Hybrid-manufactured silicon nitride coated CFR-PEKK: A candidate biomaterial for trauma plate applications?

Continuous carbon fiber-reinforced polyetherketoneketone (CCF-PEKK) is a thermoplastic composite with properties suitable for trauma plate applications (elastic modulus, strength, radiolucency, and inertness). However, components manufactured by fused filament fabrication (FFF) often display non-uniform (anisotropic) mechanical properties and contain microstructural voids. To address these limitations, we investigated a hybrid-manufacturing approach, combining FFF with continuous carbon fiber reinforcement followed by uniaxial compression molding. Here, we asked: 1. Can layup orientation be tuned to replicate the mechanical stiffness of cortical bone in flexion? 2.) How does the structure of the different layups influence fracture behavior? and 3.) Do silicon nitride (SiN) (a bioactive ceramic with antimicrobial properties) embedded particulate coatings affect flexural or fracture behavior? To answer these research questions, we fabricated CCF-PEKK plates with three fiber layups (0°/90°, +45°/-45°, and 0°/90°/+45°/-45°) with the goal of approaching the flexural modulus of cortical bone (1.7-16.3 GPa). Next, half of the hybrid-specimens were spray-coated with submicron SiN powder. Four-point bending tests demonstrated that fiber orientation significantly influenced flexural modulus and strength. The 0°/90° layup exhibited the highest flexural modulus (67.6 GPa) and strength (1020 MPa), while the +45°/-45° configuration showed the lowest values (15.6 GPa, 217 MPa), but displayed superior load dissipation in axial fiber orientations and was able to reproduce moduli values akin to those of cortical bone range. SEM analysis confirmed uniform SiN coating distribution, with no observable impact on crack initiation or propagation. No difference (p > 0.01) in flexural modulus or strength was observed between the uncoated and coated specimens, suggesting that SiN is not associated with static flexural properties of CCF-PEKK. These findings support the feasibility of hybrid-manufactured CCF-PEKK trauma plates as potential alternatives to conventional metallic implants. Further investigations into the long-term fatigue behavior and bioactivity of SiN coatings are warranted.