Fabrication Techniques and Properties of Scaffolds - Biodegradable Polymer-Based Scaffolds for Bone Tissue Engineering - page 28

Table 2.2 Indexes for degradation (Tsuji 2008 )
Material-based indexes Non-material based indexes

1. Weight remaining 1. Dissolved organic carbon (DOC) or total

organic carbon (TOC)

2. Molecular weight and distribution 2. Biochemical oxygen demand (BOD)

3. Amount of released carbon dioxide

3. Physical properties 4. Amount of release biogas

5. pH

4. Material morphology 6. Absorbance (Turbidity)

most important factors which determine the degradation of biodegradable poly-
mers. Tsuji reported that the incorporation of hydrophilic monomer units or poly-
mers, the presence of catalytic molecules, increasing or decreasing pH, increasing
temperature etc. can accelerate the hydrolytic degradation (Tsuji 2008 ). Increasing
molecular weight, crystallinity, degree of orientation reduces the hydrolytic deg-
radation rate. Table 2.2 shows the indexes for hydrolytic degradation that can be
used for tracing the hydrolytic degradation of materials depending on the erosion
mechanism.

2.4.2 General Mechanism of Degradation

It has been reported that when catalytic molecules or substances such as enzymes
and alkalis are present in the degradation media or environment, the degrada-
tion of polymer-based materials proceeds via a surface erosion mechanism (Tsuji
2008 ). Figure 2.2 is the schematic illustration of surface erosion mechanism where
the increase in brightness of the material means a molecular weight decrease.
In the surface erosion mechanism, catalytic molecules or ions act only on the
surface of materials and does 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 most of the biodegradable polymers takes
place via a bulk erosion mechanism in the absence of catalytic molecules or
ions as in a phosphate-buffered solution (Tsuji 2008 ). It was also reported that
the hydrolytic degradation mechanism depends on the thickness of biodegrad-
able materials and the critical thickness above which the degradation mechanism
changes from bulk erosion to surface erosion depends on the molecular structure
of biodegradable or hydrolysable polymers (Burkersroda et al. 2002 ). Tsuji also
reported that in the case of PLA, when the thickness of the PLA material is larger
that 2 mm, the entrapment and accumulation of hydrolysis-forming oligomers and
monomers with a high catalytic effect occur in the core part of the materials (Tsuji
2008 ). As a result, an accelerated hydrolytic degradation in the core part which is
termed as core-accelerated bulk erosion can happen.