Biomedical Engineering Reference
In-Depth Information
Soluble oligomers
H
2
O
H
2
O
t
H
2
O
H
2
O
ions?
0
5
10
15
Time in weeks
Water absorbtion + Degradation
without loss of matter
Central degradation + Formation
of the outer membrane
Beginning of the
loss of matter
Formation of hollow structure
5.4
Schematic representation of the diffusion-reaction phenomena
that occur during the hydrolytic degradation of implants, according
to the model of heterogeneous degradation (modified from Grizzi
et al.
, 1995
49
).
The second class of materials considered, category (b), are those that are
initially amorphous but crystallise with hydration and degradation. As before,
once placed in an aqueous medium, water diffuses into the sample from the
outside in and initially surface degradation exceeds the degradation in the
centre of the sample. During the second step of degradation for this class of
material, autocatalysis predominates in the centre of the sample. However,
in this instance as molecular weight decreases, factors such as preferential
degradation of some units, shorter chain length and plasticisation by water
allow crystallisation to occur in the centre of the sample. As hydrolysis
continues, the volume of crystalline material in the centre of the sample
increases, while the surface degrades at a reduced rate, becoming only
slightly crystalline. The centre of the sample reaches stable crystallinity
and becomes very resistant to further degradation. Degradation occurs in
the amorphous regions and, therefore, in the final stages, the centre is often
found to be in the form of a white powder or a porous structure. A dense
outer layer remains in which cracking may occur owing to shrinkage of the
bulk sample on degradation.
24
li
et al
.
24
found this mechanism in initially amorphous PLLA samples,
in which hydrolytic chain scission resulted in lower molecular weight and
absorbed water acted as a plasticiser, both resulting in increased chain mobility,
and the ability of the chain segments to crystallise, which in turn improved
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