Biomedical Engineering Reference
In-Depth Information
10
4
10
3
Cycle 0
10
2
Cycle 1
Cycle 2
Cycle 3
Cycle 4
Cycle 5
10
1
10
−
1
10
0
Frequency (rad s
−
1
)
10
1
10
2
G
0
versus radial frequency for 3% w/w LBG gels after each freeze/thaw cycle (
−
25°C, followed by
heating to 25°C), as indicated. From Tanaka et al.(
1998
) with permission from JohnWiley & Sons.
Figure 8.21
of hydrogen bonds in the formation of LBG cryogels by preparing water solutions
containing a chaotropic agent (urea). They observed a decrease in the mechanical
strength with a rise in urea concentration, which could be explained by a reduction in
the level of hydrogen bonding in these gels due to the chaotropic action of urea. Thus, the
junction zones in LBG cryogels are likely to be created by hydrogen bonds between the
hydroxyl groups of the galactomannan chains. It was suggested that the gelation process
occurs as in PVA, in a two-phase medium, composed of the frozen solvent polycrystals
and an unfrozen microphase. The macropores in cryogels are usually open and inter-
connected. Tanaka et al.(
1998
) showed that G
0
increases with the number of freeze
thaw
cycles, as shown in
Figure 8.21
. The freeze/thaw behaviour of LBG systems is rather
similar to the cryostructuring of starch polysaccharides, and the extreme dependence of
the properties of LBG cryogels on the frozen storage temperature is similar to that of
PVA. Hence, the same basic mechanisms may be responsible for the formation of non-
covalent polymeric cryogels from various types of polymers, either synthetic or natural
biopolymers.
Curdlan is a rather unusual microbial polysaccharide in that structurally it is a
-
β
(1
3) glucan, and can be produced by several strains of Agrobacterium and certain
strains of Rhizobium (Nishinari,
1988
). On heating to around 60°C and then cooling it
forms a gel, which on reheating to higher temperatures (>80°C) then forms irreversibly.
When either the so-called low- or high-set gels are subjected to freeze/thaw cycling, the
gel modulus is increased and the amount of syneresis is decreased. Another class of
β-
→
4) linkages derived from cereals. These are
commonly extracted from oats, although they can also be found in barley and wheat. It
has been known for many years that these form pasty products when heated solutions
are cooled. Lazaridou and Biliaderis (
2004
) prepared a number of samples in water,
typically 1
glucans are those with mixed (1
3)(1
→
→
3% w/w, by heating them to 85°C and then subjecting them to repeated
freeze/thaw cycles (N
c
=3
-
12). For the higher concentrations, soft cryogels could be
produced even after N
c
= 3 cycles, but the effect was magni
-
ed by increasing N
c
.
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