Electroactive polymers (EAPs) are a fairly old development within the polymer field, but they have been getting a lot of attention recently. An EAP is a material that exhibits a change in size and/or shape when stimulated by an electric field. The first idea of an EAP was established in 1880 when Wilhelm Röntgen pulsed an electric current through a rubber band that was fixed at one end and attached to a mass at the other. The length of the rubber band was then studied as the current pulsed on and off. Another advancement occurred in 1990 when ionic polymer-metal composites were developed and shown to exhibit electroactive properties far superior to previous EAPs. This lowered the activation energy for the EAPs to as low as 1 or 2 volts and allowed the material to exhibit as much as 380% strain.i In 2008 a company known as Artificial Muscle(tm) began producing a new EAP that used silicone and acrylic polymers to produce various new types of EAPs.
Types of EAPs
There are two main subcategories of EAP, known as “dry” (Electronic) and “wet” (Ionic), with each having their own advantages and disadvantages. Electronic EAPs can operate for long periods of time, have a rapid response time (a few mSec), can hold strains under DC activation and can induce fairly large actuation forces. However, generally speaking electronic EAPs require a higher voltage and their transition temperature is inadequate for low temperature actuation assignments. An ionic EAP requires a low actuation voltage, is exceptional at, and used primarily in, bending actuation and exhibits very large bending displacements. However, the ionic EAPs do nothold under strain of DC voltage, have a slow response time (about half a second), a fairly low actuation force due to their bending abilities, and besides a few exceptions it is very difficult to produce a consistent material.ii
Innovations and Applications
At the moment there are many early development applications that have potential. The first goal is to mimic insect, animal and human systems in a multitude of facets. By mimicking insect movement the goal is to create micro robots that can be used in long term military surveillance missions. Creating animal and human systems would lead to more advanced prosthetics and orthotics that are able to respond similarly to actual muscle movement as opposed to the static prosthetics that exist now. However both of these applications have been stalled due to a variety of reasons including the material consuming too much energy, not generating enough force, and not lasting long enough. In a recent breakthrough however researchers at Pennsylvania State were able to create an electroactive actuator that requires one-tenth the voltage that was previously required for actuation. The power of this new material is severely less than previous materials and it is estimated that the most it could contribute to is a low voltage pharmaceutical insulin pump.iii Another application that is currently emerging is using EAPs for refreshable Braille displays which serve as a faster computer assisted communication device for the visually impaired. By using rows and columns of electrodes the computer interface becomes a dynamic system that creates a tactile pattern of dots that can change faster than a regular page. The final application that is being attempted is using modular actuators in optical membranes. Due to an EAP’s ability to generate displacements that range from micrometers to centimeters, they are useful in shape correction and jitter suppression. However, many computational fluid dynamic simulations have shown that the surface created from the EAP is not a water impermeable surface and therefore allows evaporation of the water contained in the EAP and increases the potential loss of the ions necessary for the EAP to function. As the technology in the field of EAPs increases, so too will the advancements, possibly to the point where entire body parts can be seamlessly replaced with artificial polymers.
ii Bar-Cohen, Yoseph “Electroactive Polymers as Artificial Muscles – Reality and Challenges” (2001), Proceedings of the 42nd AIAA Structures, Structures Dynamics and Materials Conferences (SDM), Gossamer Spacecraft Forum (GSF), held in Seattle WA, April 16-19.