Research on the anisotropic mechanical behavior of freeze-cast lamellar porous hydroxyapatite ceramics for orthopedic bone defect repair

Freeze-casting was employed to fabricate lamellar porous hydroxyapatite (HA) scaffolds with tunable anisotropic mechanical behavior for orthopedic bone defect repair. HA slurries with solid loadings of 10, 15, 20, and 25 vol% were directionally frozen and sintered, while the 20 vol% group was further sintered at 1250, 1300, and 1350 °C to evaluate the effect of thermal treatment. All scaffolds exhibited highly aligned lamellar pore channels. Increasing solid loading from 10 to 25 vol% increased wall thickness from 15.2 ± 2.8–41.5 ± 5.1 μm, decreased lamellar spacing from 105.4 ± 12.3–62.8 ± 8.9 μm, and reduced total porosity from 78.3 ± 1.9% to 53.6 ± 2.4%. This architectural refinement produced marked directional mechanical gains. Under loading parallel to the lamellae, compressive strength increased from 8.3 ± 1.1–92.5 ± 7.6 MPa and Young's modulus increased from 0.21 ± 0.04–2.55 ± 0.28 GPa, whereas perpendicular strength increased from 2.9 ± 0.5–18.1 ± 2.2 MPa. The strength anisotropy ratio therefore increased from 2.9 to 5.1 with increasing solid loading. Fracture toughness in the parallel orientation also rose from 0.48 ± 0.05–1.15 ± 0.12 MPa·m¹ /². For the 20 vol% scaffold, sintering at 1300 °C provided the best balance between wall densification, phase stability, and mechanical performance; sintering at 1350 °C caused partial HA decomposition to α-TCP. These results demonstrate that freeze-casting provides a controllable route for matching HA scaffold architecture and directional mechanics to bone-defect repair requirements.