Synthetic bioresorbable polymers - Durability and Reliability of Medical Polymers - page 106

6

4

2

0

0 PLA

PGA
Amount of copolymer %

100

100 0

5.6 Half-life of PLA and PGA homopolymers and copolymers (from
Middleton and Tipton 4 ).

Chu 55 found there was a greater loss in the mechanical properties of PGA
sutures in buffered than unbuffered solutions. Similarly, the presence of buffer
ions increases the degradation rate of poly(a-hydroxy acid) copolymers as a
result of the enhanced removal of degradation products through neutralisation
of the end groups. 23,54,60
Work carried out by King et al . 50 and Hurrell and Cameron 54 showed no
significant change in PGA degradation rate for increases in ionic strength and
for changes to phosphate salt concentration (from 0.01 to 0.1 m ), but for an
increase in pH (from 6 to 8) a small but significant increase was observed.
Ginde and Gupta 69 also observed alkaline media (pH 9.2 and 10.6) to have
a more drastic effect on the degradation and mechanical properties of PGA
fibres than acidic or neutral media. Makino et al . 70 and Zaikov 71 also reported
greater degradation in alkaline than in acidic conditions for homopolymers
and copolymers of polylactide and polyglycolide. Their results imply that
in alkaline conditions, surface erosion dominates over bulk erosion.
Hurrell and Cameron 54 concluded that the onset of reaction erosion fronts
and the rate at which they move through the polymer depends on the rate
at which acidic degradation products diffuse out, creating space for water
in the structure. If the solution is made more alkaline or more concentrated,
more of the acid is neutralised, driving the reaction forward. This hastens the
onset of stage III, and encourages drug release from loaded drug releasing
polymers.
Other authors 23,54,72,73 also observed that the presence of ions in solution
initially decreases water uptake into the polymer compared with the uptake
from distilled water, but, as degradation products are released from the
polymer, the presence of ions increases the erosion rate.

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