The gecko's feet are coated with a very thin layer of lipid that helps it stay sticky
Geckos are famous for having gripping feet which allow them to scale vertical surfaces with ease. They get this superpower from millions of microscopic structures like the hairs on their toes.
Now, scientists have zoomed in to take a closer look at the structures, called setae, and found that they are coated in an ultra-thin film of water-repelling lipid molecules just a nanometer, or one millionth of a meter thick.
Researchers from the National Institute of Standards and Technology (NIST) analyzed the surface of the setae using high-energy X-rays emitted by a type of particle accelerator called a synchrotron. Synchrotron microscopy shows that the lipid molecules coat the surface of the setae in a dense and regular arrangement.
Lipids can play a role in this process because they are hydrophobic, meaning they repel water. “The lipids may serve to push the water under the spatula, allowing them to make closer contact with the surface,” said physicist and co-author Tobias Weidner of Aarhus University in Denmark. “This will help the gecko maintain its grip on wet surfaces.”
Setae and spatulas are made of a type of keratin protein similar to that found in human hair and nails. They are very smooth. The researchers showed that the keratin fibers were parallel to the direction of the setae, which may have helped them resist abrasion.
“The thing that interests me most about these biological systems is that they are perfectly optimized at every scale, from macro to micro to molecular,” said biologist and co-author Stanislav Gorb of the University of Kiel in Germany. “This can help biomimetic engineers know what to do next.”
“You can imagine a gecko’s boot that doesn’t slip on a wet surface, or a gecko’s glove for holding a wet tool,” says NIST physicist and co-author Dan Fischer. “Or a vehicle that can go through a wall, or a robot that can run along a power line and check it.”
The NIST synchrotron microscope that the researchers used to analyze setae is unique in its ability to identify molecules on the surface of three-dimensional objects, measure their orientation, and map their position. It is located at the US Department of Energy’s Brookhaven National Laboratory, where the National Synced Light Source II, a half-mile-long particle accelerator, provides a high-energy X-ray source for illumination.
These microscopes are commonly used to understand the physics of advanced industrial materials, including batteries, semiconductors, solar panels, and medical devices.
“But it’s really exciting to know how gecko’s feet work,” Fischer said, “and we can learn a lot from nature when it comes to improving our own technology.”
An international team of researchers published the findings in Biology Letters. An earlier companion paper, published in Physical Chemistry Letters, used the same technique to show how the individual protein strands that make up the setae are aligned.
“Much is 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.”
Geckos have inspired many products, including adhesive tapes with microstructures such as setae. Understanding the molecular features of setae might lead inventors who find inspiration in nature – a concept called biomimicry – to come up with better designs.
The setae provide adhesion because they are flexible and take on the microscopic contours of whatever surface the gecko climbs on. Even the smaller structures at the ends of the setae, called spatulae, make such close contact with the climbing surface that the electrons in the two materials interact, creating a type of attraction called a van der Waals force. In order to release its legs, which might otherwise remain restrained, the gecko changes the angle of the setae, stopping the force and allowing the animal to take its next step.
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