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Scientists unravel mystery of geckos' sticky feet

Findings open door for myriad applications

Monday, July 07, 2003

By Byron Spice, Post-Gazette Science Editor

The gecko has been perfecting its amazing wall-crawling abilities for something like 100 million years. But scientists, with feet still planted firmly on the ground, are rapidly playing catch up.


Online chart: Making like a gecko (.pdf format)

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Only last year did researchers finally figure out what makes the gecko's microscopic foot hairs so sticky that the little tropical lizard can run upside down across ceilings. Yet they are already reporting some success at producing synthetic versions.

Made of silicone rubber, polyester or other polymers, the man-made strands appear to adhere to surfaces just as strongly as the real thing, though questions remain regarding cost and durability.

The idea that synthetic gecko hair could turn anyone into a wallcrawler like Spiderman is appealing, but might prove unrealistic. A gecko weighing a few hundred grams might be able to dangle from a single toe, noted Metin Sitti, an engineer at Carnegie Mellon University, but heavier, denser humans would require much more contact area and locomotion could be difficult.

Still, plenty of applications remain. Clothing with pockets or ornamentation that can be removed or repositioned, and perhaps seams that can be instantly tailored. Athletic shoes and car tires with unusual grip. Surgical sutures and implants. Wigs and toupees that stay put. Tools that stick to the hands of astronauts, preventing them from drifting away in the weightlessness of space.

And Sitti, who is affiliated with the Robotics Institute as well as the department of mechanical engineering, envisions robots that can crawl on the skin of space shuttles for orbital inspection and repair, or new classes of search-and-rescue robots.

Just last month, physicists at the University of Manchester reported in the online edition of the journal Nature Materials that they had used a technique called electron-beam lithography to fashion "gecko tape" -- billions of tiny plastic hairs, each just a couple of billionths of a meter long, attached to a flexible backing.

Andre Geim, director of Manchester's Centre for Mesoscience and Nanotechnology, said he and his colleagues considered making enough of the tape to let a student hang out the window of a tall building. They ultimately decided against it -- not because they were worried about the student's safety, but because they couldn't afford to make that much tape.

In an upcoming issue of the Journal of Adhesion Science and Technology, Sitti and Ronald Fearing, his colleague and mentor at the University of California, Berkeley, report on their own method for making polymer gecko hairs.

Like the Manchester group, Sitti and Fearing say their gecko adhesive works about as well as natural gecko hairs. A square centimeter of the stuff could support more than two pounds. But their production technique is similar to pouring plaster of paris into a mold. That's a lot cheaper than using electron beams to sculpt hairs and a more practical for mass production.

Sitti, a native of Turkey who earned his doctorate in electrical engineering at the University of Tokyo in 1999, joined Carnegie Mellon last fall. Before that, he had served as a postdoctoral fellow in Fearing's robotics laboratory at Berkeley.

Both Fearing and Sitti were part of the Gecko Team, a group of biologists and engineers at several universities who worked together to unravel the mystery of the gecko's sticky feet.

Geckos, which range from a half-inch to 10 inches in length, have millions of tiny hairs, called setae, on their toes. Each seta branches out into 1,000 even thinner stalks that are tipped with flat caps called spatulae, each about the size of a bacterium.

Scientists had long speculated about how these billions of tiny hairs allowed geckos to cling to almost any surface. Geckos don't dig claws into the surface, like a squirrel gripping the bark of a tree. The spatulae don't act like tiny suction cups. And, unlike wall-crawling insects such as ants, geckos don't have glands on their feet to secrete liquid, which would help them adhere to surfaces thanks to the natural attraction many molecules have to water.

Up until last year, researchers had speculated that, despite their lack of glands, geckos still might be able to use wet, capillary adhesion by relying on condensation from water vapor. But the Gecko Team published a study last summer in the Proceedings of the National Academy of Sciences that appeared to eliminate that possibility. It also confirms a 40-year-old theory that geckos are able to stick to surfaces because they can exploit weak molecular attractive forces known as van der Waals force.

