"The Gooed Foot", Input/Output, ME Departments, August 2006

input/output

by Michael Abrams, Contributing Editor

The Gooed Foot

We humans have long lorded it over the animal kingdom. We've flown higher with our airplanes than the most upwardly mobile vulture, plumbed ocean depths with submarines where no shark would dare venture. And yet the lowly slug has continued to mock us with its ability to climb vertical walls and even traverse ceilings.

Thanks to a group of researchers at MIT, headed by an assistant professor of mechanical engineering, Anette "Peko" Hosoi, humans have put an end to this embarrassment by inventing the robotic slug. (Or snail: "For what we're doing, the difference between a snail and a slug is that the snail has a shell," said Hosoi. "Now, a biological scientist would probably jump all over that statement.")

The difficulty, apparently, is not in the mechanics of the monopode's foot, which moves when a compression wave passes through it. Those were easily imitated first with artificial muscles—wires that contract five percent when a current runs through them—and later with five pads that slide along a track to imitate that wave. The trick to climbing walls, though, is not in the mechanics proper but in the ooze that the gastropods exude.

MIT's robotic slug imitates the real thing with segmented feet that slide along a track. It's harder to find the right mucus formula.

A slug's "petal mucus" is a tad more complex than you might first imagine. The key to duplicating it is in the slime's "finite yield stress." Imagine a dollop of mayonnaise on a piece of bread. It sits there, wobbly perhaps, but maintaining its amorphous shape. But once a knife is used to apply a little shearing force, the condiment becomes more liquid, spreading easily to the four corners of what is sure to be the top or bottom of a turkey sandwich. What's happening is the same thing that happens in slug mucus.

"Inside there are polymer chains that are like little springs—if you apply a lot of force and shear, you break the bonds and allow it to flow," said Hosoi. A snail keeps about four-fifths of its foot rooted to the spot while the remaining fifth slides forward. The mucus acts as an adhesive to the stationary part, but flows beneath the moving, shearing fifth. And, of course, those polymer chains also have to be able to link back up after the shearing force has been withdrawn so the mucus can re-adhere to the foot. As what was once moving becomes motionless again, it needs to stay put.

Hosoi and her team tested many fluids by putting them between two plates and applying shearing forces. "Standard technique," said Hosoi. "It would be fantastic if you could measure this at the molecular level, but it hasn't been done in invertebrate mucus."

At first, they tried adding something called Laponite platelets to their tincture—at three percent Laponite, the artificial mucus resembled Jell-O. Eventually, though, they settled on Carbopol gel. "I don't know exactly what's in it because it's made by Dow Chemical and they don't tell us what's in it," said Hosoi. "But they do say that it's what's sold in hair gel. So it basically looks like hair gel."

What interests Hosoi most are questions of optimization. Not of speed, say, but of slime production. "The fluid properties are what make it all possible. So what kind of fluid properties do you want to make a slug most efficient?" It might seem obvious that a snail would move best over mucus that's, well, mucusy. But it turns out that this is only true because it takes more energy for a snail to make mucus than it does for it to move.

"As you crawl, you want to set up a flow so that you drag as much of this stuff with you as possible," Hosoi said. Were it the other way around, and muscle movement were more costly than mucus production, the snail would favor a "sheer thickening" fluid over a "sheer thinning" one—something more like cornstarch with water. "Then I'd want to move forward as much as possible. If you have something like cornstarch, then the bulk of the robot slides forward while only one-fifth stays put."

As of now, though, the robot expends no energy-making goo—before they send the mechanical foot off on a leg of any journey, the researchers swab its path with the Carbopol gel. A real slug, on the other hand, makes crystals on its underside and moistens them on the go, as it needs mucus.

Hosoi's robotic snail may have to cart its own mucus sometime in the future. Apparently, building such machines may offer more to humankind than just new knowledge and reaffirmation of our world dominance. Shortly after the team started the project, Hosoi received a call from Schlumberger, the oil services company. It was looking to make "devices that are extremely robust and that can climb in extreme environments." We may have beaten slugs at their own game.