and porous morphology of cryogels discussed in this section can be varied over a
very wide range, so that their potential applications are many. Judging from the
published information, biomedical and biotechnological applications are the primary uses for the cryogels prepared from polymeric precursors. In addition, some
other promising implementation fields also exist for such cryogels, including the
employment of rubber-based “cryosponges” developed in Turkey as a reusable
sorbent for the removal of oil spill from water surfaces [211, 212, 214, 223],
crosslinked chitosan cryogels for the absorption of radionuclides from waste
water [224, 225], crosslinked and partially saponificated poly(vinyl alcohol)
cryogel composites containing activated carbon particles for the absorption of
dyes [29, 199, 200], and so forth.
2.4 Physical (Noncovalent) Polymeric Cryogels
The publications on this group of cryogels are the most numerous and include
several well-known and very informative reviews. The majority of these studies are
related to poly(vinyl alcohol) (PVA) cryogels, which have been known since the
1970s [226–231]. The formation mechanisms of PVA cryogels [1, 29, 232–237]
and their applications in various fields have been investigated extensively. These
gel materials are used in medicine [1, 232, 235, 238–251], in biochemistry and
biotechnology [1, 234, 252–262], in environmental protection [1, 263], in construction in the permafrost regions [1, 264, 265], etc. Such popularity of PVA cryogels is
due to the combination of a set of remarkable features they possess, such as
excellent physico-mechanical properties, a high thermal endurance compared
with other physical hydrogels, a high resistance to abrasive erosion, a
macroporosity that ensures good diffusion characteristics, the availability and
relatively low cost of PVA itself, and a comparatively simple procedure for the
preparation of such cryogels. In addition, PVA cryogels have a high biocompatibility and are nontoxic for biological objects.
Noncovalent cryogels based on other “self-gelling” synthetic and natural macromolecules have also been described for more than 40 years. In 1971, it was
demonstrated that freezing of a 17 % solution of poly(acrylonitrile) in dimethylformamide/water (95–97:5–3, v/v) mixtures at À78
C leads to the formation of
physical gels that are stable at room temperature [266]. Later, the freeze–thawinduced formation of noncovalent cryogels was also reported for other polymeric
systems. For instance, freezing of an aqueous solution of syndiotactic
poly(methacrylic acid) and poly(ethylene oxide) mixture at À78
C, followed by
its defrosting at room temperature, results in noncovalent cryogels [267]. Similarly,
freezing of semidilute aqueous solutions of agar-agar or A-type gelatine at À10
C
for 24 h followed by thawing at 25
C leads to the formation of biopolymer cryogels
[30]. The cryogels thus obtained possess macroporous morphology, whose characteristics mainly depend on the type of polymeric precursor and its initial concentration in the feed. The cryogels formed from gelatinized starch [5, 70–78]
22
V.I. Lozinsky
very wide range, so that their potential applications are many. Judging from the
published information, biomedical and biotechnological applications are the primary uses for the cryogels prepared from polymeric precursors. In addition, some
other promising implementation fields also exist for such cryogels, including the
employment of rubber-based “cryosponges” developed in Turkey as a reusable
sorbent for the removal of oil spill from water surfaces [211, 212, 214, 223],
crosslinked chitosan cryogels for the absorption of radionuclides from waste
water [224, 225], crosslinked and partially saponificated poly(vinyl alcohol)
cryogel composites containing activated carbon particles for the absorption of
dyes [29, 199, 200], and so forth.
2.4 Physical (Noncovalent) Polymeric Cryogels
The publications on this group of cryogels are the most numerous and include
several well-known and very informative reviews. The majority of these studies are
related to poly(vinyl alcohol) (PVA) cryogels, which have been known since the
1970s [226–231]. The formation mechanisms of PVA cryogels [1, 29, 232–237]
and their applications in various fields have been investigated extensively. These
gel materials are used in medicine [1, 232, 235, 238–251], in biochemistry and
biotechnology [1, 234, 252–262], in environmental protection [1, 263], in construction in the permafrost regions [1, 264, 265], etc. Such popularity of PVA cryogels is
due to the combination of a set of remarkable features they possess, such as
excellent physico-mechanical properties, a high thermal endurance compared
with other physical hydrogels, a high resistance to abrasive erosion, a
macroporosity that ensures good diffusion characteristics, the availability and
relatively low cost of PVA itself, and a comparatively simple procedure for the
preparation of such cryogels. In addition, PVA cryogels have a high biocompatibility and are nontoxic for biological objects.
Noncovalent cryogels based on other “self-gelling” synthetic and natural macromolecules have also been described for more than 40 years. In 1971, it was
demonstrated that freezing of a 17 % solution of poly(acrylonitrile) in dimethylformamide/water (95–97:5–3, v/v) mixtures at À78
C leads to the formation of
physical gels that are stable at room temperature [266]. Later, the freeze–thawinduced formation of noncovalent cryogels was also reported for other polymeric
systems. For instance, freezing of an aqueous solution of syndiotactic
poly(methacrylic acid) and poly(ethylene oxide) mixture at À78
C, followed by
its defrosting at room temperature, results in noncovalent cryogels [267]. Similarly,
freezing of semidilute aqueous solutions of agar-agar or A-type gelatine at À10
C
for 24 h followed by thawing at 25
C leads to the formation of biopolymer cryogels
[30]. The cryogels thus obtained possess macroporous morphology, whose characteristics mainly depend on the type of polymeric precursor and its initial concentration in the feed. The cryogels formed from gelatinized starch [5, 70–78]
22
V.I. Lozinsky