Types of Electroactive Polymers

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The discovery of electroactive polymers (EAP) dates back to an experiment conducted by Roentgen in 1880 using a rubber band having a fixed end and a free end that can be charged and discharged. In 1899, Sacerdote extended this experiment by introducing electric field activation in a rubber band to obtain strain response. This milestone was followed by the discovery of electret, a piezoelectric polymer, by solidifying beeswax, rosin, and carnauba wax under a direct current (DC) bias field. The shape and size changes of the polymer are generally due to intermolecular and attractive forces. These changes can be controlled by slightly changing parameters such as light, pH, temperature, gel composition, and solvent.

Nearly 50 years ago, chemically stimulated polymers were discovered when it was found that collagen filaments contract or expand reversibly upon dipping in acid/alkali aqueous solutions. This early discovery has paved the way for the evolution of synthetic polymers, which have the ability to mimic biological muscles. In the last decade, EAP materials have been able to produce up to 380% strain.

Categories of EAPs

EAP can be divided into two types namely dry (dielectric) type and wet (ionic) type.

Dry Polymers

In these polymers, actuation is caused because of an electric field generated between electrodes. These polymers require an activation voltage of more than 100 V/μm. They can be made to withstand DC voltage for robotic applications. Further, these materials possess high mechanical energy density. Ferroelectric, piezoelectric, electrostatic, and electrostrictive polymers are some of the major dry polymers.

Decker Yeadon introduce an interesting demonstration of a dielectric elastomer electro-active polymer actuator as an example of dry polymer application in the following video.

Wet Polymers

In these types of EAPs, actuation is caused by the movement of ions in the polymers. Some of the wet polymers include gels, conductive polymers, and ion exchange polymers. These polymers require a drive voltage, which is as low as 1–2 V. It is necessary for the wet polymers to maintain their wetness. However, it is difficult for the wet polymers to hold DC-sustained displacements.

In addition, another type of electroactive polymer has been prepared recently by dispersing conductive fillers in an elastomer, polydimethysiloxane and coating it with electrodes made of carbon nanotubes and carbon grease.  

Advantages and Disadvantages

The advantages and disadvantages of these two EAP types are discussed in the following section:

Advantages

Dry EAP

• Generates large actuation forces
• Functions at very high frequencies for a long time
• Requires no energy to maintain the actuator at a given position
• Response is generally rapid, about a few milliseconds

Wet EAP

• Requires low voltage.
• Supports hydrolysis at >1.23 V
• Provides large bending actuation and displacements

Disadvantages

Dry EAP

• Requires an adjustment between strain and stress
• Requires high voltage

Wet EAP

• Functions at low frequencies
• Cannot withstand strain under DC voltage
• Creates low actuation force upon bending
• Requires energy to maintain the actuator at a given position
• Response time is large

Sources and Further Reading

• Cohen. Y.B, Leary.S, JPL/Caltech, Pasadena, CA, National Space and Missile Materials Symposium, 2000.
• Dahiya. R.S, Metta.G, Valle.M, Lorenzelli.L, Adami.A, Piezo-Polymer-FET Devices Based Tactile Sensors for Humanoid Robots, Robotics, Brain and Cognitive Sciences Department, Italian Institute of Technology,Italy, Springer 2010, Volume 54:pp 369-372.
• Nicolau-Kuklinska, A. et al. “A new electroactive polymer based on carbon nanotubes and carbon grease as compliant electrodes for electroactive actuators,” Journal of Intelligent Material Systems and Structures vol. 29 (7):pp 1520-1530.

This article was updated on the 21^st September, 2018.