Biomedical Engineering Reference
In-Depth Information
phenomenon of piezoelectricity, which is found only in noncentrosymmetric mate-
rials such as poly(vinylidene fluoride) (also known as PVDF). These polymers are
partly crystalline, with an inactive amorphous phase, and have a Young's modulus
of about 1-10 GPa. This relatively high elastic modulus offers high mechanical
energy density. A large applied AC field (-200 MV/m) can induce electrostrictive
(nonlinear) strains greater than 1%. Sen et al. (1984) investigated the effect of mixing
heavy plasticizers (-65 wt%) of ferroelectric polymers hoping to achieve large strains
at reasonable applied fields. However, the plasticizer is also amorphous and inactive,
resulting in decreased Young's modulus, permittivity, and electrostrictive strains.
Recently, Zhang et al. (1998) introduced defects into the crystalline structure
using electron irradiation to dramatically reduce the dielectric loss in a PVDF tri
fluoroethylene or P(VDF-TrFE) copolymer. This copolymerization apparently per-
mits AC switching with much less generated heat. The electric-field-induced change
between nonpolar and polar regions is responsible for the large electrostriction
observed in this polymer. As large as 4% electrostrictive strains can be achieved
with low-frequency driving fields having amplitudes of about 150 V/
m.
As with ceramic ferroelectrics, electrostriction can be considered as the origin of
piezoelectricity in ferroelectric polymers (Furukawa and Seo, 1990). Unlike electro-
striction, piezoelectricity is a linear effect, where the material will be strained when
voltage is applied and a voltage signal will be induced when a stress is applied. Thus,
they can be used as sensors, transducers, and actuators. Depoling due to excessive
loading, heating, cooling, or electric driving is a problem with these materials.
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1.2.4
E
LECTRETS
Electrets, which were discovered in 1925, are materials that retain their electric
polarization after being subjected to a strong electric field (Eguchi, 1925). Piezo-
electric behavior in polymers also appears in electrets, which are essentially materials
that consist of a geometrical combination of a hard and a soft phase (Sessler and
Hillenbrand, 1999). The positive and negative charges within the material are per-
manently displaced along and against the direction of the field, respectively, making
a polarized material with a net zero charge.
1.2.5
D
E
EAP
IELECTRIC
LASTOMER
S
Polymers with a low elastic stiffness modulus and a high dielectric constant can be
packaged with interdigitated electrodes to generate large actuation strains by sub-
jecting them to an electric field. This dielectric elastomer EAP can be represented
by a parallel plate capacitor (Perline et al., 1998). The induced strain is proportional
to the square of the electric field, multiplied by the dielectric constant, and is inversely
proportional to the elastic modulus. Dielectric elastomer EAP actuators require large
electric fields (100 V/
m) and can induce significant levels of strain (10-200%).
Recently, Perline and colleagues (2000) introduced a new class of polymers that
exhibits an extremely high strain response. These acrylic-based elastomers have
produced large strains of more than 200%, but suffer from the fact that they require
gigantic electric fields in the range of hundreds of megavolts per meter.
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