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Simple mechanical models reveal how a gecko's feet are released, allowing it to walk.
A LARGE NUMBER OF CREATURES-FROM BEETLES TO FLIES AND SPIDERS-have the remarkable ability to walk on vertical surfaces. The largest of these, and thus the most interesting, is the gecko (Gekko gecko). This ability is achieved by a foot that has a complex, hierarchical structure as shown in Figure 1.
At the highest length scale, each toe has a series of ridges that comprise a mass of tiny hairs, or setae, that are made from keratin-the same substance as in fingernails. The ends of the setae terminate in multiple branched, flattened structures known as spatuiae. Each has a shaft about one micron long with a flattened end that is only about 200 nanometers wide and about five nanometers thick (a nanometer is one billionth (10'9) of a meter). This provides intimate contact between the ends of the spatulae and the surface, which interact through relatively weak and short-range van der Waals interactions. However, the large number of such interactions leads to strong adhesion between the gecko foot and the surface; it turns out to be quite difficult to pull a gecko from a flat surface.
While this model...