COLLOIDAL PROCESSING FOR POROUS CALCIUM PHOSPHATE-NIOBIUM OXIDE CERAMICS: THE PATHWAY TO ENHANCING BONE GRAFTS FOR HARD TISSUE REPAIR

• Bioinspired HAP and HAP/Nb₂O₅ scaffolds were successfully fabricated by the replica method. • Colloidal processing enabled control of microstructure and porosity (40–95%). • Scaffolds exhibited interconnected pores (∼300 µm), similar to trabecular bone. • Nb₂O₅ addition increased compressive strength by ∼3 × without reducing porosity. • Both compositions showed satisfactory cytocompatibility. The increase in life expectancy and the global ageing trend have intensified the incidence of degenerative bone diseases and trauma-related injuries, increasing the demand for effective bone substitutes. In this context, the development of bioactive ceramic scaffolds combining highly interconnected porosity with mechanical performance compatible with trabecular bone remains a scientific and technological challenge. This study reports the fabrication of porous calcium phosphate scaffolds modified with niobium oxide using an aqueous colloidal processing route combined with the polyurethane foam replica method. Synthetic hydroxyapatite and hydroxyapatite/Nb 2 O 5 composite powders were prepared with a Ca/P molar ratio of 1.67 and heat treatment at 900 °C. Suspension formulation was systematically optimized by evaluating solids content, ammonium polyacrylate dispersant concentration, and poly(vinyl alcohol) (PVA) binder content, with direct correlation to rheological behavior, impregnation efficiency, and green body consolidation. After sintering at 1200 °C, scaffolds produced with optimized formulations /content preserved the template-derived open-cell architecture, exhibiting total porosity around 90% and interconnected macropores in the range relevant to trabecular bone. The incorporation of 5 wt.% Nb 2 O 5 significantly enhanced compressive strength, yielding values approximately three times higher than those of undoped scaffolds while maintaining porosity and structural integrity. Microstructural analysis confirmed continuous and well-sintered ceramic struts without pore blockage. Extract-based in vitro assays using NIH-3T3 fibroblasts demonstrated high cell viability at biologically relevant concentrations and no inhibition of cell migration. The results demonstrate that the proposed colloidal replica-based route enables the fabrication of mechanically reinforced, highly porous calcium phosphate scaffolds and highlight Nb 2 O 5 as an effective modifier for bone graft materials intended for hard tissue repair applications.