Biomedical Engineering Reference
In-Depth Information
that the stretching out and aligning of the cells along a surface feature (greater than 5
m)
causes them to initiate bone deposition. This leads to a mechanical interlocking between the
bone tissue and the implant that increases the bond strength.
There are many techniques for producing porous biomaterials for tissue engineering, and
the field continues to develop rapidly. One early approach to creating pores within a bio-
material for use as a cell scaffold is to dissolve the polymer in a solvent and mix in particu-
late materials that are stable in the solvent but can be dissolved later (porogens). The
solvent is allowed to evaporate, leaving the polymer strands tightly encasing the porogens.
The porogens are then dissolved by washing them out with a different solvent that will not
dissolve the polymer, such as water, to form pores. This is known as solvent casting/partic-
ulate leaching. Related to this technique is coascervation, in which the polymer is dissolved
in a solvent, the porogen is added, and then a nonsolvent for the polymer is added. A poly-
mer precipitate forms, entrapping the porogen particulates. The precipitate can be collected
and then compacted prior to removal of the porogen. After washing to remove the porogen,
a porous structure is revealed, as shown in the composite bone crystal/polyhydroxybuty-
rate hydroxyvalerate (PHBHV) polymer in Figure 5.13. Organic solvents are used with
these procedures and must be thoroughly removed under vacuum or low heat to avoid kill-
ing the cells.
Phase separation/emulsification can also be used to fabricate porous polymer scaffolds.
In this technique a polymer is dissolved in a solvent, and then an immiscible liquid (such as
water) is added and mixed to form an emulsion. The polymer/water mixture is cast into a
mold, rapidly frozen, and then freeze-dried, which is known as lyophilization. The space
that was water becomes a pore, and the solvent is evaporated, leaving behind a solid struc-
ture. Scaffolds with high porosity (up to 95 percent) have been formed by this method, but
the small pore sizes (13-35
m
m) are a drawback. Fiber bonding methods in which pre-
formed fibers are layered or woven and then hot-melted or glued together by solvent expo-
sure is another technique for forming porous materials. The advantage is that the pore sizes
are controllable and interconnected. The drawback is that the pore channels are rectangular
m
FIGURE 5.13
A composite biomaterial scaffold made of
bone crystals and the resorbable polyhydroxybutyrate hydroxy-
valerate (PHBHV) copolymer. The coascervation technique was
used to precipitate the polymer from solution to produce this
scaffold. Dissolvable porogens were used to create the large
pores needed for bone repair applications.
(The scaffold was made
by Marianne Manot, a former student of Dr. Kuhn's.)
