Vibrated goo mimics slithery motions.

A physicist's hunch about snail locomotion is inspiring a new way to make robots--from goop. Experiments show that matchstick-size slivers of hydrogel, the type of material used for soft contact lenses, can ooze along like snails, slither like snakes, and creep ahead like inchworms.

Greatly miniaturized robots made of hydrogel might someday shimmy across the surfaces of microchips, acting as tiny delivery carts or movable barriers. Some incarnations might glide through a person's intestines or other internal cavities collecting medical data or dispensing medication, the experimenters say.

Biomechanics specialists have long known that snails and other limbless creatures locomote by sending waves of muscular contractions down their bodies. To convert those pulsations into directional motion, the animals typically exploit transient changes in the friction between their bodies and the underlying surface, restricting propulsion to one direction, says applied mathematician Lakshminarayanan Mahadevan of Harvard University.

A couple of years ago, Mahadevan had a hunch that the type of contractile waves that snails employ is also the basis for other types of limbless locomotion. In experiments described in an upcoming Proceedings of the National Academy of Sciences, Mahadevan and chemical engineers Manoj K. Chaudhury and Susan Daniel of Lehigh University in Bethlehem, Pa., set water-lubricated rods of hydrogel (SN: 5/25/02, p. 323) into motion by applying vibrations to the rubber-coated glass plates on which they lay. Patterns of slits cut into the rubber provided the rods with frictional contact points.

The researchers found that vibrations aligned with the rods set up contractile waves like those of a snail. In these cases, the rods slid continuously forward or backward over the slits. Adding slight side-to-side or up-and-down vibrations led to a buckling of the gel, resulting in snakelike slithering or inchwormlike motion, respectively.

"These are great experiments," comments Anette P. Hosoi of the Massachusetts Institute of Technology, a developer of snail like robots for such uses as oil exploration. The link between the different slithery gaits raises the possibility of building robots that can readily match their movements to different terrains, she adds.

Although the experiments may apply to artificial mobility, some biologists say that the work doesn't add to understanding of movement of organisms. Animal-locomotion researcher R. McNeill Alexander of the University of Leeds in England says, "I think it is a different and neat way of expressing what we already understood."

Moreover, the proposed common mechanism behind the different limbless motions is not reflected in the natural world, cautions biomechanical engineer Mark W. Denny of Stanford University. "Undulation in animals is clearly a result of asymmetrical nervous excitation of muscles, not of buckling," he says.

To devise practical robots, the Harvard-Lehigh team is investigating hydrogels that would move in response to stimuli such as electric fields or chemical reactions. The team is also looking into ways to incorporate textures into the surface of the hydrogels themselves so that future gel-based robots might carry motion-directing frictional contacts onboard instead of relying on features of the surfaces they traverse.