Robocast hydroxyapatite scaffolds: Structure, mechanical performance, and bioactivity tailored by sintering

Hydroxyapatite scaffolds with controlled porosity were fabricated using robocasting to investigate the influence of additive manufacturing and sintering on microstructure, mechanical performance, and bioactivity. A printable ink containing 50 vol% nano-acicular HAp was developed to achieve stable filament deposition and well-defined lattice architectures. Thermal analysis confirmed complete removal of organic additives below 600 °C with no phase transformation. Dilatometry revealed a two-stage sintering process characterized by densification between 900-1200 °C and grain coarsening above 1200 °C. The relative density increased from 37% to 94% as the sintering temperature increased from 900 °C to 1400 °C, accompanied by grain growth from ∼200 nm to ∼2.5 μm. Analytical modeling quantified activation energies of ∼240 kJ/mol at low temperatures and ∼740 kJ/mol at high temperatures, enabling predictive description of grain growth, densification kinetics, and shrinkage behavior in lattice-structured scaffolds. Predictive FEM modeling was developed to study the loss of shape uniformity during sintering. X-ray diffraction confirmed the phase stability of HAp up to 1400 °C without decomposition. Scaffolds sintered at 1200 °C exhibited compressive strengths in the range of 0.5-5 MPa, with the highest strength for the denser infill pattern and the shortest strand distance, comparable to trabecular bone, while maintaining an interconnected porous architecture. In vitro bioactivity evaluation in simulated body fluid demonstrated rapid apatite nucleation within 7 days and the formation of a continuous apatite layer after 21-28 days. These results demonstrate that sintering conditions assisted by additive manufacturing were systematically optimized to tailor porosity, mechanical performance, and bioactivity in robocast HAp scaffolds.