Amorphous intergranular phases in mature natural tooth enamel are found to provide better adhesion and could dramatically affect their mechanical performance as a structure reinforcing phase. This study successfully synthesized an amorphous intergranular phase enhanced fluorapatite array controlled by Mg 2+ (FAP-M) at room temperature. Furthermore, atom probe tomography (APT) observation presents that Mg 2+ is enriched at grain boundaries during the assembly of enamel-like fluorapatite arrays, leading to the formation of intergranular phases of Mg-rich amorphous calcium phosphate (Mg-ACP). APT results also demonstrated that the segregation of Mg 2+ caused the chemical gradient in nanocrystalline attachment and realignment under the drive of inherent surface stress. These results indicate that the amorphous intergranular phases served like glue to connect each nanorod to reinforce the enamel-like arrays. Therefore, the as-received FAP-M artificial enamel exhibits excellent mechanical properties, with hardness and Young's modulus of 2.90 ± 0.13 GPa and 67.9 ± 3.4 GPa, which were ∼8.3 and 2.2 times higher than those of FAP arrays without controlled by Mg 2+ , respectively.

Collagen fibrils hierarchically assemble from microscale to macroscale, which endows the natural composite bone with good mechanical properties and remodeling functions. Revealing the intrafibrillar growth process of hydroxyapatite nanocrystals of collagen will guide the research of bone repair or collagen-based composites. Herein, we investigated the mineralization of multiscale collagen matrices and strongly proved the intrafibrillar hydroxyapatite nanocrystals in the collagen fibrils. The hydroxyapatite nanocrystals were deposited within collagen fibrils with co-orientation along the (002) crystal plane, which is the longitude of the fibril. The whole growth process was captured by TEM to demonstrated the five stages of the intrafibrillar growth process of hydroxyapatite nanocrystals. The infiltration and transformation of amorphous calcium phosphate in isolated collagen fibrils are both demonstrated. The intrafibrillar growth process of hydroxyapatite nanocrystals in collagen film was also investigated, showing that the growth area of collagen films increased linearly with time and the growth process. By studying the in situ mineralization under different reaction conditions, the kinetic equation of the mineralized area of collagen film under each condition was obtained, and the optimal hydroxyapatite mineralized solution was proved to be a solution with polyacrylic acid of 50 μG/mL and a pH of 7.5. Our work provides more detailed information of the growth process of HAP nanocrystals during the mineralization of collagen at different scales and would contribute to future research on the formation process of more minerals in collagen.