Biodegradable PHBV Polymer-Based Scaffolds for Bone Tissue Engineering - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 56

The degradation mechanism and rate of biodegradable polymers can be
affected by numerous factors. Among the factors which affect degradation are:
molecular weight, structure and content of comonomer unit, crystallinity, ori-
entation, blending, porosity, pH, temperature, and catalytic molecules or ions
(Tsuji 2008 ). It was also reported that when catalytic molecules or substances
such as enzymes and alkalis are present in the degradation media or environ-
ment, the degradation of polymer-based materials proceeds via a surface erosion
mechanism (Tsuji 2008 ). In the surface erosion mechanism, catalytic molecules
or ions act only on the surface of materials and will not diffuse into the material.
As a result, the material is eroded from the surface while the core part of the
material remains unchanged. On the other hand, the degradation of biodegrad-
able polymers takes place via a bulk erosion mechanism in the absence of cata-
lytic molecules or ions as in a phosphate-buffered solution. It was also reported
that the hydrolytic degradation mechanisms depends on the thickness of biode-
gradable materials and the critical thickness above which the degradation mech-
anism changes from bulk erosion to surface erosion depends on the molecular
structure of biodegradable or hydrolysable polymers (Burkersroda et al. 2002 ).
A significant weight loss can be observed at an early stage of degradation for a
surface 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 mono-
mers are formed. In order to trace bulk erosion, molecular weight change is most
effective. On the other hand, it is quite ineffective in the case of surface erosion
(Tsuji 2008 ).
The weight loss can be caused by physical disintegration and fragmentation
of scaffolds. The hydrolytic scission of polymer backbone in the scaffolds gradu-
ally produces short chain monomer molecules that absorb more water molecules
from the aqueous medium. Figure 3.12 deals with the scaffolds prepared from
5 % (w/v) and 10 % emulsion concentration. As the scaffolds porosity increases
with decreasing emulsion concentration, so it can be demonstrated that scaffolds
weight loss increases with increasing porosity of the scaffolds. In vitro degrada-
tion experiment of phosphate glass-reinforced PHB-based degradable composites
was studied in (PBS) and the results showed that the mass loss and mechanical
property change could be closely correlated with the solubility rate of the reinforc-
ing glass (Knowles et al. 1992 ). It was also demonstrated that the polymer was
highly permeable.
Accelerated hydrolysis at elevated temperatures (55 and 70 °C) was also
(Li et al. 2005 ) investigated (Albertsson 2002 ) and it was found that a homogene-
ous process was involved which had two stages. Above 60 °C, there was no induc-
tion period and the initial random scission of the ester groups occurred throughout
the polymer (amorphous and crystalline regions) which directed to a decrease in
molecular weight but little change in the polydispersity and almost no bulk mass
loss. As the scaffolds are intended to apply for biomedical application in the cur-
rent study, experiments were conducted mainly on physiological temperature and
condition.