Roboticists at Virginia Tech, in Blacksburg, VA, have developed a novel form of locomotion for robotics based on the way the single-celled amoeba moves. Unlike any other robots, the Virginia Tech ones are designed to use their entire outer skin as a means of propulsion.

Toroidal in shape--a bit like an elongated cylindrical doughnut--robots of this new breed differ from wheeled, tracked, or legged bots in that they move by continuously turning themselves inside out, says Dennis Hong, an assistant professor of mechanical engineering at Virginia Tech. "The entire outer skin moves," he says.

This novel type of locomotion is particularly suited to search-and-rescue applications, says Hong: "They can squeeze under a collapsed ceiling or between obstacles very easily." Indeed, preliminary experiments show that the robots, with their soft, contracting bodies, are able to push themselves through holes with diameters much smaller than their normal width, Hong says. And because the robots are able to use their entire contact surfaces for traction, they can move over and through very uneven environments with ease.

The actual motion is generated by contracting and expanding actuator rings along the length of the robot's body. By contracting the rings at the rear of the robot and expanding them toward the front, they are able to generate movement.

This is very much akin to the principle of the pseudopod used by single-celled organisms such as amoebas, says Hong. This principle consists of a process of cytoplasmic streaming, in which the liquid endoplasm within the cell flows forward inside a semi-solid ectoplasmic tubular shell. As the liquid reaches the front, it turns into the gel-like ectoplasm, forming an extension to this tube and moving the organism forward. At the same time, the ectoplasm at the rear of the tube turns into the liquid endoplasm, taking up the rear.

To produce a similar sort of motion, Hong's initial experiments have used robots consisting of flexible toroidal membranes lined with propulsion rings of either electroactive polymer or pressurized hoses. But now, with funding from a new National Science Foundation grant, Hong has forsaken the use of elastic membranes in favor of more-rugged designs. He declines to discuss these designs in detail because of intellectual property issues. However, he says that this latest work involves rigid mechanical parts that are linked in such a way as to enable this sort of motion. "It's like a 3-D tank tread," he says.

"It's an interesting idea," says Henrik Christensen, professor of robotics and director of Robotics and Intelligent Machines at Georgia Institute of Technology, in Atlanta. "We really need better locomotion mechanisms for robots." Wheels and tracks work fine until the terrain becomes very uneven, while legs are slow and terribly inefficient, he says.

This is not the first time that toroids have been proposed as part of a propulsion system, says Andrew Adamatzky, a professor of unconventional computing at the University of the West of England, in Bristol, U.K. But using electroactive polymers to produce propagating waves of contractions makes this latest research very interesting, he says. "These experimental designs open new and exciting perspectives in soft-bodied robotics."

However, with soft bodies come new challenges. For example, it is not clear how one would integrate a power supply, computerised controllers, and sensors. "The principles here are good, but the engineering really needs to be worked out," says Christensen.

Hong acknowledges that there are still many practical issues to work out with his robots. One solution to many of the design issues is to carry the power supply, controllers, sensors, and other key parts in the center of the toroid. Its shape would ensure that these key parts stayed in place, while wireless controllers could be used to trigger the contractions of the rings using inductive loops for power, says Hong.

The hardest part of search and rescue is developing mechanisms that can adapt to changing terrains, says Robin Murphy, a professor of computer science and engineering at the University of Florida and former director of the Center for Robot-Assisted Search and Rescue, in Tampa, FL. However, there is more to search and rescue than just oozing through gaps, she says.