For some time now people have been trying to figure out how geckos manage to climb and cling to surfaces. Like this gecko clinging upside-down on polished glass with no problems...
Many ideas were suggested, but successively ruled out. Friction, suction, any sort of natural velcro, electrostatic attraction, and "glue" glands have all been ruled out, for various reasons. So what then causes geckos to stick? I'll tell you. All the info here is gleened from multiple studies of the Tokay gecko. The result? That geckos seemingly take advantages of van der Waals forces (for those that don't know, this is essentially an intermolecular attractive force...molecular bonding).
How? Well, let's take a closer look at things. This is a gecko foot (with a toe highlighted in red).
The bottom of a gecko's foot contains about 500,000 submicron hairs called setae and each one of these have hundreds of tiny pads called spatulae at the end. These are small enough, and close enough to the surface, to react on a molecular level. Very small...the setae are about five micrometers thick by 100 micrometers long each, and each of the spatulae on the end of a seta are only about 0.2 to 0.5 micron wide. Their "sticking" ability is actually a function of the geometry of these nanostructures rather than any surface chemistry.
And it's still not quite that simple. Simply putting the foot down on a surface doesn't engage these spatulae. Rather, geckos have a unique way of activating this that involves curling and uncurling their toes. They can do this 15 times a second while running, in fact. In their resting state, setal stalks are bent proximally, just like a claw. When the gecko puts down its toes the setae extend so their tips point away from the body. A little bit of weight and displacement of the toe way from the body (on a micron-scale) bring the spatulae - which normally point every which direction - uniformly flush with the surface, maximizing surface area of contact. At this point, the setae are able to bond usinng van der Waals forces...on this small scale, van der Walls forces are significant. Gecko toe, curling:
As a result, the setae can bear the full load of the gecko's body weight, even if upside down. In fact, one single seta provides 600 times the attraction than friction could provide - and can withstand 20 milligrams of force. The total number of setae a gecko has can theoretically support over 130 kilograms!!
So, given that amount of attractive force, one naturally wonders how they can un-stick their toes. Well, their setae are very flexible. And though the exact mechanisms are unknown, it seems that geckos can change the orientation and strain on the setae. I explained how they attached, but to detach the gecko simply increases the angle by changing the seta's shape and peeling it away as a result of increased stress on the trailing edge of the surface contact, breaking the bonds - an angle of 30 degrees results in detachment...peeling them away in small numbers instead of all at once is how things they are able to un-stick. You can liken it to a piece of Scotch Tape stuck on a table...pulling the piece of tape straight-up away from the table or sliding it parallel to the table are incredibly difficult because of that adhesive bond; instead, you peel it off at an angle from the edge.
So, this is really a remarkable mechanism. At least, I think it is. Another interesting feature of these little submicron hairs is that they seem to be self-cleaning. Scientists embedded microspheres in the setae of the geckos, only to have the spheres disappear after only a couple steps.
Given the efficacy of this system, commercial applications are being considered. Most promising is a dry adhesive modeled after this setae-spatulae system. Nanotechnologists at the University of Manchester have created a prototype dry adhesive using microfabricated polyimide hairs attached to a flexible base, which produces an adhesive force comparable to gecko to these gecko foot-hairs (in part because the flexible base ensures that as many hairs as possible come into contact with the surface). And because the adhesive is dry, it can be attached and detached repeatedly. So far it's too expensive and difficult to produce commercially, but they have created one square centimeter of the material, which was enough to suspend a Spiderman action figure from a piece of polished glass...
Calculations show that if they had enough tape to cover the palm of a person's hand, it could support the weight of the average person. They think of potential commerical use...I think of potential military use...lol.
Oh, and for those wondering if you can stick two gecko feet to each other, the answer is probably not. In order for the setae to "attach" to the surface, a large number must do so to a surface simultaneously. The probability of a spatula from one foot coming into contact with a spatula from another foot is very low. Multiplying many small probabilities by manty small probabilities and you'll see it's not very likely for two feet to become strongly attached to each other.
Fascinating stuff.
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-Marcus The Great
Owner, 1.1 Leopard Geckos (T^2) 
Master, sotik.com
Photographer, imesho.com
