An extremely thin layer of lipids helps the gecko’s feet stay sticky

Geckos are known for having prehensile feet that allow them to climb vertical surfaces with ease. They get this apparent superpower from millions of microscopic, hair-like structures on their toes. Now, scientists have zoomed in to take an even closer look at these structures, called bristles, and found that they are coated with an ultrathin film of water-repelling lipid molecules just one nanometer, or billionths of a meter, thick.

Researchers at the National Institute of Standards and Technology (NIST) analyzed the surface of the bristles using high-energy X-rays emitted from a type of particle accelerator called a synchrotron. Synchrotron microscopy showed that lipid molecules line the surface of the bristles in dense, ordered arrays.

An international team of researchers published the findings in Biology Letters An earlier companion article published in Physical Chemistry Letters used the same technique to show how the individual protein strands that make up bristles are arranged.

“A lot is already known about how setae work mechanically,” said NIST physicist and co-author Cherno Jay. “We now have a better understanding of how they work in terms of their molecular structure.”

Geckos have inspired many products, including adhesive tapes with bristle-like microstructures. Understanding the molecular characteristics of bristles could lead inventors who find inspiration in nature — a concept called biomimicry — to come up with even better designs.

The bristles provide adhesive power because they are flexible and pick up on the microscopic contours of whatever surface the gecko is climbing. Even smaller structures at the ends of the bristles, called spatulas, make such close contact with the climbing surface that the electrons in the two materials interact, creating a type of attraction called van der Waals forces. To free its leg, which might otherwise become trapped, the gecko changes the angle of the bristles, breaking these forces and allowing the animal to take its next step.

Lipids can play a role in this process because they are hydrophobic, meaning they repel water. “The lipids can function to repel any water beneath the spatulae, allowing them to make closer contact with the surface,” said physicist and co-author Tobias Weidnerof Aarhus University in Denmark. “This will help the geckos maintain their grip on wet surfaces.”

Bristles and spatulas are made of a type of keratin protein similar to that found in human hair and nails. They are extremely delicate. The researchers showed that the keratin fibers are aligned in the direction of the bristles, which may help them resist abrasion.

“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 biologist and co-author Stanislav Gorboff Kiel University in Germany. “This could help biomimetic engineers know what to do next.”

“You can imagine gecko boots that don’t slip on wet surfaces, or gecko gloves for holding tools that are wet,” said NIST physicist and co-author Dan Fischer. “Either a vehicle that can run on walls, or a robot that can run along power lines and inspect them.”

The NIST synchrotron microscope that the researchers used to analyze the bristles is unique in its ability to identify molecules on the surface of a three-dimensional object, measure their orientation and map their position. It is located at the US Department of Energy’s Brookhaven National Laboratory, where the National Synchrotron Light Source II, a half-mile-long particle accelerator, provides a source of high-energy X-rays for illumination.

This microscope is commonly used to understand the physics of advanced industrial materials, including batteries, semiconductors, solar panels and medical devices.

“But it’s amazing to understand how the gecko’s legs work,” Fischer said, “and we can learn a lot from nature when it comes to improving our own technology.”

Reference: Rasmussen MH, Holler KR, Baio JE, et al. Evidence that gecko setae are covered by an ordered nanometer-thin lipid film. Biol Lett. 18 (7): 20220093. doi: 10.1098/rsbl.2022.0093

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