Picture the scene: Athens, 350 B.C., and Aristotle is reclining in his chair in Plato’s Academy. Leaning back to scratch his unruly beard, Aristotle notices a large pink-spotted gecko on the marble ceiling above him. The gecko scampers away faster than 1 meter per second, leaving Aristotle wondering how the lizard manages to stick to the polished surface. He made a note of its climbing ability in his “Historia Animalium.”
A radical free thinker and arguably the world’s first biologist, Aristotle nevertheless had no hope of discovering the gecko’s secret.
For one thing, it would take about 1,900 years before the light microscope was invented, while the electron microscope only came along in the 1930s. Without such magnification power, Aristotle couldn’t know that the gecko’s feet and toes were covered in millions of microscopic hairs.
If he had known, he might have gotten a clue as to how the gecko sticks to smooth surfaces, but more likely he would have been even more puzzled. Only now have scientists finally worked out exactly how the gecko sticks and how it even manages to support its entire body weight using only a single toe.
In a paper in last week’s issue of the Proceedings of the National Academy of Sciences, Kellar Autumn from Lewis and Clarke College, Portland, Ore., and colleagues at the University of California in Berkeley, and Santa Barbara and Stanford universities, both in California, describe the hairs on the feet of the Tokay gecko, a southeast Asian species.
The minute hairs, called setae, are a millionth of a centimeter long (equivalent to two diameters of a human hair) and branch out to end in 1,000 even smaller pads (imagine them as bundles of microscopic broccoli). The pads, called spatulae, are two-billionths of a centimeter wide. That’s less than the wavelength of visible light, so even with the best light microscope, they can’t be seen.
“Intermolecular forces come into play because the gecko foot hairs split and allow a billion spatulae to increase surface density and come into close contact with the surface,” said Autumn, who leads an interdisciplinary team of scientists, including engineers, mathematicians and biologists.
At this scale, a type of attraction called Van der Waals’ forces operates. These are the forces that keep layers of graphite stuck together and that attract an enzyme to its substrate. There are electrical charges around all molecules, including those on a gecko’s foot and those on a smooth glass wall. The attraction between such surfaces is weak unless the distance between them is very small and the area of the two surfaces is large.
But the gecko’s foot solves both these problems — its broccoli-like structure makes the attractive force (and the gecko’s sticking power) very strong. Strong enough, the team showed, that a single hair could lift an ant, and a million hairs (covering the size of a 5 yen coin) could support the weight of a small child!
So why does a gecko, weighing far less than a child, need 2 million setae? The reason, said Autumn, is that leaves have a waxy coating that may interfere with the intermolecular attraction. In the lab, a gecko might only need a small proportion of its setae to stick to glass, but in a forest, it might need far more.
“The Van der Waals theory predicts we can enhance adhesion — just as nature has — simply by subdividing a surface into small protrusions to increase surface density,” Autumn said. “It also suggests that a possible design principle underlies the repeated, convergent evolution of dry adhesive microstructures in geckos, anoles, skinks and insects. Basically, Mother Nature is packing a whole bunch of tiny things into a given area.”
The team showed that since Van der Waals’ forces were operating, it was the geometry of the hairs that was important, not what they were made of. In other words, shape and size, not chemistry, is the key to the gecko’s sticking ability.
A competing explanation of the gecko’s stickiness relied on the role of adsorbed water molecules, but Keller and colleagues disproved this theory by showing that even though the gecko’s feet were strongly water-repellent, they could stick equally well to water-repelling and water-attracting surfaces. To test their results, the team built its own foot-hair tips using artificial materials.
“We confirmed that it is geometry, not surface chemistry, that enables a gecko to support its entire body with a single toe,” said Autumn. “This means we don’t need to mimic biology precisely. We can apply the underlying principles and create a similar adhesive by breaking a surface into small bumps. These preliminary physical models provide proof that humans can fabricate synthetic gecko adhesive,” he said.
“The artificial foot-hair tip model opens the door to manufacturing dry, self-cleaning adhesive that works underwater and in a vacuum.”
Autumn and colleagues have also showed that even when the gecko’s setae are clogged with microspheres, after five steps the hairs were clean. The team see lucrative applications for synthetic gecko adhesive.
Suzi Jarvis, a Science Foundation Ireland principle investigator working in the area of nanobiotechnology at Trinity College, Dublin, was enthusiastic about the paper. “With widespread miniaturization we need to understand how surfaces interact with each other on this tiny nanoscale,” she said. “These very small surfaces have been utilized by nature in a very clever way with geckos and the possibility of scientists now being able to mimic this opens up a wide range of exciting nanotechnology applications.”
Not surprisingly, the U.S. military is interested. The Defense Advanced Research Projects Agency, the Pentagon’s central research and development body, provided financial support for the research. What do they have planned? No one knows, but watch out for real-life spidermen climbing walls near you soon.
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