How geckos stick to walls and ceilings

Settled in 2000 by Kellar Autumn, Yiching A. Liang, S. Tonia Hsieh, Wolfgang Zesch, Wai Pang Chan, Thomas W. Kenny, Ronald Fearing, Robert J. Full.

Aristotle noted around 350 BC that geckos can run up and along surfaces 'like a bird flies,' and for over two thousand years the mechanism was debated, with suction, glue, friction and electrostatics all proposed at various points and mostly ruled out by simpler observations (e.g. sticking in a vacuum rules out suction). In 2000, Autumn, Liang, Hsieh, Zesch, Chan, Kenny, Fearing and Full made the first direct force measurements on a single isolated gecko foot-hair (seta) using a MEMS force sensor, finding it produced about ten times more adhesive force than predicted from whole-animal measurements, and that the force profile matched a van der Waals adhesion model rather than any wet or chemical mechanism. This measurement, later corroborated by additional studies on humidity and capillary contributions, is considered the definitive resolution of the mechanism and directly inspired a large field of synthetic gecko-adhesive materials.

One of the oldest recorded animal-behavior puzzles in Western science turned out to hinge on the same intermolecular force that holds a stack of paper together, just multiplied across millions of microscopic hairs.

Explanations that circulated

• Geckos stick via suction cups on their toes — disproven; they adhere equally well in a vacuum, ruling out suction.
• Geckos stick via a sticky glue-like secretion — also disproven; their feet leave no residue and the adhesive force turns on and off purely through the angle of contact.
• Geckos stick via static electricity or simple friction/interlocking — the 2000 direct force measurements on single setae instead showed the adhesion is dominated by van der Waals forces acting over the huge combined surface area of millions of microscopic branched hairs (setae/spatulae).

References

• Adhesive force of a single gecko foot-hair, K. Autumn, Y. A. Liang, S. T. Hsieh, W. Zesch, W. P. Chan, T. W. Kenny, R. Fearing, R. J. Full, Nature, 2000