluid-filled one, says biologist Jon Barnes of the University of Glasgow in Scotland, who studies tree frog adherence.

The leading bio-inspired adhesive, however, is probably still gecko tape, which has proved its sticking power: Carnegie Mellon University researchers designed a robot that climbed straight up a smooth wall on strips of custom tape. And Messersmith's group unveiled a reusable adhesive in July that combined geckolike hairs with an imitation zebra mussel glue. Voila: the resulting "geckel" tape stuck to wet surfaces as well as dry ones.

Messersmith says the catch with the tree frog adhesive is that the parallel channels work in only one direction, and it could be pricey to manufacture tape with a network of fluid-filled tubes.

So what are the prospects of a real-life, wall-scaling Spider-Man? Messersmith does not rule it out, but notes it is still a long way off. Without self-cleaning, gecko tape would quickly get gunked up, and reusability remains a rare commodity among bio-inspired adhesives. "It's remarkable really," he says, "since the essence of tree frog and gecko adhesion is that it is temporary and reversible."

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