Finite element evaluations of the mechanical properties of polycaprolactone/hydroxyapatite scaffolds by direct ink writing: effects of pore geometry

Osteochondral (OC) defects usually involve the damage of both the cartilage and its underneath subchondral bone. In recent years, tissue engineering (TE) has become the most promising method that combines scaffolds, growth factors, and cells for the repair of OC defects. An ideal OC scaffold should have a gradient structure to match the hierarchical mechanical properties of natural OC tissue. To satisfy such requirements, 3D printing, e.g., direct ink writing (DIW), has emerged as a technology for precise and customized scaffold fabrication with optimized structures and mechanical properties. In this study, finite element simulations were applied to investigate the effects of pore geometry on the mechanical properties of 3D printed scaffolds. Scaffold specimens with different lay-down angles, filament diameters, inter-filament spacing, and layer overlaps were simulated in compressive loading conditions. The results showed that Young's moduli of scaffolds decreased linearly with increasing scaffold porosity. The orthotropic characteristics increased as the lay-down angle decreased from 90° to 15°. Moreover, gradient transitions within a wide range of strain magnitudes were achieved in a single construct by assembling layers with different lay-down angles. The results provide quantitative relationships between pore geometry and mechanical properties of lattice scaffolds, and demonstrate that the hierarchical mechanical properties of natural OC tissue can be mimicked by tuning the porosity and local lay-down angles in 3D printed scaffolds.