Fabrication and characterization of biodegradable Zn-Ni spinel ferrite/ β-TCP composite ceramics exhibiting enhanced cell colonization

Via a solid-state reaction route, magnetic composites of chicken eggshell-derived β-tricalcium phosphate (β-TCP, referred to as TCP in the composite system) and zinc-nickel spinel ferrite (ZNF; Zn_xNi_1‒xFe₂O₄, x = 0.2, 0.4, 0.6, or 0.8) were successfully fabricated. Discs were prepared by uniaxial pressing of milled ZNF/TCP powders and sintered at 1200 °C. Cytocompatibility of all composites was confirmed by SEM observations of human osteoblasts (h-OBs) and MTT assays. At 4-wt% ZNF addition, the composites containing Zn_0.8Ni_0.2Fe₂O₄ (Z8NF) exhibited the greatest extent of early cell spreading and were selected for further investigation. For Z8NF/TCP composites containing 4-12 wt% Z8NF, the 8-12 wt% samples demonstrated the highest levels of cell colonization, while MTT assays suggested non-cytotoxic behavior, with cell viabilities comparable to β-TCP. High-temperature sintering induced partial transformation of β-TCP to β-calcium pyrophosphate (β-CPP), as evidenced by XRD and Rietveld refinement. Increasing Z8NF content promoted β-CPP formation and increased composite porosity, whereas densification and Vickers hardness decreased accordingly. Rietveld refinement further indicated that the detectable crystalline Z8NF phase persisted as a minor yet stable secondary phase ( < 2 wt%) and did not participate in Ca-P lattice substitution. For the 8-12 wt% composites, saturation magnetization decreased with increasing Z8NF because of higher porosity and dilution by the non-magnetic β-TCP/β-CPP matrix, while coercivity increased owing to enhanced effective magnetic anisotropy in the more porous microstructure. Overall, the Z8NF/TCP composites combined biodegradability, bioactivity, and tunable soft-magnetic properties, suggesting their potential for bone repair and bone tissue engineering applications.