Gecko Feet: Natural Marvel

Thanks to the National Science Foundation for the picture

Traveling in Southeast Asia, geckos on the ceiling were a common occurrence. They would stake out territory, chasing each other away with vehemently waving tails, as if they did not notice they were in the middle of a rough, lumpy, peeling ceiling, hanging upside-down.

I would lay in my bed and watch them fight, wondering why one of them did not fall on my face. As it turns out, no one else knew either until relatively recently. Discussion of their toes as if they were suction cups were fairly common among us plebs, and even the scientific community assumed there was some capillary action at work, keeping them stuck by the moisture in the surface they were walking on. But in the past few years they have actually tested gecko feet and found that they work even in super-dry environments.

If you look at a gecko's foot, they have these weird flaps on them that look suspiciously like long, narrow suction cups. They spread when they walk in a soft, mushy way that is utterly fascinating, and if you've ever held a gecko you will understanding my saying that their amazing toes are almost velvety (in a rubbery sort of way) to the touch. And, if you did agree, you would be right. The bottoms of a gecko's feet are, in fact covered with millions of tiny foot-hairs on each toe, called setae, each about as long as the width of two human hairs (about 100 millionths of a meter). Each seta, in turn, is divided at the end into approximately a thousand tiny spatulae (yes, you guessed why they are called that), which are about 200 billlionths of a meter wide, which is smaller than the wavelength of visible light.

It seems the geckos' toes create so much surface area in this way, with such tiny endings, that they are able to make use of Van der Waal's force - a weak attractive force which is present between molecules - to stick themselves to the ceiling.

Geckos' feet are naturally ultrahydrophobic - in this case, hydrophobic referring to a molecule which is repelled by water - and have been tested by sticking them to a GaAs semiconductor, which is also hydrophobic. The only thing known to make two hydrophobic surfaces adhere in air is Van der Waal's force, so the old question of capillary action has been disproved.

Universität des Saarlandes

Kellar Autumn, probably the leading researcher on gecko's feet, says they are strong, as well:

"We took a single gecko foot hair (seta) and made the first direct measurement of its adhesive function...We used a microscopic force sensor designed by Tom Kenny at Stanford to measure the tiny forces of adhesion of the gecko seta.

We discovered that the seta is 10 times more adhesive than predicted from prior measurement on whole animals. The adhesive is so strong that a single seta can lift the weight of an ant 200 µN = 20 mg. A million setae could lift the weight of a child (20kg, 45lbs). A million setae could easily fit onto the area of a dime [$0.10 coin]. The combined attraction of a billion spatulae is a thousand times more than a gecko needs to hang from the ceiling. Maximum potential force of 2,000,000 setae on 4 feet of a gecko = 2,000,000 x 200 micronewton = 400 newton = 40788 grams force, or about 90 lbs! This is 600 times greater sticking power than friction alone can account for...

Our discovery explains how it is that any gecko can hold up its entire body weight with only a single finger. If the adhesive is so strong, how do they get their feet off? ...We found that if we increased the angle the seta makes to the surface, it just pops off" [source]

This is helped, of course, by the fact that geckos' toes are backwards-jointed, meaning that when they flex their toes they curl upward rather than downward, allowing them to peel their toes from the surface to which they are stuck.

Gecko feet are self- cleaning as well. Because the same force applies between dirt particles and the surfaces the geckos walk on, the dirt prefers to stick to the relatively large and attractive surface rather than the tiny spatulae on the gecko's feet. So when a gecko's feet get dirty, all they have to do is walk a few steps and they are clean again.

"Geckos' adhesive microstructure requires minimal attachment force, leaves no residue, is directional, detaches without measurable forces, is self-cleaning, and works underwater, in a vacuum, and on nearly every surface material and profile."

Perfect, in other words, for synthesis.

Synthetic setae have been in the works for several years now, with ideas like micro-robots that could climb walls or be sent to places like Mars, where self-cleaning sticky feet might be greatly useful; or setae-covered clothing, which would allow people to literally crawl about on the walls and ceiling. And most adhesives don't work in the vaccuum of space, while Van der Waal's forces work everywhere. But keratin, the hydrophobic protein that gecko's setae are made of, is hard to reproduce: hydrophobic substances, such as silicone and polyester, are harder to mold into setae-like shapes than other substances - and the hydrophobic qualities are key to the reproduction of the setae's effect, or water molecules stick to it as well, making it soggy and eventually destroying its sticking power.


But in June of last year, researchers at Rensselaer Polytechnic Institute and the University of Akron came up with a flexible "gecko tape" that they claim has four times the sticking power of the real thing. This tape is made by covering polymer surfaces with carbon nanotube hairs, which use a Van der Waal's effect to adhere. The tape is designed to be as easily-removed as a gecko's foot, so the tape does not get damaged and can be used over and over. They even have pictures, which show that on a microscopic scale they are admittedly much less attractive than the original.

And now, "at Stanford University have created a robot which uses synthetic setae in order to scale even extremely smooth vertical surfaces just as a gecko would." [wiki] So here we are.

I am curious, with all the hype around this technology, to see if it is indeed self-cleaning, as the original idea went. If it is, then we have once again successfully learned from nature, and created a whole new era of fun things to play around with. But if not, then it's not much different than any other tape, because it'll get dirty and stop working.

And part of me is really excited that we, too, could crawl around like geckos someday. Rock-climbing will take on a whole new meaning; skyscraper-climbing could become a new sport; or hanging onto the bottoms of airplanes - the extreme sport possibilities are endless. My travels in Southeast Asia would have been much different if I had been able to put on my special gloves and socks and join the geckos on the ceiling ("break it up, you guys!").

But part of me will regret the loss of one more part of the unknown. In the not-too-distant future, you can imagine the Southeast Asian traveler exclaiming to his or her travel partner, "Hey, look at that lizard! It looks just like you when you're in your ceiling suit!"


Somehow it loses some of the quiet magic of lying there, and watching them, and wondering.


Links:
- Paper on the gecko tape

- How geckos' feet stay clean

- Geckel, a super-sticky adhesive combining the setae concept with the "glue" used by mussels

- The Global Gecko Association, dedicated to gecko enthusiasts world-wide.

- Pictures of a Western Banded Gecko cleaning his eye, which has no moveable lids (this genus has no setae).