Additive-manufactured porous β-Ti21S hip implant: Balancing mechanics and biocompatibility

This thesis investigates the mechanical and biological performance of additively manufactured (AM) β-Ti21S lattice structures for potential application in orthopedic implants. In detail, it focuses on the manufacturability, mechanical behavior, and biological response of two lattice architectures fabricated by Laser Powder Bed Fusion (LPBF): an auxetic re-entrant bow-tie design (AUX) and a gyroid triply periodic minimal surface (TPMS) design. These geometries were selected for their potential to mitigate stress shielding and promote osteointegration through tailored mechanical properties and favorable pore architectures. The work first examines the influence of material properties and lattice geometry on mechanical performance, followed by an investigation of porosity through functionally graded porous structures (FGPS). Mechanical testing of both single-density and FGPS demonstrated that all designs achieved elastic moduli within the range of human bone. Compared to their single-density counterparts, the FGPS exhibited increased elastic modulus and yield strength, indicating that longitudinal porosity gradients enhance the mechanical response of both lattice types. The effect of surface post-processing by electropolishing is then evaluated with the aim of fatigue resistance improvement. From a biological perspective, cytotoxicity, metabolic activity, and osteogenic differentiation were assessed using as-built and surface-polished samples. Initial cytotoxicity testing confirmed material biocompatibility, and MG-63 (osteosarcoma bone cell) viability was proven for both TPMS and AUX geometries. Subsequent studies showed that larger pore sizes improved cytocompatibility across both lattice types. Surface post-processing significantly reduced surface roughness and removed partially melted powder particles, without diminishing cytocompatibility and osteogenic differentiation of mesenchymal stem cells (MSCs). Overall, the results demonstrate that β-Ti21S lattice structures produced by LPBF offer excellent biological compatibility and tunable mechanical properties suitable for orthopedic applications. Both TPMS and AUX designs emerged as promising candidates, providing a favorable balance between structural accuracy, mechanical performance, and biological response. Although electropolishing showed limited effectiveness on internal surfaces, it proved to be a valuable post-processing technique for accessible regions, significantly reducing surface asperities and powder-related defects while preserving and in some cases enhancing the cellular bone-specific activity. Finally, motivated by challenges in surface roughness quantification and limitations in existing standards for AM components, this study presents a novel methodology for surface and porosity characterization of miniaturized AM specimens. Current gaps in AM standards and regulatory frameworks are discussed, and the proposed workflow is demonstrated on four samples, yielding promising results.