Biomedical Engineering Reference
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* junctions between the rods which are probably much less
flexible than gelatin loops or
coils. The non-helical content of agarose gels is probably very low, although this is not
discussed in the literature, and may be related to defects in the structure of the
polysaccharide (
(i.e. enthalpic) effects
will tend to contribute to the greater modulus of agarose gels even if the data is
replotted in terms of c/c 0 .
'
rogue residues
'
). Both of these
'
stiffening
'
For comparison, data for another helical type network, the gellan gel presented in
Chapter 5 , is shown in Figure 7.24 . The data shows the storage moduli derived from
Young
'
s modulus E with the simple assumption G= E/3 at room temperature, and after
24 h maturation (Milas and Rinaudo, 1996 ). E values were calculated from the linear
region of the stress
strain diagram, for gels cut into small cylinders. The counterions
from commercial samples were carefully replaced and the results shown in Figure 7.24
contain either K + or Na + counterions with M w = 2.5 × 10 5 g mol 1 . The authors also
explain the role of cooling history on gel properties. It is clear that under these conditions
very high shear moduli were observed in the gellan networks compared to the other
helical networks. Here the gellan concentration did not exceed 1.3 wt% but the moduli
are extremely high, K + counterions giving the highest values.
Most helical type networks show very high moduli, compared for instance to chemi-
cally cross-linked gels, and ionic effects give additional rigidity to the helical aggregates.
In the case of gelatin, if we choose the master curve ( Figure 7.15 ), then helix concen-
tration is the only parameter that controls the storage modulus of the gels. In this case the
gelatin plot is shifted to lower concentrations because the minimum concentration of
helices is about 0.3 wt%. For agarose and gellan gels, OR data suggests that the helix
fraction is very high, so the polymer concentration is very close to the helical concen-
tration. Again, however, whether the high modulus from the high helix fraction is due
solely to the same factors as for agarose or whether the effect of ions is simply to increase
the concentration of junction zones is currently too dif
-
cult to establish.
7.5
Conclusions
A gelatin network is made of portions of triple-helical rods of constant cross-section and
variable length, and along the chain contour there are more
flexible coiled sequences. The
conformational change from ordered to disordered (triple helix to coil) occurs coopera-
tively, at a speci
c temperature. Taking into account the complexity of gelatin samples,
a
speci
cannot be a single temperature. Such a gel is never at equilibrium, and
experimentally the growth in helix fraction continues inde
c temperature
nitely, as can be observed by
several techniques. There is only a small degree of thermal hysteresis between gel
formation and gel melting temperatures, even though this depends on many factors.
Overall, the parameter controlling the gel elasticity is the helix concentration.
By contrast, an agarose network is made of aggregated
fibrils of double helices, with
variable aggregation numbers and/or variable cross-section of
fibrils. There is no clear
indication of the cross-linking mechanism, but it is most likely to be bridging by
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