“SRI INTERNATIONAL began work on artificial muscles in 1992 under contract to the Japanese micromachine program,” says Ron Pelrine, the physicist-turned-mechanical engineer who leads the SRI team. Japanese officials were looking for a new kind of microactuator technology. A few SRI scientists started searching for a motion-generating material that resembled natural muscle in terms of force, stroke (linear displacement) and strain (displacement per unit length or area).
“We looked at a whole bunch of possible actuation technologies,” Pelrine recalls. Eventually, however, they considered electrostrictive polymers, a class of materials then being investigated by Jerry Scheinbeim of Rutgers University. The hydrocarbon molecules in those polymers are arranged in semicrystalline arrays featuring piezoelectriclike properties.
When exposed to an electric field, all insulating plastics, such as polyurethane, contract in the direction of the field lines and expand perpendicularly to them. This phenomenon, which differs from electrostriction, is called Maxwell stress. “It had been known for a long time but was regarded generally as a nuisance effect,” Pelrine says.
He recognized that polymers softer than polyurethane would squash more under electrostatic attraction and thus would provide greater mechanical strains. Working with soft silicones, the SRI scientists soon demonstrated quite acceptable strains of 10 to 15 percent. With further research those numbers rose to 20 to 30 percent. To distinguish the new actuator materials, silicones and other softer plastics were christened dielectric elastomers (they are also called electric-field-actuated polymers).
Having identified several promising polymer materials, the group focused for much of the remainder of the 1990s on the nuts and bolts of building devices for specific applications. Much of the SRI team's new external funding support and research direction came at the time from the Defense Advanced Research Projects Agency (DARPA) and the Office of Naval Research, whose directors were primarily interested in using the technology for military purposes, including small reconnaissance robots and lightweight power generators.
As the elastomers began to exhibit much larger strains, the engineers realized that the electrodes would have to become expandable as well. Ordinary metal electrodes cannot stretch without breaking. “Previously, people didn’t have to worry about this issue, because they were working with materials that provided strains of 1 percent or so,” Pelrine notes. Eventually the team developed compliant electrodes based on carbon particles in an elastomeric matrix. “Because the electrodes expand along with the plastic,” he points out, “they can maintain the electric field between them across the entire active region.” SRI International patented this concept, one of the keys to subsequent artificial-muscle technology.
Eager to demonstrate, Pelrine holds out what looks like a six-inch-square picture frame with plastic sandwich wrap stretched tautly across it. “See, this polymer material is very stretchy,” he says, pushing a finger into the transparent film. “It's actually a double-sided adhesive tape that's sold at low cost in large rolls.” On both sides of the middle of the sheet are the black, nickel-size compliant electrodes, trailing wires.
Pelrine turns a control knob on the electric power supply. Instantly, the dark circle of the paired electrodes grows to the diameter of a quarter. When he returns the knob to its original position, the disk shrinks back immediately. He flashes a grin and repeats the sequence a few times, explaining: “Fundamentally, our devices are capacitors—two charged parallel plates sandwiching a dielectric material. When the power is on, plus and minus charges accumulate on the opposite electrodes. They attract each other and squeeze down on the polymer insulator, which responds by expanding in area.”
Although several promising materials had been identified, achieving acceptable performance in practical devices proved to be a challenge. A couple of breakthroughs in 1999 drew considerable interest from government and industry, however. One arose from the observation that stretching the polymers before electrically activating them somehow vastly improved their performance. “We started to notice that there seemed to be a sweet spot at which you get optimum performance,” remembers engineer Roy Kornbluh, another team member. “No one was sure exactly why, but prestretching the polymers increased breakdown strengths [resistance to the passage of current between electrodes] by as much as 100 times.” Actuation strains improved to a similar degree. Although the reason is still unclear, former SRI chemist Qibing Pei believes that “prestretching orients the molecular chains along the plane of expansion and also makes it stiffer in that direction.” To achieve the prestraining effect, SRI's actuator devices incorporate an external bracing structure.
The second key discovery came about primarily because the researchers “were testing every stretchy material we could find—what we call an Edisonian approach,” Pelrine says with amusement. (Thomas Edison systematically tried all kinds of materials for suitability as lightbulb filaments.) “At my home, we had placed a polymeric door lock on the refrigerator to keep my toddler from getting in. As he got older, we didn’t need the lock anymore, so I removed it. But since it was made of a stretchy material, I decided to test its strain properties. Surprisingly, it gave very good performance.” Tracking down the material and determining its composition took no small effort, but in the end the mystery polymer “turned out to be an acrylic elastomer that could provide tremendous strains and energy output—as much as 380 percent linear strain. These two developments allowed us to start applying the dielectric elastomers to real-world actuator devices,” the researcher says.