Biomedical Engineering Reference
In-Depth Information
temperature of the polymer (thermally induced shape-memory effect). Exemplary
shape-memory biodegradable polyesters have been prepared as networks of star-
shaped polymers crosslinked by diisocyanates [50], or as photocrosslinkable mac-
rodimethacrylates [51] . Biodegradable shape - memory polymers will be covered in
more detail in Chapter 8 .
1.4
Degradation Mechanisms
In general, two different mechanisms for the biodegradation of polyesters are
discussed in literature: bulk degradation and surface erosion [52]. In the bulk
degradation process, water diffuses into the polymer matrix faster than the polymer
is degraded. The hydrolyzable bonds in the whole polymer matrix are cleaved
homogeneously. Therefore, the average molecular weight of the polymer decreases
homogeneously. In the case of surface erosion, the diffusion rate of water into the
polymer matrix is slower than the degradation rate of the macromolecules. The
degradation only takes place in the thin surface layer while the molecular weight
of the polymer in the bulk remains unchanged. Surface erosion is a heterogeneous
process, with a rate strongly dependent on the shape of the test sample (e.g., size
of the surface) [53].
The majority of polyester materials undergo bulk hydrolysis, as will be explained
in the following section. Polyanhydride materials differ from the common polyes-
ters by the fact that they undergo linear mass loss by surface erosion mechanisms
[54]. The hydrophobic chains preclude water penetration into the bulk of the mate-
rial, thus negating bulk erosion mechanisms.
1.4.1
Determining Erosion Kinetics
Erosion rates can be determined
in vitro
and
in vivo
[55] . For
in vitro
experiments,
the polymers are exposed to an aqueous solution, in which ionic strength, pH-
value, and temperature can be varied. The degradation products of the polymer
can be isolated from the aqueous solution and characterized. The addition of
enzymes is also possible. Furthermore, the polymers can be exposed to cell- and
tissue cultures. By suitable selection and systematic variation of the
in vitro
test
conditions, the infl uence of single parameters on the degradation behavior of the
polymer can be determined. Accelerated degradation tests at elevated temperature
(usually 70°C) serve as preliminary experiments for planning the 37°C experi-
ments and to give reference to the extended degradation behavior of the materials.
Another method to accelerate hydrolysis tests is the elevation of the pH-value of
the degradation medium to the alkaline region (as a rule 0.01 or 0.1M NaOH
solution). The reaction of cell- and tissue cultures in contact with the material gives
information on the compatibility of the partially degraded polymer samples and
their degradation products.
