Geckos are known to be skilled climbers, able to stick to any surface thanks to small hair-like structures at the base of the feet. Together with colleagues in Oregon, Denmark and Germany, researchers at the National Institute of Standards and Technology (NIST) took a closer look at these structures that use high-energy synchrotron, revealing that they are coated with an ultra-thin layer of lipid molecules in an upright orientation. recent article published in the journal Biology Letters.
These small microscopic hairs are called setae, each of which divides into hundreds of even smaller bristles 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 significant.
Essentially, the tufts of tiny hairs on gecko feet come so close to the contours of walls and ceilings that electrons from the gecko hair molecules and electrons from the wall molecules interact with each other and create an electromagnetic attraction. This is what enables geckos to climb effortlessly on slippery surfaces such as glass. Spiders, cockroaches, beetles, bats, frogs and lizards all have sticky foot pads of varying sizes that use the same forces.
Geckos and their unusual feet have long been of great interest to scientists. In 2013, for example, researchers at the University of California, Santa Barbara, designed a reusable dry glue inspired by gecko’s feet that easily adhered to slippery surfaces, adhered strongly when pushed forward and slid off when pulled back. The secret behind that directional ability was the angle and shape of the semi-cylindrical fibers produced in the silicon-based adhesive. Pushing the flat side down gave a larger surface area to adhere to a glass surface. Pulling the fibers down with the rounded side reduced the surface so that the glue could easily slip off.
In 2020, Berkeley researchers investigated why soft, hairy geckos only “stick” in one direction. Pull one foot in one direction and the gecko toes will engage a surface. Release the foot and the toes will “bottle” in the opposite direction, even if it does not stop the agile gecko from moving in any way it chooses. The researchers found that geckos could run sideways as fast as they climbed, thanks to the ability to adjust their toes. Having multiple toes helps geckos adapt to stick to slippery or irregular surfaces. The toes that kept in contact with the surface were able to change orientation and better distribute the load. And because the toes are soft, the animals could more easily adapt to rough surfaces.
Despite everything we have learned, little is known about the detailed surface chemistry of gecko pillows, especially setae. So the authors of this latest article set out to learn more, with particular interest in the possible prominent role that water can play in surface adhesion. “Much was already known about how setae work mechanically,” said 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 indicated the presence of water-repellent lipid molecules in gecko-footprints and arrays of gecko-setae (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 the molecular structure because it is able to not only identify molecules on the surface of three-dimensional objects, but also to reveal exactly where they are and how they are oriented.
The authors speculate that the thin film of lipids (only a nanometer thick) can serve to push away water under the spatulas, allowing the spatulas to come into closer contact with the surface, thus helping the geckos to maintain their grip on wet surfaces. Furthermore, the setae and spatula are composed of keratin protein, much like the proteins in human hair and fingernails. The analysis showed that the adjustment of the keratin fibers is in the direction of the setae, which may be how they resist wear.
Gecko feet have inspired many exciting applications lately, including a sticky tape, the aforementioned glue, a “stickybot” climbing robot with synthetic setae, and even (I do not burn) a strapless bra design. Jaye et al. Imagine “gecko boots” that can adhere 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 on all scales, from macro to micro to molecular,” said co-author Stanislav Gorb, a biologist at Kiel University in Germany. “This can help biomimetic engineers know what to do next.”