Biomedical Engineering Reference
In-Depth Information
2000
1000
0
-20
-15
-10
-5
0
Temperature (°C)
Figure 14.4.
Linear fl ow velocity of water through supermacroporous monolithic cryogels pre-
pared at different temperatures: plain-pAAm (open triangles), epoxy-pAAm (closed squares).
The fl ow rate of water passing through the monolithic columns was measured at the constant hy-
drostatic pressure equal to 100 cm of water -column corresponding to a pressure of circa 0.01 MPa.
For each sample the average of three measurements was taken. Reproduced from [Plieva
et al., 2004a] with permission.
A crystallization plateau near 0 °C was observed for the system that con-
tained both free and bound water (collagen dispersion at pH 12). The change of
slope for the curve two (collagen suspension at pH 3) at a temperature of about
−
15 °C was treated as the freezing of the bound water [Podorozhko et al., 2000].
A different cooling (freezing) rate of the reaction mixture directly affected
the porous properties of MGs. Namely, the pAAm MGs formed at different fi nal
freezing temperatures (T
f
) had different fl ow resistances when they were pro-
duced as monolith columns [Plieva et al., 2004a]. The water fl ow resistance (esti-
mated as water fl ow path through the cryogel monolith column at a given
hydrostatic pressure equal to one meter of water column) decreased signifi cantly
when decreasing the freezing temperature (T
f
) from
15 ° C (Figure 14.4 ).
At lower freezing temperatures smaller ice crystals are formed, yielding
smaller cross-section area of each macropore, and, therefore, fl ow resistance of
the cryogel monolith increases. Besides, the gel fraction yield as well as compres-
sive strength were decreased for the pAAm-MGs prepared at T
f
of
−
10 to
−
−
30 ° C (Table
14.1 ) [Plieva et al., 2006a ].
The low gel fraction yield for the pAAm MGs prepared at T
f
of
−
30 ° C com-
pared to that prepared at
12 °C was due to the quenching of the polymerization
process when the system was completely frozen (no NFLMP remained) at tem-
peratures below the eutectic point (which for AAm solution is about
−
18 ° C). In
agreement with high fl ow resistance of the pAAm-MGs prepared at T
f
of
−
30 ° C,
scanning electron microscopy (SEM) analysis revealed no macroporous structure
formed after polymerization at
−
−
30 ° C [Plieva et al., 2006a ].
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