Effect of Salinity on Hydroxyapatite Dissolution Studied by Atomic Force Microscopy

The complexity of bone tissue and the lack of techniques for directly probing bone surfaces in vivo have hindered studies on the fundamental mechanisms of bone mineral remodeling. Here, we addressed these issues by using single crystal hydroxyapatite (HAP) as a well-defined bone surface model and directly observe its surface using in situ atomic force microscopy. Specifically, we investigated the effects of NaCl concentration on the dissolution of HAP (100) surfaces and found that NaCl strongly suppressed HAP dissolution kinetics, including at physiological ion concentrations. These findings indicate that local ion concentrations in vivo may contribute significantly to the stability of bone mineral. Moreover, our analysis of HAP surfaces over a broad range of pH conditions suggests that only one kind of surface termination exists exclusively in the solutions we used. This observation is important for understanding the surface chemistry of HAP. Our molecular level, real-time observations of HAP dissolution are significant for understanding bone resorption and provide useful insights for the design of novel therapies for treating osteoporosis and other bone related diseases.