Biomedical Engineering Reference
In-Depth Information
Scheme 1.5
Ring-expansion mechanism for the preparation of ABA triblock polyesters.
1.3.3
Covalently Crosslinked Polyesters
A further method to produce elastomeric polyester-based materials is the prepara-
tion of crosslinked amorphous polyesters. In this case, crystalline regions are
absent but mechanical strength is given by the inherent rigidity of the network.
Such materials are often described as “cured” polymers [46]. Due to the absence
of crystalline regions, erosion occurs more homogeneously, and properties can be
tailored by composition. Such elastomers were prepared by the photopolymeriza-
tion of methacrylate functionalized star- shaped
- caprolactone] -
co
- [
rac
-
lactide]) (see Section 1.5.2). Networks suitable for implant materials were obtained,
with physical properties adjustable by selecting the molecular weight of the pre-
cured polymers [47]. Poly(diol citrates) were synthesized by reacting citric acid with
various diols to form a covalent crosslinked network via polycondensation [48].
The physical properties and degradation characteristics could be controlled by
choosing different diols and by controlling the crosslink density of the polyester
network. Biocompatible materials with elongations at break as high as 500% could
be obtained. Other common crosslinkers used for curing polymers are multifunc-
tional isocyanates and acid chlorides.
poly([
ε
1.3.4
Networks with Shape-Memory Capability
Polymer networks can be designed in a way that they become capable of a shape-
memory effect [49]. Such materials possess the ability to memorize a permanent
shape, which can substantially differ from their temporary shape. The transition
from the temporary to the permanent shape could be initiated by an external
stimulus such as a temperature increase above a characteristic switching
