Biomedical Engineering Reference
In-Depth Information
presence of hydroxyl ions (OH
-
) causes the polymeric network to elongate the fibers.
An assortment of such PAN muscles is shown in figure 4.1.
PAN has been studied for more than a half century in many institutions and
now is popularly used in the textile industry as a fiber form of artificial silk. The
useful properties of PAN include the insolubility, thermal stability, and resistance
to swelling in most organic solvents. Such properties were thought to be due to the
cross-linked nature of the polymer structure. However, a recent finding has shown
that a number of strong polar solvents can dissolve PAN and thus has raised a
question that its structure could be of the linear zigzag conformations
(Umemoto
et al., 1991; Hu, 1996) with hydrogen bonding between hydrogen and the neigh-
boring nitrogen of the nitrile group. Yet, the exact molecular structure of PAN is
still somewhat unclear. (We expect that it can be atactic with possibly amorphous
phases in pure state.)
Also, it should be noted that, as received, PAN fibers are a somewhat different
formulation from that of the pure PAN that is usually in a powder state. Generally,
the PAN fibers used in the textile industry are often copolymerized with acrylamide
(approximately 4-6% by weight) and manufactured through a spinning process.
Therefore, changes in material properties of such PAN fibers are expected in a sense
that it can be reorganized into a semicrystalline structure. Therefore, improved
strength of the PAN fibers is a definite advantage over other types of ionic gels that
are usually very weak.
Activated PAN fibers are highly elastic or hyperelastic like rubber bands. The
term “activated PAN fibers” refers to the PAN fibers acting as the elastic fibers of
which length varies depending upon the ionic concentration of cations in the solution.
They contract at low cationic concentration and expand at high cationic concentra-
tion. At high cationic concentration, they appear to reject water out of the polymer
network, resulting in shrinkage due to their elastic nature. Conversely, they elongate
at low cationic concentration, attracting water from outside into the polymer network.
In our laboratory, it has been observed that activated PAN fibers can contract more
than 200% relative to that of the expanded PAN. The use of PAN as artificial muscles
is promising since they are able to convert chemical energy to mechanical motion,
possibly acting as artificial sarcomeres or muscles.
Other types of materials that have a capability to be electroactive are polyelec-
trolyte gels such as polyacrylamide (PAM), polyacrylic acid-polyvinyl alcohol
(PAA-PVA), and poly (2-acrylamido-2-methyl propane) sulphonic acid (PAMPS).
Under an electric field, these gels are able to swell and de-swell, inducing large
changes in the gel volume. Such changes in volume can then be converted to
mechanical work. Artificial sarcomeres or muscles made from PAN, unlike poly-
electrolyte gels, have much greater mechanical strength than polyelectrolyte gels,
thus having a greater potential for application as artificial sarcomeres and muscles
(or soft actuators). Figure 4.65 shows mechanical strength of the activated PAN
bundle in a contracted state and an elongated state. The different mechanical behavior
of the ionically active PAN fibers clearly establishes the fundamental difference
between the network structures in the presence of H
+
ions or protons as compared
to the presence of hydroxyl OH
-
ions.