Van der Waals attraction is an electrodynamic force that acts when molecules come into very close proximity to each other. The van der Waals force is too small for people to even notice under normal circumstances, but if enough of the setae could make close contact with a surface, the thinking goes, the collective van der Waals force could be significant.

To prove the point, Fearing and Sitti made synthetic setae of different materials -- including some that naturally repel water. They found that the setae all adhered equally well to surfaces, regardless of whether they were made of materials that attracted or repelled water. That showed that the attraction didn't depend on any kind of wetness, but on van der Waals force. The experiments also demonstrated that it was the size of the setae that had the most to do with adhesion, again suggesting the van der Waals mechanism is at work.

In addition, team researchers found that the toes of live Tokay geckos were highly hydrophobic -- water-hating -- and that the lizards adhered as well to surfaces that repelled water as they did to surfaces that attracted water.

Since then, a survey of other creatures has shown that many animals use this dry adhesion technique, said Robert Full, a professor of integrative biology at Berkeley and one of the team leaders.

"It turns out that these have evolved independently in other lizards, insects and spiders," Full said. Because of these different evolutionary paths, the adhesive pads on these animals may not always resemble each other -- the appendages on spiders look like feathers, rather than hairs or brushes -- but seem to work in the same way.

"I suspect that part of the diversity we see is that some of the animals stick better to certain surfaces than to others," Full added. Some may also make use of wet adhesion as well as dry adhesion. Work is continuing to better understand these principles, which in turn could provide important hints about how best to design synthetic versions.

To adhere to a surface, the hair-covered pads are pressed against a surface and then dragged along it, Sitti said. This helps the tips of the hairs conform as closely as possible to the shape of the surface. The hairs are oriented in one direction, so they can be detached by pushing in the other direction or twisting them.

Attachment takes about 6 thousandths of a second, detachment takes 16 thousandths of a second -- fast enough to allow a gecko to run, Sitti said.

To make synthetic versions, Sitti has used nano-pore membranes -- filters with pores about 200 billionths of a meter, or 200 nanometers, in diameter -- as a mold into which liquid polymers can be pored. Once the dried polymers are peeled out of the membranes -- or the membranes themselves are dissolved away -- what is left is a series of hair-like appendages the size of the pores.

Sitti said the branched shape of gecko setae can be mimicked by combining a membrane with 200 nanometer pores with a second membrane with even smaller pores. Polymer injected into the larger pores would also seep into the smaller pores of the adjoining membrane, he explained, resulting in brush-like setae.

Both Sitti and the Manchester researchers emphasize that the length and spacing of the setae can be critical to performance. If the setae are too long or too close, they tend to stick to each other rather than to surfaces. If the hair-like projections are too sparse or too short, too few setae may make contact with the surface to achieve strong adhesion.

Properly designed, however, the synthetic gecko hairs would act much like a piece of Velcro, albeit one-sided Velcro.

Though Velcro requires two complementary patches to adhere to each other, the gecko glue could work when pressed against almost anything. Teflon is about the only thing to which geckos won't stick.

One issue to be addressed is durability -- how many times could the synthetic pads be used before they fall apart? The answer, Sitti said, may be in the choice of materials. Because it is the size and shape of the hairs that appear to determine performance, not the material, using a tough polymer such as Kevlar might allow them to last longer.

Though many applications are possible, Sitti is particularly interested in making use of the new adhesion technology in his Nanorobotics Laboratory. For instance, gastroenterologists now use a capsule-size camera that can be swallowed to view some parts of a patient's intestine; Sitti said that by adding gecko adhesion to the exterior of the capsule, it may be possible to control the camera's movement by selectively attaching to the intestinal lining.

Sitti and William "Red" Whittaker, director of the Field Robotics Center, also have developed a "WaalBot" concept -- a legged robot that might be used for space and extraterrestrial applications. In low or no gravity, the van der Waals force might be sufficient to support even a heavy robot, allowing a WaalBot to scurry over the skin of a space shuttle for in-flight inspections and repair, or to scale rock walls during extraterrestrial exploration.

Byron Spice can be reached at bspice@post-gazette.com or 412-263-1578.

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