Uncovering the role of ionic doping in hydroxyapatite: the building blocks of tooth enamel and bones

Hydroxyapatite (HAp) is the primary mineral component of various mineralized tissues in the human body, including bone and teeth, where it performs critical roles of structural support and load transmission. In the context of dental health, the two most crucial properties of HAp are mechanical stability, which ensures resistance to forces, and chemical stability, which preserves surface integrity in acidic environments. During the early stages of human evolution, e.g. when teeth were used to crush uncooked food, mechanical stability was of paramount importance. However, with changes in diet and lifestyle, the principal origins of tooth damage and loss shifted towards bacterially mediated chemical attack, known as tooth decay, or caries. To enhance the chemical stability, ion doping has emerged as a particularly significant approach, and it lies at the focus of the present study. A molecular dynamics (MD) framework was developed to investigate the effects of ion doping on the chemical and mechanical stability of HAp and to identify optimal doping candidates. The framework combines conventional MD with steered molecular dynamics (SMD), thermodynamic integration (TI) and uniaxial compression test simulations to provide comprehensive insights into the doping process. The findings revealed surface atoms as the most viable candidates for doping, as demonstrated by SMD and conventional MD simulations. Notably, TI calculations have identified magnesium ions as a better candidate among the ions considered here for enhancing the chemical stability of HAp. However, uniaxial compression simulations revealed that increasing Mg²⁺ concentration simultaneously reduces the mechanical stability of HAp, highlighting a trade-off between chemical resistance and mechanical performance. By contrast, fluoride and carbonate substitutions showed comparatively minor effects on both properties. The results presented in this study offer valuable guidelines for synthesizing HAp-based substituent materials with properties tailored to meet the demands of modern dental applications such as implant coatings, enamel remineralization agents and restorative materials.