Copper-doped β-tricalcium phosphate: synthesis, characterization, and in vitro osteoclastic resorption

This study investigated how copper (Cu) incorporation and grain-boundary segregation in β-tricalcium phosphate (β-TCP) influence microstructure, surface chemistry, and <i>in vitro</i> osteoclastic resorption. β-TCP powders containing 0-2.4 wt% Cu were synthesized via precipitation of Cu-substituted calcium-deficient hydroxyapatite (CDHA) followed by thermal conversion. Phase composition, chemistry, and microstructure were characterized using X-ray diffraction (XRD), inductively coupled plasma mass spectrometry (ICP-MS), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Dense β-TCP cylinders were fabricated, polished, and incubated with primary murine osteoclasts. Osteoclastic resorption was quantified by white-light interferometry (WLI) and ion release analysis. Cu incorporation in CDHA and β-TCP increased proportionally with the Cu concentration in the precipitation solution. At low concentrations, Cu substituted for Ca at the Ca(5) site in the β-TCP lattice, whereas at higher Cu contents, CuO segregated at grain boundaries. Tartrate-resistant acid phosphatase (TRAP) positive osteoclasts were observed on all samples; however, Cu content strongly modulated resorptive activity. β-TCP containing 0.1 wt% Cu exhibited deeper and more localized resorption pits, indicating enhanced localized osteoclastic activity. In contrast, higher Cu levels progressively reduced resorption, with complete inhibition observed at 2.4 wt% Cu. These results identify an optimal Cu doping level near 0.1 wt%. While grain-boundary segregation of Cu as CuO was confirmed at higher doping levels, its specific contribution to the biological response could not be resolved in this study.