It turns out that opposable thumbs aren't critical for getting a good grip. Neither are fingers. Scientists have created a robotic arm that can do everything from serve drinks to draw pictures even though it has no digits.
Fingers and thumbs work perfectly well for humans, says Eric Brown, a physicist at the University of Chicago in Illinois. But on a robot they can be clumsy. The fingers slip. They grip too hard and break whatever they're trying to hold. And sometimes they don't grasp it at all. Then there are the complexities of manipulating 20-odd joints with a computer.
So Brown and his colleagues took a different tack. Their robotic hand, which they describe online today in the Proceedings of the National Academy of Sciences, is a thin rubber sack filled with coffee grains or small glass spheres. When this hand comes in contact with an object, a small pipe sucks air from the sack, causing it to contract and mold to the object's shape. The contraction is small—a mere 1% change in volume–but was enough to grab most objects the researchers tested. "It's very simple to control," notes Brown. "You don't have all these joints."
The hand works best on hard, dry, geometrically complex objects like screwdrivers and toy jacks. It has more trouble with flat objects like plastic discs and porous objects like cotton balls, because the air holes weaken the suction. It also can't grip anything bigger than half its size—the biggest items the team picked up were two one-gallon jugs of water. But the hand's true strength, according to engineering student and co-author John Amend of Cornell University, is its versatility. Aside from the limitations noted above, he says, as long as the gripper can fold about one-fourth of the object's surface, it can pick up just about any shape thrown in its path.
Scientists have been searching for a "universal gripper" for decades, and the idea of using grain sacks instead of fingers is not new, says Yale University physicist Corey O'Hern, who was not part of the study. But this is the first time the idea has been tested and quantified in so much detail. Compared with robotic fingers, he says, "this seems like a much better way to go."
O'Hern suggests solving the porous-object problem by making the sack stickier. But the problem there, he adds, is that letting go would be hard.
Amputees could benefit most from the technology, says Brown. Having a moldable hand that could hold a fork or swipe a credit card could drastically improve the quality of life for the tens of thousands of patients in the United States who have lost an arm. And without the need to manipulate eight fingers and two thumbs, he says, it would be much easier to operate than many of the more complex prosthetic hands currently on the market.