Monday, June 25, 2007

Climbing Walls with Carbon Nanotubes

A new kind of tape mimics the qualities of gecko feet.

By Kevin Bullis

Researchers have mimicked the hairs on gecko feet (bottom image) by growing carbon nanotubes in bundles. Credit: Ali Dhinojwala

Multimedia •  See a slide show about the technology.

Researchers have developed a carbon-nanotube-based tape that could prove useful for creating robots that climb walls and special adhesive gloves for astronauts. Unlike ordinary tape, which eventually loses its stickiness, this new material sticks like a permanent glue, but it can be removed and reused. It can also stick to a wider variety of materials, including glass and Teflon.

Dubbed "gecko tape" by researchers, the material works by imitating the nano- and microscale structures on geckos' feet that allow them to quickly scale walls and run across ceilings. The tape is reusable and will not dry up or slide off the wall because, unlike ordinary tape, it does not use viscoelastic glues. Instead, it employs carbon nanotubes to make use of microscale van der Waals forces that occur at very short ranges between surfaces. Bundles of nanotubes conform to the slightest microscopic variations on a surface, the same way that the bundles of nanoscopic keratin fibers that make up the hairs on gecko feet allow them to conform to walls.

Like ordinary tape, gecko tape clings strongly when pulled parallel to a surface; it can support just under 10 pounds per square centimeter. But the tape can be peeled off relatively easily when pulled perpendicular to a surface.

As nanoscale control over materials has grown, so has the number of groups developing versions of gecko tape. But the new tape, made by researchers at the University of Akron and Rensselaer Polytechnic Institute (RPI), is much stronger than those versions and is able to support four times more weight per area than gecko feet. Other researchers have tried using carbon nanotubes in the past, in part because they are very strong and can withstand much higher temperatures than plastic, hence would be useful in more applications. But those nanotubes were too stiff to be practical, says Manoj Chaudhury, a professor of chemical engineering at Lehigh University, who has worked on similar projects. So the Akron and RPI researchers modified the nanotubes by exploiting the fact that they can be chemically grown to have various numbers of walls. The researchers struck a balance between having too many walls, which makes the nanotubes too rigid, and having too few, which makes them weak, Chaudhury says.