Biomedical Engineering Reference
In-Depth Information
Bioactive Molecules
A
B
Microparticles
Pre-cultured Cells
Pre-cultured Cells
Fig. 7.1.4-2 Localized delivery of bioactive molecules from scaffolds. (A) Release directly from the supporting matrix. (B) Microparticles
or nanoparticles loaded with bioactive molecules are impregnated into scaffolds and serve as delivery vehicles.
complex 3D scaffolds since only a thin layer at the sur-
face may be engineered by coating.
onto glass petri dishes. After evaporating the solvent, the
PLA/salt composite membranes are heated above the PLA
melting temperature and then quenched or annealed by
cooling at controlled rates to yield amorphous or semi-
crystalline foams with regulated crystallinity. The salt
particles are eventually leached out by selective dissolution
in water to produce a porous polymer matrix.
Highly porous PLA foams with porosities up to 93%
and median pore diameters up to 500 m m have been
prepared using the above technique ( Mikos et al. , 1994b;
Wake et al. , 1994 ). Porous PLGA foams fabricated by the
same method have been shown to support osteoblasts
growth both in vitro and in vivo ( Ishaug et al. , 1997;
Ishaug-Riley et al. , 1997, 1998 ). The porosity and pore
size can be controlled independently by varying the
amount and size of the salt particles, respectively. The
surface-area-to-volume ratio depends on both initial salt
weight fraction and particle size. In addition, the crys-
tallinity, which affects both degradation and mechanical
strength of the polymer, can be tailored to a particular
application. A disadvantage of this method is that it can
only be used to produce thin wafers or membranes with
uniform pore morphology up to 3 mm thick ( Wake et al. ,
1996 ). The preparation of thicker membranes may result
in the formation of a solid skin layer characteristic of
asymmetric membranes. The two controlling phenomena
are solvent evaporation of the surface and solvent diffu-
sion in the bulk.
This method has been modified to fabricate tubular
scaffolds (Mooney et al., 1995a, 1994b). Porous PLGA
membranes prepared using SC/PL are wrapped around
Teflon cylinders, and the overlapping ends are fused to-
gether with chloroform. The Teflon core is then removed
to leave a hollow tube. Because of the relatively brittle
nature of the porous membranes used, this method is
Solvent casting and particulate leaching
In order to overcome some of the drawbacks associated
with fiber bonding, a solvent-casting and particulate-
leaching (SC/PL) technique has been developed to prepare
porous scaffolds with controlled porosity, surface-area-
to-volume ratio, pore size, and crystallinity for specific
applications ( Mikos et al. , 1994b ). This method can be
applied to PLA, PLGA, and any other polymers that are
soluble in a solvent such as chloroform or methylene
chloride. For example, sieved salt particles are dispersed in
a PLA/chloroform solution that is used to cast a membrane
Table 7.1.4-3 Examples of scaffolds processed by various techniques
Processing technique
Examples
Fiber bonding
PGA fibers; PLA-reinforced PGA
fibers
Solvent casting and particulate
leaching
PLA, PLGA, PPF foams
Superstructure engineering
PLA, PLGA membranes
Compression molding
PLA, PLGA foams
Extrusion
PLA, PLGA conduits
Freeze-drying
PLGA foams
Phase separation
PLA foams
High-pressure gas foaming
PLGA, P(PF-co-EG) scaffolds
Solid free-form fabrication
Complex 3D PLA, PLGA structures
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