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Bar-Cohen : Biomimetics: Biologically Inspired Technologies DK3163_c010 Final Proof page 273 21.9.2005 11:46am

Artificial Muscles Using EAP 273

et al., 2004). This electrostrictive polymer consists of two components, a flexible backbone
macromolecule and a grafted polymer that can form a crystalline structure. The grafted crystalline
polar phase provides moieties in response to an applied electric field and cross-linking sites
for the elastomer system. This material offers high electric-field-induced strain (~4%), relatively
high electromechanical power density, and excellent processability. Combination of the elec-
trostrictive-grafted elastomer with a piezoelectric poly(vinylidene fluoride-trifluoroethylene)
copolymer yields several compositions of a ferroelectric–electrostrictive molecular composite
system. Such a combination can be operated both as a piezoelectric sensor and as an electro-
strictive actuator. Careful selection of the composition allows the creation and optimization of the
molecular composite system with respect to its electrical, mechanical, and electromechanical
properties.

10.3.1.4 Electrostrictive Paper

The use of paper as an electrostrictive EAP actuator was demonstrated at Inha University, Korea
(Kim et al., 2000). Paper is composed of a multitude of discrete particles, mainly of a fibrous nature,
which form a network structure. Since paper is produced in various mechanical processes with
chemical additives, it is possible to prepare a paper that has enhanced electroactive properties. Such
an EAP actuator has been prepared by bonding two silver laminated papers with silver electrodes
placed on the ouside surface. When an electric voltage is applied to the electrodes the actuator
produces bending displacement, and its performance depends on the excitation voltages, frequen-
cies, type of adhesive, and the host paper. Studies indicate that the electrostriction effect that
is associated with this actuator is the result of electrostatic forces and an intermolecular interaction
of the adhesive. The demonstrated actuator is lightweight and simple to fabricate. Various appli-
cations that are currently being considered include active sound absorbing materials, flexible
speakers, and smart shape-control devices. The energy density of these materials and electromech-
anical coupling is relatively small.

10.3.2 Ionic EAP

Ionic EAP materials consist of a polymer with electrolyte and two electrodes where the phenom-
enon of electroactivation involves diffusion of ions through the thickness of the polymer (Bar-
Cohen, 2004). This group includes of the following types of EAP materials:

10.3.2.1 Ionic Polymer Gels

Polymer gels can be synthesized to produce significant actuation forces potentially matching
the force and energy density of biological muscles (Calvert, 2004). These materials (e.g., poly-
acrylonitrile) are generally activated by a chemical reaction, changing from an acidic to an
alkaline environment causing the gel to become dense or swollen, respectively. This reaction can
be stimulated electrically as was shown by researchers at the University of Arizona, U.S.A. (Liu
and Calvert, 2000). When activated, these gels bend as the cathode side becomes more alkaline and
the anode side more acidic. However, the response of this multilayered gel structure is relatively
slow because of the need to diffuse ions through the gel. A significant amount of research and
development efforts and application considerations using ionic gel polymers (IPG) were explored
at the Hokkaido University, Japan (Osada and Kishi, 1989). The polymers that were explored
include electrically induced bending of gels and electrical induced reversible volume change of
gel particles. Further, Schreyer and his coinvestigators at the University of New Mexico,
Albuquerque used a combination of ionic gel and conductive polymer electrodes to demonstrate
an effective EAP actuator (Schreyer et al., 2000; Section 10.6).