Artificial Muscles [Preview]

Novel motion-producing devices--actuators, motors, generators--based on polymers that change shape when stimulated electrically are reaching commercialization.
Materials That Move

BAR-COHEN HAS SERVED as the unofficial coordinator for the highly diverse community of international EAP researchers since the mid-1990s. Back during the field's infancy, “the electroactive polymer materials I read about in scientific papers didn’t work as advertised,” he recalls, chuckling slyly. “And as I already had obtained NASA funding to study the technology, I was forced to look around to find who was working in this area to find something that did.” Within a few years Bar-Cohen had learned enough to help establish the first scientific conference on the topic, start publishing an EAP newsletter, post an EAP Web site and edit two books on the nascent technology.

Sitting among arrays of lab tables strewn with prototype actuation devices and test apparatuses in a low-slung research building on the JPL campus, Bar-Cohen reviews the history of the field he has come to know so well. “For a long time,” he begins, “people have been working on ways to move objects without electric motors, which can be too heavy and bulky for many applications. Until the development of EAPs, the standard replacement technology for motors were piezoelectric ceramics, which have been around for some time.”

In piezoelectric materials, mechanical stress causes crystals to electrically polarize, and vice versa. Hit them with electric current, and they deform; deform them, and they generate electricity.

Bar-Cohen lifts a small grayish disk off one of the lab benches, saying, “This one's made of PZT—lead zirconate titanate.” He explains that electric current makes the piezoelectric PZT shrink and expand by a fraction of a percent of its total length. Not much motion but useful nonetheless.

In an adjoining room, Bar-Cohen shows off foot-long impact drills driven by PZT disks that he is building with his JPL colleagues and Cybersonics, Inc., in Erie, Pa. “Inside this cylinder is a stack of piezoelectric disks,” he states. “When activated with alternating current, the stack beats ultrasonically on a mass that hops up and down at a high rate, driving a bit into solid rock.” To one side sit piles of stone blocks into which drill bits have cut deep holes.

As a demonstration of how effectively piezoceramics can perform as actuators, it is impressive. But many applications would demand electroactive materials that grow by more than just a fraction of a percent.

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