Preparation And Properties Of Selenium Doped Hydroxyapatite Biocoatings

Hydroxyapatite（HA）,the chemical composition of which is very similar to the mineral component of human bone tissue,has remarkable biocompatibility and bioactivity.Coating HA on metal substrate has become a research focus of bone implant materials field.Especially,coating HA on titanium（Ti）substrate by plasma spraying technology,used as artificial hip joint,has been commercially applied.However,HA coatings still face some problems in clinical use.Easy infection after implantation into organisms,and poor repair effect on bone defects in patients with pathological conditions such as fluorosis and osteoporosis are the typical problems in its clinical use.In order to solve these problems,ion doped HA has received increasing attention.Selenium（Se）,as one of the essential trace elements in the human body,has biological effects such as anti-tumor,antibacterial,and anti fluorosis.Therefore,this study introduced Se element into HA to explore its additional biological properties.The main research work and results are as follows:（1）Synthesis of Se-doped HA（Se-HA）nano-powders by chemical coprecipitation method.In order to systematically compare the impact of Se doping on the microstructure and properties of HA materials,first necessary step is to synthesize Se-HA powders with different Se doping amounts,so as to provide a basis for the structural regulation of the Se-HA coating in the next step.In this part of research,HA and Se-HA powders with different Se doping amounts were synthesized by chemical coprecipitation method using Ca（NO₃）₂·4H₂O,（NH₄）₂HPO₄,Na₂Se O₃ as reactant.The synthesized powders were nano-rod-like particles.It was clear that Se entered the HA lattice by Se O₃^2-replacing PO₄^3-,and no other impurities were generated.With the increase of Se doping amount,the lattice volume increased,the grain size decreased,and the crystallinity of HA gradually decreased.（2）Preparation of Se-HA coatings by suspension plasma spraying（SPS）technology.Ultrasonic and ball milling methods were adopted,and the stability of suspension was studied by adjusting the type and concentration of dispersant.The results showed that when the solid content of the suspension is 10 wt.%,using 2 wt.%Na₄P₂O₇ dispersant,after ball milling for 2 h,Se-HA water-based suspension with low viscosity,uniform dispersion,stability and good rheology was obtained.Then the Se-HA coating was prepared on the surface of Ti substrate by SPS technology.Compared with the powder,the main crystal phase of the coating was HA phase,and there were trace decomposition phase,the lattice size of the coating was larger,and the crystallinity was improved.SEM results showed that the thickness of the coatings were relatively uniform,and the coating was closely combined with the substrate.There were a certain number of microholes and cracks inside of the coating.The surface of the coating was composed of fused flat particles,unfused and semi-fused spherical particles,and it had a certain roughness.（3）Research on the mechanics,corrosion resistance,and solubility of Se-HA coatings.This part focuses on the hardness,bonding strength,electrochemical behavior,and solubility of the coatings.The results showed that the porosity of the Se-HA coatings prepared by SPS technology was low（15.27%,13.76%,13.84%,and 14.03%,respectively）,the cross-sectional microhardness（1.69 GPa,1.76 GPa,1.79 GPa,and 1.75 GPa,respectively）and the bonding strength（30 MPa,34.33 MPa,33.67 MPa,and 33 MPa,respectively）was high.The existence of coatings has different degrees of protection on the Ti substrate,improving its corrosion resistance and slowing down its corrosion rate.The dissolution experiment showed that with the increase of Se content,the release rate of Ca²⁺and Se⁴⁺in the coating was accelerated in the phosphate buffer solution（simulating the human tumor microenvironment）with p H=4,which was conducive to killing bacteria and tumor cells.（4）Study on the biological effects of Se-HA coating in vitro.Through the establishment of pathological microenvironment of fluorosis in vitro,the key indicators such as cell adhesion,proliferation and mineralization were investigated to clarify the regulatory effect of Se-HA coating material on cell behavior,and the antibacterial properties of Se-HA coatings were studied.The results show that,like HA coating,Se-HA coating can continuously exchange ions with SBF,induce the formation of bone-like apatite on its surface in a short time,and has good in vitro mineralization performance.Cell experiments showed that the introduction of Se was beneficial to the proliferation and growth of osteoblasts,and the proliferation capacity of osteoblasts on the surface of coated samples was enhanced with the increase of Se dosage,which was related to the participation of Se in the removal of excess harmful reactive oxygen species generated by excess fluoride in cells.The antibacterial experiment showed that Se-HA coating had certain antibacterial activity against S.aureus.Se could change the permeability of cell membrane through oxidation reaction with the cell membrane of bacteria,so as to achieve bactericidal purpose.