Biomedical Engineering Reference
In-Depth Information
field, these ionic polymeric networks undergo substantial contraction accompanied
by exudation of the liquid phase contained within the network.
Under these circumstances, there are generally four competing forces acting on
such ionic networks: rubber elasticity, polymer-liquid viscous interactions due to
the motion of the liquid phase, inertial effects due to the motion of the liquid through
the ionic network, and electrophoretic interactions. These forces collectively give
rise to dynamic osmotic pressure and network deformation and subsequently deter-
mine the dynamic equilibrium of such charged networks.
On the other hand, there are situations in which a strip of such ionic polymeric
gels undergoes bending in the presence of a transverse electric field with hardly any
water exudation. Under these circumstances, there are generally three competing
forces acting on the gel polymer network: rubber elasticity, polymer-polymer affin-
ity, and the ion pressure. These forces collectively create the osmotic pressure, which
determines the equilibrium state of the gel. The competition between these forces
changes the osmotic pressure and produces the volume change or deformation.
Rubber elasticity tends to shrink the gel under tension and expand it under compres-
sion. Polymer-polymer affinity depends on the electrical attraction between the
polymer and the solvent. Ion pressure is the force exerted by the motion of the
cations or anions within the gel network. Ions enter the gel attracted by the opposite
charges on the polymer chain while their random motions tend to expand the gel
like an ionic (fermionic) gas.
Kuhn et al. (1948) originally reported on the possibility that certain copolymers
can be chemically contracted or swollen like a synthetic muscle (pH muscle) by
changing the pH of the solution containing them. As originally reported by Kuhn et
al. (1950), a three-dimensional network consisting of polyacrylic acid can be
obtained by heating a foil of polyacrylic acid containing a polyvalent alcohol such
as glycerol or polyvinyl alcohol. The resulting three-dimensional networks are insol-
uble in water but swell enormously in water on addition of alkali and contract
enormously on addition of acids. Reversible dilations and contractions of the order
of more than 1000% have been observed for ionic gel muscles made with polyacry-
lonitrile (PAN) fibers (Shahinpoor and Mojarrad, 1996). Chemically stimulated
pseudomuscular actuation has also been discussed recently by Li and Tanaka (1989),
De Rossi et al. (1986), and Caldwell and Taylor (1990). Hamlen et al. (1965) were
the first to report that contraction and swelling of these gels can also be obtained
electrically by placing fibrous samples of PAA-PVA in an electric field.
One of the earlier mathematical modelings of deformation of ionic gels in an
electric field was presented by Tanaka and coworkers at MIT as early as 1978,
proposing a phase transformation phenomenon responsible for such electrically
induced contraction. Other experimental and theoretical investigations addressing
the electrically induced contractile behavior of ionic polymeric gels have been
presented by Osada and Hasebe (1985), De Rossi et al. (1986), Osada and Kishi
(1989), Grodzinsky and Melcher (1974), Grimshaw, Nussbaum, Grodzinsky, et al.
(1990), Shiga and Kurauchi (1990), and Kishi et al. (1990). Applications of electri-
cally activated ionic polymeric gel muscles to swimming robotic structures have
been discussed by Shahinpoor.
