Biomedical Engineering Reference
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at about 3.3 minutes for the thermogram obtained at T f
30 ° C (a temperature
which is below the eutectic point of about
20 ° C for water/acrylamide system
[Plieva et al., 2006a] indicated probably the complete crystallization of the liquid
microphase that is not frozen at higher temperatures.
When freezing a dispersed system (as collagen dispersion), the amount of
bound solvent (in addition to the free solvent) should be taken into account when
choosing the freezing regime to provide the conditions for ice crystallization. Col-
lagen (one of the most important extracellular matrix (ECM) proteins) is known
to undergo gel formation on cooling at positive temperatures [Podorozhko
et al., 2000]. The swelling of collagen particles in aqueous collagen dispersions in
acidic and basic media governed the ratio of the amount of free solvent to that
bound by protein, thus promoting the favorable conditions for the effi cient non-
covalent interactions and gel-formation of this protein. High swelling of collagen
particles in dispersions in basic and especially in acidic medium was accompanied
by irreversible change in the morphology of the dispersed particles [Podorozhko
et al., 2000]. The freezing curves obtained for the 7.5% aqueous dispersions of
collagen in acidic and basic media at fi nal freezing temperature T f of
25 ° C
showed how the temperature changed during the cooling of the system that
contained both free and bound water (collagen dispersion in basic medium at
pH 12) or only bound solvent (collagen dispersion in acidic medium at pH 3)
(Figure 14.3 ).
20
10
0
2
1
-10
-20
-30
0
5
10
15
20
Time, min
Figure 14.3. Freezing thermograms of 7.5% aqueous collagen dispersions at freezing tem-
perature of 25 °C. Curve 1 for collagen dispersions at pH 12 and curve 2 for collagen disper-
sions at pH 3. Reproduced from [Podorozhko et al., 2000] with permissions.
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