Sticky Secrets of the Gecko

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HEADLINE: Sticky Secrets of the Gecko;

Researchers have finally unlocked the mystery of the small lizard's remarkable climbing ability

BYLINE: Carl T. Hall, Chronicle Science Writer

BODY:
One small step for a gecko is proving to be one giant leap for robot designers -- and Spiderman may not be far behind.
Geckos are a superhero's envy when it comes to their ability to climb rapidly up just about any vertical surface. Unlike other climbing animals and insects, the small lizards have no need for suction cups, Velcro-like hooks, electrostatic attraction or sticky secretions.
A gecko can dash up a smoothly polished glass surface as easily as we can fall off a log, sticking and unsticking its feet 15 times a second. It can hang from a ceiling, if it feels like it, by a single toe. Even in a laboratory-created vacuum, where suction would do little good, a gecko's foot will still stick. Despite a century of anatomical studies, nobody could ever figure out just how the lizard does it.
Now, researchers at the University of California at Berkeley, Stanford University and Lewis & Clark College in Portland, Ore., appear to have stumbled upon the answer by taking a close look at the stylish Tokay gecko's remarkable feet and toes.
A report on the research appeared earlier this month in the journal Nature. Animated pictures and graphics can be found on the Internet at http://www.lclark.edu/ autumn/private/u38j47a0t/
Whimsical-Looking Appendages
Each five-toed foot of the Tokay is tipped with about 500,000 microscopic foot-hairs, whimsical-looking little appendages scientists call "setae" (pronounced see-tee). Each seta is no more than about 5 microns in diameter -- just 5 millionths of a meter, or about one-tenth the diameter of a human hair.
Even more astounding, the hairs blossom at the ends into hundreds of teeny split ends. The submicroscopic, funnel-shaped pads, or spatulae, measure a scant 200 nanometers across -- 200 billionths of a meter, so small they cannot even be seen with visible light.
A climbing Tokay moves in such a way that when it presses a foot down, the toe hairs splay out and the spatulae unfurl, shooting out and recoiling like blow-through party toys as the lizard scuttles along.
This allows the spatulae to get extraordinarily close to the target surface -- so close that subtle intermolecular attractions kick in. Essentially, the lizards may be grabbing onto walls with the same atomic-scale "glue," known as van der Waals forces, that make enzymes biochemically 'sticky."
The reptilian toe-hairs are much smaller than ant feet. And yet a single seta, when pressed against a surface at the right angle, packs enough adhesive punch to hold up a typical ant's full 20 milligrams of body weight. Changing the angle allows the seta to pop right off again.
A million of these suckers could fit onto a dime-sized patch capable of holding up 45 pounds. In theory, a teacher could use one of these patches to stick an unruly kindergartner to the classroom ceiling.
Conquering El Capitan
The new calculations might inspire some fantastic possibilities in climbing gear. Stores might someday stock gecko-style climbing gloves, for starters, with hundreds of little setae woven into the fingertips to help vertically challenged hominids conquer El Capitan.
"It would be a challenging thing to manufacture, but it's not all that far off," said UC Berkeley's Robert Full, one of the lead authors of the Nature report.
Collaborators included Lewis & Clark biologist Kellar Autumn, a former post-doctoral researcher in Full's laboratory, which specializes in studying how animals move about.
Engineers Ronald Fearing at Berkeley and Thomas Kenny at Stanford helped the researchers achieve the first direct measurements of a single gecko foot-hair, the key step in unlocking the animal's adhesive secrets, by constructing a delicate icro-electromechanical force sensor.
Many other gecko species sport similar footgear, but the Tokay is considered to have the most elaborate feet of any lizard known, possibly the most advanced dry-adhesive technology in nature outside the insect world.
Ants and cockroaches are well-equipped, too. Their feet sport tiny spines or hooks, supplemented by sticky secretions in some species, that work extremely well on nearly any rough or porous surface, even if the surface appears perfectly smooth to the naked eye.
Some beetles and kissing bugs have setae very similar to those found in lizards, and their relatively low body weights give them a leg up on the heavier geckos. The Tokays could rely on their claws and muscles to grab on, as do birds and most big climbing animals, but that only would take them so far. Because of their unique foot apparatus, the geckos can generate adhesive forces 600 times greater than mere friction alone.
Such stick-on overkill appears to be nature's strategy for overcoming the fact that a gecko has to contend with all kinds of rough, irregular surfaces in its native Southeast Asia habitat. In fact, with each step it takes, a Tokay grabs on with only about 2 percent of its available toe-hairs.
Mecho-Gecko'
Proper attachment and detachment happen only when the feet hairs are angled and pressed correctly. And the findings, researchers say, reveal a new strategy for making robots with gecko-like feet capable of scampering up and down walls, say in a burning building to help locate trapped victims.
Smaller devices could use these same dry-adhesive tricks for medical applications and computer architecture. A prototype robot, called the "mecho-gecko," is already in the works at a Massachusetts company called IS Robotics.
The fact that gecko adhesion works in a vacuum means the system could be used to design setae-studded tape suitable for use in space. "Geckos can do things that we just can't do with current robotics and adhesive technology," Autumn said.
Some key details are still being kicked around by the researchers. It's not quite clear, for example, just what kinds of molecular interactions might be going on to explain the lizard's climbing prowess. Besides van der Waals forces, another factor that might be significant has to do with the water content of the setae and attractive forces between the water molecules and the surface.
Another puzzle is how the setae -- made of the same keratinous material as human toenails -- manage to stay clean, even when the lizards walk up dirty surfaces.
"You stick a piece of tape to a dirty surface and the tape is worthless after the first stick," Autumn noted. "As the geckos walk, their feet get cleaner, not dirtier, and we don't know exactly how they do that."
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HOW THE GECKO STICKS TO A WALL
Researchers have discovered that a Tokay gecko can climb a vertical wall, stick upside down or even cling to polished glass without benefit of suction, sticky secretions, electrostatic forces, friction or Velco-like interlocking. Instead, the lizard uses an elaborate array of toe-hairs and a peeling motion to form intermolecular bonds with the surface as it walks.
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GECKO FOOT STRUCTURE
The gecko's foot has vast numbers of setae fibrous structures of keratin, the material of fingernails about 500,000 per foot.
These setae are lined up in rows on the gecko's toes. Each seta is about a tenth the diameter of a human hair.
A single seta, depicted at left, ends in what appears to be an array of split ends; these are clumps of 400 to 1,000 tiny structures called spatulae.
These spatulae, numbering perhaps a billion per gecko, are around 10 millionths of an inch across, and when spread out, begin to interact with a surface at a molecular level.
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HOW THE GECKO MOVES AND ADHERES (AN APPROXIMATION)
The gecko moves with a peculiar motion of curling and uncurling its toes, up to 15 times a second.
When the spatulae reach the proper angle and pressure against the surface, they get so close that they generate the same binding force that holds molecules together.
The gecko tries to find the proper angle and pressure by moving its toes. The adhesive force of one spatula may not amount to much, but collectively they generate an impressive force.
As the gecko begins curling its feet again, an angle of around 30 starts to break the molecular force and the bond is released, leaving no residue.
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Source: Kellar Autumn, Lewis & Clark College and Robert J. Full, UC Berkeley; Nature magazine
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GRAPHIC: GRAPHIC, Steve Greenberg/The Chronicle