Assembly Mechanism of Highly Crystalline Selenium-Doped Hydroxyapatite Nanorods via Particle Attachment and Their Effect on the Fate of Stem Cells

Hydroxyapatite is the main inorganic component of human bone. Synthetic hydroxyapatite and its different modified forms, which have been shown to be biocompatible and osteoconductive, have been widely used in bone tissue engineering. It is still challenging to controllably synthesize hydroxyapatite with a targeted morphology. In this work, we synthesized highly crystalline selenium-doped hydroxyapatite nanorods (SeHAN) via a two-step alcohol thermal method and provided a complete explanation of the synthesis mechanism. Tracing the crystals obtained from the solvated phase to the crystal phase with high-resolution microscopy, the nanorod formation route can be briefly described as follows: as a basic unit, similar to 30 nm amorphous apatite initially formed in the first step and partly crystallized in early part of the second step; after a period of alcohol thermal reaction, immature nanorods appeared, which were composed of nanocrystals; finally, immature nanorods transformed into single-crystal nanorods through crystallization by particle attachment. Since few works have focused on the osteogenesis ability of SeHAN, whose antitumor effect has been widely studied, we investigated the influences of SeHAN on rat-bone-marrow-resident mesenchymal stem cells (MSCs). Surprisingly, SeHAN exhibited excellent biocompatibility for MSCs, enhanced their osteogenic differentiation, and inhibited their adipogenic differentiation. This work provides not only a general method to controllably synthesize hydroxyapatite nanorods/SeHAN but also an insight into understanding the hydroxyapatite formation mechanism. The current study also highlights the effects of SeHAN on MSCs that could furnish a significative strategy for manufacturing functional biomaterials for biomedical applications and tissue engineering by enhanced ossification and reduced marrow adiposity.