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274 Biomimetics: Biologically Inspired Technologies

Figure 10.4 IPMC in relaxed (left) and activated states (right).

10.3.2.2 Ionomeric Polymer–Metal Composites
Ionomeric polymer–metal composites (IPMC) is an EAP that bends in response to an electrical
activation (Figure 10.4) as a result of mobility of cations in the polymer network (Nemat-Nasser
and Thomas, 2004). In 1992, IPMC was realized to have this electroactive characteristic by three
groups of researchers: Oguro et al. (1992) in Japan, as well as Shahinpoor (1992) and Sadeghipour
et al. (1992) in the U.S. The operation as actuators is the reverse process of the charge storage
mechanism associated with fuel cells (Heitner-Wirguin, 1996; Holze and Ahn, 1992). A relatively
low voltage is required to stimulate bending in IPMC, where the base polymer provides channels
for mobility of positive ions in a fixed network of negative ions on interconnected clusters. Two
types of base polymers are widely used to form IPMC: Nafion 1 (perfluorosulfonate made by
DuPont, U.S.A.) and Flemion 1 (perfluorocarboxylate, made by Asahi Glass, Japan). In order to
chemically electrode the polymer films, metal ions (platinum, gold, or others) are dispersed
throughout the hydrophilic regions of the polymer surface and are subsequently reduced to the
corresponding zero-valence metal atoms.
Generally, the ionic content of the IPMC is an important factor in the electromechanical
response of these materials (Bar-Cohen et al., 1999; Nemat-Nasser and Li, 2000). Examining the
bending response shows that using low voltage (1 to 5 V) induces a large bending at frequencies
below 1 Hz, and the displacement significantly decreases with the increase in frequency. The
bending response of IPMC was enhanced using Liþ cations that are small and have higher mobility
or large tetra-n-butylammonium cations that transport water in a process that is still being studied.
The actuation displacement of IPMC was further increased using gold metallization as a result of
the higher electrode conductivity (Abe et al., 1998; Oguro et al., 1999).

10.3.2.3 Conductive Polymers

Conductive polymers (CP) typically function via the reversible counter-ion insertion and expulsion
that occurs during redox cycling (Otero et al., 1995; Sansin ˜ena and Olazabal, 2004). Oxidation and