Biomedical Engineering Reference
In-Depth Information
FIGURE 4.62
Exploded view of the electric PAN muscle apparatus.
combinations of chemically doped polyacrylic acid plus polyvinyl alcohol (PAA-PVA)
can be easily observed in our laboratory. Such deformation gives rise to an internal
molecular network structure with bound ions (polyions) and unbound or mobile ions
(counter-ions) when submerged in an electrolytic liquid phase.
In the presence of an electric 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: the rubber elasticity, the 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 the electrophoretic inter-
actions. These forces collectively give rise to dynamic osmotic pressure and net-
work deformation and subsequently determine 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: the rubber elasticity, the polymer-polymer
affinity, 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. Poly-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.
