Boffins’ gecko dekko decodes secrets of sticking power

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Boffins’ gecko dekko decodes secrets of sticking power

Gripped by their abilities, scientists say gecko-inspired adhesives could transform production lines and homes

Cape Town bureau chief
The toe pads of a Tokay gecko. They have about 15,000 hairs per foot, each of which has split ends that maximise contact with the surface and support the lizard's weight by interacting with surface molecules.
Something's afoot The toe pads of a Tokay gecko. They have about 15,000 hairs per foot, each of which has split ends that maximise contact with the surface and support the lizard's weight by interacting with surface molecules.
Image: Yi Song

The secrets of geckos’ amazing ability to stick to just about anything have been unravelled in two new studies.

Now scientists say mass production of gecko-inspired adhesive materials is within reach, potentially transforming production lines and homes.

The materials will use the tiny protrusions that cover geckos’ feet for static gripping and the thousands of hairs that cover toes for robots that need to move while defying gravity.

Polymers with “gecko adhesion” surfaces could be used to make versatile grippers to pick up different objects on the same assembly line, said researchers who did the first study, published in the journal, Applied Materials & Interfaces.

They could also make hanging pictures easy by sticking to the frame and the wall at the same time, and robots with gecko superpowers could scoot up tall buildings to clean facades.

Principal Georgia Institute of Technology investigator Michael Varenberg said the grippers, which are dry and contain no glue, would adhere to anything, with the exception of non-stick substances such as Teflon.

Flexible walls a few hundred microns high emulate the surface of a gecko’s foot to make the same kind of adhesive force, but much stronger than that of the lizard’s toe.
Small wonder Flexible walls a few hundred microns high emulate the surface of a gecko’s foot to make the same kind of adhesive force, but much stronger than that of the lizard’s toe.
Image: Georgia Tech / Varenberg lab

“This is a clear advantage in manufacturing because we don’t have to prepare the gripper for specific surfaces we want to lift,” he said.

“Gecko-inspired adhesives can lift flat objects such as boxes, then turn around and lift curved objects such as eggs and vegetables.”

The stickiness comes from protrusions of less than half a millimetre that often look like sections of short, floppy walls running parallel to each other across the material’s surface.

Until now, the surfaces have been created by pouring polymers into moulds, but the process is expensive, time-consuming and unreliable.

The inset on the upper right illustrates how the gecko-adhesion surface is made by pushing razor blades into a setting polymer. The razor blades are pulled out, leaving indentations and stretching some of the polymer up, resulting in flexible walls that produce the gecko-adhesion effect.
Cutting edge The inset on the upper right illustrates how the gecko-adhesion surface is made by pushing razor blades into a setting polymer. The razor blades are pulled out, leaving indentations and stretching some of the polymer up, resulting in flexible walls that produce the gecko-adhesion effect.
Image: Georgia Tech / Varenberg lab

In the new method, the polymer is poured onto a smooth surface and allowed to partly set before rows of razor blades are dipped into it. When the blades are drawn out, they leave indentations surrounded by walls.

In the second study, experiments suggested that climbing robots could benefit from flexible, hairy toes, like those of geckos, that can adjust quickly to accommodate shifting weight and slippery surfaces.

Biologists observed geckos running horizontally along walls to learn how they used their five toes to compensate for different types of surfaces without slowing down.

A Tokay gecko was used by biologists to understand how the lizard’s five sticky toes help it climb on many types of surface.
Sticky patch A Tokay gecko was used by biologists to understand how the lizard’s five sticky toes help it climb on many types of surface.
Image: Yi Song

Previous research by Robert Full, from the University of California, Berkeley, found geckos’ toes use “intermolecular forces” to stick to the smoothest surfaces, uncurling and peel in milliseconds.

Their toes have up to 15,000 hairs per foot and each hair has “an awful case of split ends, with as many as a thousand nano-sized tips that allow close surface contact”, he said.

These discoveries have spawned research on new types of adhesives that stick almost anywhere, even under water.

One puzzle, Full said, was that gecko toes grab when pulled in one direction, but release when peeled in the opposite direction. Yet, geckos move agilely in any orientation.

Yi Song, from Nanjing University of Aeronautics and Astronautics in China, made high-speed video recordings of geckos’ toes as they ran sideways along a vertical surface. He also measured the area of contact of each toe.

He found that geckos ran sideways just as fast as they climbed upward, easily and quickly realigning their toes against gravity. The toes of the front and hind top feet during sideways wall-running shifted upward and acted like toes of the front feet during climbing.

Researchers were able to tell which parts of the toe pad (bright spots) were in contact with the surface and supporting the gecko’s weight.
lit up Researchers were able to tell which parts of the toe pad (bright spots) were in contact with the surface and supporting the gecko’s weight.
Image: Yi Song

Then researchers, who reported their findings last week in Proceedings of the Royal Society B, added slippery patches and strips, as well as irregular surfaces.

To deal with these hazards, geckos took advantage of having multiple, soft toes. Toes that retained contact with the surface were able to reorient and distribute the load, while their softness let them conform to rough surfaces.

“Toes allowed agile locomotion by distributing control among multiple, compliant, redundant structures that mitigate the risks of moving on challenging terrain,” Full said.

“Distributed control shows how biological adhesion can be deployed more effectively and offers design ideas for new robot feet, novel grippers and unique manipulators.”

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