Credit: MARK MOFFETT

Humid nights in cheap hotels in the tropics are not complete without a gecko scuttling up the walls or across the ceiling. The adhesive properties of the feet of these fleet lizards are proverbial, yet decades of investigation have still not revealed exactly how they do it. In their report elsewhere in this issue (Nature 405, 681–685; 2000), Autumn et al. come the closest yet. Their force measurements on adhesive setae (foot-hairs) from the Tokay gecko, Gekko gecko, are consistent with the rapid formation and breaking of intermolecular bonds — van der Waals forces.

Suction and friction — two other hypotheses — have been ruled out, as gecko feet can work in a vacuum, and the lizards are quite content on surfaces as smooth as polished glass. Electrostatic attraction is also unlikely, as the feet can still adhere in ionized air. And there aren't any gland cells that might produce some kind of glue. The role of adsorbed water, however, has yet to be studied. Nevertheless, the observation that geckos get stickier with increasing surface energy of the substrate suggests that they are tapping directly into the molecular structure of the surfaces they walk on.

Viewed through a microscope, the foot of a gecko is densely packed with fine setae, shown here in the inset. There are around 5,000 per square millimetre — around half a million on each foot. The end of each seta is further subdivided into between 400 and 1,000 structures called spatulae. The setae tend to point towards the heel: as the gecko takes a step, driving the sole into the substrate and pushing it backwards, the setae become maximally engaged. If all the setae were simultaneously stuck to the surface, the feet of a gecko could produce an adhesive force equivalent to ten atmospheres. The gecko releases each foot by ‘peeling’ off the setae at a critical angle, rather like peeling adhesive tape. Engineering a structure as exquisite as the foot of the gecko is probably beyond human technology, say Autumn et al., but the principles on which it operates could inspire the design of new kinds of dry adhesive.