Biomedical Engineering Reference
In-Depth Information
subjected to electric fields. However, if PVC is plasticized with dioctyl phthalate (DOP),
a typical plasticizer, it can maintain its shape and behave as an elastic nonionic gel.
1.2.9
I
P
-M
C
(IPMNC
)
ONIC
OLYMER
ETAL
OMPOSITES
S
An ionic polymer-metal composite is an EAP that bends in response to a small
electrical field (5-10 V/mm) as a result of mobility of cations in the polymer network.
In 1992, the IPMNC was realized based on a chemical-plating technique developed
by Merlet, Pinneri, and coworkers in France and Kawami and Takanake in Japan in
the 1980s. The first working actuators were built by Oguro and colleagues (1992),
in Japan, and Shahinpoor (1992) and Mojarrad (2001) in the United States. The first
working sensors of this kind were fabricated in the United States by Shahinpoor
(1992) and Sadeghipour et al. (1992).
The operation as actuators is the reverse process of the charge storage mechanism
associated with fuel cells (Heitner-Wirguin, 1996; Kim et al., 1998, 2000). A rela-
tively low electric field is required (five orders of magnitude smaller than the fields
required for PVDF-TrFE and dielectric elastomers to stimulate bending in IPMNCs,
where the base polymer provides channels for mobility of positive ions in a fixed
network of negative ions on interconnected clusters. In order to electrode the polymer
films chemically, metal ions (platinum, gold, palladium, or others) are dispersed
throughout the hydrophilic regions of the polymer surface and are subsequently
reduced to the corresponding zero-valence metal atoms. These methodologies and
characteristics will be fully explained in this chapter.
1.2.10
C
P
(CP
)
S
M
ONDUCTIVE
OLYMERS
S
OR
YNTHETIC
ETALS
Conductive polymers operate under an electric field by the reversible counter-ion
insertion and expulsion that occurs during REDOX cycling (Otero et al., 1995;
Gandhi et al., 1995). Oxidation and reduction occur at the electrodes, inducing a
considerable volume change due mainly to the exchange of ions with an electrolyte.
When a voltage is applied between the electrodes, oxidation occurs at the anode and
reduction at the cathode. The presence of a liquid electrolyte containing conjugated
ions or a solid polyelectrolyte medium in close proximity to conductive polymers,
such as polypyrrole (pPy), is often necessary to cause charge migration into and out
of the conductive polymer.
Ions (H+) migrate between the electrolyte and the electrodes to balance the
electric charge. Addition of the ions causes swelling of the polymer and, conversely,
their removal results in shrinkage. As a result, the sandwich assembly bends. Con-
ductive polymer actuators generally require small electric fields in the range of 1-5
V/
m; the speed increases, with the voltage having relatively high mechanical energy
densities of over 20 J/cm
ยต
, but with low efficiencies at the level of 1% or less. In
recent years, several conductive polymers have been reported, including polypyrrole,
polyethylenedioxythiophene, poly(p-phenylene vinylene)s, polyanilines, and poly-
thiophenes. Operation of conductive polymers as actuators at the single-molecule
level is currently being studied, taking advantage of the intrinsic electroactive prop-
erty of individual polymer chains.
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