Molecular Dynamics Characterization of Sr-Doped Biomimetic Hydroxyapatite Nanoparticles

Hydroxyapatite (HAP), which is the main inorganic component of human bones and teeth, has received much attention for its use as an implant material. Doping foreign ions into the HAP lattice structure is a valuable approach to endow and augment its biological characteristics. In this study, based on first-principles calculations, the force-field parameters of strontium ions substituted in HAP were first fitted by a genetic algorithm. The corresponding Sr-doped HAP bulk and surface properties were then characterized by molecular dynamics simulations. The calculated lattice structure, melting temperature, infrared spectra, and elastic parameters of 0-100 at.% Sr-doped HAP are in good agreement with the experimental observations. Several models with fully Sr-substituted HAP were constructed to understand the effects of different pH values. Our simulations indicate that Sr-substituted HAP might exhibit higher solubility along with decreasing pH values. Meanwhile, Sr ions tend to adsorb onto the HAP surface, and the amount of adsorption increases with increasing pH. Finally, the substitution of Sr ions might weaken the adsorption of Glu side-chain analogues but strengthen the adsorption of Lys side-chain analogues on the HAP surface. The force-field parameters developed for Sr in this work could be applied to subsequent studies for interactions between Sr-doped HAP and other biomolecules.