The greater the adhesive strength, the better, says Ali Dhinojwala, a professor of polymer science at the University of Akron. However, says Dhinojwala, who works on carbon-nanotube adhesives as well, "we also need to solve other problems before they're commercially viable." Wall-climbing robots will require adhesives that work again and again without wearing out or getting clogged with dirt. "We want a robot to take more than 50 steps in a dirty environment," says Dhinojwala. No one has demonstrated strong gecko-inspired adhesives that can do this. And nanotube adhesives will need to be grown on different substrates than those used so far. Carbon nanotubes are easy to grow on silicon wafers; creating large areas of the adhesive wouldn't be a problem. But "we're not going to stick silicon wafers to robot feet," says Dhinojwala.

Dai says that carbon nanotubes' versatility may help overcome the dirt problem. These structures can readily be functionalized with proteins and other polymers. Dai is developing adhesive nanotube arrays coated with proteins that change their shape in response to temperature changes. A robot could have feet that heat up when they get clogged, shedding dirt so that it can keep walking.

Other applications of the adhesive may take better advantage of carbon nanotubes' properties than robotics would. Carbon nanotubes are highly conductive to electricity and have promising thermal properties, Dai notes. Nanotube adhesives created to replace solder for holding together electronics components could also act as heat sinks. Other gecko-inspired adhesives made of polymers can't hold up to high temperatures, says Metin Sitti, who heads the nanorobotics lab at Carnegie Mellon. Spacecraft using nanotube adhesives instead of polymers could go to hotter areas.