Biomedical Engineering Reference
In-Depth Information
(which are homophase systems where solvent is bound to the polymer network),
the MGs are heterophase systems where solvent (water) is presented both inside
interconnected pores and bound to the polymer network. Depending on the gel
precursors and chemical reaction used, the micro- and macroporous structure of
MGs can be varied to a large extent [Plieva et al., 2007a]. In fact, the cryogelation
technique allows for the formation of biocompatible macroporous materials with
unique properties such as open and highly permeable porous structure, tissue-like
elasticity, and excellent mechanical strength of the MGs. It is somewhat diffi cult
to achieve these properties with other techniques that are being used for the
preparation of macroporous biomaterials.
The MGs are produced via gelation processes at subzero temperatures when
most of the solvent is frozen while the dissolved substances (monomers or
polymer precursors of a cryogel) are concentrated in small non-frozen regions
(NFLMP), where the gel-formation proceeds. While all reagents are concentrated
in NFLMP, some part of the solvent remains non-frozen and provides the solutes
accumulated into NFLMP with suffi cient molecular or segmental mobility for
reactions to perform. An acceleration of chemical reactions performed in NFLMP
compared to the chemical reaction in bulk solution is often observed within a
defi ned range of negative temperatures [Lozinsky, 2002]. After melting the
solvent crystals (ice in case of aqueous media), a system of large continuous inter-
connected pores is formed (Figure 14.1).
Thus the shape and size of the crystals formed determine the shape and size
of the pores formed after defrosting the sample. In general, the size of ice crystals
depends on how fast the system is frozen, provided other parameters (such as
Freezing
Polymerization
in frozen state
Thawing
Monomers and initiators
Ice crystals
Initially forming polymer
Cross-linked polymer gel
Supermacropore
Figure 14.1.
Schematic presentation of the formation of macroporous gels. Reproduced
from [Plieva et al., 2004b] with permission.
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