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25 November 2011

Mystery of Geckos' sticky feet

The mystery of how geckos manage to scurry up walls and stick to ceilings may have been solved by scientists.

It seems the little lizards have a network of tiny hairs and pads on their feet which produce electrical attractions that literally glue the animals down.

With millions of the hairs on each foot, the combined attraction of the weak electrical forces allow the gecko to stick to virtually any surface - even polished glass.

Californian researchers believe the reptile's sticky toes could now help them to develop a novel synthetic adhesive that is both dry and self-cleaning.

If a human hand had the equivalent "sticking power", it could lift huge weights. "If the hands were maximally attached, we estimate that kind of size would be able to hold about 90 pounds (40 kilos) or so," Professor Autumn Kellar, one of the researchers in the gecko study, told the BBC.


Geckos are small, insect-eating, and often very noisy creatures that have become popular pets.

Close up on the gecko's setae
Biologists have long admired the animals' ability to walk up smooth surfaces but have never really understood how it was done.

Suction was regarded as an unlikely explanation since geckos can cling on to a wall even in a vacuum. That astonishing trick of walking upside down on the ceiling would seem to rule out friction.

Furthermore, without any glands on their feet, it would be hard for geckos to produce their own natural glue.

But a team of researchers, led by Professor Robert Full, now think it may all come down to van der Waals forces - the weak attraction that molecules have for one another when they are brought very, very close together.

Outstanding adhesives

The scientists looked closely at the feet of a Tokay gecko (Gekko gecko) which is native to South-East Asia. Close-up pictures reveal about two million densely packed, fine hairs, or "setae", on each toe.

The end of each seta is further subdivided into hundreds to thousands of structures called spatulae.

Professor Full's team of biologists and engineers calculated the combined adhesive force of all the tiny hairs lining the gecko's toes is 10 times greater than the maximum force reportedly needed to pull a live gecko off the wall.
The spatulae - scale bar: one thousandth of a millimetre
"These billion spatulae, which look like broccoli on the tips of the hairs, are outstanding adhesives," said Professor Full, head of the Poly-PEDAL (Performance, Energetics, Dynamics, Animal Locomotion) Laboratory at the University of California, Berkeley.

He said: "Geckos have developed an amazing way of walking that rolls these hairs onto the surface, and then peels them off again, just like tape. But it's better than tape."

Professor Full's team believe the stickiness of the gecko can now be attributed to intermolecular forces so weak they are normally swamped by the many stronger forces in nature.

Unbalanced charges

These forces come into play, though, because the gecko foot hairs get so close to the surface.

He said: "The hairs allow the billion spatulae to come into intimate contact with the surface, combining to create a strong adhesive force.

Hanging on to glass: The gecko's party piece
 "Our calculations show that van der Waals forces could explain the adhesion, though we can't rule out water adsorption or some other types of water interaction."

Van der Waals forces arise when unbalanced electrical charges around molecules attract one another.

They are responsible for the attraction between layers of graphite, for example, and the attraction between enzymes and their substrate.

Though the charges are always fluctuating and even reversing direction, the net effect is to draw two molecules together, such as molecules in a gecko foot and molecules in a smooth wall.

Geckos will stick to metal, plastics or glass, in air or under water, Kenny says. Engineers predicted the force of a single gecko foot hair by measuring the adhesive force of a whole gecko and dividing that by the number of foot hairs per animal. They were surprised when their measurements revealed a single hair to be 10 times more forceful at adhesion than they had estimated.

It turns out a gecko foot exerts a force of 10 Newtons (about a kilogram, or 2.2 pounds). A million hairs could fit onto a dime-sized patch capable of supporting about 20 kilograms (about 44 pounds).

Attachment takes about 6 thousandths of a second, detachment takes 16 thousandths of a second -- fast enough to allow a gecko to run, said. Metin Sitti, an engineer at Carnegie Mellon University

In yet-to-be published work the gecko hairs have been shown to be self-cleaning, unlike any other known adhesive. Work has also begun on building a mechanical gecko that Professor Full hopes will lead researchers to a new, synthetic, dry adhesive.

"There are signs for the believing nation in the creation of their (own) selves, and the creation of the animals He has scattered (across the world)." [Quran 45:4]

Courtesy: BBC Science, Stanford University 

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