Geckos' grip finally grasped


Sticky: Scientists say the lizards owe their climbing ability to tiny hairs and a quirky principle of quantum mechanics.

August 27, 2002|By Michael Stroh | Michael Stroh,SUN STAFF

For centuries, geckos have been revered as one of nature's coolest climbers.

Whether they're skedaddling up smooth glass at 3 feet per second or dangling from a hotel ceiling by a single toe, there's almost nothing the little lizards can't scale. Aristotle marveled at the gecko's ability to "run up and down a tree in any way, even with the head downwards." And for more than a century, the gecko has tormented scientists trying to divine the secret of its grip.

Now a group of biologists and engineers who call themselves the Gecko Team has solved the long-standing mystery and created the first artificial adhesive based on the gecko's sticky secret. Their research, published today in the Proceedings of the National Academy of Sciences, could lead to such things as exotic new Post-It notes and sure-footed space robots capable of climbing Martian cliffs.

The secret to the gecko's grip, it turns out, hinges on one of the animal world's worst cases of split ends and a quirky property of quantum mechanics known as the van der Waals forces.

Geckos have millions of microscopic hairs sprouting from the bottom of their feet. The hairs, called setae (SEE-tee), split into as many as 1,000 tinier hairlets, each capped with a triangular pad whose shape has been likened to a hamburger flipper. These tips, called spatulae, are about the size of a small bacterium.

Scientists have debated their function over the years.

"The initial thought was maybe they were little suction cups," says Kellar Autumn, a biologist at Lewis and Clark College in Portland, Ore., and member of the Gecko Team.

Others speculated the tiny pads functioned more like spikes - the so-called "climber's boot" hypothesis. Still others thought geckos, like some species of ants and beetles, secreted a natural glue onto their hairy toes to hold onto leaves and prevent predators from prying them lose.

Autumn became interested in the problem on a trip to Hawaii several years ago. Lying on his hotel bed one evening, he watched a gecko dart from the corner of the ceiling to ambush a spider. "The spider fell off," he recalls. The gecko stayed put.

Upon his return, he discovered he was far from the first to pursue the problem. Over the years scientists have plunged geckos into beakers of water to see if they stick (they do). They've dropped the animals into vacuum chambers, killing the geckos but not diminishing the adhesive power of their tacky toes.

Scientists have bombarded geckos with X-rays and blasted them with ionizing air guns to neutralize static electricity, another force once thought to be helping the animals hold on.

"Little by little, theories were eliminated," says Anthony Russell, a zoologist at the University of Calgary who has been studying geckos for more than three decades.

Fleeting attraction

In the 1960s, German scientist Uwe Hiller proposed another idea: Perhaps the creatures stick using the van der Waals forces.

Named after the 19th-century Dutch physicist who first described them, the van der Waals forces occur when atoms come close to one another and - for a fleeting instant - display a weak electrical attraction.

The forces are too subtle to be felt by humans when, for example, they press a palm to a wall. But could a gecko, with millions of microscopic pads to make contact with a surface, exploit the effect?

Autumn and a team from Stanford University and the University of California set out to see.

The work wasn't always easy. They chose the tokay gecko, one of the largest - and most ornery - of the 800 or so gecko species, to study. The foot-long tokay hiss. They bark. They bite.

"Their bite is actually worse than their bark," Autumn says.

A motivated tokay, he says, can chomp halfway through a pair of heavy gardening gloves. And once a tokay latches on, it rarely lets go. "They prefer revenge to freedom."

Still, the effort led to the first measurement of the holding power of gecko hair. A single seta, the researchers found, could lift an ant. If all the hairs worked simultaneously, they could theoretically hoist a 280-pound man.

The research strongly suggested the van der Waals forces were at the heart of the gecko's grip. But Autumn and his group needed to rule out one last alternative explanation: water.

Just about every surface that comes into contact with the air is covered by a thin film of water molecules. "Even if it's dry to the touch," Autumn says.

Water molecules have a strong attraction to one another. That is what causes a pair of glass microscope slides to stick when pressed together. Scientists know certain frogs exploit that phenomenon to get a grip.

To see whether the gecko does, too, they designed an experiment with gallium arsenide, a material used in military night-vision scopes. But what drew Autumn and his team to the material was something else: water molecules don't stick to it.

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