Biomedicine

Electroactive Polymers

Artificial muscles made of electroactive polymers impart lifelike movements to biomedical and robotic devices.

In the Artificial Muscle Research Institute at the University of New Mexico, electricity is in the air. When lab director Mohsen Shahinpoor applies a voltage to an artificial “hand” made of a polymer-metal composite, its fingers curl into a fist. Poke around the lab and you’ll see robotic fish swimming, wings flapping, and arms lifting-all gaining their muscle from electrically activated polymers. You’ve seen robots before, but there is something different about these. They look alive.

Since the early 1990s, materials scientists and engineers have been developing electroactive polymers for use as sensors, actuators, and artificial muscles. An applied voltage changes the polymer’s composition or molecular structure so that it expands, contracts, or bends. The motion is smoother and more lifelike than movement generated by mechanical devices: like muscles, polymers are flexible, unhampered by the clunky rigidity of gears and bearings. Scientists believe that with this similarity to natural motion, electroactive polymers could revolutionize robotics and biomedical devices. Such materials could make it possible to design robots that maneuver with the grace of a human, prosthetic legs that move and feel real, and implantable microdelivery systems that smoothly and quietly pump drugs to where they’re needed.

Until recently, however, electroactive polymers have presented practical problems. They consumed too much energy. They couldn’t generate enough force. And they didn’t last long enough. But researchers in academia and industry have found ways to make the polymers stronger, more robust, and more efficient. These improvements, says Yoseph Bar-Cohen, a senior research scientist at NASA’s Jet Propulsion Laboratory and one of the field’s pioneers, “will enable faster implementation of science fiction ideas into engineering reality.”

Last September, in a breakthrough that could lead to lower-power medical devices, Qiming Zhang and his colleagues at Pennsylvania State University reported that they had created an electroactive actuator that requires one-tenth the voltage previously needed. Zhang’s key advance: a polymer-semiconductor composite that gets more electric bang for the buck and remains very flexible. The advantages of this class of device are its high efficiency and fast response. But “this is just the start,” says Zhang. He predicts that pharmaceutical products based on the technology-for example, small wearable insulin pumps powered by low-voltage batteries-could be available within five years.

Benjamin Mattes, CEO of Santa Fe Science and Technology, is building strong, long-lasting artificial muscles out of conducting polymers that expand and contract in response to changes in the flow of ions into the materials. These electroactive polymers generate huge forces at low voltages. Because chemical reactions break down the polymer, earlier versions were slow and able to survive only a few cycles. Mattes’s latest device, however, smashes previous records for speed and durability. Its coaxial structure-tiny fibers threaded through a hollow tube and engulfed in liquid electrolyte-allows ions to flow rapidly into the fibers in response to applied voltage. Because he uses a highly stable and conductive ionic liquid as the electrolyte, Mattes says he has achieved “millions of cycles without degradation.”

Thanks to such advances in materials science, electroactive polymers are starting to yield useful biomedical devices. At the University of New Mexico, Shahinpoor has demonstrated thin, durable artificial muscles that can lift many times their own weight. Shahinpoor is using the materials to develop implantable aids such as a pump that works like a mechanical pacemaker to compress the heart and a tiny device that corrects vision by gently squeezing the eyeball. His team is commercializing the devices through a spinoff, Environmental Robots in Albuquerque, NM.

There is plenty of work to do before the technology will be ready for market, however. To be successful, Shahinpoor says, the company will need to ensure that the materials are compatible with living tissue and that their functions can be precisely controlled. He will also need to cut manufacturing costs by a factor of 10.

Although the next five years should see electroactive polymers used as components in microsurgical tools, drug delivery systems, and corrective aids, such advances may be only a beginning. To achieve more lifelike robots and prosthetic devices, scientists will need to make materials that are smarter and more interactive. Within 10 years, researchers aim to develop artificial limbs that provide feedback to the user, graceful autonomous robots that are powered by musclelike polymers, and even suits that enhance the strength and endurance of soldiers and rescue personnel. If the research is successful, robotics may truly come to life.

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