Biomedical Engineering Reference
In-Depth Information
12
10
8
6
HiG
160
LoG
230
4
0
200
400
600
800
G
′
(Pa)
Correlation between mean cross-sectional radius and storage modulus for 1% alginate gels: (
□
)
high guluronic acid content, M
w
= 16×10
4
g mol
−
1
(HiG
160
), [Ca
2+
] = 10 mM using CaEGTA; (
○
)
low guluronic acid content, M
w
= 23 ×10
4
g mol
−
1
(LoG
230
), [Ca
2+
] = 10 mM using CaEGTA.
Reprinted with permission from Yuguchi et al.(
2000
) © 2000 Elsevier.
Figure 5.17
calcium ions are very effective gel inducers. This ion selectivity can be explained by
introducing a selectivity coef
cient (Smidsrød and Haug,
1968
).
nity of alginate for divalent cations increased with
increasing content of G unit, and that the physical properties of these polymers in aqueous
media depend not only on the M/G ratio but on the distribution of the M and G units along
the chain. Overall, then, the egg-box model is acceptable for describing the major features
of the formation of alginate gels in the presence of the larger alkaline earth metals.
A recent study by Zhao and co-workers (Zhao et al.,
2010
) has contrasted the
behaviour of chemically cross-linked and Ca
2+
cross-linked alginate gels using compres-
sion and subsequent stress relaxation (Ferry,
1980
). The chemical gels show a predom-
inantly elastic response with a
It was con
rmed that the af
finite equilibriummodulus, whereas the ionic gels show an
almost monotonic decrease in log stress (or modulus, since strain is constant) when
plotted against log (time).
At the same time, Zhao et al. monitored the mass of the samples. The chemical gel
showed a decrease in mass as water was driven out, but this was recovered when the gel
was re-swollen to equilibrium. By contrast, the ionic gel mass remained almost constant
under compression. The authors explain this by assuming that the gel with covalent
cross-links relaxes stress simply by migration of the water (compressive de-swelling),
while in the ionic gel cross-links relax by breaking and then re-forming, as suggested by
others, including Pines and Prins (
1973
) in their work on agarose gels (
Chapter 7
).
However, the strain (15%) and strain rates used in the work are comparatively high for
stress relaxation studies of physical gels, and it is not clear that all measurements are
entirely within the linear viscoelastic region.
As a
final comment, alginate gels can also be prepared by lowering pH below ~2.5.
This produces an alginic acid gel, which has equilibrium swelling properties and seems to
involve hydrogen bonding (Draget et al.,
1994
).
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