Biomedical Engineering Reference
In-Depth Information
achieve active PAN muscles, first, fibers are cut to a desired length and number of
fibers from a spool of Orlon materials. Additional length is added to allow for
shrinkage due to annealing, end connections, and bundling. The fibers are carefully
placed in a Pyrex or similar nonsticking container so as not to contaminate the
fibers with any types of oil or corrosive contaminant and placed in a laboratory
oven to be annealed and oxidized for 6 h at about 200
C. It is best to use a convection
oven to circulate the chamber air to maintain uniform temperature throughout the
fibers. The time of annealing depends on the oven type and density of the fibers'
and filaments' diameter; it can vary from 200 to 240
°
C for 6-2 h, respectively. At
this point, the fiber color changes to dark brown to black, indicating oxidation of
the polymer chain. Appearance of lighter color fibers indicates an incomplete
annealing process.
The fibers then are bundled at both ends via a nylon line and epoxy adhesive to
secure the ends and for later attachment to any structure to be moved or manipulated.
Depending on the application and the types of muscles needed, the end connection
can be varied. For example, in the case of encapsulated fiber bundles, we used rubber
stoppers with through holes to allow for injection of pH solutions (weak acids and
alkalides) and pure water into and out of the muscle (figs. 4.1 and 4.2). The rubber
stoppers also provided a means of securing fibers by adhesives and nylon wrapping
lines. Each was carefully constructed for a specific application. We then hydrolyzed
by boiling the resulting bundle in a 1-
°
N
solution of NaOH for 30 min to complete
the gelation and activation of the fibers.
At this point, the bundle appears at its elongated or expanded state because of
the effect of the OH ions on the active ionic polymer chain. After letting the muscle
cool to room temperature, the fiber bundles are thoroughly rinsed of any residual
alkali solution and further expansion is observed because of the increased availability
of water molecules to penetrate within gel-like fibers. (Note that PAN gel is a
hydrophilic material like most ionic gels.) The PAN muscle is now ready to be stored
in pure water (or slightly acidic solution since any alkalinity for a long duration will
eventually degrade the PAN muscle) for any number of future experiments. In this
way, a variety of different types of PAN muscles can be manufactured, as shown in
figure 4.1, that can be attached to other structures for robotic manipulation. Thus,
an optimum manufacturing procedure turned out to be
1.
oxidation at 210
°
C for 75 min (annealed and cross-linked-PAN)
2.
hydrolysis with 1
N
NaOH at 95
°
C for 30 min
causes contraction of the bundle. The
muscle can be infinitely contracted and expanded using weak acids and bases with a
rinse cycle in between. Typical contraction of 100-200% from the original length is
commonly achieved. Depending on the packaging of the fibers, one could improve the
response time by further segmentation of the fibers. This in effect divides the total
length of each fiber into smaller pieces, allowing rapid diffusion of pH fluids along
and within each segment of the fiber. In comparison to a long fiber, where fluid will
take a longer time to reach the entire fiber, this method proved useful in fabricating
fast-response muscles similar to biological muscle fibers with each filament comprising
Addition of weak acids such as HCl 1
M
