Sticky Feet

Up against der Waals

The hairs on geckos' feet stick to most all surfaces because of what are called "van der Waals forces"— attractive forces between the molecules in gecko feet and the molecules of the surface they are sticking to. The molecules have areas of slight positive or negative charge that attract each other, sort of like mini magnets. Since gecko toes are covered with as many as 2 million tiny hairs, and each hair then splits into 100 to 1,000 tiny branches, the number of mini magnets is immense.

"The reason we got so excited is that these van der Waals forces depend on geometry, not chemistry," says Kellar Autumn, lead researcher on a recent study published in the Proceedings of the National Academy of Sciences. "Which means that if we want to fabricate a synthetic gecko adhesive, we don't have to copy the precise biochemical nature of the very tips of those hairs."

In other words, van der Waals forces will occur in just about any material, as long as it is split up into tiny enough ends.

Ron Fearing, professor of engineering at University of California Berkeley, proved just that with the tiny synthetic gecko hairs he molded with an atomic force microscope. Two types of hairs he created— one from silicone rubber and one from polyester— stick to surfaces as well as gecko hairs do.

Synthetic gecko adhesives would be unique for several reasons. They stick very strongly yet detach easily, which makes them a "smart adhesive" that's sticky when you want it to be and not sticky when you don't. It is also a "dry" adhesive that doesn't leave a residue, and can work underwater or in a vacuum.

"You can imagine larger patches of gecko adhesive being used for mundane things like post-it notes that are reusable, or band aids that are waterproof and don't cause your child pain when you pull them off," says Autumn. "Geckos live in some of the dirtiest places on earth— sandy deserts, dusty places— and their feet don't get dirty."

Another advantage is that they can be made so small.

"You could also think about using them for nanosurgery, since these are like the finest bristle brush that you can have," says Autumn. Other nano-applications include micro-parts manipulation (in the silicon industry), or moving optical fibers around.

At the moment, the team of researchers has only imitated the fine hair tips of the gecko toe. Autumn compares it to building a tree from the leaves back. "Right now we have the leaves, but need to find a way to make the branches and the shaft," which he says is another few years away.