Biomedical Engineering Reference
In-Depth Information
2.0
L1 5% w/w
L2 2% w/w
H 2% w/w
1. 5
1. 0
0.5
0.0
0
20
40
60
80
100
Sucrose (wt%)
Dependence of the failure strain on sucrose concentration for various molecular masses
(11.6 × 10 4 g mol 1 for sample L1, to 16.2 × 10 4 g mol 1 for sample L2 and 16.2 × 10 4 g mol 1 for
sample H) and two concentrations. Adapted with permission from Normand et al.( 2003 ) © 2003
Elsevier.
Figure 7.23
of helical aggregates, increases the number of cross-links and decreases the length of
the helices (Watase et al., 1990 ;Nishinariet al., 1992a , 1992b ).
Agarose helices are known to be stabilized by hydrogen bonds with water molecules,
some occupying the inner part of the helix (Foord and Atkins, 1989 ) as in collagen
(although we note that their model for agarose was single helical). The reduction of
available water molecules decreases their contribution to the stabilization of the helices,
which explains the decrease in the coil
helix conversion degree. While the aggregation
number of the double helices decreases, the gels can be deformed to a much higher
extent, an effect ascribed to increased internal
-
flexibility of the ordered chains, possibly
as a consequence of the decrease in cross-sectional thickness.
The effect of sucrose on the strain at failure is very important: increasing the sucrose
concentration leads to a transition from brittle gels (about 40% strain at rupture) to highly
deformable gels (about 200%), somewhat unusually for polysaccharide gels; see
Figure 7.23 . As the sucrose concentration increases, the strain at failure increases as
well, although mainly above 60 wt% sucrose when a larger proportion of coils are in the
non-helical conformation.
7.4
Comparison between helical type networks
To conclude, it is interesting to compare the small-strain limit shear moduli of gelatin and
agarose, both of which, in aqueous solution, can be considered as
fibrillar type networks
constructed of helices. Such a comparison was made by Clark et al.( 1983 ), and is
reported in Figure 7.24 over a wide concentration range, up to 25 wt% in the case of
gelatin. The plot highlights the fact that agarose gels have a higher modulus than gelatin
at low concentrations. For instance, at c
2 wt% the modulus is about 40 Pa for gelatin
and 35 kPa for agarose.
As has been pointed out in Chapter 3 , the shapes of modulus versus concentration
curves are very similar but, as can be judged from Figure 7.24 , they differ in both
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