A Gecko's Toes Showed Scientists How to Stick Without Glue

On a warm evening in the tropics, a small gecko strolls up a window pane, pauses upside down on the ceiling, and trots off again as if the room had no top or bottom. It leaves no sticky trail behind it, and its feet are perfectly dry. For a very long time, nobody could quite say how it managed it.

The answer is in the toes. Turn a gecko over and the underside of each toe is lined with neat ridges, a little like the tread of a tyre. Under a microscope those ridges turn out to be covered in a dense carpet of tiny hairs called setae, far too small to see with the naked eye, whose tips spread out flat when the foot is pressed against a surface.

The hairs are not glue and they are not suction cups. They rely on van der Waals forces, a very weak electrical pull that exists between any two molecules that come close enough together. A single one of those pulls is next to nothing. But when an enormous number of tiny hair tips press right up against a wall, the pulls add up, and the whole animal hangs on.

For years, engineers tried to copy the hairs themselves, building surfaces covered in microscopic artificial fibres, and had only limited success in making them big and strong. The turn came when a team at the University of Massachusetts Amherst looked at the whole foot instead. Polymer scientist Alfred Crosby, his doctoral student Michael Bartlett and biologist Duncan Irschick noticed something unusual about gecko anatomy: the tendons run from the bones right out into the skin of the toe. The grip is not only in the hairs. It is in the way the entire foot holds its gripping skin flat against the surface.

So they built a foot. Their pad, called Geckskin, is a stiff fabric with a soft rubbery layer on its face, and a second strip of fabric sewn into it to act as a tendon. A pad about the size of an index card held a pull of about 700 pounds, roughly 320 kilograms, on smooth glass. And like a gecko's foot, it lets go politely: a gentle peel from one edge frees it, and it can be stuck somewhere else as often as needed, leaving no residue and losing none of its strength.

Why does this work with no glue involved? A glue makes a chemical bond, or fills tiny gaps and hardens there, so pulling it off means breaking something. Van der Waals attraction breaks nothing. It only exists while two surfaces are very, very close, so the whole trick is about contact. A pad that is stiff along its length but soft on its face, pulled straight by its tendon, keeps a large area pressed close all at once, and that is when it holds hardest.

Peeling is the opposite. Lift one corner and only a thin line of contact lets go at a time, so the pad comes away with hardly any effort. The gecko uses exactly the same trick every time it takes a step, gripping and releasing without a thought. The engineers only had to stop staring at the hairs and look at the whole foot to see how.