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Scientists get new insight into the secret of how gecko feet stay sticky

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Increase / Close-up of the toe pads of a Tokay gecko. They have many tiny hairs per foot called setae, each of which splits into hundreds of even smaller setae called spatulas. They help maximize contact with the surface.

Yi Song

Geckos are known to be skilled climbers, able to stick to any surface thanks to tiny hair-like structures on their feet. Together with colleagues from Oregon, Denmark and Germany, researchers at the National Institute of Standards and Technology (NIST) took a closer look at these structures using a high-energy synchrotron and found that they are coated with an ultrathin layer of lipid molecules. upright, according to a recent article published in the journal Biology Letters.

These tiny microscopic hairs are called setae, each of which splits into hundreds of even smaller setae called spatulas. It has long been known that, on a microscopic scale, the so-called van der Waals forces, the forces of attraction and repulsion between two dipole molecules, become significant.

In fact, the tufts of tiny hairs on the gecko’s legs fit so close to the contours of the walls and ceilings that the electrons from the gecko’s hair molecules and the electrons from the wall molecules interact with each other and create an electromagnetic attraction. This is what allows geckos to effortlessly climb smooth surfaces like glass. Spiders, cockroaches, beetles, bats, tree frogs, and lizards all have sticky pads of different sizes that use the same powers.

Geckos and their unusual legs have long been of great interest to scientists. For example, in 2013, scientists at the University of California, Santa Barbara developed a reusable gecko-foot-inspired dry adhesive that sticks easily to smooth surfaces, sticks firmly when pushed forward, and slips off when pulled back. The secret of this direction lay in the angle and shape of the semi-cylindrical fibers produced in the silicon-based adhesive. Pressing the flat side down resulted in more surface area for bonding to the glass surface. Pulling the fibers with the rounded side down reduced the surface area so the adhesive could slip off easily.

In 2020, scientists at Berkeley investigated why the gecko’s soft, hairy toes only stick in one direction. Pull the leg in one direction and the gecko’s toes will grab onto the surface. Release the paw and the toes will “peel off” in the opposite direction, although this does not prevent the agile gecko from moving in any way it chooses. Scientists have found that geckos can run sideways as fast as they climb up thanks to the ability to rearrange their toes. Having multiple toes helps geckos adapt to slippery or uneven surfaces. Those fingers that remained in contact with the surface could change orientation and better distribute the load. And because the toes are soft, animals can more easily adjust to rough surfaces.

Despite everything we have learned, little is known about the detailed surface chemistry of the gecko’s toe pads, especially the bristles. Therefore, the authors of this latest article decided to find out more, with particular interest in the possible important role that water can play in surface adhesion. “A lot was already known about how bristles work mechanically,” said NIST physicist and co-author Cherno Jay. “Now we have a better understanding of how they work in terms of their molecular structure.”

According to the authors, recent studies have shown the presence of water-repellent lipid molecules in gecko tracks and gecko bristle arrays (these can also be found in the epidermis of reptiles arranged in a brick-and-mortar pattern). The NIST Synchrotron Microscope is well suited for closer examination of molecular structure because it can not only identify molecules on the surface of 3D objects, but also pinpoint exactly where they are and how they are oriented.

This thin film of lipids (only a nanometer thick) could serve to repel any water under the shoulder blades, the authors suggest, allowing the shoulder blades to come into closer contact with the surface, thus helping the geckos hold on to wet surfaces. In addition, the bristles and spatulas are made up of a protein called keratin, very similar to the proteins in human hair and nails. The analysis showed that the keratin fibers are oriented in the direction of the bristles, which may be why they resist abrasion.

Gecko feet have inspired many intriguing uses in the past, including sticky tape, the aforementioned glue, a climbing “stickybot” robot with synthetic bristles, and even (I’m not kidding) a strapless bra design. Jay and others. present “gecko boots” that can stick to wet surfaces or “gecko gloves” for better grip on wet tools as potential applications of their latest research.

“The most interesting thing for me about this biological system is that everything is perfectly optimized at all scales, from macro to micro to molecular,” said co-author Stanislav Gorb, a biologist at the University of Kiel in Germany. “This could help biomimetic engineers figure out what to do next.”

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