THE SRI TEAM'S general approach is flexible, encompassing many designs and even different polymers. As Pei says, “This is a device, not a material.” According to Pelrine, the team can produce the actuation effect using various polymers, including acrylics and silicones. Even natural rubber works to some extent. In the extreme temperatures of outer space, for example, artificial muscles might best be made of silicone plastics, which have been demonstrated in a vacuum at –100 degrees Celsius. Uses that require larger output forces might involve more polymer or ganging up several devices in series or in parallel.
“Because the dielectric elastomers can be purchased off the shelf and we’d use at most only a few square feet of material in each device, the actuators would be very low cost, particularly in volume production,” SRI's von Guggenberg estimates.
The voltages required to activate dielectric elastomer actuators are relatively high—typically one to five kilovolts—so the devices can operate at a very low current (generally, high voltage means low current). They also use thinner, less expensive wiring and keep fairly cool. “Up to the point at which the electric field breaks down and current flows across the gap [between the electrodes], more voltage gives you greater expansion and greater force,” Pelrine says.
“High voltage can be a concern,” Kornbluh comments, “but it's not necessarily dangerous. After all, fluorescent lights and cathode-ray tubes are high-voltage devices, but nobody worries about them. It's more of an issue for mobile devices because batteries are usually low voltage, and thus additional electric conversion circuits would be needed.” Moreover, at Pennsylvania State University, Qiming Zhang and his research group have managed to lower the activation voltages of certain electrostrictive polymers by combining them with other substances to create composites.
When asked about the durability of SRI's dielectric elastomer actuators, von Guggenberg acknowledges a need for more study but attests to a “reasonable indication” that they continue to work sufficiently long for commercial use: “For example, we ran a device for one client that produces moderate, 5 to 10 percent strains for 10 million cycles.” Another generated 50 percent area strains for a million cycles.
Although artificial-muscle technology can weigh significantly less than comparable electric motors—the polymers themselves have the density of water—efforts are ongoing at SRI to cut their mass by reducing the need for the external structure that prestrains the polymers. Pei, for instance, is experimenting with chemical processing to eliminate the need for the relatively heavy frame.