Walking effortlessly across vertical surfaces, or hanging comfortably from the ceiling, gecko lizards are capable of performing feats we haven't quite figured out how to replicate yet. The adhesive strength of gecko feet is higher than that of most artificial adhesives, and can latch onto almost any surface.
Attempts to design a mimetic system have met with little success. Until recently, that is, as a collaboration between different US research groups has now produced a material with remarkable properties [Qu et al, Science (2008) doi:10.1126/science.1159503].
At the core of this material are vertically-oriented carbon nanotubes with a twist: the ends of the tubes consist of a coiled and entangled segment, which can engage in strong interactions due to its efficient side contact.
“These designs match the structure of real gecko feet, which have microscopic hairs that branch off in different directions,” explains Liming Dai of the University of Dayton. “For the first time, they also demonstrate anisotropic adhesion forces: while the strong shear adhesion force (about 100 N/cm2) keeps the nanotube adhesive attached very strongly to the vertical surface, the normal adhesion force is only about 10 N/cm2, so it can be easily removed by pulling away from the surface in a normal direction. The very strong shear binding force is about ten times stickier than natural gecko feet, and three times stickier than any other gecko-inspired glues.”
The key to this mechanism lies in the alignment of the nanotube ends, Liming continues. “When the adhesive is pulled in a direction parallel to its surface, the tangled portion of the nanotubes aligns with the substrate, drastically increasing the sticking interaction. In contrast, when lifting in a direction normal to the substrate surface, as one would peel off a piece of Scotch tape, the nanotubes lose contact point by point, minimizing the normal adhesion force.”
The applications could range from low-tech fridge magnets to electronics or even airplane parts. For instance, rather than soldering components into electronic devices, using these adhesives would make the parts easy to remove and replace. As a dry adhesive, the carbon nanotube material would also have many uses in space, where the vacuum causes traditional adhesives to dry out. More futuristic applications might include climbing robots, super-grip tires and rapid-repair systems.