DESIGN OF SUPERMACROPOROUS BIOMATERIALS VIA GELATION AT SUBZERO TEMPERATURES —CRYOGELATION - Advanced Biomaterials: Fundamentals, Processing, and Applications - page 502

MGs (cryogels)
Conventional gels

40

20

0

pAAm PVA Dex-MA Ags PEG HEMA

Figure 14.8. The degree of swelling of macroporous gels prepared at subzero temperatures
(gray bar) and conventional gels prepared at room temperature of 20 °C (white bar) from
the same initial reaction mixture. pAAm (polyacrylamide), PVA (polyvinyl alcohol), Dex-MA
(dextran-methacrylate), Ags (agarose), PEG (polyethylene glycol) and HEMA (hydroxyethyl
methacrylate). Reproduced from [Plieva et al., 2007a] with permission.

cross - linking ratio vinyl(monomers)/divinyl (cross - linker) of 5 - 10, mol/mol)
had lower fl ow resistance compared to that of pAAm-MGs formed from
the feedstock with low content of the cross-linker (MBAAm) in the reaction
mixture (vinyl/divinyl ratio of 20-30, mol/mol). The swelling degree of highly
cross-linked network of pAAm-MGs was much less compared to the swelling
degree of the pAAm-MGs with low MBAAm content [Plieva et al., 2007a].
In general, the swelling degree for the MGs is much lower compared to that of
conventional gels prepared from the same feedstock (Figure 14.8). The MGs
prepared from both monomer or macromer gel precursors swell much less
compared to the conventional gels (Figure 14.8). Lower swelling ability of MGs
compared to conventional gels is one of the characteristic features of these mac-
roporous materials.
For the MGs with “ smart ” properties (as poly - N - isopropylacrylamide MGs,
pNIPAAm-MGs), faster stimuli response to the change of environmental condi-
tions compared to that of conventional pNIPAAm gels was observed [Zhang and
Chu, 2003; Galaev et al., 2007]. It is known that in aqueous solutions, pNIPAAm
undergoes hydrophobic aggregation followed by the transition from soluble to
insoluble state when the temperature is increased above a critical temperature of
32 °C [Schild, 1992]. The swelling degree of pNIPAAm-MGs prepared at subzero
temperatures was much less compared to the conventional pNIPAAm gels pre-
pared at 22 ° C (Figure 14.9 ) [Zhang and Chu, 2003 ].
The swelling of pNIPAAm-MGs depended on the concentration of mono-
mers in the initial reaction mixture and cross-linking degree [Galaev et al., 2007].
The sharp transition around 30 °C was observed for pNIPAAm-MGs in the tem-
perature interval studied from 4 to 40 °C [Galaev et al., 2007]. The stimuli response