formation between the ice crystals. Inter- and intramolecular hydrogen bonds are
also formed that connect the crystallites together and create an amorphous polymer
network. Thawing of the ice crystals leaves behind regions of low polymer concentration solutions. Subsequent FTCs will repeat the process, reinforcing the PVA
crystallites and tying up an increasing amount of the PVA molecules in the solution.
At the end of the process, the final thawing of the ice crystals leaves behind
micropores in the hydrogel body that are filled with the original solvent, typically
water [9].
The structure of PVA hydrogels by repeated freezing and thawing cycles has
been studied by several groups and the consensus is that they consists of a polymerrich region and a polymer-poor region, which is consistent with Willcox’s model [8,
10, 12, 15, 49, 50].
Based on micro-/nanodimension characterization, several structural models
have been proposed [8, 9, 32]. One of the most recent and up-to-date studies used
SANS. It was determined that the PVA-C structure consists of polymer crystallites
of ~3 nm in size dispersed in the polymer-rich region with a spacing of ~19 nm
between them [10]. During thawing, melting ice crystals create water-filled
micrometer-sized macropores that make up the polymer-poor regions. The structural evolution during the FT process is illustrated in Fig. 1a.
Interestingly, the FT process can be altered by adding one additional step to the
freeze–thaw procedure to create anisotropic PVA-C with orientation-dependent
mechanical properties. Since most natural tissues are anisotropic in structure and
properties, it is beneficial to be able to achieve this with polymer materials for tissue
replacement applications. Millon et al. produced the first PVA-C flat sheet and
conduit displaying anisotropic mechanical behavior similar to that of the porcine
aorta. Structural anisotropy was created by applying an orientational-specific strain
to the PVA sample after the initial FTC, and performing further thermal cycling on
it. It was suggested that the applied strain forces the polymer mesh and polymerpoor phase to elongate in the direction of the strain. Subsequent FTCs then produce
ice crystals that freeze and thaw in the strained pores that are already present,
reinforcing the structure in the direction the strain is applied. The pores semioriented in the direction of strain can increase in size with the additional cycling.
Additional crosslinking can also occur [10]. Figure 1b shows a model constructed
on the basis of the SANS data.
The unique poro-viscoelastic mechanical property of PVA-C when subjected to
external forces has been modeled using the finite element method. It has been
shown to be a direct result of the part-solid and part-liquid biphasic structure
[51]. This result is consistent with the model shown in Fig. 1b.
As the number of FTCs increases, the network mesh becomes denser and, hence,
the strength of the PVA-C increases with the number of FTCs. However, after seven
cycles, all available PVA materials in the initial solution are tied up in the mesh and
the mechanical strength of PVA-C levels off [45].
As the initial concentration of the PVA solution increases, more polymers are
available for network formation. The crystallinity increases and the polymer mesh
becomes denser, but the pore size becomes smaller [23]. The denser polymer mesh
Poly(Vinyl Alcohol) Cryogels for Biomedical Applications
291
also formed that connect the crystallites together and create an amorphous polymer
network. Thawing of the ice crystals leaves behind regions of low polymer concentration solutions. Subsequent FTCs will repeat the process, reinforcing the PVA
crystallites and tying up an increasing amount of the PVA molecules in the solution.
At the end of the process, the final thawing of the ice crystals leaves behind
micropores in the hydrogel body that are filled with the original solvent, typically
water [9].
The structure of PVA hydrogels by repeated freezing and thawing cycles has
been studied by several groups and the consensus is that they consists of a polymerrich region and a polymer-poor region, which is consistent with Willcox’s model [8,
10, 12, 15, 49, 50].
Based on micro-/nanodimension characterization, several structural models
have been proposed [8, 9, 32]. One of the most recent and up-to-date studies used
SANS. It was determined that the PVA-C structure consists of polymer crystallites
of ~3 nm in size dispersed in the polymer-rich region with a spacing of ~19 nm
between them [10]. During thawing, melting ice crystals create water-filled
micrometer-sized macropores that make up the polymer-poor regions. The structural evolution during the FT process is illustrated in Fig. 1a.
Interestingly, the FT process can be altered by adding one additional step to the
freeze–thaw procedure to create anisotropic PVA-C with orientation-dependent
mechanical properties. Since most natural tissues are anisotropic in structure and
properties, it is beneficial to be able to achieve this with polymer materials for tissue
replacement applications. Millon et al. produced the first PVA-C flat sheet and
conduit displaying anisotropic mechanical behavior similar to that of the porcine
aorta. Structural anisotropy was created by applying an orientational-specific strain
to the PVA sample after the initial FTC, and performing further thermal cycling on
it. It was suggested that the applied strain forces the polymer mesh and polymerpoor phase to elongate in the direction of the strain. Subsequent FTCs then produce
ice crystals that freeze and thaw in the strained pores that are already present,
reinforcing the structure in the direction the strain is applied. The pores semioriented in the direction of strain can increase in size with the additional cycling.
Additional crosslinking can also occur [10]. Figure 1b shows a model constructed
on the basis of the SANS data.
The unique poro-viscoelastic mechanical property of PVA-C when subjected to
external forces has been modeled using the finite element method. It has been
shown to be a direct result of the part-solid and part-liquid biphasic structure
[51]. This result is consistent with the model shown in Fig. 1b.
As the number of FTCs increases, the network mesh becomes denser and, hence,
the strength of the PVA-C increases with the number of FTCs. However, after seven
cycles, all available PVA materials in the initial solution are tied up in the mesh and
the mechanical strength of PVA-C levels off [45].
As the initial concentration of the PVA solution increases, more polymers are
available for network formation. The crystallinity increases and the polymer mesh
becomes denser, but the pore size becomes smaller [23]. The denser polymer mesh
Poly(Vinyl Alcohol) Cryogels for Biomedical Applications
291