Biomedical Engineering Reference
In-Depth Information
distribution of fi ller and low cost. In melt molding, the optimization of the
processing parameters such as compounding, compression and injection
molding and extrusion, composition and shape are important [17]. The
technique gives uniform particle distribution [18], particularly when twin-
screw extrusion is used in the processing of polymer and nanocomposites.
5.2.3 Gas-Foaming Processes
Gas foaming has been developed as a method for producing porous 3D
polymer scaffolds without the use of organic solvents. This happens to be
very cost effective and environmentally friendly. This is desirable, as the
residual solvent can have toxic effects
in vitro
and elicit an infl ammatory
response
in vivo
. In this method the molded samples are exposed to high
pressure CO
2
to saturate the material. Subsequent reduction in pressure
causes the nucleation and formation of pores in the composites of PLA and
ceramic nanoparticles (HAp or
b
-TCP) from the CO
2
gas [19]. The main
disadvantage of this method is that it yields a nonporous surface and
closed-pore structure, with only 10-30% of interconnected pores. In order
to improve porosity and interconnectivity of the pores, a method combin-
ing particulate leaching with the gas-foaming process has been developed
(Figure 5.2a), albeit not being able to completely eliminate closed pores [20].
(
a
)
(
b
)
5 mm
(
c
)
(
d
)
1 mm
Figure 5.2
Representative examples of the structure of 3D scaffolds fabricated
using various methods. (a) Stereomicroscope image of PLA-HA scaffold
fabricated by gas foaming. (b) SEM image of PLA/HAp fi brous scaffold
fabricated by electrospinning method. (c) SEM image of PLGA/n-HAp
composite scaffold fabricated by microsphere sintering method. (d) A 3D
periodic structure with a tetragonal symmetry of a scaffold fabricated using
rapid prototyping method. Reprinted with permission from [20-23].
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