E-beam sterilization of aerospace materials: Microbiological & mechanical property evaluations

NASA missions to Mars, the Galilean moons, and planetary sample return missions have to meet stringent Planetary Protection (PP) specifications. A bioshield or a partial sterilization of the hardware having direct contact with the target body may be required for these missions. Thermal blankets, solar sail, and epoxy adhesives such as Eccobond-9™ are examples of spacecraft materials that are often used in planetary missions, but yet cannot withstand Dry Heat Microbial Reduction (DHMR) treatment, the only NASA approved PP technique. We have previously demonstrated that high energy (10 MeV) electron beam (E-beam) is a sterilization technology that has significant PP application. E-beam doses as low as 12 kGy can achieve over 6-log reduction of bacterial spores of thermal blankets and solar sail material. In this study, we evaluated the mechanical properties of three different heat-sensitive aerospace materials (solar sail, thermal blankets and epoxy adhesive, Eccobond-9™) to E-beam sterilization1,2. E-beam (12 kGy) irradiated and non-irradiated (control) thermal blankets and solar sail material were subjected to quasi-static tensile strength tests at room temperature using a TA Instruments DMA Q800 equipped with a tension clamp. The ASTM Test Method D1002 was used to test the shear strength using an Instron Model # 4411. The load (lbs) and extension (in) properties of 88 mm metal-metal bonded plates were determined. Nine blanket layers (including representative “mesh” and “film” layers) were studied using non-irradiated (control), irradiated, and week-old irradiated materials. From the tensile tests, stress (MPa) versus strain (%) charts was obtained and mechanical properties such as tensile strength, tensile modulus and % elongation at break were determined. There was no statistically significant (p ≫0.05) difference between the irradiated and non-irradiated Layer 1 “mesh” samples for tensile strength and % elongation at break. However, there was a significant difference (p ≪0.05) in the tensile modulus between the non-irradiated and freshly irradiated “mesh” samples. There was, however, no significant difference (p ≫ 0.05) when the irradiated “mesh” samples were analyzed a week later. There were statistically significant (p ≪0.05) differences in the tensile strength, tensile modulus and % elongation at break between the non-irradiated and irradiated samples. There was no statistically significant (p≥0.05) difference in the tensile strength at break and the tensile modulus between the non-irradiated (control) solar sail film, the irradiated, and the week-old irradiated solar sail film. However, there was a statistically significant difference (p≪0.05) when the % elongation at break was compared between the control and the irradiated samples. There was no statistically significant (p ≫0.05) difference in the irradiated and non-irradiated Eccobond™ materials. These results highlight the importance of analyzing irradiated materials at defined timeframes post E-beam irradiation, and identifying specific E-beam doses that does not compromise material properties yet achieving desired levels of microbial sterility.