Biomedical Engineering Reference
In-Depth Information
In another approach, the repetitive freezing-thawing technique during the
preparation is used for the reinforcement of the PVA cryogels [Fray et al., 2007;
Hassan et al., 2000; Peppas and Stauffer, 1991; Stauffer and Peppas, 1992; Watase
and Nishinari, 1988]. Hassan and co-authors presented a novel group of PVA
laminate cryogels that could be used for different pharmaceutical applications
prepared through the additional freezing thawing technique [Hassan and Peppas,
2000; Hassan et al., 2000]. In that work, the fresh PVA solution was added to
a previously formed gel and was then subjected to additional freeze-thawing
cycles to create a layered structure. PVA-MGs found different applications
in biomedicine due to the biocompatibility of PVA and excellent mechanical
properties and elasticity which allow for withstanding extensive pressures [Jiang
et al., 2004 ].
14.4.2 Covalently Cross-Linked MGs
Covalently cross-linked MGs are produced via two main approaches: through
cross-linking reaction using an appropriate cross-linker; or through free radical
cross-linking polymerization reaction using the respective initiators (Table 14.2).
According to the fi rst approach, the cross-linking reaction is performed in par-
tially frozen media using the available cross-linkers such as, for example, glutar-
aldehyde (GA) and epichlorohydrin (ECH). Thus, agarose-based MGs were
prepared through cross-linking of agarose chains with ECH at high pH value
[Plieva et al., 2007b] and the chitosan- and PVA-based MGs were prepared
through cross-linking reaction with GA in acidic medium [Noir et al., 2007; Plieva
et al., 2006b] (Table 14.2).
According to the second approach, the covalently cross-linked MGs can be
produced through free-radical cross-linking polymerization reaction using ammo-
nium persulfate (APS) and
N,N,N¢ ,N ¢
- tetra - methyl - ethylenediamine (TEMED)
initiating system [Dinu et al., 2007; Plieva et al., 2005, 2006; Yao et al., 2006a,b] or
by UV irradiation of both aqueous solutions and moderately frozen aqueous
systems [Petrov et al., 2006; Vlierberghe et al., 2007].
The APS/TEMED initiating system is one of the rare initiating systems that
can be used to initiate the free-radical polymerization reaction of the vinyl mono-
mers at subzero temperatures. In the free radical polymerization reactions, the
concentrations of the initiator (APS) and activator (TEMED) have a great infl u-
ence on the polymerization rate as well as on the molecular weight of the result-
ing polymers. The polymerization reaction starts with the reaction between APS
and TEMED to form free radicals to initiate the polymerization reaction of the
vinyl monomers/or macromonomers as well as the cross-linking of polymer
chains with the cross-linking monomer (as MBAAm or polyethylene diacrylate,
PEG-DA). Different MG systems were produced using the APS/TEMED initiat-
ing systems as pAAm - , pDMAAm - , pHEMA - , PEG - , pNIPAAm - dextran - MA -
and allyl-agarose based MGs (Table 14.2). Among the chemically cross-linked
monolithic MGs prepared through the free-radical polymerization reaction
using the APS/TEMED initiating system, the pAAm based MGs are the most
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