Fabrication Techniques and Properties of Scaffolds - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 29

Fig. 2.2 Hydrolytic degradation mechanisms (Tsuji 2008 ). a Surface erosion. b Bulk erosion.
c Core-accelerated bulk erosion
It was also reported that in the case of crystallized polymers, numerous spher-
ulites are contained in them (Tsuji 2008 ). Tsuji also reported that the chains in
amorphous regions in crystallized polymers are more susceptible to hydrolytic
degradation than those in crystalline regions which leave the chains in crystal-
line regions intact. “Crystalline residues” that are the remaining crystalline
regions have the structure of “extended chain crystallites” (Tsuji 2008 ). A sig-
nificant weight loss can be observed at an early stage of degradation for a sur-
face erosion mechanism. On the other hand, the weight loss occurs only at a
late stage of degradation for a bulk erosion mechanism when a large decrease in
molecular weight takes place and when water soluble oligomers and monomers
are formed. In order to trace a bulk erosion, molecular weight change is most
effective. On the other hand, molecular weight measurement is quite ineffective
in the case of surface erosion (Tsuji 2008 ). The in vivo and in vitro degrada-
tion of aliphatic polyesters were studied and it was reported that degradation is
catalyzed by carboxyl end groups formed by chain cleavage and the amorphous
regions are preferentially degraded (Li 2006 ).
2.4.3 Kinetics of Degradation
The hydrolytic degradation of semicrystalline high molecular weight PLLA gen-
erally proceeds through random bulk hydrolysis in two distinct stages. The first