The Effects of E-beam Sterilization on Poly-L-lactic acid (PLLA) Mechanical, Material and Thermal Properties

Abstract In this work, the effects of E-beam sterilization on the mechanical, chemical and thermal properties of poly(L-lactic acid), or PLLA, were evaluated. In addition to quantifying the effects of radiation dose, the impact of a cold storage factor called dwell was also evaluated. High molecular weight medical-grade PLLA (PL38) was injection-molded, packaged under a nitrogen atmosphere and kept in a freezer. Three levels of radiation dosage were tested: 30 kGy, 50 kGy and 80 kGy. For the dwell condition, the total dose was delivered to the samples as a set of two consecutive split doses, delivered 24 hours apart. Both Mn and Mw decreased significantly after irradiation as determined by gel permeation chromatography. Thermogravimetric analysis data showed an increase in the thermal stability of the material after irradiation despite decreases in molecular weight, indicating that PLLA was undergoing cross-linking. Differential scanning calorimetry results showed a statistically significant increase in crystallinity as a function of both dwell and increasing radiation dose. Tensile strength was preserved at the 30 kGy dose level (compared to unirradiated samples), and changes in the elastic modulus between sample conditions were not statistically significant. Statistically significant decreases were seen in percent elongation at break of the material, and were attributed to the increases in crystallinity. While increases in crystallinity present challenges associated with decreased material toughness, this work supports the potential of E-beam irradiation as a sterilization treatment for high molecular weight PLLA.