Biomedical Engineering Reference
In-Depth Information
10
5
10
4
c
=
4.5-22
wt % - cooling
c
=
4.5-22
wt % - heating
c
=18-22.5
wt % - maturation
10
3
10
2
Percolation regime
10
1
Gelation threshold 0.03
g cm
−
3
10
0
10
−
1
0.00
0.04
0.08
0.12
0.16
0.20
c
helix
(g cm
−
3
)
Figure 7.14
Correlation between storage modulus and helix concentration during various thermal treatments
(bovine bone gelatin, M
w
= 157 000 g mol
−
1
). Adapted from El Harfaoui et al.(
2007
) with
permission from John Wiley & Sons.
more concentrated solutions, at equal values of c
helix
, G
00
is higher, presumably because of a
larger proportion of coils and free chains, which contribute to the viscous dissipation. Also,
when gelatin concentration is higher, c
helix
can reach larger values and accordingly the
storage moduli reach larger values, as shown on the plot. The threshold or critical concen-
tration of helices is identical for the three gelatin concentrations, at c
critical
0.003 g cm
−
3
.
helix
Thermal treatment, molecular mass, solvent effects
Helix formation is reversible, but with some hysteresis between cooling and heating.
Does the correlation between storage modulus and helix concentration apply to different
stages of gel formation and melting?
Figure 7.14
shows the correlation between storage modulus and helix concentration
during cooling, annealing and heating steps at gelatin concentrations between 4.5% and
22% w/v. The superposition of the data is very good for all measurements of the
mammalian gelatin. The plot is based on frequency-independent storage moduli, which
occur later in the course of gelation for higher concentrations, after which all the moduli
follow the same general trend, whether helix concentration is derived from microcalori-
metric measurements or from optical rotation. Various extracts of this type of gelatin were
also reported by El Harfaoui (
2007
) with various average molecular masses and distri-
butions. It was shown that the correlation is also valid in this case. The correlation is also
valid when the solvent is changed from water to mixtures of water and glycerol, as is
often the case in pharmaceutical applications (Joly-Duhamel et al.,
2002a
).
Source of gelatin
Collagen composition re
ects the biodiversity of gelatin samples from various species.
The variability in imino acid content has a direct effect on gelation temperatures,
although the fundamental assembly remains the triple helix. This is an additional
parameter which can be tested against data for G
0
. The result is shown in
Figure 7.15
,
which, from now on, may be considered a master curve for the storage moduli of all
gelatin gels. The data in
Figure 7.15
includes
fish skin gelatin from cold and warm seas
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