Biomedical Engineering Reference
In-Depth Information
occurring biopolymers are typically degraded by enzymatic means at a rate that
may be diffi cult to predict clinically. Furthermore, natural polymers may have
unwanted side effects arising from inherent biological activity. This has led to the
widespread use of biodegradable synthetic polymers in therapeutic applications.
Of this class, biodegradable aliphatic polyesters, which are degraded hydrolytically,
are by far the most employed.
1.1.2
Poly(Hydroxycarboxylic Acids)
All polyesters are, in principle, hydrolytically degradable. However, only (co)poly-
esters with short aliphatic chains between ester bonds typically degrade over the
time frame required for biomedical applications. The major group of this material
are the poly(hydroxycarboxylic acids), which are prepared via ring- opening polym-
erization of lactones or cyclic diesters. Indeed, the fi rst biodegradable polyester
used as a medical suture in the 1960s was based on the polyglycolide. Scheme 1.1
shows the most common monomers and the polymers they produce. These
can be summarized as diglycolide, stereogenic dilactides, lactones such as
ε
caprolactone and stereogenic
-butyrolactone, the cyclic trimethylene carbonate,
and
p
-dioxanone. As the polymerization methods of these monomers are broadly
applicable to each, copolymers such as poly(lactide-
co
- glycolide)
β
are
readily
produced.
Another source of poly(hydroxycarboxylic acids) is from bacteria, which store
polyesters as their energy source [7]. These polymers are known as polyhydroxy-
alkanoates (PHAs) in the literature. The most common polymer derived from
bacteria is poly(3-hydroxybutyrate), which has the same structure as the polymer
which can be obtained from optically active
- butyrolactone [8] . Poly(3 -
hydroxybutyrate) formed in this way is strictly stereoregular, showing the (
R
)
confi guration. Biotechnologically produced polymers are discussed in more details
in Chapter 2 of this handbook.
β
Scheme 1.1
Common cyclic monomers for the preparation of polyester derivatives.
