A newly created robot improves upon a gecko's sticking power.Climbing the walls: As the robot’s motor turns, its tail presses against the surface, and its triangular legs rotate forward (a). As its front feet come into contact with the surface, the motor torque caused by the tail’s contact with the surface presses the front feet against the surface while pulling away the rear feet (b). When the force acting on the rear foot reaches a critical point, it peels away from the surface, and the robot steps forward (c).
Credit: Courtesy Michael P. Murphy and Metin Sitti
Researchers have created a robot that can run up a wall as smooth as glass and onto the ceiling at a rate of six centimeters a second. The robot currently uses a dry elastomer adhesive, but the research group is testing a new geckolike, ultrasticky fiber on its feet that should make it up to five times stickier.
It's not the first robot to use fiberlike dry adhesives to stick to surfaces, says Metin Sitti , an assistant professor of mechanical engineering, who led the research at the Robotics Institute at Carnegie Mellon University (CMU), in Pittsburgh. But this robot should prove to have far greater sticking power, thanks to fibers that are twice as adhesive as those used by geckos.
Such robots could, among other applications, be used to inspect the hulls of spacecraft for damage, their stickiness ensuring that they would stay attached.
In addition to its sticky feet, the robot uses two triangular wheel-like legs, each with three foot pads, and a tail to enable it to move with considerable agility compared with other robots, says Sitti. Not only can it turn very sharply, but its novel design allows it to transfer from floor to wall and wall to ceiling with great ease.
"It is very compact and has great maneuverability," says Mark Cutkosky, a professor of mechanical engineering and codirector of the Center for Design Research at California's Stanford University. "It is a practical solution for climbing."
Geckos are able to stick to surfaces thanks to very fine hairlike structures on their feet called setae. These angled fibers split into even finer fibers toward their tips, giving the gecko's foot a spatula-like appearance. These end fibers have incredibly weak intermolecular forces to thank for their adhesiveness: the attractive forces act between the fiber tips and the surface they are sticking to. Individually, the forces are negligible, but because the setae form such high areas of contact with surfaces, the forces add up.
In the past few years, a number of research groups have fabricated fiber structures designed to emulate setae. But Sitti's group has tried to improve upon the gecko's design. Using microfabrication techniques, Sitti and his colleagues created fibers just four micrometers in diameter--two orders of magnitude smaller than those used in any other robots. "This size difference makes a significant difference," says Sitti. This is because scaling down the fibers increases their surface contact and hence enhances adhesion.
Using the commercial elastomer adhesives, the robot can already climb far more nimbly than any other robot. But the fibers should make it possible for the robot to climb even rough surfaces, says Sitti. However, having only just integrated them into the robot, the researchers have yet to demonstrate this.
One of the challenges in making a robot stick to walls lies in finding a way to apply sufficient pressure to make them stick. The new CMU robot handles this using a tail. At any one moment, at least two of its six foot pads are in contact with the surface, as is the tail, which is spring-loaded so that it will always push against the surface, even when on the ceiling.
However, in developing these materials, the researchers still need to resolve some issues, says Andre Geim, a professor of condensed-matter physics at the University of Manchester, in the United Kingdom, who has also fabricated setaelike structures. "No one has yet explained why geckos can first run on a dirt road picking up dust and then somehow climb up walls," he says. "This is a major obstacle."
Cutkosky agrees that more research needs to be done into the self-cleaning abilities of geckos. "The world is dirty, and robots cannot be stopping to wash their feet every few meters," he says.
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