For many years, engineers have strived to develop efficient actuators that transform energy from one form to another. For example, to transform electrical energy into rotational mechanical energy, inventors developed the electric motor. Most of these devices, however, have the problem of wasting energy in other forms, such as heat. Their many moving parts also make them prone to breaking or malfunction. Furthermore, they are often noisy.
Scientists are now developing a solution to these problems. Electroactive polymers, or EAPs, are materials that change shape in response to electricity. EAPs use no moving parts, being solid-state devices, and can produce a variety of movements ranging from elongation to bending. They also make no noise, and are very efficient, wasting little to no energy.
Electroactive polymers are a relatively new technology. Mototaro Eguchi developed the first true electroactive polymer, the beeswax electret, in 1925. Few advancements were made, however, until the 1970s, when scientists began discovering more electroactive polymers, such as carbon nanotubes. However, EAP research did not fully take off until the 1990s. At this time, developers, in addition to discovering new electroactive polymers like the dielectric elastomer, refined already existing EAP technology.
Electroactive polymers can be sorted into two main groups: ionic EAPs and electronic EAPs. Ionic EAPs include carbon nanotubes, conductive polymers, and ionic polymer gels. These electroactive polymers can operate with low voltages, and can achieve large strains. However, their response time is slow, and these EAPs do not provide much mechanical force. The other group, known as electronic EAPs, contains materials like ferroelectric polymers, electrostrictive graft elastomers, and perhaps the most focused-on electroactive polymer technology today, the dielectric elastomer. These electroactive polymers are much stronger than the ionic EAPs and respond more quickly, but they require high voltages to work.
Dielectric elastomers are one of the most commonly discussed electroactive polymers, as well as one of the most likely candidates for use in future devices. These electroactive polymers work through the electromagnetic attractive force between two compliant electrodes. Between these electrodes is placed a flexible polymer such as an acrylic or silicone (this part is the dielectric elastomer). When electric current is applied, the positive and negative electrodes attract each other, putting pressure on the dielectric. The elastomer responds by expanding outward, allowing the electrodes to move toward each other. Thus a dielectric elastomer actuator will decrease in thickness while increasing in area.
Use as an actuator is only one of the potential applications of dielectric elastomers. They also show great promise in being used as generators and as sensors. Just as putting electricity through the material generates movement, subjecting the material to mechanical stress will generate electricity. Not only is this an ideal method of power generation, but dielectric elastomers will also generate different amounts of electricity based on the force they are subjected to. This opens up many opportunities for use as force sensors.
Dielectric elastomers are used in another electroactive polymer technology. If a dielectric elastomer is stretched over a circular frame and biased on one side by a slight amount of pressure, then when electricity is applied, the elastomer will expand outward in one direction and swell, acting like a diaphragm. This type of EAP, called an electrostrictive polymer, serves two purposes. The device shows potential for creating sound if the diaphragm oscillates up and down at the right frequency, opening the door for possible speaker technology. In addition, this diaphragm can create a vacuum in a chamber and function as a pump.
Dielectric elastomers also exhibit potential for use as batteries. The basic structure of the dielectric elastomer is similar to the makeup of a capacitor. There are two electrodes, and in the middle, the dielectric, separating the electrodes and preventing current from flowing through the device. When connected to a power source, electrons are forced through the circuit to one of the electrodes, creating a negative charge. The battery’s voltage also draws electrons away from the other electrode, creating a positive charge. These opposite charges attract each other, and cause the elastomer to compress. If, while the elastomer is activated, it is connected to a circuit as the power source, the electrons stored in the dielectric elastomer will travel through the circuit, powering whatever they are connected to. The device is thus capable of storing electrical energy. Electroactive polymers, because of their simple structure and low density, could one day replace today’s battery technology.
Dielectric elastomers, as well as other electroactive polymers, have found a variety of applications in today’s technologies. Many companies are already developing EAP products. For example, the Japan-based company Eamex distributes an aquarium of swimming EAP fish that mimic the movements of actual fish. In addition, Artificial Muscle, Inc. has developed a camera lens auto-focuser that relies on EAP technology. DARPA has also funded research and development for a heel-strike generator using electrostrictive polymers and that can be placed in a boot or shoe to generate approximately one watt of power during normal walking.
Electroactive polymer technology still has far to go before it reaches its full potential. Today’s dielectric elastomers and electrostrictive polymers require high voltages to work, which is challenging for use with batteries, which characteristically provide low voltages. However, researchers in Pennsylvania discovered that by using elastomers with a high dielectric constant, lower voltages could be used to produce motion. Current EAPs are also relatively weak when compared to traditional actuation devices. Researchers are striving to develop stronger, more powerful EAP actuators.
The future remains open to a great deal of electroactive polymer devices and technologies. Electrostrictive polymer tweeters could replace conventional high-frequency speakers. New force sensors based on dielectric elastomer technology could be developed. Wind and wave-powered generators may utilize electroactive polymers. Whatever the application may be, EAP technology will most likely become ubiquitous in the next few decades.
Perhaps the greatest use of all for EAP technology will be the very nickname often given to these materials: artificial muscles. The low density, scalability, and energy efficiency of electroactive polymers make them an ideal candidate for use in replacing biological tissue. Dielectric elastomers are particularly promising, because they have strain levels and output forces comparable to natural muscle tissue. Medical professionals could implant a stack of dielectric elastomers and attach it to tendons in place of a damaged natural muscle, connect it to some type of battery device, and wire it into the human nervous system to restore movement to individuals with muscle diseases. This could improve the lives of tens, if not hundreds, of thousands.
The number of individuals researching EAP technology today is steadily increasing. Dr. Yoseph Bar-Cohen of Jet Propulsion Laboratories informally heads the electroactive polymer research community, and has established scientific conferences dedicated solely to EAP technology. New companies are springing up to help develop this technology, like Artificial Muscle, Inc. A great deal of literature on electroactive polymers has been published.
To help further EAP research, Bar-Cohen and other scientists came up with a challenge to researchers: build an EAP-powered robotic arm capable of beating a human in an armwrestling match. Teams of engineers entered the competition, and all the arms entered were easily beaten by a 16-year-old girl. The prize remains unclaimed, but engineers draw closer to it every year.
This little-known field of electroactive polymers has grown greatly in the last fifteen years. As these specialized actuators have grown from a mere concept to patented commercially available products, one imagines what the future will hold for EAPs. Whatever developments occur, one can all but be assured they will open up new doors to twenty-first century devices, and from this, society will benefit.
