Biomedical Engineering Reference
In-Depth Information
14.4 PREPARATION OF MACROPOROUS GELS (MG S ) FROM
DIFFERENT GEL PRECURSORS
MGs with a broad range of porosities are produced at moderately low tempera-
ture, namely in a temperature range of
20 ° C. The MGs prepared through
the cryogelation technique can be divided in two main groups: thermo-reversible
physically cross-linked MGs (where the MG network is formed through hydro-
gen-bond formation) and thermo-irreversible covalently cross-linked MGs (when
the MG network is formed through covalent bond formation). Different macro-
porous cryogel systems were prepared through cryogelation approaches as poten-
tial materials for biomedical applications (Table 14.2). The porosity of such
biomaterials can be varied to a large extent depending on the physical-chemical
properties of the used gel precursors and particular application of MG.
10 to
14.4.1 Physically Cross-Linked MGs
The physically cross-linked MGs were prepared through physical gelation at
subzero temperatures of both natural biocompatible polymers such as agarose
[Plieva et al., 2007b] and collagen [Podorozhko et al., 2000] or synthetic polymer
such as PVA [Bajpai and Saini, 2005a,b; Kokabi et al., 2007; Lozinsky, 1998;
Lozinsky and Plieva, 1998; Nho and Park, 2001].
Among the physically cross-linked MG biomaterials, the MGs prepared from
polyvinyl alcohol (PVA) are the most utilized (Table 14.2). PVA is known to be
non-toxic [DeMerlis and Schoneker, 2003] and a readily available synthetic
polymer widely used in biomedical [Hassan and Peppas, 2000; Hassan et al., 2000;
Peppas and Mongia, 1997] and biotechnological [Lozinsky and Plieva, 1998]
applications. PVA with high deacetylation degree (DH more than 97%) is a
hydrophilic polymer that in concentrated aqueous solution can form weak gels
during prolonged storage at room temperature because of the formation
of hydrogen bonds between polymer molecules. The cryogelation technique
facilitates the gelation process as a result of the increased concentration of the
dissolved macromolecules in the unfrozen liquid microphase.
As a result of cryoconcentraton, PVA chains form ordered structures known
as microcrystallinity zones which function as physical cross-links and are formed
only when -OH groups are free to participate in such interactions [Hassan et al.,
2000 ; Lozinsky, 1998 ].
Typically, the PVA with DH more than 97% is used for the preparation of phys-
ically cross-linked elastic cryogels with high mechanical strength and chemical
stability. Two main approaches are used for the reinforcement of physically cross-
linked PVA cryogels. First, the porosity and mechanical strength of PVA cryo-
gels is regulated by the thawing rate; the lower the thawing rate, the higher the
mechanical strength of the prepared PVA cryogels [Lozinsky, 1998]. The reason
is that the slow thawing ensures the prolonged duration of the PVA sample at the
temperature range of
1 °C. In this temperature region, the amount of
unfrozen water is already suffi cient to allow the movements of polymer chains
5 to
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