IMAGE: BRYAN CHRISTIE

IONIC MUSCLE: Ionic polymer metal composites are a form of artificial muscle that depends on the movement of ions for motion. Flexible metal foils sandwich a wet polymer filling. With the foils charged, free ions flow toward one side, expanding it and bending the actuator. The only consumer items based on artificial muscles are toy fish made by Eamex of Osaka, Japan, which use such polymer metal composites.

In the mid-1990s, Qiming Zhangand co-workers at Pennsylvania State University in University Park demonstrated strains in their ferroelectric polymers of 4 percent—not a lot, but it comes with a lot of force: about a gigapascal. But Zhang's technology is not quite ideal, in part because it needs high voltages to deliver musclelike power and energy.

In the graft elastomer, a long backbone molecule is engrafted with elements that respond to an electric field. A rather high voltage contracts the entire structure. One such material, developed by Ji Su at NASA Langley, in Hampton, Va., produced a strain of about 4 percent and a rather strong force.

Liquid-crystal muscles work by undergoing a sudden phase change from an ordered crystalline phase to a disordered soup when electrically heated through the phase transition temperature. Groups, particularly teams in the U.S. Navy and in Germany, have been reporting great forces, as well as greater stretch than that found, say, in ferroelectrics. But the need to heat and cool the muscles makes them slow to respond and inefficient.

Electrostrictive paper is a type of artificial muscle discovered serendipitously by Jaehwan Kim at Inha University, in Inchon, South Korea, who began his experiments, essentially, with cellophane tape. Glue two layers of silvered tape together and, surprisingly, the end product will shrink when charged. After varying the numbers of layers and the materials in the electrodes and adhesives, he found a reasonable, and very cheap, technology for large-area applications. One idea people have for using electrostrictive paper is to make an electronically active form of acoustic tile. The tile would broadcast antinoise to cancel out sound in a room.

JUST AS VARIOUS AN ASSORTMENT of ionic muscles exists. In general, the ionics are less energy efficient—less than 30 percent, even under the best conditions, compared with the 80 percent seen in some of the electronic muscles. But they have the advantage in at least two respects: they react to drive voltages as low as 1-5 volts, compared with the electronics' tens of volts per micrometer of thickness. Even better, they readily produce bending motions, rather than just expanding or contracting.

Polymer gels—materials formed of chainlike molecules dissolved in a solvent to form a semisolid—have been investigated for years as sensing devices as well as actuators. Chemical stimulation is possible—changing the acidity of the surrounding liquid can, for instance, cause a movement of ions into or out of the gel, forcing it to contract or expand. But electrical stimulation can produce the same effect and is more convenient for most machine designs. Environmental Robots in Albuquerque, N.M., plans to use this type of polymer to drive its wrestling robot.

Polymer metal composites, developed independently by Keisuke Oguro, Keya Sadeghipour, and Mohsen Shahinpoor, shuttle ions from one side of a band of muscle to the other, causing contraction on one side and expansion on the other. The resulting bending can be reversed easily and lends itself to movements that engineers have, in the past, achieved only by Rube Goldberg-type heroics. The Japanese toy fish, mentioned earlier, are driven by this device; so is the front end of a catheter, developed in Osaka, that steers itself through the circulatory system.

Conductive polymers achieve the same sort of complementary stresses by another means. If two conductive polymer films are separated by a polymer electrolyte and are independently connected—one as a working electrode and the other one as a counterelectrode—one of the conductive films will expand and the other one will contract. In general, the one expanding will be the one ions are entering, and the one contracting will be the one ions are exiting.

Carbon nanotubes, the darlings of nanotechnology research, are a work in progress, more promising for their potentially spectacular results than for any achievements so far. These tubes, chemical cousins to the famous buckminsterfullerene molecule—a soccer-ball-shaped cage of 60 carbon atoms—show the highest tensile strength known. Theoretically, they are some 100 times as strong as an equivalent mass of steel, and they also survive at temperatures up to 1000 oC. Electric charging causes ions to attach to the carbon cage, shortening the carbon-carbon bond lengths and creating strains of 1 percent—not very much, but potentially very powerful.

Electrorheological liquids change their viscosity dramatically when an electric field aligns suspended particles, setting up a logjam of material. The effect can create a virtual valve, throttling the flow of fluid between the electrodes. Unlike most ionic systems, this one is quick off the mark, reacting in as little as a 10th of a second.

IN PRACTICE, IF NOT THEORY, all these artificial muscles are rather weak and inefficient. Nobody is going to use them to punch sheet metal into industrial dies or to drive home rivets. Moreover, they are not standardized yet and therefore are difficult to incorporate into mass-produced devices, a problem stemming from the very richness of choice that the field enjoys. No one actuator class has gained an ascendancy that would grant it the lion's share of R&D investment.

Finally, some of the materials tend to exhibit short working lifetimes, breaking down from material fatigue or electrochemical forces. To find a use beyond toy fish, the actuators must become tough enough to last through millions of cycles.