Biomedical Engineering Reference
In-Depth Information
1.2.6
L
C
E
(LCE) M
IQUID
RYSTAL
LASTOMER
ATERIALS
Liquid crystal elastomers were pioneered at Albert-Ludwig Universitat in Freiburg,
Germany, (Finkelmann et al., 1981). These materials can be used to form an EAP
actuator by inducing isotropic-nematic phase transition due to temperature increase
via joule heating. LCEs are composite materials that consist of monodomain nematic
liquid crystal elastomers and conductive polymers that are distributed within their
network structure (Shahinpoor, 2000d; Finkelmann and Shahinpoor, 2002; Ratna et
al., 2002). The actuation mechanism of these materials involves phase transition
between nematic and isotropic phases over a period of less than a second. The reverse
process is slower, taking about 10 sec, and it requires cooling to cause expansion
of the elastomer to its original length. The mechanical properties of LCE materials
can be controlled and optimized by effective selection of the liquid crystalline phase,
density of cross-linking, flexibility of the polymer backbone, coupling between the
backbone and liquid crystal group, and coupling between the liquid crystal group
and the external stimuli.
1.2.7
I
EAP
/I
P
G
(IPG
)
ONIC
S
ONIC
OLYMER
ELS
S
Polymer gels can be synthesized to produce strong actuators with the potential of
matching the force and energy density of biological muscles. These materials (e.g.,
polyacrylonitrile [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
Shahinpoor and Mojarrad (1994, 1996, 1997a, 1997b, 1997c, 1997d, 2000).
Current efforts are directed toward the development of thin layers and more
robust electroding techniques. Progress was recently reported by researchers using
a mix of conductive and PAN fibers at the University of New Mexico (Schreyer et
al., 2000). The mechanism responsible for the chemomechanical behavior of ionic
gels under electrical excitation is described by Osada and Ross-Murphy (1993) and
a model for hydrogel behavior as a contractile EAP is described in Gong et al.
(1994). A significant amount of research and development has been conducted at
the Hokkaido University, Japan, and applications using ionic gel polymers have been
explored. These include electrically induced bending of gels (Osada and Hasebe,
1985; Osada et al., 1992) and electrically induced reversible volume change of gel
particles (Osada and Kishi, 1989).
1.2.8
N
P
G
/EAP
ONIONIC
OLYMER
ELS
S
Nonionic polymer gels containing a dielectric solvent can be made to swell under a DC
electric field with a significant strain. Hirai and his coworkers (1995, 1999) at Shinshu
University in Japan have created bending and crawling nonionic EAPs using a
poly(vinyl alcohol) gel with dimethyl sulfoxide. A 10-
2-mm actuator gel was
subjected to an electrical field and exhibited bending at angles greater than 90
×
3-
×
at a
speed of 60 msec. This phenomenon is attributed to charge injection into the gel and a
flow of solvated charges that induce an asymmetric pressure distribution in the gel.
Another nonionic gel is poly(vinyl chloride) (PVC), which is generally inactive when
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