RoboSnail Tackles Any Terrain -- Slime Not Included

First Robotic Snails Developed at MIT


Jan. 26, 2006 —

The snail may seem like a humble creature, but it turns out walking with one foot on a trail of slime is quite a feat.

For the first time, researchers have developed a robotic snail that can do just that. Anette Hosoi at the Massachusetts Institute of Technology and her team, including graduate student Brian Chan, have developed RoboSnails I and II that move the way snails do. The battery-powered devices are larger than the average snail and don't exactly resemble the mollusks, but they crawl using two modes of locomotion that snails employ to travel across their trails of slime. The team published the results of its work in the most recent issue of the Physics of Fluid.

Why bother building a robot that moves like a snail? According to Hosoi, the possibilities are endless.

Oil Explorers, Surgical Aids

"There are a lot of interesting applications," she said. "Snails may be slow and may require a slime trail, but the good thing is they can crawl over anything -- sand, mud, twigs. If they encounter a wall or ceiling, they keep moving."

In fact, Hosoi's research is partly funded by the oil company Schlumberger because some version of the robotic snails might someday aid in oil exploration in harsh, hard-to-reach environments. The devices might also be used in the medical field. Hosoi said that the devices could help surgeons reach interiors of a patient's body more accurately.

Hosoi imagined the robotic devices could "crawl to the point where the doctor needs to operate and then crawl back out" once the surgeon was finished.

Such applications are still just theory, but the two prototypes created in the MIT lab are the first step toward creating such exploratory tools.

RoboSnail I and II move using two different crawling methods. One undulates a flexible, rubbery foot front to back while the other can move up walls by pushing backward to build up pressure in the slime, which propels the creature forward when released.

"It's like when you want to move a carpet," Hosoi explained about the pressure crawling method. "You can grab one end and pull, but that's hard. To move it a couple feet, you can go to one end and make a bump and then push that bump along the carpet until it has shifted. The waves are analogous to that bump in the carpet."

The Challenge of Slime

Figuring out the motion of the robotic snails' feet is one step, the other major challenge is finding a way to equip the devices with a steady source of slime.

Mark Denny, a marine scientist based at Stanford University's Hopkins Marine Station in Pacific Grove, Calif., has studied the locomotion of slugs extensively. He has calculated that the creatures, which also travel on trails of slime, expend 70 percent of their energy making their mucous trails. In fact, he found the cost of making mucus was 10 times greater than the energy other animals expended in running, swimming or even flying.

Hosoi said that real snails stored their mucous slime in a crystal form and then mixed it with liquid as they dispensed it. So far the robotic snails can't make their own slime, but rely on a pre-provided trail. Steven Vogel, a biologist at Duke University in Durham, N.C., speculates the slime factor may be a major hurdle.

"Snail feet are interesting because they're versatile," Vogel said. "But they leave that slime trail, so it can be an expensive form of locomotion mile for mile."

Hosoi said the slime the robots used was a suspension of clay particles in water that looked like clear gel and acted a little like mayonnaise.

"It has a finite yield stress," she said. "That means, like mayonnaise, if you dump it on bread, it doesn't move, but as soon as you exert force and start to spread it, it flows easily."

She admits that finding a way for the robots to make their own mayonnaiselike slime is a considerable challenge. Another lies in making the robots smaller. Right now the robotic snails are about 6 inches long and 1 inch wide in size; the MIT researchers are working on miniaturizing the devices for possible medical applications.

The practice of borrowing from nature to develop technology is known as biomimicry. Vogel said that sometimes nature provided clues that lead to revolutionary technology. But, he cautions, nature doesn't always have the perfect solution.

"Nature isn't always the ultimate model," he said. "Think about aviation -- all the early, birdlike planes crashed. But then if birds didn't fly, we probably never would have even tried anything so preposterous."

Likewise, future versions of robotic snails may end up moving using very different methods than their natural counterparts. But, if not for the snail, Hosoi said that medical teams certainly would have never attempted to make one-footed robots that crawl on slime.

"My background is in studying fluids," she said. "It was my graduate student who then thought of snails. The snail was our inspiration."