Tissue Engineering: Focus on the Musculoskeletal System - Biomaterials Science: An Integrated Clinical and Engineering Approach - page 211

another important issue. The speed of scaffold manufacturing impacts
fabrication cost and the feasibility for clinical applications. Longer manu-
facturing time also impacts cell viability when making cell-encapsulating
scaffolds. Because CW-SLS, fs-SLS, and microsyringe stereolithography are
3-D volumic scanning methods, the time for making one scaffold using these
platforms is proportional to the scaffold volume, or the 3rd order of scaffold
dimension; these platforms become very inefficient for building scaffolds of
centimeter size. Compared to other platforms, PPS-developed microstruc-
tures through the aforementioned 1-D scanning have the advantage of build-
ing large scaffolds at a much higher fabrication rate. Table 9.2 summarizes
the different aspects of the aforementioned platforms: their working prin-
ciples, fabrication speed, structure resolutions, and suitable monomer types.
Supercritical CO 2 and Gas-foamed Scaffolds
Gas foaming is a technique that allows for processing polymers such as
PLGA into highly porous scaffolds without using organic solvents or high
temperature, which facilitates incorporating sensitive biological signals such
as growth factors or nucleic acids. Supercritical CO 2 has been employed to
make polymeric scaffolds at approximately 31°C and 1500 psi. Under these
conditions, the biomolecules remain intact while the polymer transforms
into a liquid stage, and therefore encapsulation of biomolecules within the
polymer can be achieved. When the pressure is decreased to ambient condi-
tions, the polymer reverts to a solid state and swells during the removal of
CO 2 ; a sponge-like scaffold is formed that is directly related to the rate of
CO 2 removal (Figure 9.4). Howdle and colleagues have performed extensive
research on this topic using a variety of polymers, biomolecules, and even cells
to form porous scaffolds for non-viral gene delivery and tissue engineering
applications [90-93]. Plasmid DNA was complexed with a cationic poly-
mer and deposited on the PLA powder before freeze drying. The mixture
was then exposed to supercritical CO 2 at 35°C and 2500 psi. The pressure was
gradually released to form porous constructs encapsulating the polyplexes.
The resulting scaffolds showed a sustained release of functional polyplexes
from the scaffolds, with a moderate level of transfection over 60 days. This
study demonstrates the potential of using supercritical CO 2 -sustained deliv-
ery of functional polyplexes. Ginty et al. have evaluated viability of multiple
cell lines (C2C12 cell line, 3T3 fibroblasts, chondrocytes, and hepatocytes)
under supercritical conditions (35°C and 1070 psi) [94]. Cell/polymer com-
posites were exposed to supercritical CO 2 for 30 seconds with an additional
80 seconds for pressurization and depressurization of the cylinder. Cell
viability was confirmed and osteogenic differentiation of C2C12 cells was
shown by alkaline phosphatase staining. Combining the supercritical
technique of scaffold fabrication with biomolecule encapsulation provides
a versatile platform for delivering biological cues from 3D depot to guide
tissue regeneration. High-pressure CO 2 (800 psi) has also been demonstrated