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Scientists have discovered the secret behind geckos' ability to walk up walls and dangle from the ceiling, and to prove it, have synthesized the very tips of the toe hairs geckos rely on to stick.
The minute artificial hair tips, though rudimentary, work nearly as well as the millions that line the geckos' own feet, showing that it is possible to mimic nature and build nano-scale structures that stick to many different surfaces and in environments where glue-like adhesives fail.
"This is the first step, this is the pathway to synthesizing the first self-cleaning dry adhesive," said Robert J. Full, professor of integrative biology at the University of California, Berkeley. "It's going to take a lot of work to figure out the best way to make a biologically inspired adhesive, but this demonstrates that the path to manufacturing them is there."
A self-cleaning dry adhesive would have many uses, such as moving semiconductors around in a vacuum chamber, and could stick to surfaces underwater or in space.
Full, along with colleagues at Lewis & Clark College in Portland, Ore., UC Santa Barbara and Stanford University, report their findings this week in the on-line edition of the Proceedings of the National Academy of Sciences. The article will be published on the PNAS Web site sometime during the week of Aug. 26.
"We have solved the puzzle of how geckos use millions of tiny foot-hairs to adhere to even molecularly smooth surfaces such as polished glass," said Kellar Autumn, assistant professor of biology at Lewis & Clark College and lead author of the study. "Our new data prove once and for all how geckos stick."
The proof is in the pudding, though. Engineers at UC Berkeley created synthetic hair tips that stick almost as well as the geckos' own.
"We've synthesized the smallest part of the entire foot, but now we've got to make billions of them to get significant adhesive force," said UC Berkeley's Ron Fearing, professor of electrical engineering and computer science. "We don't have a Post-it(r) yet."
Full and Kellar reported two years ago the secret of getting gecko toe hairs to stick without the use of suction, glue or static electricity. They found that the angle the hair makes with a surface is critical for controlling both sticking and release. The hundreds or thousands of pads at the end of each hair interact on a molecular level with the surface, generating a combined attraction that keeps the gecko attached.
The tiny pads, called spatulae, are like split ends, Full said, each only 200 billionths of a meter wide - below the wavelength of visible light. With up to 500,000 hairs per foot and hundreds to a thousand split ends per hair, the sum total of intermolecular forces, known to chemists as van der Waals forces, can amount to 1,000 times the weight of a gecko.
"Intermolecular forces come into play because the gecko foot hairs split and allow a billion spatulae to increase surface density and come into close contact with the surface. This creates a strong adhesive force," Autumn said.
A single seta can lift the weight of an ant, he said. A million setae, which could easily fit onto the area of a dime, could lift a 45-pound child. If a gecko used all of its setae at the same time, it could support 280 pounds.
At the time of its earlier paper, the team could not rule out several physical effects that might also play a role in gecko adhesion. For example, various animals, including frogs, insects and some mammals, stick to surfaces by capillary adhesion, taking advantage of the surface tension of a film of liquid. Many of these animals have glands on their feet that secrete liquids that help them stick. Geckos, though, have no such glands. Nevertheless, the spatulae at the ends of the hairs on their toes could be interacting with the thin water film - only a molecule thick - that covers almost all surfaces.
To explore this adhesive mechanism, the two biologists expanded their team to include Stanford engineer Thomas Kenny, who precisely measured the forces exerted by toe hairs; theoretical chemist Jacob Israelachvili at UC Santa Barbara, who modeled toe hair adhesion to see if prediction matched measurement; and UC Berkeley's Fearing, who made synthetic spatulae.
They tested the sticking power of gecko feet and toe hairs on different types of polarizable surfaces, i.e., surfaces where the molecules can shift around to attract or repel charged molecules, such as water. Those that repel water are called hydrophobic surfaces, while those that attract water are called hydrophilic. If capillary adhesion were partly responsible, gecko feet would stick better to water-loving or hydrophilic surfaces than to hydrophobic surfaces.
Using nine separate Tokay geckos - one of the largest geckos and a native of Southeast Asia - the researchers found that the geckos' toes, though hydrophobic, stuck equally well to the hydrophobic semiconductor gallium arsenide on silicon and to the hydrophilic semiconductor oxidized silicon. Kenny's lab showed also that single setae stick equally well to hydrophobic and hydrophilic MEMS (microelectromechanical systems) sensors. The two experiments confirmed that van der Waals forces are the most likely adhesive mechanism.
Using a well-known theory of adhesion, Israelachvili predicted the size of spatulae required to make geckos stick, and obtained an answer exactly in the range observed with gecko hairs.
The clincher was creation of synthetic split ends that stick almost as well as the real spatulae of Tokay geckos. With the help of an atomic force microscope to punch wax molds of the right size, Fearing cast little spatulae of two separate materials - silicone rubber and polyester resin. He then used the microscope to measure the adhesion force of these rubber feet.
"One bump at the end sticks with 200 nanoNewtons of adhesive force," at least half of which is due to van der Waals forces, Fearing said. "We confirmed that it's geometry, not surface chemistry, that enables a gecko to support its entire body with a single toe."
A nanoNewton is the weight of a single blood cell, that is, the force exerted on a blood cell by Earth's gravity. The force exerted by sunlight on a satellite orbiting the Earth is on the order of 200 nanoNewtons.
Since fabricating the initial nanobumps, Fearing and post-doctoral fellow Metin Sitti have found a way to make arrays of 10,000 rubber bumps. Though the centimeter-square arrays don't yet adhere to surfaces better than rubber without bumps does, the team sees this as proof of concept.
"We can apply the underlying principles and create a similar adhesive by breaking a surface into small bumps," Fearing said. "These preliminary physical models provide proof that humans can fabricate gecko glue."
Added Full, "We've shown directly that something manufactured out of different materials works by the same mechanism proposed for gecko toe hairs. We think that the deformable nature of these tiny tips, and perhaps the whole hair, allow you to get very close to any kind of surface. That's the advantage."
Geckos aren't the only creatures to evolve this sticking technique. Anoles, skinks and insects use dry adhesion also, which means that nature converged on the same solution several times, Autumn said.
The key design principle, discovered numerous times during evolution, is that you can get very effective adhesion by simply taking advantage of skin structure - keratin in the case of geckos, anoles and skinks; chitin in the case of insects - and subdividing it to create an array of microstructures that stick phenomenally, he said. The animals didn't have to develop structures with specialized surface chemistry, they simply took advantage of the geometry.
Fearing continues to explore the synthetic possibilities, including what types of hair shafts make the best synthetic pads. Meanwhile, Autumn and Full are exploring the natural variety of hairs and pads to see what characteristics seem critical to a successful dry adhesive.
The research was funded by grants from the federal government's Defense Advanced Research Projects Agency.
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