Biomedical Engineering Reference
In-Depth Information
1.2.3.1 Poly(
α
-hydroxy ester)s
α
Thefamilyofpoly(
-hydroxyacid)ssuchasPGA,PLA,anditscopoly-
mer PLGA that are among the few synthetic polymers approved for
human clinical use by the Food and Drug Administration (FDA) are
extensively used or tested for the scaffold materials as bioerodi-
ble materials due to good biocompatibility, controllable biodegrad-
ability, and relatively good processability.
14
It has been used for
three decades as sutures of PGA, bone plate, screw, and reinforced
materials for PLA and drug delivery devices of PLGA in surgi-
cal operation and whose safety has been proved in many medical
applications.
15
These polymers degrade by nonspecific hydrolytic scission of
their ester bonds. PGA biodegrades by a combination of hydrolytic
scission and enzymatic (esterase) action, producing glycolic acid,
which can either enter the tricarboxylic acid (TCA) cycle or be
excreted in urine and be eliminated as carbon dioxide and water.
The hydrolysis of PLA yields lactic acid that is a normal by-product
of anaerobic metabolism in the human body and is incorporated
in the TCA cycle to be finally excreted by the body as carbon diox-
ide and water. With an additional methyl group to glycolide, PLA
is much more hydrophobic than the highly crystalline PGA. As a
result, PLA has a much slower rate degradation rate for over one
year. The degradation time of PLGA as copolymers of these two
polymers can be controlled from weeks to over a year by varying
the ratio of monomers, its molecular weight, and the process-
ing conditions. The synthetic methods and physicochemical prop-
erties such as melting temperature, glass transition temperature,
tensile strength, Young's modulus, and elongation were reviewed
elsewhere.
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The mechanism of biodegradation of poly(
α
-hydroxy acid)s is
bulk degradation, which is characterized by a loss in the polymer
molecular weight, while mass is maintained. Mass maintenance is
useful for tissue engineering applications of those specific shapes.
However, loss in molecular weight causes a significant decrease
in mechanical properties. Degradation is depending on chemical
history, porosity, crystallinity, steric hindrance, molecular weight,
water uptake, and pH. Degradable products such as lactic acid and
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