Translating 3D-printed PEEK for orthopaedic implants: nonlinear effects of gamma sterilization on time- dependent mechanical integrity andin vivobiocompatibility.

While fused deposition modelling (FDM) of polyether ether ketone (PEEK) has achieved regulatory acceptance and is utilized for patient-specific orthopaedic implants, the obligatory terminal sterilization via gamma irradiation raises concerns regarding polymer aging and structural degradation. This study investigates the spatiotemporal effects of gamma sterilization on the macroscopic mechanical integrity and medium-termin vivobiocompatibility of 3D-printed PEEK. Standard specimens were fabricated using a Taguchi L9 orthogonal array, manipulating key FDM parameters (layer thickness, print speed, number of shells, and raster width). To simulate a worst-case scenario of accelerated aging and free radical-induced degradation, specimens were subjected to 50 kGy of gamma irradiation and mechanically evaluated over a two-month period. In parallel, to assess clinical safety, an extensivein vivostudy was conducted utilizing 81 BALB/c rats over 12 weeks. Specimens sterilized at the standard clinical dose of 25 kGy were implanted subcutaneously and compared with autoclaved controls. Mechanical testing revealed complex, nonlinear interactions between printing parameters and post-irradiation time, indicating simultaneous cross-linking and delayed oxidative chain scission mechanisms that ultimately preserved functional strength. Crucially, histological evaluation at 4, 8, and 12 weeks demonstrated excellentin vivobiocompatibility, characterized by minimal acute inflammation, rare foreign body giant cells, and the formation of a healthy, organized fibrous capsule, irrespective of the sterilization method. These findings suggest that 3D-printed PEEK maintains its structural resilience and favourable immunomodulatory profile post-gamma sterilization, successfully bridging the gap between additive manufacturing parameters and safe clinical translation.