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

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 bristles called spatulas.  These help maximize contact with a surface.
enlarge / 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 bristles called spatulas. These help maximize contact with a surface.

Yi Song

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

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

Essentially, the tufts of tiny hairs on gecko legs come so close to the contours in walls and ceilings that electrons from the gecko hair molecules and electrons from the wall molecules interact and create an electromagnetic attraction. That’s what allows geckos to effortlessly climb smooth surfaces like glass. Spiders, roaches, beetles, bats, tree frogs, and lizards all have sticky pads of different sizes that use the same powers.

Geckos and their unusual feet have long been of great interest to scientists. For example, in 2013, scientists at the University of California, Santa Barbara, designed a reusable dry glue inspired by the gecko’s legs that stick easily to smooth surfaces, adhere strongly when pushed forward and slip when pulled back. . The secret to that directivity was the angle and shape of the fabricated half cylinder fibers in the silicone-based adhesive. Pushing the flat side down created a larger surface to stick to a glass surface. Pulling the fibers with the rounded side down reduced the surface area so that the adhesive could slide off easily.

In 2020, Berkeley scientists investigated why soft, furry geckos only “stick” in one direction. Pull a foot in one direction and the geckos will grab onto a surface. Release the foot and the toes will “peel off” in the opposite direction, although that doesn’t stop the nimble gecko from moving any way they want. The scientists found that geckos could run sideways just as fast as they climbed up, thanks to the ability to realign their toes. Having multiple toes helps geckos adapt to stick to smooth or irregular surfaces. Those toes that kept contact with the surface were able to shift orientation and better distribute the load. And because the toes are soft, the animals can more easily adapt to rough surfaces.

Despite all we’ve learned, little is known about the detailed surface chemistry of gecko toe pads, especially the setae. So the authors of this latest paper wanted to learn more, with particular interest in the possible prominent role that water could play in surface adhesion. “Much was already known about how setae work mechanically,” says NIST physicist and co-author Cherno Jaye. “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 footprints and the arrays of geckosetae (they can also be found in the epidermis of reptiles, arranged in a brick-and-mortar pattern). NIST’s synchrotron microscope is well suited for a closer look at molecular structure, as it can not only identify molecules on the surface of three-dimensional objects, but also reveal exactly where they are and how they are oriented.

That thin film of lipids (just a nanometer thick) could serve to push water out from under the spatulas, the authors speculate, allowing the spatulas to make closer contact with the surface, allowing the geckos to maintain their grip on wet surfaces. In addition, the setae and spatulas are composed of keratin protein, much like the proteins in human hair and fingernails. The analysis revealed that the alignment of the keratin fibers is in the direction of the setae, which is possibly how they resist wear.

Gecko feet have inspired many intriguing uses in the past, including adhesive tape, the aforementioned glue, a “stickybot” climbing robot with synthetic setae, and even (I’m not kidding) a strapless bra design. Jaye et al. see “gecko boots” that can stick to wet surfaces, or “gecko gloves” for getting a better grip on wet tools as possible uses of their latest research.

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