Henry Gee looks at the surprising secrets of one of nature’s stickiest surfaces – the gecko’s atom-powered feet
Robert Full’s laboratory is more than usually full of gizmos, even for a modern biologist. Full is a biological engineer. Like an industrial espionage outfit, he and colleagues at the University of California, Berkeley, look at nature and try to reverse-engineer it – taking nature’s solutions to pieces, learning how they work, then putting them together in robot form, in new and exciting ways.
A big man with big enthusiasm, Full shows me a video of his team’s latest robot. This six-legged machine moves with all the natural assurance of an insect; the cartoon face painted on the front makes the creature (for that’s what it is) look animated. It crosses terrain that overturns other robots or stops them dead in their caterpillar tracks. How does it do it? By doing what insects do naturally: a bit of springiness engineered into the legs, combined with mechanical responses at a local level (rather than always being centralised through a “brain”) make for a flexible, robust system.
Some feet, however, are the very opposite of springy: they are sticky with a vengeance. Full’s latest mission has been to understand what makes the feet of geckos so adhesive.
No hot tropical night is complete without one of these small lizards darting up walls and across ceilings. For decades scientists have studied this trick. The anatomy is incredible. Each foot of a Tokay gecko (Gekko gecko) is clothed with a fine pile of microscopic hairs, about 5 000 a square millimetre – or about two-million a gecko. These hairs have split ends, each dividing into as many as 1 000 frayed strands – the total, per lizard, running into billions.
Full and colleagues have measured the force acting on isolated hairs as a gecko runs. The hairs on the foot point backwards. As the gecko takes a step and gains a purchase, the hairs are driven backwards and down, engaging with the surface. The force on each hair is minuscule, but the effect is cumulative: a totally stuck gecko would clamp on to a surface with a force equivalent to 10 atmospheres of pressure. No wonder they can stick to ceilings. But how does a gecko not get bogged down?
The answer is that the gecko doesn’t yank its feet off the surface all at once, but peels off its toes at a certain angle, rather like peeling off a strip of adhesive tape or Velcro.
The puzzle is what makes them stick. Simple suction is out, because gecko feet work perfectly, even in a vacuum. Clinging to very fine cracks or imperfections is also out: geckos can cope with polished glass. It could be electrostatic attraction, like balloons that, when rubbed vigorously on your shirt or sweater, can be suspended from a ceiling. But gecko feet work in ionised air, which would cancel out the electrostatic effect. That leaves old- fashioned glue: except that gecko feet have no suitable glue-secreting glands.
In the 1960s a German scientist, Uwe Hiller, wondered whether gecko feet might work by tap-dancing their way into the molecular landscape of the surfaces they crossed. Many molecules have a slight electrical polarisation. Rather than being balanced throughout, they have spots of excess negative or positive charge. These allow different molecules to bind together very loosely, through a weak kind of electrostatic interaction called the Van der Waals force. Although very weak, this force is highly important. It holds the strands of DNA together, and some of the properties of water – such as its high boiling point – are connected with the fact that its molecules form loose, ever-changing associations, bound by the Van der Waals force.
Hiller wondered if the fine hairs of gecko feet might exploit these forces. Supporting evidence came from an unusual finding: that geckos got stickier in proportion to the “free energy” – the degree to which surfaces have loose molecular ends – of the surfaces they crossed. Measurements by Full and colleagues on single gecko foot hairs are consistent with this idea: geckos make their way by a kind of atomic energy.
How to reverse-engineer gecko feet into something useful? Forget Duck tape – Gecko tape could stick anything to anything else as securely as molecular bonds would allow, yet peel off gently, leaving no stain or trace. Sounds wonderful. Except that Full and colleagues admit defeat, at least for now: human technology is just too crude to manufacture structures as exquisitely tiny and precise as gecko foot hairs.
But, they suggest, the very contemplation of such structures could inspire those souls driven to create the perfect dry adhesive. After all, stranger things have happened: legend has it that the hooked burrs of plants inspired the invention of Velcro. And, these days, we would certainly come unstuck without that.
Henry Gee is a senior editor of Nature. His book Deep Time: Cladistics, the Revolution in Evolution is published by Fourth Estate