The Preparation And Characterization Of Phytic Acid/hydroxyapatite Bioactive Coatings On Magnesium Alloy

Magnesium(Mg)and its alloys have been considered as the candidate for the application as the biodegradable bone repair materials due to their desirable mechanical property,biocompatibility,biodegradability and bone induction ability.However,the rapid degradation of Mg alloy implants in containing chloride physiological environment results in their fast loss of mechanical integrity,the excess of Mg2+ ions,H2 release and abrupt p H value increase,all of which are harmful to tissue healing and limit their biomedical applications.Therefore,it is important to develop magnesium alloys with controllable degradability and developing surface modification to induce protective coatings is proven to be an effective method to control the degradation rate and maintain the desired mechanical strength of magnesium alloys.In this work,a phytic acid/hydroxyapatite composite coating has been prepared on AZ31 magnesium alloy substrate.The influence of preparation parameters on the surface morphology and composition of the coatings was studied by SEM and FTIR.The corrosion resistance and bioactivity were investigated by electrochemical testing and in vitro simulated body fluid immersion test.The results showed that homogeneous and crack-free phytic acid/ hydroxyapatite composite coatings with the thickness of 1.4 ?m were successfully fabricated on AZ31 magnesium alloys when the content of hydroxyapatite is 45 wt.%.Electrochemical measurements in simulated body fluid(SBF)revealed that the Ecorr of the composite coated sample increased 190 mV,the icorr decreased to 2.28 ?A/cm2 and the value of impedance was six times larger than the bare magnesium alloy.During the immersion,the composite coating could induce the generation of hydroxyapaite,enhancing the corrosion resistance and improving the bioactivity of magnesium alloy.To further improve the corrosion resistance of AZ31 magnesium alloy,a crack-free hydroxyapatite/phytic acid(HA/PA)hybrid coating was synthesized on AZ31 substrate via immersing the substrate into reaction solution of phytic acid,calcium nitrate tetrahydrate and phosphorus pentoxide,and followed hydrothermal treatment in the saturated CaO solution.The hydrothermal treatment temperature has a great influence on the composition and properties of coatings.With the increase in hydrothermal temperature,the phosphate and calcium ions diffused to the interface of the precursor coating/solution to form amorphous CaP compound and subsequently crystallized into HA particles that repaired the defects in the precursor coating.Moreover,the hybrid coating prepared in 120 ?C for 4 h exhibited a high bonding strength of 24.3 ± 1.7 MPa with the thickness of 8.0 ?m.The results of electrochemical test and immersion test showed that compared with the precursor coating,hybrid coating greatly improved the corrosion resistance of magnesium alloy.After 16 days of immersion,the hybrid coating could still provide effective protectiveness to the magnesium alloy substrate.Meanwhile,the hybrid coating could induce the deposition of hydroxyapatite,thus improving the corrosion resistance and bioactivity of magnesium alloy.