Appropriate regulation of magnesium on hydroxyapatite crystallization in simulated body fluids

Amorphous calcium phosphate (ACP), as a precursor phase of hydroxyapatite (HAP, the main inorganic mineral of bone) crystallization in a biological system is well documented. Numerous studies have shown that Mg2+ can chemically inhibit the transformation of ACP to HAP. Importantly, the inhibition effect is closely related to the addition time of Mg2+ for bone mineralization in vivo. However, Mg2+ is also widely used to promote bone growth and regeneration in the biomedical field. So, the regulatory role of Mg2+ in HAP crystallization remains elusive. In this study, the detailed effect of Mg2+ (especially the adding time) on ACP-mediated HAP crystallization is revealed in simulated body fluids. We reported that adding Mg2+ before ACP formation or in ACP stable stage can effectively inhibit HAP crystallization; however, in the near phase transformation stage (ACP is unstable), Mg2+ does not affect HAP crystallization. In particular, surface associated Mg2+ on ACP (compared to Mg2+ incorporation) is more suppressive to HAP crystallization. This results indicates that appropriate regulation of Mg2+ adding time is vital for amorphous mediated biomineralization, which sheds new light on HAP crystallization regulation.