Geckos' feet are right up there with adhesive tape, when it comes to being able to stick to things. Unlike tape, however, those feet retain their adhesive qualities even after many, many uses. Now, thanks to research being conducted at Carnegie Mellon University and Germany's Karlsruhe Institute of Technology, we may one day be using self-cleaning reusable gecko-inspired tape.
The feet get their stickiness from millions of microscopic hair-like projections known as setae. These temporarily bond with surfaces at a molecular level, thanks to the effect of Van der Waals forces. When a gecko walks forward, the friction created by its foot dragging laterally against the surface causes larger dirt particles to roll off of the setae, while smaller particles fall down between them into folds in the lizards' skin. This is what is largely responsible for the self-cleaning aspect of the feet.
The scientists copied this effect by creating mushroom-shaped elastic microhairs modeled after setae, in three sizes. Instead of dirt, the researchers spread tiny glass spheres on a plate. A piece of tape covered in the microhairs was then pressed down onto that plate, slid laterally, and then pulled off again – in the same fashion in which a gecko might step on it.
In cases where the microhairs were smaller in diameter than the spheres, the tape initially lost its adhesive force after its first contact with the plate, but then regained 80 to 100 percent of it after eight to ten subsequent applications. This was due to the self-cleaning effect kicking in, as the spheres rolled off the microhairs.
When the microhairs were larger in diameter than the spheres, however, the spheres tended to fall down between them instead rolling off. Because there were no skin folds for the spheres to disappear into, the self-cleaning effect wasn't as pronounced – only one third of the tape's original adhesive force came back after repeated applications.
Because of this, the scientists believe that smaller microhairs (in the nanometer-wide range) should work best at repelling dirt, as they would be smaller in diameter than most dirt particles. The team has already artificially reproduced the skin folds, which could be used for trapping dust particles, and plan on testing a new-and-improved version of the tape on actual dirt in the near future. It is hoped that once perfected, it could be used in applications including "reusable tapes, clothing closures and medical adhesives."
Perhaps not surprisingly, this isn't the first time that reusable gecko-foot tape has been created. Other versions have previously been developed at the University of Massachusetts Amherst and the University of Kiel.