Fabrication Techniques and Properties of Scaffolds - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 23

PLGA/gelatin microsphere composites (Mooney et al. 1996 ). According to this
method, a fine PLGA powder was mixed with previously sieved gelatin micro-
spheres and poured into a Teflon mold, which was then heated above the glass
transition temperature of the polymer. The PLGA/gelatin microsphere compos-
ite was then removed from the mold and placed in distilled-deionized water. The
water soluble gelatin was leached out leaving a porous PLGA scaffold with the
geometry identical to the shape of the mold. It was possible to construct PLGA
scaffolds of any shape simply by changing the mold geometry by using this
method. The porosity could be controlled by varying the amount of gelatin used
to construct the composite material and the pore size of the scaffold could also
be altered independently of the porosity by using different diameters of micro-
spheres. Another advantage of this method was that it does not utilize organic sol-
vents and is carried out at relatively low temperatures. For this reason, it had the
potential for the incorporation and controlled delivery of bioactive molecules. This
scaffold manufacturing technique could also be applied to other polymers such
as PLLA and PGA. Many of the scaffold preparation design criteria were satis-
fied by this technique and offer an extremely versatile means of scaffold prepara-
tion. Alternative leachable components such as salt or other polymer microspheres
could also be used other than gelatin microspheres.
2.1.7 Gas Foaming
In order to eliminate the need for organic solvent in the pore-making process, a
new technique involving gas as a porogen was introduced (Harris et al. 1998 ). The
process started with the formation of solid discs of PGA, PLLA or PLGA by using
compression molding with a heated mold. The discs were placed in a chamber
and exposed to high pressure CO 2 (5.5 MPa) for three days. At this time, pres-
sure was rapidly decreased to atmospheric pressure. Up to 90 % porosities and
pore sizes of up to 100 μ m could be obtained using this technique. But the disad-
vantage was pores are largely unconnected, especially on the surface of the scaf-
fold. Although the fabrication method required no leaching step and used no harsh
chemical solvents, but the high temperature which was involved in the disc forma-
tion, prohibited the incorporation of cells or bioactive molecules. Also the uncon-
nected pore structure made cell seeding and migration within the scaffold difficult.
In order to produce an open pore morphology using this technique, both gas foam-
ing and particulate leaching technique were developed. According to this, salt
particles and PLGA pellets were mixed together and compressed to scaffold solid
disks which were saturated with high pressure gas and the pressure was subse-
quently reduced. The salt particles were removed then by leaching. This combina-
tion guided to a porous polymer matrix with an open, interconnected morphology
without the use of any organic solvents. This technique might have widespread use
in cell transplantation applications of many types of cells, including hepatocytes,
chondrocytes, and osteoblasts.