Degradation and calcification in vitro of new bioresorbable terpolymers of lactides with an improved degradation pattern

Experimental implants produced from poly(l-lactide/ɛ-caprolactone/glycolide) 80/10/10% (polymer 1), poly(l-lactide/dl-lactide/glycolide) 80/10/10% (polymer 2) and poly(l-lactide/dl-lactide/glycolide) 80/5/15% (polymer 3) were subjected to in vitro degradation in a buffer solution at 37 °C and pH = 7.4 and calcification in vitro was performed at 37 °C in a simulated body fluid for 4, 8, 12, 16, 24, 28, 32 and 36 weeks. In vitro degradation was performed in static and pseudodynamic modes. Samples from poly(l-lactide) were used as a control. The changes in the materials during the course of degradation were assessed from the measurements of molecular weight, mechanical properties and crystallinity. The changes in the appearance of the materials upon degradation and calcification were observed using a scanning electron microscope with an EDAX attachment. The decrease of molecular weight at 4 weeks was 66% for polymer 1, 56% for polymer 2 and 20% for polymer 3. Samples retained 55% of their bending strength at 4 weeks (polymer 1), 50% at 12 weeks (polymer 2) and 99% at 12 weeks (polymer 3). The bending modulus of polymer 3 remained practically unchanged during the first 12 weeks of degradation. Subsequently it increased by 44% at week 16 and remained unchanged up to 24 weeks and next decreased to 33% of the initial value at the end of the experiment at week 32. The bending modulus of polymer 2 decreased 35% at week 8 and subsequently increased to 44% of the initial value at week 16 and remained at this level until week 20. Next the modulus decreased to 84% of the initial value at week 24. The bending modulus of polymer 1 progressively decreased over the first 12 weeks of degradation to 40% of the initial value. The maximum crystallinity attained by the samples at the end of the experiments was 60% for polymer 1 and 38% for polymers 2 and 3. In the static mode the pH remained constant up to week 8 for polymer 1, week 20 for polymer 2 and week 28 for polymer 3. It decreased to 3.8 at weeks 12, 20 and 36 for polymers 1, 2 and 3, respectively. All the samples underwent calcification from week 16 of the experiments with the Ca/P ratio ranging from 0.92 to 1.20.