increase in PVA concentration from 10 % to either 15 or 20 %, the tangent elastic
modulus increased by 69 and 137 %, respectively, and the secant elastic modulus
increased by 83 and 180 %, respectively, at a strain of 0.25 [23].
2.3 Solvent
The use of different solvents in the processing of PVA can dramatically alter the
PVA-C properties. Hassan and Peppas have reviewed this area well [6]. Hyon and
Ikada showed that the use of organic solvents such as dimethyl sulfoxide (DMSO),
glycerin, ethylene glycol, propylene glycol, and ethyl alcohol provides excellent
light transmittance, along with good tensile strength and high water content,
making a gel material with good potential for contact lens applications [24]. The
addition of DMSO has been studied extensively because it imparts transparency to
the PVA-C. Ohkura et al. showed that DMSO/water solutions with PVA exhibit
transparency, high elasticity and higher gelation rates because gelation occurs
without phase separation at temperatures below À20
C [25]. Murase et al. showed
that crystallinity increased over time due to the interaction between water and
DMSO in the PVA-C [6].
Additives are sometimes put into the PVA solution. Lozinsky et al. showed that
by adding triethylene glycols and its higher oligomers, the strength and thermal
stability of the PVA cryogels increased and the gelation process was altered [26,
27].
For biomedical applications, the presence of salts or other solutes in the environment while producing PVA-C can be beneficial because the biomaterial will
eventually be used in an electrolyte-containing physiological environment. Due to
freezing point depression as a result of increased solute concentration, the freezing
point of PVA solution decreases with the addition of salts. Gordon showed that the
freezing point of PVA in water was À18.7
C [28], whereas Shaheen
et al. demonstrated a decrease of freezing point to À30
C when PVA, theophylline,
and 11 % NaCl was used [29]. The intermolecular and intramolecular hydrogen
bonds that are so important to the formation of crystallites in PVA-C are disrupted
by the presence of salts [30]. The crystallinity of PVA-C was shown to decrease and
the cryogel became weaker when prepared in the presence of NaCl (0.0125–
0.0625 M) compared to when distilled water was used [31].
2.4 Freeze–Thaw Cycling
As described above, the freeze–thaw cycling process allows the formation of
amorphous and crystalline regions to form a physically crosslinked matrix of
PVA. The number of FTCs, rate of freezing and thawing, and the time for which
the gel is held frozen (freezing holding time) all have direct impact on the structure
288
W. Wan et al.
modulus increased by 69 and 137 %, respectively, and the secant elastic modulus
increased by 83 and 180 %, respectively, at a strain of 0.25 [23].
2.3 Solvent
The use of different solvents in the processing of PVA can dramatically alter the
PVA-C properties. Hassan and Peppas have reviewed this area well [6]. Hyon and
Ikada showed that the use of organic solvents such as dimethyl sulfoxide (DMSO),
glycerin, ethylene glycol, propylene glycol, and ethyl alcohol provides excellent
light transmittance, along with good tensile strength and high water content,
making a gel material with good potential for contact lens applications [24]. The
addition of DMSO has been studied extensively because it imparts transparency to
the PVA-C. Ohkura et al. showed that DMSO/water solutions with PVA exhibit
transparency, high elasticity and higher gelation rates because gelation occurs
without phase separation at temperatures below À20
C [25]. Murase et al. showed
that crystallinity increased over time due to the interaction between water and
DMSO in the PVA-C [6].
Additives are sometimes put into the PVA solution. Lozinsky et al. showed that
by adding triethylene glycols and its higher oligomers, the strength and thermal
stability of the PVA cryogels increased and the gelation process was altered [26,
27].
For biomedical applications, the presence of salts or other solutes in the environment while producing PVA-C can be beneficial because the biomaterial will
eventually be used in an electrolyte-containing physiological environment. Due to
freezing point depression as a result of increased solute concentration, the freezing
point of PVA solution decreases with the addition of salts. Gordon showed that the
freezing point of PVA in water was À18.7
C [28], whereas Shaheen
et al. demonstrated a decrease of freezing point to À30
C when PVA, theophylline,
and 11 % NaCl was used [29]. The intermolecular and intramolecular hydrogen
bonds that are so important to the formation of crystallites in PVA-C are disrupted
by the presence of salts [30]. The crystallinity of PVA-C was shown to decrease and
the cryogel became weaker when prepared in the presence of NaCl (0.0125–
0.0625 M) compared to when distilled water was used [31].
2.4 Freeze–Thaw Cycling
As described above, the freeze–thaw cycling process allows the formation of
amorphous and crystalline regions to form a physically crosslinked matrix of
PVA. The number of FTCs, rate of freezing and thawing, and the time for which
the gel is held frozen (freezing holding time) all have direct impact on the structure
288
W. Wan et al.