Evolution of phase, morphology, physicochemical properties, and biological properties of HA ceramic with the increase of crystallinity

Hydroxyapatite (HA), a commonly used material for bone repair, can enhance its biological properties through the modulation of its crystallinity. Exploring the variations in physicochemical and biological properties associated with different degrees of crystallinity is crucial for advancing the applications of HA. In this study, stoichiometric nano-apatite precursors were initially synthesized using chemical precipitation. Subsequently, four groups of HA ceramics with varying degrees of crystallinity (24 %, 42 %, 72 %, 100 %) were produced by adjusting the calcination temperature, as confirmed through comprehensive characterizations. A systematic investigation was then conducted to examine the impact of crystallinity on the physicochemical, cytological, and in vivo osteogenic properties of HA ceramics, elucidating the influence of crystallinity on the osteogenic potential of HA. Results indicated that higher crystallinity in HA ceramics enhanced mechanical strength and hardness while reducing surface roughness, hydrophilicity, and protein affinity. Cytological studies revealed that decreased crystallinity led to lower cell proliferation rates but facilitated cell adhesion, spreading, and increased expression of osteogenic genes. Moreover, cell experiments with extracts further supported the significant role of surface properties of HA ceramics in modulating cell behavior. In vivo osteogenic assays demonstrated that reduced crystallinity notably stimulated new bone formation. Overall, by integrating the results of in vitro and in vivo experiments, it can be concluded that reducing the crystallinity of HA ceramics, the biological properties of HA ceramics can be significantly enhanced while maintaining a certain level of mechanical strength, such as HA2 group (42 % crystallinity). This study highlights the substantial impact of crystallinity and surface characteristics of HA ceramics on cell behavior, offering valuable insights and strategies for enhancing and optimizing bone repair materials in future applications.