PVA formed in mixed solvents [1, 20, 23, 234, 264, 373], complex PVA cryogels
[1, 234, 239, 374], and composite PVA cryogels [1, 20, 23, 29, 234, 236, 237, 252,
254, 265, 375–377].
2.5 Ionic (Ionotropic) Cryogels
As pointed out above, the accomplishment of gel formation through crosslinking of
polyelectrolyte chains with suitable counterions within the space of the unfrozen
liquid microphase is a difficult task because of the high rate of the ionic processes.
As a rule, conventional ionotropic gelation occurs before freezing of the gelling
system. This is the reason why there is only a limited number of successful
examples of preparation of ionically crosslinked cryogels by this “direct” route
(e.g., [33, 34]). However, the cryogenically structured macroporous gel-like matrices can be fabricated by using roundabout pathways.
One way is the freezing of the partially (incompletely) formed gel, followed by
its frozen storage in order to complete the ionic crosslinking, and then sublimation
of the frozen solvent crystals. Such a sequence of operations results in macroporous
dry cryostructurates that are transformed upon swelling in macroporous gels. This
approach was used, for instance, for the preparation of Ca-alginate-based wound/
burn dressings [34], or wide-pore scaffolds for tissue engineering [378–380].
Another method includes freezing the polyelectrolyte solution, followed by the
removal of frozen solvent crystals without their thawing. The latter can be done
either via freeze-drying, or via cryoextraction, i.e., by rinsing the frozen sample at a
negative temperature with a liquid that acts as a solvent for the crystalline phase but
a nonsolvent for the polyelectrolyte. The resulting macroporous cryostructurate is
then treated with an agent capable of either ionically crosslinking the polymeric
chains, or recharging their ionogenic groups. This stage is also carried out in a
nonsolvent for the macromolecular material. A macroporous gel is obtained upon
subsequent swelling of the resulting material in the respective solvating liquids.
This approach was employed for the fabrication of wide-pore cryostructured matrices based on both polyacids and polybases [1]. For example, in order to prepare
Ca-alginate sponges, the initial aqueous sodium alginate solution was frozen, and
ice polycrystals were then removed via vacuum sublimation or via cryoextraction
with cold ethanol. The resultant cryogenically structured polymer was then
immersed in an ethanolic solution of calcium salt for ionic crosslinking leading to
water-insoluble sponge-like Ca-alginate cryogel [381]. Such sponges possessing a
system of interconnected gross pores of capillary size are of interest as cell culture
scaffolds in biotechnological systems [382–384]. The preparation of cryostuctured
polyelectolytes through recharging of their ionic groups can be exemplified by
chitosan-based sponges. Here, after removal of ice crystals from the frozen
chitosan–aqueous acetic acid system, the resulting cryostructurate was treated
with alkaline acetone, thus causing formation of water-insoluble unprotonated
36
V.I. Lozinsky
[1, 234, 239, 374], and composite PVA cryogels [1, 20, 23, 29, 234, 236, 237, 252,
254, 265, 375–377].
2.5 Ionic (Ionotropic) Cryogels
As pointed out above, the accomplishment of gel formation through crosslinking of
polyelectrolyte chains with suitable counterions within the space of the unfrozen
liquid microphase is a difficult task because of the high rate of the ionic processes.
As a rule, conventional ionotropic gelation occurs before freezing of the gelling
system. This is the reason why there is only a limited number of successful
examples of preparation of ionically crosslinked cryogels by this “direct” route
(e.g., [33, 34]). However, the cryogenically structured macroporous gel-like matrices can be fabricated by using roundabout pathways.
One way is the freezing of the partially (incompletely) formed gel, followed by
its frozen storage in order to complete the ionic crosslinking, and then sublimation
of the frozen solvent crystals. Such a sequence of operations results in macroporous
dry cryostructurates that are transformed upon swelling in macroporous gels. This
approach was used, for instance, for the preparation of Ca-alginate-based wound/
burn dressings [34], or wide-pore scaffolds for tissue engineering [378–380].
Another method includes freezing the polyelectrolyte solution, followed by the
removal of frozen solvent crystals without their thawing. The latter can be done
either via freeze-drying, or via cryoextraction, i.e., by rinsing the frozen sample at a
negative temperature with a liquid that acts as a solvent for the crystalline phase but
a nonsolvent for the polyelectrolyte. The resulting macroporous cryostructurate is
then treated with an agent capable of either ionically crosslinking the polymeric
chains, or recharging their ionogenic groups. This stage is also carried out in a
nonsolvent for the macromolecular material. A macroporous gel is obtained upon
subsequent swelling of the resulting material in the respective solvating liquids.
This approach was employed for the fabrication of wide-pore cryostructured matrices based on both polyacids and polybases [1]. For example, in order to prepare
Ca-alginate sponges, the initial aqueous sodium alginate solution was frozen, and
ice polycrystals were then removed via vacuum sublimation or via cryoextraction
with cold ethanol. The resultant cryogenically structured polymer was then
immersed in an ethanolic solution of calcium salt for ionic crosslinking leading to
water-insoluble sponge-like Ca-alginate cryogel [381]. Such sponges possessing a
system of interconnected gross pores of capillary size are of interest as cell culture
scaffolds in biotechnological systems [382–384]. The preparation of cryostuctured
polyelectolytes through recharging of their ionic groups can be exemplified by
chitosan-based sponges. Here, after removal of ice crystals from the frozen
chitosan–aqueous acetic acid system, the resulting cryostructurate was treated
with alkaline acetone, thus causing formation of water-insoluble unprotonated
36
V.I. Lozinsky