Geckos, nature's supreme climbers, can race up a polished glass wall at a meter per second and support their entire body weight from a wall with only a single toe. But the gecko's remarkable climbing ability has remained a mystery since Artistotle first observed it in the fourth century B.C.
Now a team of biologists and engineers has cracked the molecular secrets of the gecko's unsurpassed sticking poweropening the door for engineers to fabricate prototypes of synthetic gecko adhesive.
"Two millennia later, we have solved the puzzle of how geckos use millions of tiny foot hairs to adhere to even molecularly smooth surfaces such as polished glass," said Kellar Autumn, lead author of an article in this week's Proceedings of the National Academy of Sciences. Our new data prove once and for all how geckos stick."
Working at Lewis & Clark College, the University of California at Berkeley, the University of California at Santa Barbara, and Stanford University, the interdisciplinary team:
confirmed speculation that the gecko's amazing climbing ability depends on weak molecular attractive forces called van der Waals forces,
rejected a competing model based on the adhesion chemistry of water molecules, and
discovered that the gecko's adhesive depends on geometry, not surface chemistry. In other words, the size and shape of the tips of gecko foot hairsnot what they are made ofdetermine the gecko's stickiness.
To verify its experimental and theoretical results, the gecko group then used its new data to fabricate prototype synthetic foot-hair tips from two different materials.
"Both artificial setal tips stuck as predicted," said Autumn, assistant professor of biology at Lewis & Clark College in Portland, Oregon. "Our initial prototypes open the door to manufacturing the first biologically inspired dry, adhesive microstructures, which can have widespread applications."
The project required an interdisciplinary team, according to Autumn. Engineers Ronald Fearing and Metin Siiti at the University of California at Berkeley built prototype synthetic gecko foot-hair tips that stick like the real thing. Engineer Jacob Israelachvili at the University of California at Santa Barbara provided the mathematics that led to the prototype's design. Other team members include biologist Robert Full at the University of California at Berkeley and engineer Thomas Kenny of Stanford University.
Van der Waals Force versus Capillary Adhesion
The team tested two competing hypotheses: one based on van der Waals force and a second on capillary (water-based) adhesion.
"Our results provide the first direct experimental verification that a short-range molecular attraction called van der Waals force is definitely what makes geckos stick," Autumn emphasizes.