Stretching the Horizon for Electrical Devices

Hunting for materials that change shape when zapped electrically, researchers have found a new champion literally hanging out in the kitchen. A rubbery acrylic used as an adhesive on kitchen gadgets triples its length when subjected to high voltage, shattering the standard for electrically induced elongation. The acrylic and its stretchy silicone brethren might someday control video displays, animate small robots, or power artificial limbs.

Materials that deform when juiced are used to drive high-tech speakers, guide the tips of scanning tunneling microscopes, and otherwise convert electrical energy into mechanical work. But such materials generally don't change length by more than a few percent. So a team from SRI International in Menlo Park, California, set out to find a real mover among a class of superstretchy substances called dielectric elastomers. "Basically, we've tried everything we can try," says engineer Ron Pelrine. Indeed, Pelrine even tested the adhesive on a child safety latch for his refrigerator. The stuff turned out to be the star acrylic.

As reported in tomorrow's Science, Pelrine and colleagues fashioned films of elastomers into charge-collecting devices called capacitors. They mounted each film on a frame and painted both sides with electrodes made of conductive grease. They then charged the electrodes to several thousand volts. Opposite charges from the two electrodes squished the elastomer film in the middle and spread it like a hamburger beneath a spatula. Meanwhile, like charges along each surface of the film repelled each other, forcing it to spread even more.

To get still bigger strains, the researchers added a twist: They stretched the film in one direction before applying the voltage. The prestraining toughened the film so the researchers could apply higher voltages without shorting out the system. It also funneled energy into stretching the film in the perpendicular direction. Thanks to the extra boost, the width of the acrylic capacitor increased 215%.

"I was very excited by these results," says Ray Baughman, a materials scientist with Honeywell International in Morristown, New Jersey. "The strains you get here are just giant." But some researchers aren't bowled over by the numbers. Materials scientist Qiming Zhang of Pennsylvania State University, University Park, says the real question is what makes the newfound elastomers so much stretchier than other substances.