Biomedical Engineering Reference
In-Depth Information
macropores. However, freeze-dried materials can be produced only as relatively
thin objects, such as fi lms, plates, or small beads [O'Brien et al., 2004]. Besides,
the freeze-drying procedure is well-known to be a highly energy- and time-
consuming process. However, the macroporous gels ( cryogels ) are prepared at
moderately low temperatures and can be produced in any desirable shape, that is,
blocks, rods, disks, beads and so on. The cryogels can be produced from practi-
cally any gel forming system with a broad range of porosity, from 0.1 to 200
μ
m
[Lozinsky et al., 2002; Plieva et al., 2005, 2006a; Srivastava et al., 2007].
14.3 PREPARATION OF SUPERMACROPOROUS BIOMATERIALS
THROUGH CRYOGELATION TECHNIQUE
14.3.1 General Concept of Cryogelation
Cryogels are formed as a result of cryogenic treatment (freezing, storage in the
frozen state for a defi nite time, and defrosting) of low- or high-molecular-weight
precursors, as well as colloid systems, all capable of gelling. Cryotropic gelation
(or cryostructuration) is a specifi c type of gel-formation that takes place as a
result of cryogenic treatment of the systems potentially capable of gelation. The
essential feature of the cryogelation is compulsory crystallization of the solvent,
which distinguishes cryogelation from chilling-induced gelation when the gela-
tion takes place on decreasing temperature (as, for example, gelation of agarose
or gelatin solutions on cooling, which proceeds without any phase transition of
the solvent).
The processes of cryogelation have some unique characteristics:
1. Cryotropic gelation proceeds in a non-frozen liquid microphase existing in
a macroscopically-frozen sample. At moderately-low temperature below
the freezing point of solvent (often water), some part of solvent remains
non-frozen. All reagents are concentrated in this non-frozen part (so-
called non-frozen liquid microphase, NFLMP) where the chemical reac-
tion or process of physical gelation proceeds with time.
2. Crystals of the frozen solvent grow until they merge and after melting
leave behind the interconnected pores, thus playing a role of porogen. As
the size and alignment of solvent crystals is varied to a large extent, the
pores in the prepared cryogels are in the range of 1-200
m.
3. Typically, the critical concentration of gelation (CCG) is decreased
for cryogels as compared to the conventional gels prepared at room
temperature due to the concentrating of reagents in NFLMP (so-called
cryoconcentration).
μ
The cryogels (referred here as macroporous gels, MGs) are synthesized in
semi-frozen aqueous media where ice crystals perform as porogen and template
the continuous interconnected pores after melting. Contrary to conventional gels
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