4 Concluding Remarks and Outlook
Poly(vinyl alcohol) cryogels are strong physical gels that form as a result of
different crosslinking processes that occur simultaneously in synergy or in competition. We demonstrate here that the concept of circumstantial metastability and the
hierarchy of metastable states may turn out to be useful in unraveling the complex
mechanisms subtending their formation. In particular, we have shown that kinetic
analysis of the cryotropic gelation from PVA solutions using time-resolving SANS
has allowed an unprecedented level of comprehension of this process. At least three
elementary steps have been identified:
• Incomplete solvent crystallization occurring at subzero temperatures, with consequent formation of an unfrozen liquid microphase of eutectic composition
• Incomplete PVA crystallization in the unfrozen liquid microphase, with consequent formation of a microgel fraction
• Coarsening of the dense and diluted regions imprinted by the cryogenic treatment, occurring at room temperature
A wealth of applications have been proposed for these gels in many different
fields, including biomedicine and diagnostics. Applications include the building of
artificial muscles [84, 85] or phantom organs for NMR imaging and mammography
[86, 87], controlled drug release [11, 88–90], and biotechnology in which these gels
act as carriers of immobilized bioaffinity ligands, enzymes, and cells [4–6, 91].
More recently, it has been pointed out that aged gels possess self-healing
properties at room temperature, without the need for any stimulus or healing
agent [10]. Welding occurs spontaneously via chain diffusion across the interface
provided that a sufficient number of PVA free hydroxyl groups survive at the
contact between the two cut surfaces [10].
A further interesting property of PVA cryogels is that, whereas a long aging in
sealed vials at room temperature induces large variations in their structure and
properties, the porous structure imprinted by cryotropic treatment is not greatly
altered upon drying and during the successive rehydration step. Rehydrated gels
almost completely recover the volume, shape, and physical properties of the
as-formed freeze–thaw PVA hydrogels [42, 57, 62, 63]. It has been shown that
the outstanding physical and mechanical properties of freeze–thaw PVA hydrogels
in the as-prepared state can be preserved, even for a long time, by drying the
samples immediately after preparation and then restoring when needed by rehydration of the dried samples [42, 57, 63].
Finally, it is worth mentioning that PVA cryogels are able to incorporate
different soluble and insoluble additives inside the pores to obtain composite
materials of both scientific and applied interest [4–6]. In particular, PVA-based
cryogels have been obtained that include inside the porous structure nanoparticles
of solid crystalline compounds, co-elastic gels, microorganisms, gas bubbles, or
microdroplets of liquids immiscible with PVA solutions [4–6, 92–98].
192
C. De Rosa et al.
Poly(vinyl alcohol) cryogels are strong physical gels that form as a result of
different crosslinking processes that occur simultaneously in synergy or in competition. We demonstrate here that the concept of circumstantial metastability and the
hierarchy of metastable states may turn out to be useful in unraveling the complex
mechanisms subtending their formation. In particular, we have shown that kinetic
analysis of the cryotropic gelation from PVA solutions using time-resolving SANS
has allowed an unprecedented level of comprehension of this process. At least three
elementary steps have been identified:
• Incomplete solvent crystallization occurring at subzero temperatures, with consequent formation of an unfrozen liquid microphase of eutectic composition
• Incomplete PVA crystallization in the unfrozen liquid microphase, with consequent formation of a microgel fraction
• Coarsening of the dense and diluted regions imprinted by the cryogenic treatment, occurring at room temperature
A wealth of applications have been proposed for these gels in many different
fields, including biomedicine and diagnostics. Applications include the building of
artificial muscles [84, 85] or phantom organs for NMR imaging and mammography
[86, 87], controlled drug release [11, 88–90], and biotechnology in which these gels
act as carriers of immobilized bioaffinity ligands, enzymes, and cells [4–6, 91].
More recently, it has been pointed out that aged gels possess self-healing
properties at room temperature, without the need for any stimulus or healing
agent [10]. Welding occurs spontaneously via chain diffusion across the interface
provided that a sufficient number of PVA free hydroxyl groups survive at the
contact between the two cut surfaces [10].
A further interesting property of PVA cryogels is that, whereas a long aging in
sealed vials at room temperature induces large variations in their structure and
properties, the porous structure imprinted by cryotropic treatment is not greatly
altered upon drying and during the successive rehydration step. Rehydrated gels
almost completely recover the volume, shape, and physical properties of the
as-formed freeze–thaw PVA hydrogels [42, 57, 62, 63]. It has been shown that
the outstanding physical and mechanical properties of freeze–thaw PVA hydrogels
in the as-prepared state can be preserved, even for a long time, by drying the
samples immediately after preparation and then restoring when needed by rehydration of the dried samples [42, 57, 63].
Finally, it is worth mentioning that PVA cryogels are able to incorporate
different soluble and insoluble additives inside the pores to obtain composite
materials of both scientific and applied interest [4–6]. In particular, PVA-based
cryogels have been obtained that include inside the porous structure nanoparticles
of solid crystalline compounds, co-elastic gels, microorganisms, gas bubbles, or
microdroplets of liquids immiscible with PVA solutions [4–6, 92–98].
192
C. De Rosa et al.