Biomedical Engineering Reference
In-Depth Information
be accomplished in various ways. These processes represent a cross-section of
the general principles of matrix fabrication. In the following examples, porosity
is introduced by dissolving a dispersed solid particulate, sublimating a frozen
solvent, forcing gas into a solid polymer, or laying a polymer in a manner that
leaves holes. Slightly more time will be spent describing electrospinning and
computational methods of polymer fabrication due to their complexity. Other
fabrication techniques that will not be described in more detail include fiber
bonding, membrane lamination, extrusion, phase separation and high internal-
phase emulsion.
10.5.1 Solvent casting and particulate leaching
Solvent casting and particulate leaching use a solvated liquid polymer combined
with a solvated porogen. Particulate leaching is a simple method for introducing
porosity into a tissue engineered scaffold. To achieve this goal, the matrix
material solution is initially mixed with a suspended porogen. This suspension is
then allowed to solidify leaving the porogen dispersed in the solid polymer. The
entire construct is then soaked in a solvent specific to the porogen agent used
leaving empty space that was previously occupied by the solid porogen. The
pore size, geometry and density can be adjusted by altering the size, geometry
and concentration of the porogen in the polymer solution (Lee et al., 2003;
Mano et al., 2007).
10.5.2 Melt molding
Melt molding is an easy way to adjust the three-dimensional morphology of
polymer scaffolds without the use of harsh chemical solvents. Many of the
synthetic polymers used are thermoplastics, so they can be easily heated and
cooled into various solid forms. For this fabrication technique, the polymers are
heated above their glass transition temperature or melting point, so they can
assume a liquid form. The resulting polymer is then allowed to cool and
solidifies in the form of the mold. Melt molding is used in tandem with particle
leaching to introduce porosity into the scaffold. Unlike solvent casting, melt
molding does not use polymer solvents, so residual toxic solvents in the matrix
are not an issue. Unfortunately, the heat needed to liquefy the polymer restricts
the incorporation of active biomolecules.
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10.5.3 Emulsion freeze-drying
Freeze-drying is a method of introducing pores into a polymer scaffold without
the use of particulate leaching. Emulsion freeze-drying uses a solvated polymer
emulsified in a non-miscible liquid. The homogeneous mixture is then poured
into a mold and freeze-dried. Varying the weight percentage of the emulsifying
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