Optimizing biocomposite formulations: Hydroxyapatite and zinc-doped β-TCP ratios for enhanced bone regeneration performance – Structural, mechanical and biological insights

Calcium phosphate-based bioceramics are highly valued in biomedical engineering for their remarkable biocompatibility and ability to promote bone regeneration. Hydroxyapatite (HAp) and zinc-doped β-tricalcium phosphate ( x Zn-TCP) have both been the subject of extensive research as biomaterials. However, the performance of composites combining these two materials still needs to be optimized. In this study, the HAp- and x Zn-TCP-based composites, where x = 3 mol% and 5 mol%, were carried out by mixing in water the two powders previously synthesized through a classical coprecipitation process. The stabilized slurries were slip cast on a plaster support to prepare thin pellets to be sintered. The study focused on two series of Zn-doped HAp/TCP composites containing HAp as the main phase (90 and 80 wt%) and x Zn-TCP as the main phase (90 and 80 wt%). Structural analyses confirmed the presence of β-TCP with a rhombohedral structure and HA with a hexagonal structure. Zinc doping significantly improved sinterability, yielding relative densities of up to 99% at 1200 °C, particularly for the lower Zn content. Furthermore, zinc stabilized the β-TCP phase, resulting in an increase in the β-TCP → α-TCP transition temperature to approximately 1420 °C. The mechanical study highlighted a positive relationship between increased sample density and optimization of material properties. Indeed, the composite made up of 80% (3 mol% Zn-TCP) and 20% HAp exhibited superior performance, showing the highest value of Young's modulus – 135 GPa and the Vickers hardness – 5.4 GPa. Along with these mechanical properties, cytotoxic tests performed by means of MTT assay on HEK293 cells showed that all compositions supported cell viability above 100%, thus indicating the absence of any cytotoxic effects. In addition to that, the ability to stimulate cell proliferation is another important property of these materials. Thanks to these unique properties, the materials under discussion have excellent prospects for usage in clinical practice, namely, as dental implants, spinal cages, and bone defect repair sites.