A Theoretical Study On Hydroxyapatite Doping System And Its Composite Interface Properties

Background:For a long time,the biomedical community has been interested in the biological materials hydroxyapatite（HAp）and anatase（A-TiO₂）.In recent years,theoretical calculations based on density functional theory have made it possible to easily control variables and study atomic level interactions that were previously difficult to observe directly experimentally.As a result,it has become a useful tool for studying the properties of biological materials such as hydroxyapatite and anatase,as well as chemical reactions at microscopic interfaces.It makes it much easier for us to understand the internal mechanisms of physical and chemical changes in the human body.Based on DFT,some scientific problems in medical science such as how fluorine doping enhances hydroxyapatite crystals and the microscopic interface between hydroxyapatite and anatase will be explained theoretically in this paper.Objective:1.Use the density functional theory（DFT）method to assess the geometric electronic structure and mechanical characteristics of fluorine-doped hydroxyapatite in order to investigate the enhancing effect and microscopic mechanism of fluorine-doping in hydroxyapatite crystals.2.By theoretical computation and simulation,examine the microscopic interface contact mode of the interface between hydroxyapatite and anatase,and the effect of strontium doping on interfacial bonding.Methods:The calculations involved in this work are carried out by using the DFT method at Perdew-Burke-Ernzerhof（PBE）level.After relaxation,the Ca and OH vacancy defect models of HAp,fluorapatite（FAp）,and fluorine-doped hydroxyapatite（HFA）systems were built to analyze their geometric electronic structures and energy characteristics.Several interfacial contact modes of hydroxyapatite（001）and anatase（101）surfaces were tested to create a stable interfacial model,then their geometric structure and charge transfer were investigated.Following that,the impact of the strontium substitution interface model on chemical bonding between surfaces will be investigated.Results:1.The lattice parameters of HAp,FAp and HFA are similar,and the energy of FAp（001）surface is lower and more stable than that of HAp.Owing to these hydrogen bonds which form between the F dopants and their neighboring hydroxyl groups in the HA lattice,the dissociation energy barrier of the small anions（i.e.,OH^–and F^–）increased,and therefore,protect the fluorine anion and the hydroxyl groups in the lower layer.Mechanical analysis showed that the mechanical properties of FAp,HAp and HFA were not significantly different.2.Hydroxyapatite（001）and anatase（101）can produce chemical bonding between the surface,the chemical bond at the interface is mainly composed of Ca-O ionic bond,the interface tends to be thermodynamic stability;Sr doping on HAp（001）surface has no significant effect on the geometry of A-TiO₂/HAp interface,but can slightly increase the amount of charge transfer at the interface.Conclusions:1.By forming hydrogen bonds with HAp hydroxyl groups,the doping of fluoride ions into HAp crystals helps preserve or restore the lattice structure of HAp.2.Chemical bond between the surfaces of A-TiO₂（101）and HAp（001）can provide a stable interface.Sr doping on HAp（001）surface has no substantial enhancement or attenuation effect on A-TiO₂/HAp interface.