Biomedical Engineering Reference
In-Depth Information
1.
solution tendency of polymeric molecules and osmotic pressure of the
cations of the alkali bond by the gel
2.
rubber-like contraction tendency of the stretched polymer molecules
They observed that swelling capacity of the gel decreased with increasing
degrees of cross-linking of the polymer. The degree of ionization depended on charge
distribution of the polymer chain. For about 50% neutralization of these charge
groups, maximum swelling could be achieved.
Later Hamlen and coworkers (1965), three research scientists at General Electric
Company, were able to stimulate copolymer of PVA-PAA (polyvinyl alcohol-poly-
acrylic acid) electrically by making the polymer conductive with chemical treatment
by solution of platinic chloride and sodium borohydride. Thus, they were able to
actuate it by electricity instead of pH variation of surrounding liquid environment.
Although the resulting ionic polymer was slow in response relative to its pH-activated
counterpart, it nevertheless proved the possibility of electrical stimulation of these
polymers, making it attractive for robotics controls and manipulation. Because
PVA-PAA copolymer is a negatively charged gel (due to the COOH
-
side group in
PAA), it swells or shrinks osmotically depending on total ionic concentration inside
the polymer; this is determined by the degree of ionization of the weak carboxylic
acid group (COOH
-
).
When the external environment (electrolyte solution) is acidic, degree of dissoci-
ation is low and the polymer shrinks; in alkaline solution, it expands similarly to the
pH muscles mentioned earlier. In order to produce the same effect electrolytically, a
conductor such as platinum can be included in the polymer and made to have low
overvoltage for the evolution of hydrogen and oxygen. This class of ionic polymers
is called electrically activated ionic polymer gels (or simply electroactive muscles). In
these researchers' experiment, PVA-PAA fibers were treated eight times in platinum
solution and submerged in 0.01-
N
solution of NaCl (1% concentration). A counter-
electrode of platinum wire was used and placed in a container holding the muscle and
salt solution. A square wave of
5 V amplitude at 40 mA and a 20-min period (
f
=
0.0008 Hz) were then applied. When fiber is negative, hydrogen evolves, causing the
solution to become alkaline, and the fiber expands. When the fiber is positive, the
solution surrounding and within fibers becomes acidic, causing its contraction.
Fragala and colleagues (1972) of the GE-Direct Energy Conversion Program
experimented with weak acidic contractile polymeric membranes in a setup that
forced a change in pH of the solution surrounding the membrane by electrodialysis
process. They developed mathematical formulations that adequately described the
response of the artificial muscle in relation to applied field current. They concluded
that a dissociation constant in excess of 10
-3
g-eq/L for weak acidic groups within
the material was needed to obtain large deformation in the muscle membrane. Their
apparatus consisted of a three-compartment container with weak acidic polymeric
muscle membrane immersed in a mixture of weak salt and acid solution in the middle
compartment. This was separated by two weak cationic and anionic ion-exchange
membranes, respectively, to make the other compartments fill with the same con-
centration of weak salt and acids as the muscle membrane.
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