Direct powder extrusion 3D printing of poly(ɛ-caprolactone)-carbonate substituted hydroxyapatite powder composites: Influence of powder content and multi-scale spatial organization on scaffold morphology and mechanical properties

Poly(ɛ-caprolactone) (PCL) is widely used in bone tissue engineering but lacks intrinsic bioactivity. In this study, carbonate substituted hydroxyapatite (CHA) was incorporated into PCL using direct powder extrusion (DPE) three-dimensional (3D) printing to fabricate scaffolds with controlled CHA contents. Structural and thermal analyses confirmed that CHA retained its apatite crystal structure and acted as a mild nucleating agent without significantly altering PCL thermal behavior. Micro-computed tomography (micro-CT) combined with multi-scale spatial statistical analysis revealed scale-dependent particle distribution, with near-random arrangements at the nearest-neighbor scale and increasing mesoscale clustering at higher CHA contents. Mechanical testing showed that compressive and tensile elastic moduli increased with CHA addition up to 10 wt%, followed by a decrease at higher loadings. Annealing enhanced elastic modulus by improving interlayer bonding and internal structural homogeneity. These findings demonstrate that CHA content and spatial particle organization govern the mechanical performance of DPE 3D printed PCL-CHA scaffolds for bone tissue engineering.