Whether zipping up walls or hanging out over the porch light, geckos have got it down when it comes to defying gravity. While scientists have studied the feet of these lizards for years in an attempt to understand their adhesive abilities, no one could figure out how they hold on.
|Tokay inverted on glass|
image: Kellar Autumn
Now a study published in the journal Nature reveals the secret of their stick. The geckoís toes donít just grab, they chemically bond to ceilings, walls and even windows.
Geckos have real sticking power, according to Robert Full, a professor of Integrative Biology at the University of California Berkeley, who led the study. If you were to take skin from a geckoís foot and spread it over the size of a quarter, it could hold the weight of a 45-pound child. But geckos have piqued the interest of scientists, not just because of what they can do, but how they seem to do it.
"Previous researchers have looked at the mechanisms by which geckos grab onto walls for many years, and they showed that they can still adhere even in a vacuum, so itís not suction; thereís no glue residue left over so itís not some noticeable adhesive; they donít hold on by electrostatics, because you can test that with an anti-static gun," says Full. About the only mechanism left over is an interaction between the molecules in the geckoís feet and the molecules of the surface they are sticking to.
By looking closely at the geckoís feet, Full and his colleagues hoped to solve the mystery. Gecko toes are covered with as many as 2 million tiny hairs, or setae, arranged in rows (see image at right) and each hair then splits into 100 to 1000 tiny branches. (see image below, left) Each foot has as many as one billion of these tiny endings, called spatulae and about 50 times smaller than a human hair, and it is these that are the key to the geckoís grip. "These billion spatulae, which look like broccoli, on the tips of the hairs are outstanding adhesives," says Full.
|Rows of setae|
image: Kellar Autumn & Ed Florance
But although the anatomy of the geckoís foot fit the criteria for a state-of-the-art sticking device, Full couldnít get it to work in the lab. Simply pushing the hairs against a surface wouldnít make them stick. "We first tried to take the individual hair and look only at the molecular level, but we couldnít figure out how the hair functioned."
To solve the mystery, Full and his colleagues stepped back to look at how the geckos used their feet. "We discovered that geckos actually do something quite unusual with their feet," says Full. By filming geckos running up walls using a high speed camera (see movie below) the researchers found that geckos carefully uncurl their toes as they take each step, "just like you would when you blow into a party favor and it uncurls," says Full. And when they take their feet off the wall geckos reverse this process: "They actually do the same thing as you do when you maneuver a piece of tape," says Full. "You pull off the tape by peeling it away from a surface. Their toes actually extend up and pull away."
image: Kellar Autumn & Ed Florance
Practical applications afoot Having seen the adhesive process in action, Full was then able to mimic it in the lab, and measure the sticking strength of an individual hair. "We made the first ever measurement of these forces by taking the single hairs and pulling on them and we discovered that these forces were large enough to be Van der Waals forces, or forces that result from molecular interactions," says Full.
Now that the researchers understand the geckos grip, they are looking at ways to harness the technology. Because of its immense sticking power and the fact that it requires no solvent or liquid to make it stick, Full and his colleagues think that the geckos hairs will make the ideal "dry" adhesive. "It doesnít require any glue or moisture to grab on, it doesnít require another surface to attach to like velcro, and it can do it on smooth surfaces," says Full.
According to Full, his study teaches us more than how geckosí feet work and the potential for creating adhesives of the future. It also serves to remind us of the need to conserve biological diversity: "This projectís really a great example of how studying very diverse and unusual creatures in nature allows us to make discoveries that we could never predict would lead to some relevant application. So in general, itís really important to save even the strangest and most disgusting type of creature, because you never know what you can learn from it."Elsewhere on the web:
Global Gecko Association
A Look at Geckos
Robert Fulls lab