The distribution of water in degrading polyglycolide. Part II: magnetic resonance imaging and drug release.

This paper reports the use of magnetic resonance imaging (MRI) on polyglycolide disks to monitor the change in water ingress with degradation time. Very little response was measured before 13 days, but after this time, water began to penetrate the disks as fronts, starting from the sample surface and moving inwards towards the centre. These results provide more direct evidence in support of the four-stage degradation model for PGA outlined in previous literature, and in particular, that fairly sharp reaction-erosion fronts move in from the sample surface to the centre when the polymer is undergoing significant mass loss and water gain. A combination of MRI and drug release data suggest that fronts originate at the surface at about 7 (+/-2) days, and proceed at a rate of 0.033 (+/-0.002) mm/day. These results agree with results obtained from cumulative drug release profiles for different sample thicknesses presented in Part I. They support the hypothesis that drug releases quickly from the swollen regions behind the fronts where the polymer is open and porous, and that release finishes when the fronts meet in the centre of the sample.