Formation of Stable Zinc-Rich Amorphous Calcium Phosphate

Metal ion-rich amorphous calcium phosphate (ACP) is an essential mineral component in biogenic hard tissues, such as bones and teeth. However, the formation mechanism of ion-doped ACP and the role of metal ions in this process remain elusive. Herein, taking Zn as an example, we develop a series of Zn-substituted calcium phosphate materials that serve as models for investigating the formation process of Zn-ACP. It is found that entirely pure Zn-ACP can be successfully achieved when the precursor Zn/Ca ratio is maintained between 0.1 and 0.2. The resulting Zn-ACP nanoparticles exhibit a homogeneous distribution of Zn at the nanoscale, excellent cytocompatibility, and exceptionally high amorphous stability in aqueous media, including water and simulated body fluid. Furthermore, we fabricate monolithic Zn-ACP bioceramics through the application of pressure, resulting in remarkable hardness (1.7 GPa) and modulus (25.5 GPa) that exceed those of human bones. This work presents a novel approach to producing Zn-ACP monoliths and advances our understanding of the biomineralization processes involving Zn and ACP, thus opening potential applications in biomedicine.