Biomedical Engineering Reference
In-Depth Information
Stress vs. strain curve: PAN muscle (50 strands)
0.9
Contracted PAN muscle
Elongated PAN muscle
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Strain
Contracted muscle: Length under 0 load = 6.9 cm; maximum load applied = 650 gm (did not break); max length = 24.2 cm.
Elongated muscle: Length under 0 load = 15.5 cm; maximum load applied = 200 gm (broke under load); max length = 20.3 cm.
Estimated cross-sectional area of one strand = 1.57
×
10^-7 meters squared; for 50 strands = 7.85
×
10^-6 meters squared.
FIGURE 4.65
Normal stress-strain relationship for the contracted and the expanded state of
PAN muscles.
C
C
C
C
N
O
N
OH
O
O
N
OH
OH
O
OH
N
O
OH
C
N
FIGURE 4.66
Possible structure of activated polyacrylonitrile.
The activation of PAN requires inducing cross-linking by the formation of
pyridine rings by low-temperature annealing and, subsequently, converting nitrile
groups to carboxylic acid groups by saponification with sodium or lithium hydroxide.
The degree of cross-linking depends on annealing temperature and time, which in
turn determines the amount of free nitrile groups left to be converted to carboxylic
acids during saponification.
A possible structure for activated PAN is given in figure 4.66,
as discussed by
Umemoto et al. (1991) and Hu (1996). In figure 4.67, SEM micrographs show raw
fibers as well as activated fibers. Typically, one strand is composed of about 2,000
fibers. Each fiber has an approximate diameter of 9
m.
The strength of activated PAN and its ability to change length of up to 100% or
more makes it an appealing material for use as linear actuators and artificial muscles.
An attractive alternative is electrical activation. During the electrolysis of water, hydro-
gen ions are generated at the anode while hydroxyl ions are formed at the cathode in
an electochemical cell. Electrochemical reactions can then potentially be used to
control the length of a PAN artificial muscle. This may be achieved by locating a PAN
µ
