Biomedical Engineering Reference
In-Depth Information
for the desired application, while equipping the respective systems with ap-
propriate physical properties. Relevant physicochemical properties include
the rheology and mechanism of gel formation, degradation behavior, and me-
chanical properties of the system. These features are specified by the polymer
and fabrication technique used to form the polymeric vehicle.
Polymers for VEGF Delivery
Synthetic polymers are readily available and exhibit well-defined chemical
and physical characteristics allowing for the reproducible fabrication of VEGF
delivery systems. Thermoplastic, aliphatic polyesters including poly(glycolic
acid) (PGA), poly(lactic acid) (PLA), and their copolymers (PLGA) are
the most widely used synthetic polymers for this purpose (Fig. 6). These
polyesters are FDA approved and considered biocompatible [76]. When
placed in an aqueous environment the otherwise water-insoluble materi-
als degrade through hydrolysis yielding naturally occurring metabolic by-
products (lactic and glycolic acid). In order to provide systems that allow for
the controlled release of bioactive factors for extended periods of time, these
aliphatic polyesters are most commonly processed into microspheres and
porous polymer scaffolds [77, 78]. VEGF incorporation into microspheres
is typically accomplished with emulsion techniques involving organic sol-
vents, and yields delivery systems that may be readily injected into the
body [77, 79, 80]. However, the stability of growth factors incorporated into
polymer in this manner may be negatively affected by the organic solvents,
and the microclimate in the microspheres may deteriorate the bioactivity of
the VEGF [69, 81]. Efforts have been made to overcome these limitations by
appropriately modifying the microsphere composition, and developing tech-
nologies that do not require organic solvents [69, 82].
Porous polymer scaffolds made from aliphatic polyesters often serve as
three-dimensional (3-D) cell carriers in tissue engineering [83-85]. Recently,
a gas foaming particulate leaching procedure has been established to allow for
scaffold fabrication without the use of the organic solvents or high tempera-
Fig. 6 Chemical structure of the aliphatic polyesters poly(glycolic acid) (PGA) and
pol(lactic acid) (PLA), sodium alginate, and oxidized sodium alginate
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