The race to find a material as "sticky" as a gecko's foot could soon be over as stronger and more practical materials are reported in the latest issue of Science by a team based at four US institutions.

Scientists have long been interested in the ability of gecko lizards to scurry up walls and cling to ceilings by their toes. It has been eight years since the discovery that geckos climb by the hair of their toes. Microscopic branched elastic hairs on their toes take advantage of atomic-scale attractive forces to grip surfaces and support surprisingly heavy loads. Now, researchers have found a way to mimic those hairy gecko feet using polymers or carbon nanotubes.

The new material has a grip almost three times stronger than the previous record and ten times as sticky as the gecko foot itself. Research published in Science, by a team from the University of Dayton, the Georgia Institute of Technology, the Air Force Research Laboratory, and the University of Akron describes an improved carbon nanotube-based material that for the first time creates a directionally varied adhesive force. The new material could give artificial gecko feet the ability to tightly grip vertical surfaces while being easy to lift off when needed.

Beyond possibly giving researchers the ability to walk up walls, the material could have more serious technological applications such as connecting electronic devices together and substituting for conventional adhesives in the dry vacuum of space.

"The resistance to shear force keeps the nanotube adhesive attached very strongly to the vertical surface, but you can still remove it from the surface by pulling away from the surface in a normal direction," explains Dayton's Liming Dai. "This directional difference in the adhesion force is a significant improvement that could help make this material useful as a transient adhesive."

The key to the new material is that the team used multi-walled carbon nanotubes, which they could form into arrays with "curly entangled tops". The tops, which Georgia Tech's Zhong Lin Wang compares to spaghetti or a jungle of vines, mimic the hierarchical structure of real gecko feet, which include branching hairs of different diameters.

When this tangled mass is pressed on to a vertical surface, the tangled portion of the nanotubes becomes aligned in contact with the surface. This greatly increases the surface area coming into contact with the atoms that make up the surface. Electrostatic forces that exist between any two surfaces coming into contact are then so powerful at the atomic scale that the material "sticks". By pulling the material parallel to the direction of the nanotubes, only the tips remain in contact and so these attractive forces are reduced and the stickiness is lost.

"The contact surface area matters a lot," Wang explains. "When you have line contact along, you have van der Waals forces acting along the entire length of the nanotubes, but when you have a point contact, the van der Waals forces act only at the tip of the nanotubes. That allows us to truly mimic what the gecko does naturally."

Now, the team is working on how much nanotube adhesive they will need to support a scientist wearing tights, a red mask, and a penchant for arachnids.

Science, 322: 238-242, 2008
University of Dayton—Prof. Liming Dai/