Polyesters - Biodegradable Polymers - page 13

In vivo experiments are performed with different species such as dogs, monkeys,
rats, mice, and sheep [56]. To investigate the degradation behavior, the implants
are typically placed subcutaneously or intramuscularly. The tissue compatibility of
the polymer can be determined by histological investigations. There are several
characteristics to follow in implant material degradation, for example, height,
weight, and mechanical properties of the test device. An additional method for in
vivo tests is marking of the implant with 14 C or by fl uorescent chromophores. In
this case, it can be observed where the fragments of the degraded polymer and
the degradation products in the test animal remain. Furthermore, the change in
thermal properties, crystallinity, and the surface properties (wettability, rough-
ness), depending on the degradation- and implantation time duration, can be
determined.

1.4.2
Factors Affecting Erosion Kinetics

Poly(hydroxycarboxylic acid)s degrade via the bulk process [57]. The degradation
process can be divided into three parts. In the fi rst step, water is absorbed and the
polymer swells. Several ester bonds are cleaved already, but there is no mass loss.
In the second step, the average weight is signifi cantly reduced. As ester bonds are
cleaved, carboxylic groups are formed, which autocatalyze the hydrolysis. During
this period, the polymer loses mechanical strength. The third step is characterized
by mass loss of the test sample and an increase in degradation rate. The degrada-
tion of an implanted material is completed when oligomeric and low-molecular-
weight fragments are dissolved in the surrounding medium. The dissolved
polymer fragments are then hydrolyzed to the free hydroxycarboxylic acids. Deg-
radation products, many of which occur naturally within the metabolic cycle (e.g.,
lactic acid), are typically removed from the body without toxic effect. To some
extents, smaller crystalline segments may remain, which are eliminated from the
body by phagocytosis [58].
The degradation times of several poly(hydroxycarboxylic acids) are summarized
in Table 1.4. The differences in degradation rates may be rationalized mainly by
the ability of water to permeate the polymers (crystallinity and hydrophobicity),
and in the case of polylactides the presence of an - methyl group which hinders

hydrolysis on steric grounds. Copolymers, due to the greater prevalence of amor-
phous regions, are generally degraded faster than homopolymers [59].
The higher degradation rate of poly( rac - lactide) compared to poly( l - lactide) is
due to the higher crystallinity of the isotactic poly(l - lactide). Polyglycolide, being
less hindered at the scission site, is degraded relatively quickly. The degradation
rate of poly(lactide- co - glycolide) can be fi nely tuned by varying the monomer
content. Polydioxanone and poly(

α

-caprolactone) are also less sterically hindered
but increased hydrophobicity hinders erosion. Poly(

ε

- caprolactone), with a penty-
lene ( - C 5 H 10 -) chain, erodes an order of magnitude slower than polyglycolide.
Due to the high crystallinity, poly(3- R - hydroxybutyrate) is relatively slowly
degraded. In this case, a certain amount of surface erosion takes place fi rst. With

ε