Biomedical Engineering Reference
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characterized [Dinu et al., 2007; He et al., 2007; Plieva et al., 2004a, 2006a, 2007c;
Yao et al., 2006a,b].
The advantage of the UV irradiation is the cost effi ciency and the very short
time needed for an effi cient gel formation. Cellulose based MGs were prepared
through irradiation of the frozen aqueous solution of HEC with a Dymax 5000-
EC UV curing equipment for one to fi ve minutes [Petrov et al., 2006]. The frozen-
thawed samples were opaque spongy materials, while all samples irradiated at
room temperature were still liquid-like.
Porous gelatin materials with a gradient in pore-size were prepared by cryo-
genic treatment using temperature gradient between the top and the bottom
phase of the sample [Dubruel et al., 2007; Vlierberghe et al., 2007]. For this
purpose, gelatin was modifi ed fi rst with methacrylic anhydride to incorporate the
polymerizable double bonds [Bulcke et al., 2000]. Then hydrogels were formed
by gelation of an aqueous methacrylamide-modifi ed gelatin solution, followed by
radical cross-linking using a UV-active photoinitiator and chemically cross-linked
hydrogels were subjected to a cryogenic treatment [Dubruel et al., 2007] (Table
14.2). By varying the conditions of the cryogenic treatment, gelatin MGs with dif-
ferent pore morphologies were prepared [Vlierberghe et al., 2007].
14.5 CHARACTERIZATION OF MG
S
14.5.1 Pore Volume of MGs
The determination of absolute values of pore size in macroporous hydrophilic
MGs presents a challenging problem. First, a signifi cant amount of water is bound
to the hydrophilic polymer backbone occupying some space in between polymer
chains. This water is not exchanged freely with bulk water present in the macro-
pores. Different methods were used for the determination of pore volume of
pAAm-MGs [Plieva et al., 2005]. In the most typical case, more than 90% of
pAAm-MGs (prepared from 6% monomer concentration) composed of large
and highly interconnected pores, while the polymer bound water present less than
10% (Figure 14.11 ).
More than 70% of liquid can be squeezed from pAAm-MGs by mechanical
compression. The total pore volume of pAAm-MGs was estimated using three
different techniques (Table 14.3), namely: from water vapor adsorption experi-
ments assuming that the pore volume is equal to the water volume inside the
cryogel-minus volume of polymer-bound (adsorbed) water; from uptake of a
good solvent, water and a poor solvent, cyclohexane. (As the hydrophilic polymer
did not swell in cyclohexane, the uptake of cyclohexane refl ected only the pore
volume however, in the dry sample.); and from comparison of the densities of the
dry and completely swollen cryogel samples.
All three methods gave consistent results with a clear tendency of decreasing
total pore volume from ~90% for pAAm-MGs prepared from 6% monomer
concentration to ~70% for pAAm-MGs prepared from 20% monomer
concentration.
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