The gecko's ability to “run up and down a tree in any way", was firstly observed by Aristotle in his Historia Animalium, almost 25 centuries ago. A comparable adhesive system is found in spiders and in several insects. In general, when two solid (rough) surfaces are brought into contact with each other, physical/chemical/mechanical attraction occurs. The developed force that holds the two surfaces together is known as adhesion. A simple example is suction. Suction cups operate under the principle of air evacuation, i.e., when they come into contact with a surface, air is forced out of the contact area, creating a pressure difference. The adhesive force generated is simply the pressure difference multiplied by the cup area. Thus, in our (sea level) atmosphere the achievable suction strength is coincident with the atmospheric pressure, i.e. about 0.1MPa. Such an adhesive strength is of the same order of magnitude of those observed in geckos and spiders, even if their adhesive mechanisms are different, mainly due to van der Waals attraction and also capillarity. Thus, although several insects and frogs rely on sticky fluids to adhere to surfaces, gecko and spider adhesion is fully dry.
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