An EPR and ENDOR study of γ- and β-radiation sterilization in poly (lactide-co-glycolide) polymers and microspheres

EPR/ENDOR spectroscopy was used to characterise the free radicals generated in a series of PLGA raw polymers and microspheres (with lactide : glycolide compositions of (75 : 25), (65 : 35) and (50 : 50)) after exposure to gamma (γ-) and electron beam (β-) irradiation at room temperature. Both sets of irradiated samples produced analogous EPR spectra, indicating that the type and distribution of free radicals generated by γ-irradiation are similar to those generated by β-irradiation. The radicals were identified by EPR simulations as the chain scission species -CHO-(≈ 27% abundance),-C(CH3)O- (≈ 23% abundance) and the terminal-C(CH3)-OR fragment (≈ 50% abundance), and these assignments were supported by the ENDOR analysis. The latter two radical species were demonstrated to originate from the lactide component of the PLGA polymer. Overall systematically higher radical concentrations were found as the lactide content of the PLGA raw polymer and microspheres increases (ie., 75 : 25 > 65 : 35 > 50 : 50) for both γ- and β-irradiation. However, while the relative concentrations of free radicals was similar in the raw polymer samples after exposure to γ- or β-irradiation, a substantial difference was found for the microsphere samples; an approximate doubling of the radical content was found in the γ-irradiated PLGA microspheres compared to the identical β-irradiated microspheres.