Muscular Biopolymers - Engineered Biomimicry - page 143

soft phase [8] . The positive and negative
charges within the material are permanently
displaced along and against the direction of
the field, respectively, making a polarized
material with a net zero charge.
4. Dielectric elastomer EAPs . Polymers with low
elastic stiffness modulus and high dielectric
constant can be packaged with flexible and
stretchable electrodes to generate large
actuation strain by electrostatic attraction
between the stretchable electrodes, like a
parallel plate capacitor [9] . However,
Roentgen [10] appears to have been the first
to discover this effect as early as 1880 by
observing the stretching of a rubber band
that could change its shape by being
charged or discharged electrostatically.
5. Liquid crystal elastomer (LCE) materials .
Liquid crystal elastomers as artificial
muscles were pioneered by Finkelmann and
coworkers [11, 12]. These materials can be
used to form an EAP actuator by inducing
isotropic-nematic phase transition due to
temperature increase via Joule heating.
6. Ionic EAP/ionic polymer gels (IPG) . Polymer
gels can be synthesized to produce strong
actuators with the potential to match the
force and energy density of biological
muscles. These materials (e.g., polyacryloni-
trile, PAN) are generally activated by a
chemical reaction, changing from an acid to
an alkaline environment and causing the gel
to become dense or swollen, respectively.
This reaction can be stimulated electrically,
as was shown by Osada et al. [13] , Osada
and Ross-Murphy [14] , Osada et al. [15] ,
and Osada and Matsuda [16] .
7. Nonionic polymer gels/EAPs . Nonionic
polymer gels containing a dielectric solvent
can be made to swell under a DC electric
field with a significant strain. Hirai and
coworkers at Shinshu University in Japan
created bending and crawling nonionic
EAPs using a poly(vinyl alcohol) gel with
dimethyl sulfoxide [17, 18].

At that point, all motor units will be firing at
their maximum frequencies. For a detailed
discussion of mammalian muscles, the reader
is refered to Bobet and Stein [1] and Ding et al.
[2, 3].
In the following section we present a brief
review of electroactive polymers (EAP) as artifi-
cial muscles, in general.

6.1.4 Electroactive Polymers and
Artificial/Synthetic Muscles

For a recent history on EAPs, the reader is
referred to Bar-Cohen [4] and Shahinpoor
et al. [19] . Some specific EAPs are as follows:

1. Magnetically activated polymers . Magnetically
activated gels, also called ferro-gels , are
chemically cross-linked polymer networks
that change shape in the presence of a
magnetic field [5] .
2. Electronic EAP/ferroelectric relaxer polymers .
Zhang et al. [6] have introduced defects into
the crystalline structure of Poly(vinylidene
fluoride) (also known as PVDF) using
electron irradiation to dramatically reduce
the dielectric loss in a PVDF Tri Fluoroeth-
ylene, or P(VDF-TrFE), copolymer. This
copolymerization apparently permits AC
switching with much less heat generated. It
is the electric-field-induced change between
nonpolar and polar regions that is responsi-
ble 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/ μ m.
3. Electrets . Electrets, which were discovered in
1925, are materials that retain their electric
polarization after being subjected to a strong
electric field [7] . Piezoelectric behavior in
polymers also appears in electrets, which
are essentially materials that consist of a
geometrical combination of a hard and a