1 Introduction
Polyvinyl alcohol (PVA) is a hydrogel with desirable properties for biomedical
applications [1, 2]. Ivalon™, a highly porous PVA sponge crosslinked with formaldehyde, was probably one of the first medical products marketed [3]. It was used
extensively in duct replacement, articular cartilage replacement [4], as a pharmaceutical release agent [1], and in reconstructive (vocal cord) surgery [5]. In addition
to the use of chemical crosslinking agents such as formaldehyde and glutaraldehyde, PVA can also be crosslinked using several other methods, such as the use of
electron beam,γ-irradiation, and physical crosslinking. For biomedical applications,
physical crosslinking has the advantage of not leaving residual amounts of toxic
crosslinking agents, as well as providing higher and more tunable mechanical
strength than the PVA gels crosslinked by either chemical or irradiative techniques
[6]. The physical crosslinking methods have generated the most interest because no
new chemicals are introduced that could complicate their use in the biomedical
environment.
Physical crosslinking can be accomplished using a freeze–thaw (FT) cycling
method in which a solution of PVA is allowed to undergo repeated freezing and
thawing cycles. For obvious reasons, the product hydrogel is popularly called a
PVA cryogel (PVA-C). Using this approach, and by carefully controlling the
process parameters used in the hydrogel preparation procedure, material properties,
including mechanical and diffusion properties, can be tailored. Moreover, with the
incorporation of nanomaterials into the PVA solution, nanocomposites of interesting mechanical properties (tensile and compressive) relevant to a range of medical
device applications can be created. We will focus on the preparation and properties
of PVA-C and its composites by the freeze–thaw method. Biomedical applications
in the areas of medical device and drug delivery will be used to illustrate the range
of biomedical applications possible for this class of hydrogel material.
2 Processing Parameters for PVA-C Preparation
PVA-C is today one of the most commonly investigated cryogels for biomedical
applications. PVA, which is synthesized through hydrolysis of polyvinyl acetate
synthesized via free radical polymerization of vinyl acetate [6], consists of a
secondary alcohol group attached to a linear carbon chain. The alcohol group
allows for hydrogen bonding and, therefore, PVA dissolved in an aqueous solution
is able to produce a hydrogel with high water content. Subsequent thermal cycling
leads to physical crosslinking via formation of structured crystalline domains of the
polymer chains through phase separation. Several phases occur during the thermal
cycling process. First, the gel is brought down to a temperature of between À5 and
À20
C [7], during which time the water phase freezes. This creates regions of high
polymer concentration, where crystallites are formed, as well as regions of low
Poly(Vinyl Alcohol) Cryogels for Biomedical Applications
285
Polyvinyl alcohol (PVA) is a hydrogel with desirable properties for biomedical
applications [1, 2]. Ivalon™, a highly porous PVA sponge crosslinked with formaldehyde, was probably one of the first medical products marketed [3]. It was used
extensively in duct replacement, articular cartilage replacement [4], as a pharmaceutical release agent [1], and in reconstructive (vocal cord) surgery [5]. In addition
to the use of chemical crosslinking agents such as formaldehyde and glutaraldehyde, PVA can also be crosslinked using several other methods, such as the use of
electron beam,γ-irradiation, and physical crosslinking. For biomedical applications,
physical crosslinking has the advantage of not leaving residual amounts of toxic
crosslinking agents, as well as providing higher and more tunable mechanical
strength than the PVA gels crosslinked by either chemical or irradiative techniques
[6]. The physical crosslinking methods have generated the most interest because no
new chemicals are introduced that could complicate their use in the biomedical
environment.
Physical crosslinking can be accomplished using a freeze–thaw (FT) cycling
method in which a solution of PVA is allowed to undergo repeated freezing and
thawing cycles. For obvious reasons, the product hydrogel is popularly called a
PVA cryogel (PVA-C). Using this approach, and by carefully controlling the
process parameters used in the hydrogel preparation procedure, material properties,
including mechanical and diffusion properties, can be tailored. Moreover, with the
incorporation of nanomaterials into the PVA solution, nanocomposites of interesting mechanical properties (tensile and compressive) relevant to a range of medical
device applications can be created. We will focus on the preparation and properties
of PVA-C and its composites by the freeze–thaw method. Biomedical applications
in the areas of medical device and drug delivery will be used to illustrate the range
of biomedical applications possible for this class of hydrogel material.
2 Processing Parameters for PVA-C Preparation
PVA-C is today one of the most commonly investigated cryogels for biomedical
applications. PVA, which is synthesized through hydrolysis of polyvinyl acetate
synthesized via free radical polymerization of vinyl acetate [6], consists of a
secondary alcohol group attached to a linear carbon chain. The alcohol group
allows for hydrogen bonding and, therefore, PVA dissolved in an aqueous solution
is able to produce a hydrogel with high water content. Subsequent thermal cycling
leads to physical crosslinking via formation of structured crystalline domains of the
polymer chains through phase separation. Several phases occur during the thermal
cycling process. First, the gel is brought down to a temperature of between À5 and
À20
C [7], during which time the water phase freezes. This creates regions of high
polymer concentration, where crystallites are formed, as well as regions of low
Poly(Vinyl Alcohol) Cryogels for Biomedical Applications
285