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Flex That Muscle

Electroactive polymers represent the actuators of the future.

01 January 2007

Erin M. Schadt

Actuators are everywhere. From cars to cameras, if a device requires components that move, it uses actuator technology. While this used to require motors, drive trains, or hydraulics, over the past several years a newer technology has been gaining ground. Electroactive polymers (EAPs)?often called artificial muscles?have a number of variations and a multitude of potential applications.

There are two primary types of EAPs: electronic and ionic. Electronic EAP materials activate electromechanically in response to electric stimulation and include ferroelectric polymers, dielectric polymers, electrostrictive graft elastomers, and liquid crystal elastomers.

Ionic polymers are, with rare exception, hydrated, and actuate when ions move along the material once a charge is applied. These take the form of ionic gels, ionic polymer metal composites (IPMCs), conductive polymers, carbon nanotubes, and electro-rheological fluids.

Ionic's main advantage is the low voltage required for actuation, and electronic's greatest assets are large strains, rapid response, and good efficiency. Disadvantages for ionic are that most materials require constant hydration and induce low torque, while electronic's main drawback is the high amount of voltage it requires.

In general, EAP technology needs to create greater force, greater efficiency, and be more robust, as well as be more easily scalable.

Researchers are tackling these challenges and have seen marked improvement in EAP development in the last few years. Yoseph Bar-Cohen, one of the chairs of the SPIE symposium Smart Structures and Materials & Nondestructive Evaluation and Health Monitoring (see sidebar this page), outlines some of the most notable achievements in the field:

"Advances were reported in every area of EAP including improved understanding of the material electro-activation mechanism. This includes the work by Sia Nemat-Nasser on understanding the mechanism that makes IPMCs bend; and development of dielectric elastomer actuators that don't require preload allowing increased operation life currently affected by creep, which causes relaxation of the preload. Also, there were improvements in the methods of processing the materials, the electrodes, and the drive electronics as well as improved test methods for material characterization."

Biomimetic muscles are just one application of EAP technology. In this example, four actuator banks (left) are combined and placed inside the arm wrestling robot (right). Photos courtesy of the Laboratory for Materials and Engineering, EMPA Dübendorf.

Promising Materials

New materials, as well, are being tested for EAPs. Dielectric elastomers are seen as the most promising material for artificial muscle applications because of their large deformation, high energy density, good efficiency, and quick response. Typically silicones or acrylics are used in the elastomer film, but other materials are being explored. Synthetic rubber, in particular, exhibits higher dielectric constant, elastic strength, and less creep.

Electroactive paper (EAPap), made with cellulose material, is also being developed. Being ultra light weight, inexpensive, requiring low actuation voltage, and consuming a low amount of power, EAPap material is a promising technology for biosensors, MEMS, flexible displays, and small flying objects.

"Development of new material with high actuation performance is the next big development step for the next two to three years," explains Gabor Kovacs, of the Swiss Federal Laboratories for Materials Testing and Research (Dübendorf, Switzerland) and cochair of the EAP Actuators and Devices conference. He says that challenges to this step are "increasing the relative dielectric constant and decreasing the film thickness."

"In our lab we are working in the field of research of newly synthesized dielectric elastomer-materials with high actuation performance at low actuation voltage," says Kovacs. "Large-scale active structures represent our main engineering activities using EAP technology. In general we are working on active implants and on large-scale shell-like active structures."

Revolutionizing Actuators

Often called artificial muscles, it's natural to just think of the possible biomimetic functions of EAP materials. The technology has found commercial application, however, as an actuator in such areas as medical, automotive, and aerospace.

"Since they are energy efficient and noiseless, as well as [exhibit a] simple and low-cost structure, this material-deformation-based actuation represents a very promising technology," says Kovacs.

There are several companies commercializing EAP devices and components; leading companies include Artificial Muscle Inc. (Menlo Park, CA), Micromuscle AB (Linköping, Sweden), and ERI-Environmental Robots Inc. (Albuquerque, NM).

In fact Frost & Sullivan awarded its Actuator Technology Product Innovation of the Year Award to Artificial Muscle for its Universal Muscle Actuator, citing the technology's "potential to revolutionize the small actuator industry because of its market-changing light weight, high power density characteristics."

These are precisely the characteristics that researchers hope to exploit in future applications.

"If the material durability and actuation force can be increased, they can become the actuators of choice for engineers of mechanisms that require fracture tolerance, quiet, light weight, easy-to-shape actuators with many other benefits that polymers offer," says Bar-Cohen of NASA's Jet Propulsion Lab (Pasadena, CA). "This includes robots where quiet is needed for military application, smart prosthetics, or even large gossamer structures for potential use in space applications.

"Examples of applications that are in various stages of development include a catheter steering mechanism, a power harvester embedded in a shoe, controlled drug release, rotation mechanisms, actuators for flapping wings and fins, actuators of micro-optical shutter arrays, an active Braille display, speakers, and many others."

Biomimetic Applications

Indeed, there are many applications being explored, including biomimetic possibilities. For example, the U.S. Navy has been developing a self-contained sonobuoy composed of EAPs modeled after the jellyfish for covert surveillance. The small device would be made mostly of polymers, the body encapsulating electronic processing and communications package, and the tentacles housing the sonar sensors. Use of EAPs as artificial muscles would allow precise control of its jellyfish-like motion.

Researchers in Italy are working on actuators that could be activated by electrophysical signals, allowing the body to control the artificial system, which could allow active prosthetics or orthotics for humans.

Wearable dielectric elastomer actuators are another application of the technology. Using a new type of fiber-prestrained composite actuator, researchers from MIT and Artificial Muscle were able to achieve good actuator performance without a rigid frame. In other words, actuator material could be worn on the body like fabric for virtual tactile feedback gloves, and to improve devices such as automatic blood pressure cuffs.

Of course, there is also the science fiction of today that Bar-Cohen envisions as the science fact of tomorrow.

"Eventually, EAP materials will become very strong and robust to produce such robots as biomimetic legged ones that can possibly run as fast as a cheetah, carry mass like a horse, climb steep cliffs like a gecko, reconfigure its body like an octopus, fly like a bird, and dig tunnels like a gopher," speculates Bar-Cohen.

This may sound a bit far fetched today, but with improved EAP capability, it's a strong possibility.