Biomedical Engineering Reference
In-Depth Information
concentration of the dissolved substances, volume, and geometrical shape of the
sample) remain the same [Lozinsky et al., 2002]. The pore size depends on the
initial concentration of precursors in solution, their physicochemical properties
and the freezing conditions [Plieva et al., 2005, 2006b, 2007a]. Specifi cally, cryogels
differ from traditional gel materials due to the system of interconnected macro-
pores (frequently enduring the cryogels with sponge morphology) and due to the
structure of pore walls formed when a compulsory increase in polymer concen-
tration in non-frozen regions (NFLMP) takes place (enduring the MGs with a
relatively higher mechanical strength as compared to traditional gels with the
same formal bulk concentration of the polymer). One of the principal differences
of MGs compared to other macroporous materials (with the same range of pore
size) is that the MGs possess tissue-like elasticity and withhold large deforma-
tions without being collapsed.
14.3.2 Parameters Infl uencing Cryostructuration (Cryogelation) of
Polymeric Systems
Cryotropic gelation allows for the formation of polymeric materials with essen-
tially different morphology compared to gels prepared in non-frozen media.
Several parameters should be taken into account when preparing the polymeric
materials at sub-zero temperatures. Two main processes, namely the chemical
reaction (which proceeds in NFLMP) and solvent crystalization (which generate
the interconnected pores) proceed in the semifrozen systems. In order to obtain a
reproducible freezing pattern, a careful control of all experimental conditions is
required. Many parameters infl uence the cryostructuration of polymeric systems,
such as cooling (freezing rate), concentration and composition of gel precursors
in the initial reaction mixture, thermal prehistory of the reaction mixture, sample
size, the presence of nucleation agents, and so on.
14.3.2.1 Freezing Rate and Freezing Temperature.
The cooling (freez-
ing) rate is one of the crucial parameters to be controlled during the preparation
of MGs. The lower the freezing rate (or the higher the freezing temperature), the
bigger the size of growing ice crystals and, as a result, the cryogels with bigger
pore size are prepared [Plieva et al., 2004a,b]. However, at high freezing tempera-
ture there is the risk that the solution to be frozen will be in an overcooled (super-
cooled) state. Overcooling (or supercooling) is defi ned as cooling below the initial
freezing point of the water without forming ice crystals. This is a non-equilibrium,
metastable state of water. The nucleation temperature of water is affected by
both the cooling (freezing) rate, volume of the sample and the addition of a nucle-
ation agent [Chen and Lee, 1998; Chen et al., 1999]. So the temperature should be
low enough to securely freeze the reaction mixture. The faster the freezing (or the
lower the freezing temperature), the more spontaneous nucleation is promoted,
and the greater the number of crystals of smaller size [Lozinsky et al., 2002].
During freezing of an aqueous solution, one could distinguish at least three essen-
tial temperatures: the fi nal freezing temperature, T
f
(defi ned as temperature set in
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