Artificial muscles pump up everything from military gear to drug delivery.
Mohsen Shahinpoor used to amuse visitors to his University of New Mexico lab with a sinewy robot named Mr. Bony. Bolted onto an exercise bike, the android would pedal laboriously for as long as he was plugged in to an electrical outlet, pumping with muscles made of specially engineered polymer.
The demonstration mainly proved that artificial quads and hamstrings could survive a spinning workout, but lately Shahinpoor's been building devices that do more than entertain. His new company, Environmental Robots, sells tiny sensors and actuators made of polymer strands. They're a bit mundane compared with a robocyclist, but eminently practical.
Researchers have been tinkering with electroactive polymers - or EAPs, a type of artificial muscle - for years, but basic materials research seems to have finally paid off with some useful products. Among them: powerful pumps and motors, nearly silent propulsion technologies, and novel drug-delivery systems. After years of hype, EAPs may soon live up to their potential.
The new polymers work a lot like biology. Real muscles are bundles of fibers that contract in response to electrical signals from adjacent nerves, with a sensitivity and fluidity of motion unmatched by machines. But EAPs come close: Electricity causes a controlled, precise change in the polymer's shape. By contrast, piezoelectric crystals - a material commonly used in devices like watches and inkjet printers - do the same thing, but they undergo length changes of just a fraction of a percent. One EAP used by SRI International can grow to five times its original size. The polymer is wedged between two flexible carbon plates; when the electricity is turned on, negative charge moves to one plate and positive to the other. The plates attract each other, squeezing the EAP until it expands.
Many early EAP applications didn't pan out because the polymers weren't durable enough. So SRI tried every stretchy material available until finding an acrylic elastomer that met its specs. One device made with the elastomer withstood 10 million cycles of testing. SRI spinoff Artificial Muscle thinks that could be tough enough to power the cooling fans in laptops, or to replace the electric motors in car seats and windows. The elastomer already moves the legs of Flex, the first self-powered EAP robot. Flex walks like a cockroach: Each spring-loaded leg operates independently. The bot was designed with battlefield apps, such as detecting mines, in mind.
The US Navy hopes to use EAPs in stealthy underwater vehicles that move like fish. Ian Hunter, a biological engineer at MIT, has synthesized a semiconducting polymer that shrinks when ions - electrically charged particles - leave the material and cling to attached electrodes. When the electric potential is reversed, the ions diffuse back into the polymer, causing it to swell. Hunter's lab is dissecting and modeling sunfish fins, each controlled by more than 50 muscles, to try to create artificial fins that reproduce the sunfish's complex motions. The team expects to have a working model this summer.
In medicine, EAPs could fundamentally alter drug delivery. Marc Madou of UC Irvine is developing implantable, matchstick-sized capsules with microscopic pores. When sensors detect that a patient needs, say, more insulin, artificial muscles open valves under the pores, releasing the drug. The capsules could be inserted in the shoulder or upper arm - similar to, but far more precise than, the Norplant birth control system.
If the earliest uses of artificial muscles prove to be boring, that's just fine with Yoseph Bar-Cohen, a Jet Propulsion Laboratory researcher and a leader in EAP R&D. The hype of the 1990s, with talk of prosthetic limbs and other exotica, helped bring attention to the field, but now researchers are focusing on performance and longevity, "not on how cool a device looks," says Bar-Cohen. "All you need is one successful product. That will open the door to all sorts of possibilities." Mr. Bony may have a future after all.
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