Scientists discover the secret of how gecko feet stay sticky

Scientists discover the secret of how gecko feet stay sticky

Close up of the toes of a Tokay gecko.  They have many tiny hairs per foot called bristles, each of which divides into hundreds of even smaller hairs called spatulas.  These help maximize contact with a surface.
Enlarge / Close up of the toes of a Tokay gecko. They have many tiny hairs per foot called bristles, each of which divides into hundreds of even smaller hairs called spatulas. These help maximize contact with a surface.

Song Yi

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

These tiny microscopic hairs are called bristles, each separating into hundreds of even smaller hairs called spatulas. It has long been known that at microscopic size scales, the so-called van der Waals forces – the attractive and repulsive forces between two dipole molecules – become important.

Essentially, the tufts of tiny hairs on the gecko’s feet come so close to the contours of 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 varying sizes that use these same forces.

Geckos and their unusual legs have long interested scientists. In 2013, for example, scientists at the University of California, Santa Barbara designed a reusable dry adhesive inspired by the gecko’s feet that adheres easily to smooth surfaces, sticks strongly when pushed forward, and slips when pushed. it is pulled back. The secret to this directionality was the angle and shape of the semi-cylindrical fibers made into the silicone-based adhesive. Pushing the flat side down produced a larger area to stick to a glass surface. Pulling the fibers with the rounded side down reduced the surface area so that the adhesive could slide easily.

In 2020, Berkeley scientists investigated why soft, furry gecko toes only “stick” in one direction. Pull one foot in one direction and the gecko’s toes will latch onto a surface. Release the foot and the toes will “peel” in the opposite direction, although this does not prevent the agile gecko from moving as it sees fit. Scientists have found that geckos can run sideways as quickly as they climb, thanks to the ability to realign their toes. Having multiple toes helps geckos adapt in order to stick to slippery or uneven surfaces. The toes that maintained contact with the surface could change orientation and better distribute the load. And because the toes are soft, animals could more easily conform to rough surfaces.

Despite all we have learned, little is known about the detailed surface chemistry of gecko pads, especially the setae. The authors of this last article therefore set out to find out more, with a particular interest in the important role that water could play in surface adhesion. “A lot was already known about the mechanical functioning of setae,” said NIST physicist and co-author Cherno Jaye. “We now have a better understanding of how they work in terms of molecular structure.”

According to the authors, recent studies have indicated the presence of water-repellent lipid molecules in gecko footprints and gecko bristle networks (these can also be found in the epidermis of reptiles, arranged in a brick-and-mortar pattern) . NIST’s synchrotron microscope is well suited for taking a closer look at molecular structure because it is able to not only identify molecules on the surface of three-dimensional objects, but also reveal precisely where they are and how they are oriented.

This thin film of lipids (just a nanometer thick) could serve to repel water under the spoonbills, the authors speculate, allowing the spoonbills to make closer contact with the surface, helping the geckos maintain their grip. on wet surfaces. Additionally, bristles and spatulas are made up of keratin proteins, just like the proteins in human hair and nails. The analysis revealed that the alignment of the keratin fibers is in the direction of the bristles, which could be how they resist abrasion.

Gecko feet have inspired many intriguing applications in the past, including sticky tape, the aforementioned sticky, a “stickybot” climbing robot with synthetic bristles, and even (I’m not kidding) a bra design without straps. Jaye et al. imagine “gecko boots” that can stick to wet surfaces, or “gecko gloves” to get a better grip on wet tools as potential applications of their latest research.

“The most exciting 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. “It can help biomimetic engineers know what to do next.”

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