PORTLAND, Ore. -- Geckos, nature's supreme climbers, can race up a polished glass wall at a meter per second and support their entire body weight from a wall with only a single toe. But the gecko's remarkable climbing ability has remained a mystery since Artistotle first observed it in fourth century B.C.
Now a team of biologists and engineers has cracked the molecular secrets of the gecko's unsurpassed sticking power--opening the door for engineers to fabricate prototypes of synthetic gecko adhesive.
"Two millennia later, 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," says Kellar Autumn, lead author of an article in this week's Proceedings of the National Academy of Sciences. Our new data prove once and for all how geckos stick."
Working at Lewis & Clark College, the University of California at Berkeley, the University of California at Santa Barbara, and Stanford University, the interdisciplinary team:
* confirmed speculation that the gecko's amazing climbing ability depends on weak molecular attractive forces called van der Waals forces,
* rejected a competing model based on the adhesion chemistry of water molecules, and
* discovered that the gecko's adhesive depends on geometry, not surface chemistry. In other words, the size and shape of the tips of gecko foot hairs--not what they are made of--determine the gecko's stickiness.
To verify its experimental and theoretical results, the gecko group then used its new data to fabricate prototype synthetic foot-hair tips from two different materials.
"Both artificial setal tips stuck as predicted," notes Autumn, assistant professor of biology at Lewis & Clark College in Portland, Ore. "Our initial prototypes open the door to manufacturing the first biologically inspired dry, adhesive microstructures, which can have widespread applications."
The project required an interdisciplinary team, according to Autumn. Engineers Ronald Fearing and Metin Siiti at the University of California at Berkeley built prototype synthetic gecko foot-hair tips that stick like the real thing. Engineer Jacob Israelachvili at the University of California at Santa Barbara provided the mathematics that led to the prototype's design. Other team members include biologist Robert Full at the University of California at Berkeley and engineer Thomas Kenny of Stanford University.
Van der Waals force vs. capillary adhesion
The team tested two competing hypotheses: one based on van der Waals force and a second on capillary (water-based) adhesion.
"Our results provide the first direct experimental verification that a short-range molecular attraction called van der Waals force is definitely what makes geckos stick," Autumn emphasizes.
Van der Waals forces, named after a Dutch physicist of the late 1800s, are weak electrodynamic forces that operate over very small distances but bond to nearly any material.
Geckos have millions of setae--microscopic hairs on the bottom of their feet. These tiny setae are only as long as two diameters of a human hair. That's 100 millionth of a meter long. Each seta ends with 1,000 even tinier pads at the tip. These tips, called spatulae, are only 200 billionths of a meter wide--below the wavelength of visible light.
"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," says Autumn.
A single seta can lift the weight of an ant. 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.
"Our previous research suggested that van der Waals force could explain gecko adhesion. But we couldn't rule out water adsorption or some other types of water interaction. With our new data, we can finally disprove a 30-year-old theory based on the adhesion of water molecules."
The team's previous research ruled out two other possible forms of adhesion: suction and chemical bonding.
Geometry vs. chemistry
"The van der Waals theory predicts we can enhance adhesion--just as nature has--simply by subdividing a surface into small protrusions to increase surface density," Autumn explains. "It also suggests that a possible design principle underlies the repeated, convergent evolution of dry adhesive microstructures in geckos, anoles, skinks, and insects. Basically, Mother Nature is packing a whole bunch of tiny things into a given area."
If van der Waals adhesion determines setal force, then geometry and not the material make-up that should dictate the design of setae, the team predicted.
Jacob Israelachvili at the University of California at Santa Barbara applied a mathematical model--the Johnson-Kendall-Roberts theory of adhesion--to predict the size and shape of the setae.
Ronald Fearing at the University of California at Berkeley took the empirical results and nanofabricated synthetic foot-hair tips from two different materials.
"We confirmed it's geometry, not surface chemistry, that enables a gecko to support its entire body with a single toe," Autumn says.
"This means we don't need to mimic biology precisely," he explains. "We can apply the underlying principles and create a similar adhesive by breaking a surface into small bumps. These preliminary physical models provide proof that humans can fabricate synthetic gecko adhesive," he says.
"The artificial foot-hair tip model opens the door to manufacturing dry, self-cleaning adhesive that works under water and in a vacuum," according to Autumn, who foresees countless applications for synthetic gecko adhesive--from vacuum areas of clean rooms to outer space.
Defense Advanced Research Projects Agency (DARPA) supports the research.
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