An electron spin resonance analysis on γ-irradiated poly(glycolic acid) and its copolymers with lactic acid

Perspectives on Modification of Aliphatic Polyesters by Radiation Processing

Aliphatic polyesters have been studied mainly for medical applications, but also for other uses. Although the properties of different aliphatic polyesters are broad, there is a need for modifications to achieve desired material properties. Plasma modification of polymers has been applied to surface modification of polymers, while the bulk of the polymer remains unchanged. High energy radiation induced processing is an established technique that is used in polymer science. Radiation is used to initiate radical polymerizations and for modifications such as degradation, cross-linking, and graft-copolymerization. In this paper article radiation processing of aliphatic polyesters is reviewed. Included are synthetic polyesters, like polyglycolide, polylactide, poly(ε-caprolactone), poly(lactide-co-caprolactone) and two commonly occurring bacterial polyesters, polyhydroxybutyrate and poly(hydroxybutyrate- co-valerate). Relevant processing parameters as well as the effects and applications of the radiation modification are presented.

Finalizing the properties of porous scaffolds of aliphatic polyesters through radiation sterilization

Porous scaffolds made of various L,L-lactide (LLA), 1,5-dioxepane-2-one (DXO) and epsilon-caprolactone (CL) copolymers were sterilized by EB- and gamma-irradiation. Differences in the comonomers, composition and the microstructure of the starting materials were used to influence the degradation mechanism and susceptibility towards irradiation and by this means to achieve sterilized scaffolds with predicted end-properties. The chemical changes and the formation of low-molecular-weight products were determined by SEC, 1H nuclear magnetic resonance (NMR), 13C NMR and gas chromatography-mass spectrometry (GC-MS). The degradation mechanism changed from random chain scission to cross-linking depending on the choice of monomers, the copolymer composition and the monomer sequences. Copolymerization of LLA with small amounts of CL or DXO increased the stability compared to that of the LLA homopolymer. Changing DXO to CL in a LLA copolymer also increased the stability. The type of radiation and the microstructure of the copolymer chains determined which of the monomer sequences were more prone to degrade. The most abundant low-molecular-weight product identified after sterilization was DXO monomer. Traces of LLA and CL monomers were also identified. Modification of the copolyester microstructure changed the degradation mechanism and the susceptibility towards irradiation. This allows the use of radiation sterilization to finalize the scaffold properties.

Bioresorbable polymers: heading for a new generation of spinal cages

The use of polymer-based bioresorbable materials is now expanding to the realm of spinal interbody fusion. Bioresorbable polymers have important advantages over metals, because they are temporary, much less stiff, and radiolucent. Most promising is a group of alpha-polyesters, in particular polylactide acids (PLAs). Their biocompatibility is excellent, and they have sufficient stiffness and strength to provide initial and intermediate-term stability required for bone healing. However, polylactides have characteristics that make them vulnerable to complications if not properly controlled. Degradation rate strongly depends on polymer type, impurities, manufacturing process, sterilization, device size, and the local environment. The fact that larger implants degrade faster is contra-intuitive, and should be considered in the design process. Also optimal surgical techniques, such as careful bone bed preparation, are required for a successful application of these materials. The purpose of this paper is to highlight the specific properties of these bioresorbable polymers and to discuss their potential and limitations. This is illustrated with early preclinical and clinical data.Bioresorbable cage technology is just emerging: their time-engineered degradation characteristics allow controlled dynamization in interbody applications, facilitating spinal fusion. Their radiolucency improves image assessment of fusion healing. Acceptance and use of bioresorbable implants may increase as further research and clinical studies report on their safety, efficacy, and proper usage.

An EPR and ENDOR study of gamma- and beta-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 (gamma-) and electron beam (beta-) irradiation at room temperature. Both sets of irradiated samples produced analogous EPR spectra, indicating that the type and distribution of free radicals generated by gamma-irradiation are similar to those generated by beta-irradiation. The radicals were identified by EPR simulations as the chain scission species -(CHO-(approximately 27% abundance),-C(CH(3))O- ( approximately 23% abundance) and the terminal-C(CH(3))-OR fragment (approximately 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 gamma- and beta-irradiation. However, while the relative concentrations of free radicals was similar in the raw polymer samples after exposure to gamma- or beta-irradiation, a substantial difference was found for the microsphere samples; an approximate doubling of the radical content was found in the gamma-irradiated PLGA microspheres compared to the identical beta-irradiated microspheres.

Electron magnetic resonance study of gamma-irradiated poly(lactide-co-glycolide) microspheres

A series of poly(lactide-co-glycolide) samples of compositions ranging from (75:25) to (65:35) to (50% (65:35):50% (50:50)) were gamma-irradiated under ambient conditions in air. The irradiation doses used were 15, 20, 25, and 30 kGy. The generation of radicals resulting from the gamma-irradiation was confirmed using EPR. Two major radical species were observed and identified as centered at alkyl and alkyl peroxy groups. The indication from the X-band (9 GHz) frequencies showed that alkyl radicals gave rise to a quartet hyperfine pattern. However, measurements performed at higher W-band frequencies (90 GHz) showed that the X-band spectra are actually a composite profile arising from a series of overlapping individual resonances. Using combined EPR and ENDOR (Electron-Nuclear DOuble Resonance) measurements, an alkyl peroxy radical was identified. For increasing glycolide concentration from 75:25 to the 50:50 blend, there was a factor of 7 increase in the concentration of radicals A and B. Furthermore, both radical species were found to be stable for several weeks after storage at ambient temperature conditions. At elevated temperatures and humidities, radical stability decreased--the decay rate was estimated at approximately 3x10(-8) mol K(-1). The stability characteristics of the radicals under different conditions are attributed to changes in the morphology of the polymer.