4 PVA Composite Cryogels
PVA-C composites are prepared by the addition of fillers into the PVA solution
before the freeze–thaw process. The main purposes of filler addition are for
improving mechanical properties and for controlled release and delivery. A wide
variety of filler materials that vary in dimensions from micro- to nanometers have
been reported, but here we focus on two widely studied filler materials for their
relevance in biomedical applications: bacterial cellulose and chitosan.
4.1 Cellulose-PVA Composites
Bacterial cellulose (BC), a biocompatible natural polymer, has been studied significantly for the production of composite materials for biomedical applications. Its
polyfunctionality, hydrophilicity, and biocompatibility [67] make it a desirable
material for many different biomedical applications. It is a nanomaterial of roughly
50 nm diameter [68]. Cellulose produced by bacteria has high crystallinity,
mechanical strength, the capacity to absorb water, and a large aspect ratio [69,
70]. The use of BC as a composite material for PVA that has been crosslinked using
freeze–thaw cycling has so far been limited. The fabrication and use of this
composite material with its advantageous properties for biomedical applications
will be discussed here.
Wan et al. found that the addition of BC nanofibers to a PVA solution that was
then thermally cycled resulted in a PVA-BC composite cryogel that could be tuned
to possess the mechanical properties of cardiovascular tissue. BC and PVA-C
mimic the role of collagen and elastin, respectively, in soft tissues, making
PVA-BC a good candidate material for possible use as a cardiovascular tissue
replacement [42]. With the addition of 0.6 wt% BC, the nanocomposite material
had higher strength (for FTCs 1–3) and also possessed a broader range of mechanical property control. Because the BC has a very large surface area per unit mass,
there is opportunity for significant hydrogen bonding with the PVA matrix, both BC
and PVA being hydrophilic polymers. This bonding results in the high strength of
the nanocomposite material. The material can be tuned easily by changing
processing parameters. Particular compositions of this material were able to show
excellent matching with the stress–strain properties of porcine aorta as well as
porcine heart valve. Additionally, the material showed faster relaxation and a lower
residual stress, which are desirable characteristics for cardiovascular soft tissue
replacement applications [42].
The PVA-BC nanocomposite is able to provide a wide range of mechanical
properties, depending on the processing parameters chosen for the material’s
application. As discussed in Sect. 3.1.2, the addition of an initial strain following
the first FTC can produce a cryogel with anisotropic mechanical properties. Cardiovascular tissue is composed of the structural proteins collagen and elastin, and is
302
W. Wan et al.
PVA-C composites are prepared by the addition of fillers into the PVA solution
before the freeze–thaw process. The main purposes of filler addition are for
improving mechanical properties and for controlled release and delivery. A wide
variety of filler materials that vary in dimensions from micro- to nanometers have
been reported, but here we focus on two widely studied filler materials for their
relevance in biomedical applications: bacterial cellulose and chitosan.
4.1 Cellulose-PVA Composites
Bacterial cellulose (BC), a biocompatible natural polymer, has been studied significantly for the production of composite materials for biomedical applications. Its
polyfunctionality, hydrophilicity, and biocompatibility [67] make it a desirable
material for many different biomedical applications. It is a nanomaterial of roughly
50 nm diameter [68]. Cellulose produced by bacteria has high crystallinity,
mechanical strength, the capacity to absorb water, and a large aspect ratio [69,
70]. The use of BC as a composite material for PVA that has been crosslinked using
freeze–thaw cycling has so far been limited. The fabrication and use of this
composite material with its advantageous properties for biomedical applications
will be discussed here.
Wan et al. found that the addition of BC nanofibers to a PVA solution that was
then thermally cycled resulted in a PVA-BC composite cryogel that could be tuned
to possess the mechanical properties of cardiovascular tissue. BC and PVA-C
mimic the role of collagen and elastin, respectively, in soft tissues, making
PVA-BC a good candidate material for possible use as a cardiovascular tissue
replacement [42]. With the addition of 0.6 wt% BC, the nanocomposite material
had higher strength (for FTCs 1–3) and also possessed a broader range of mechanical property control. Because the BC has a very large surface area per unit mass,
there is opportunity for significant hydrogen bonding with the PVA matrix, both BC
and PVA being hydrophilic polymers. This bonding results in the high strength of
the nanocomposite material. The material can be tuned easily by changing
processing parameters. Particular compositions of this material were able to show
excellent matching with the stress–strain properties of porcine aorta as well as
porcine heart valve. Additionally, the material showed faster relaxation and a lower
residual stress, which are desirable characteristics for cardiovascular soft tissue
replacement applications [42].
The PVA-BC nanocomposite is able to provide a wide range of mechanical
properties, depending on the processing parameters chosen for the material’s
application. As discussed in Sect. 3.1.2, the addition of an initial strain following
the first FTC can produce a cryogel with anisotropic mechanical properties. Cardiovascular tissue is composed of the structural proteins collagen and elastin, and is
302
W. Wan et al.