Geckos have long inspired scientists and super-hero fans alike with their ability to scamper up vertical walls and cling to ceilings with a single toe. In recent years, people have attempted to create materials that match those spectacular abilities, in the hope of creating new advanced adhesives, or even car braking systems.
Now US chemists claim to have made one based on nanotubes that it is 10 times stickier than some gecko feet. Even more impressively, like a real gecko foot, it can also be easily unstuck with a tug in the right direction.
Gecko's superhero toes are covered in microscopic hairs, known as setae, with even smaller branches at the tips, called spatulae.
These ensure that a gecko's foot has a large surface area in contact with any surface, maximising the weak but ever-present attraction between adjacent molecules known as the van der Waals force.
Chemists led by Liming Dai of University of Dayton, Ohio and Zhong Lin Wang of Georgia Institute of Technology, made their artificial setae by growing nested carbon nanotubes on a silicon wafer.
They controlled the growth process to make a forest of vertical nanotube trunks turning into a canopy of tangled ends on top. The curly entangled mess acts like natural spatulae – when pressed against a surface, they have a large contact area and hence a strong hold.
The new material was tested for stickiness on surfaces ranging from Teflon to sandpaper. Attached to a glass surface, a 4mm2 piece of the material can support over 1600 grams when pulled roughly parallel to the surface (see video, right).
That is around 10 times better than some species of gecko and three times better than the best artificial competitor.
But removing a pad of the material is simple, unlike some rival materials. Pulling it perpendicular to a surface means only the tips of the nanotubes remain in contact with the surface, and the setae will easily loosen their grip. A weight of 160 grams on the tiny sample is enough to do that.
Kellar Autumn from the Lewis and Clark College, Oregon, was the first to suggest that the gecko's stickiness was down to geometry rather than the chemistry of its feet (see Gecko's gravity-defying trick explained). "The ability of this material to support large shear loads and to detach easily is very encouraging," he told New Scientist.
He points out, though, that although a person can easily stick the material to a surface, it requires much more force to apply than real gecko setae. A 4 millimetre2 piece of the new material needs about 2 kilograms of force to stick, compared to the few grams required by a real gecko or some synthetic rivals.
Liming thinks his material should still be able to replace glue and other forms of adhesion. For example, because nanotubes are excellent conductors, the carbon setae could replace solder in electronics. The material could also be valuable in the vacuum of space, where traditional adhesives dry out quickly, he says.
As for superhero suits, Liming says: "We will exploit this possibility, if there is a serious need."
Economics is likely to play a deciding role too – carbon nanotubes are not cheap to produce. But the price has already declined more than a thousand fold over the last few years as fabrication processes have improved. Liming says dropping prices will eventually make it possible to produce his material in rolls rather than one-off sections.
Journal reference: Science (DOI: 10.1126/science.1159503)