Low-Dose Gamma Irradiation of Decellularized Heart Valves Results in Tissue Injury In Vitro and In Vivo

Background Decellularized heart valves are emerging as a potential alternative to current bioprostheses for valve replacement. Whereas techniques of decellularization have been thoroughly examined, terminal sterilization techniques have not received the same scrutiny. Methods This study evaluated low-dose gamma irradiation as a sterilization method for decellularized heart valves. Incubation of valves and transmission electron microscopy evaluation after different doses of gamma irradiation were used to determine the optimal dose of gamma irradiation. Quantitative evaluation of mechanical properties was done by tensile mechanical testing of isolated cusps. Sterilized decellularized heart valves were tested in a sheep model (n = 3 [1 at 1,500 Gy and 2 at 3,000 Gy]) of pulmonary valve replacement. Results Valves sterilized with gamma radiation between 1,000 Gy and 3,000 Gy were found to be optimal with in vitro testing. However, in vivo testing showed deteriorating valve function within 2 months. On explant, the valve with 1,500 Gy gamma irradiation showed signs of endocarditis with neutrophils on hematoxylin and eosin staining, and positive gram stain resembling streptococcus infection. The 3,000 Gy valves had no evidence of infection, but the hematoxylin and eosin staining showed evidence of wound remodeling with macrophages and fibroblasts. Tensile strength testing showed decreased strength (0 Gy: 2.53 ± 0.98 MPa, 1,500 Gy: 2.03 ± 1.23 MPa, and 3,000 Gy: 1.26 ± 0.90 MPa) with increasing levels of irradiation. Conclusions Low-dose gamma irradiation does not maintain the mechanical integrity of valves, and the balance between sterilization and damage may not be able to be achieved with gamma irradiation. Other methods of terminal sterilization must be pursued and evaluated.