Synthetic bioresorbable polymers - Durability and Reliability of Medical Polymers - page 109

grizzi et al . 49 and Ginde and Gupta's 69 work suggested that the degradation
rate of PLA and PGA devices was very much dependent on size, and the
greater the thickness of the device, the faster the degradation.
in a separate experiment, visscher et al . 77 investigated the effect of size on
the release of drugs in microparticles made of a copolymer of 25% d -lactide,
25% l l-lactide and 50% glycolide. The microparticles ranged in size from
45-177 mm and, in accordance with the surface layer hypothesis, the larger
microparticles degraded first.
Tormala et al . 37 degraded SR-PGA rods with diameters ranging from
1.5-4.5 mm, which were prepared by sintering bundles of PGA sutures
(dexon) together. in these experiments, in contrast to those reported above,
tensile measurements revealed that the larger rods degrade more slowly. This
was attributed to the smaller surface area to volume ratio, which resulted in
slower diffusion of water into the samples. Another factor that would have
affected the degradation is a change in internal porosity of the rods with
sample size.
Hurrell et al . 43 found that during PGA degradation the progression of
reaction erosion fronts through the samples was linear and at the same rate
irrespective of sample size. Stage I of the degradation is diffusion-controlled
and is therefore strongly affected by the thickness of the sample. However,
because this diffusion occurred quickly in comparison with the timescale of the
degradation it did not have a strong effect. Once water had diffused through
the bulk of the sample by the beginning of stage II, hydrolysis occurred at
the same rate regardless of sample size, until the critical molecular weight
was reached. The reaction-erosion fronts (whose appearance marks the
onset of stage iii) started at the same time in samples of different size. They
concluded that it was only the onset of stage IV of degradation (explained
in section 5.3.3), when the reaction erosion fronts meet in the centre of the
sample, that showed size dependence, with the onset time increasing for
thicker samples.

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5.4.7 Sterilisation and packaging
it is necessary to sterilise all medical implants after fabrication and before
their surgical placement to reduce the risk of infection and associated
complications. The most common sterilisation techniques utilise heat, steam,
radiation, or a combination of these methods.
PLA and PGA polymers, in addition to being susceptible to damage by
moisture and radiation, are heat sensitive. Thus, selection of the correct
sterilisation technique is of crucial importance to their physical and mechanical
performance in vivo . Table 5.3 gives an overview of the sterilisation techniques
commonly used and their advantages and disadvantages. 19 Sterilisation by
g-radiation is known to cause chain scission in PLA and PGA polymers. At