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                    Artificial Muscles Using EAP                                                 269

                    were discovered over half-a-century ago when collagen filaments were demonstrated to revers-
                    ibly contract or expand upon dipping into acidic or alkaline aqueous solutions, respectively
                    (Katchalsky, 1949). Even though relatively little has since been done to apply such ‘‘chemo-
                    mechanical’’ as practical actuators, this early work pioneered the development of synthetic poly-
                    mers that mimic biological muscles (Steinberg et al., 1966). The convenience and practicality
                    of electrical stimulation, and technology progress led to a growing interest in EAP materials.
                    Following the 1969 observation of a substantial piezoelectric activity in polyvinylidene fluoride
                    (PVDF) (Bar-Cohen et al., 1996; Zhang et al., 1998), investigators started to examine other
                    polymer systems, and a series of effective materials have emerged. The largest progress in EAP
                    materials development has been reported in the last 15 years (Bar-Cohen, 2001, 2004) where
                    materials that can create linear strains that can reach up to 380% have been developed (Pelrine
                    et al., 2000; Kornbluh et al., 2004).
                      Generally, EAP can be divided into two major groups based on their activation mechanism:
                    ionic (involving mobility or diffusion of ions) and electronic (driven by electric field or Maxwell
                    Forces) (Bar-Cohen, 2001, 2004). A summary of the advantaged and disadvantages of these two
                    group of materials are listed in Table 10.1. The electronic polymers (electrostrictive, electrostatic,
                    piezoelectric, and ferroelectric) are driven by Maxwell Forces and can be made to hold the induced
                    displacement under activation of a DC voltage, allowing them to be considered for robotic
                    applications. Also, these materials have a greater mechanical energy density and they can
                    be operated in air with no major constraints. However, they require a high activation field
                    (>100-V/mm) close to the breakdown level. In contrast, ionic EAP materials (gels, polymer–
                    metal composites, conductive polymers, and carbon nanotubes) are driven by diffusion of ions
                    and they require an electrolyte for the actuation mechanism. Their major advantage is the




                    TABLE 10.1 A Summary of the Advantages and Disadvantages of the Two Basic EAP Groups
                    EAP type                  Advantages                       Disadvantages
                    Electronic EAP  .  Can operate for a long time in room  .  Requires high voltages (~100 MV/m).
                                       conditions                       Recent development allowed for (~20
                                    .  Exhibits rapid response (milliseconds)  MV/m) in the Ferroelectric EAP
                                    .  Can hold strain under DC activation  .  Independent of the voltage polarity, it
                                    .  Induces relatively large actuation  produces mostly monopolar actuation
                                       forces                           due to associated electrostriction
                                    .  Exhibits high mechanical energy  effect
                                       density
                    Ionic EAP       .  Natural bi-directional actuation that  .  Requires using an electrolyte
                                       depends on the voltage polarity  .  Requires encapsulation or protective
                                    .  Requires low voltage             layer in order to operate in open air
                                    .  Some ionic EAP like conducting   conditions
                                       polymers have a unique capability of  .  Low electromechanical coupling
                                       bi-stability                     efficiency
                                                                      .  Electrolysis occurs in aqueous
                                                                        systems at more than 1.23 V
                                                                      .  Except for CPs and NTs, ionic EAPs
                                                                        do not hold strain under DC voltage
                                                                      .  Slow response (fraction of a second)
                                                                      .  Bending EAPs induce a relatively low
                                                                        actuation force
                                                                      .  Except for CPs, it is difficult to produce
                                                                        a consistent material (particularly
                                                                        IPMC)
                                                                      .  High currents require rare earth
                                                                        electrodes such as gold or platinum
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