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Like wall-hugging geckos, tree frogs are capable of gravity-defying feats of the feet. But new research shows that the two species cling to surfaces in markedly different ways.
The "dry" grip of geckos relies on molecular bonds—firm but easily broken—between tiny fibers in the animal's toe pads and the surfaces on which they stand. But scientists found that frogs use a different approach to hold on.
Biologist Jon Barnes of the University of Glasgow in Scotland, who led the research, used an atomic force microscope (AFM), which can provide images on the scale of billionths of a meter, to scan the feet of White's tree frogs. To the naked eye, the frogs' toe pads appear patterned with flat-topped, hexagonal cells surrounded by grooves filled with mucus. On closer inspection, however, Barnes discovered that the tops were not flat at all but rather were covered by tightly packed "nanopillars," each with a small dimple in the end, which generate powerful friction against the surfaces they contact.
"The AFM can also be used to measure the stiffness of the outer layer of the foot," says Barnes, who published the findings in The Journal of Experimental Biology. "It turns out to be of the same order as silicone rubber. Soft materials are important, for they allow the pad to achieve close contact, following the contours of the surface to which the frog is adhering."
Although mucus can be a lubricant, for tree frogs the substance—only 1.5 times more viscous (resistant to flow) than plain water—serves as a "wet" adhesive. The reason: the nanopillars and larger structures on the toe pads come in direct contact with surfaces. As a result, the small amount of wet mucus between these protrusions provides adhesive forces.
Tree frogs can climb most surfaces, from sheer leaves to glass, with ease, although they do not fare so well on dry, rough materials—presumably because they cannot produce enough mucus to create a continuous fluid layer beneath their pads on such a surface, Barnes says. "In support of this idea is the fact that adhesion dramatically improves if the rough surface is wet," he notes.
Walter Federle, a zoologist at the University of Cambridge in England who studies adhesion, says the study sheds light on the material properties of frog toes at the microscopic level and clarifies that nanopillars play "an important role in adhesion." But he notes that the exact function of these tiny columns is still unclear.
Research on both geckos and tree frogs has tantalized materials scientists with visions of smart adhesives for human applications. For example, a paper in the March 2008 issue of the Journal of the Royal Society Interface estimated that a car brake equipped with a modest patch of synthetic gecko-grip could stop a 2,200-pound (1,000-kilogram) vehicle traveling 50 miles (80 kilometers) per hour in about 16 feet (five meters).
Barnes and his colleagues believe understanding the adhesive properties of tree frog feet could lead to better tire design, and perhaps even a nonslip shoe, although they first need to demonstrate that the adhesion—and, equally important, the rapid disengagement from the surface—is maintained on structures much larger than an amphibian's toe. Another possible application of the work, Barnes says, is the creation of a coating to protect nerves during surgery by holding them delicately out of the way of the scalpel.
