Factors Contributing to PLLA Biomedical Device Degradation During In-Core Irradiation

Abstract

The production of medical isotopes via neutron bombardment in a nuclear reactor is critical to support nuclear medicine treatment and diagnostic techniques. The conditions in-core are very harsh and easily degrade organic materials, sometimes used in medical isotope production. Biomedical devices of poly-L-lactic acid (PLLA) impregnated with radionuclides have gained popularity for selective internal radiation therapy in the past decade. Previous research has shown that PLLA based samples are highly sensitive to the conditions within the nuclear reactor during radioisotope production: minor changes to the environment can cause decomposition during production or immediately after the samples are suspended in media. It is believed that the PLLA is degraded by interactions between temperature, gamma-radiation, reactive oxygen species, and fast neutrons. However, the relationship between these factors and integrity of the polymer structure is not well understood. This research analyzed the degradation of PLLA/lanthanide biomedical devices from environmental factors present in-core. The degradation of PLLA was found to strongly correlate to temperature. This was seen both in-core with increased water-cooling of samples, and externally in a water bath test. Gamma radiation appeared to impact quality due to chain-scissions below 600 kGy applied dose. Higher doses showed better quality, suggesting the rate of chain-scissions was overcome by the competitive process of crosslinking at extreme doses and doserates. The link between temperature and gamma dose on PLLA degradation will be presented, along with in-core site selection characteristics, cooling during irradiation, and lead shielding effects.