Ask a bioengineer about muscle, and you´ll hear high praise and a spec sheet full of properties. First and foremost is force. A thigh muscle can generate 36 pounds of force per square inch—enough to snap a pine board. Then there´s power, the rate at which the force is applied over distance. As in automobiles, high power leads to tremendous speeds; a typical skeletal muscle produces horsepower that pound for pound is“way more than a car engine,” says bioengineer Richard L. Lieber of the University of California at San Diego. Muscles also act as brakes, springs and shock absorbers, which is why we, unlike your typical robot, can run, jump, and land softly. And finally, as a waggish British biologist once put it to a roomful of engineers, muscle is“good to eat.”
Artificial muscle will never rival a good ribeye, but it´s on its way to replicating many of muscle´s other properties. To generate force on command, a material must first be able to deform, like a rubber band, at the flick of a switch. It must contract or expand far enough to move an object a sufficient distance. And it must be stiff enough to generate sufficient force. An effective arm-wrestling robot has to match the force of human torso muscles while rotating an armlike extension and must have sufficient control to adjust its force as necessary.“It´s the ultimate in application,” Bar-Cohen says.“If I can do that, I can make something useful.” The three arms pitted against Felsen each employ a different type of artificial muscle, so the contest will double as a test of the field´s most promising technologies.
Felsen´s first opponent was built by Mohsen Shahinpoor at Environmental Robots, a small
Albuquerque-based company. It´s made of ionic polymer metal composites (IPMCs), which require low voltages but move rather slowly. IPMCs are bendable, so they can be molded into whatever sort of actuator will be most powerful.
If a bookie were laying odds on this match, the favorite would probably be Felsen´s second opponent, built by a team of Swiss government engineers. This arm is propelled by dielectric elastomers, films in which thin carbon-based electrodes sandwich a soft plastic such as silicone or acrylic. Electricity draws the electrodes together, squeezing the plastic, which expands to up to three times its normal area in about half a second. Actuators made of dielectric elastomers exert up to 30 times as much force, gram for gram, as human muscle. But they require several thousand volts of electricity—a bit of a problem if you want to use them near, or in, the body.
Felsen´s third opponent is the underdog. A team of undergraduates from Virginia Tech University, working long nights on a tight budget, created a gel fiber that shrinks when acid is added. The students couldn´t get anyone to donate the artificial muscle, so they made it themselves. Their creation is slow to get going but contracts a lot, up to 40 percent of its length, and has the additional benefit of requiring no electricity.
All three teams arrived early to prepare. As the hours tick down, the pressure mounts.