| As people’s living standard improves,traditional medical materials using for bone implants cannot meet the increasing demand.Magnesium and its alloys have attracted more attention as biomaterials due to their spontaneous degradation,nontoxicity and good mechanical properties.However,magnesium alloy degrades too fast and has poor biocompatibility.Ternary Mg alloys Mg-2Zn-0.3Ca,Mg-2Zn-0.5Ca,Mg-3Zn-0.3Ca,Mg-3Zn-0.5Ca,Mg-3Zn-0.15Sr,Mg-3Zn-0.5Sr,Mg-3Zn-1Sr were designed and prepared in this paper.After the comparison and evaluation of microstructure,mechanical properties,electrochemical testing and in vitro behavior of seven alloys,the homogenized Mg-2Zn-0.5Ca and Mg-3Zn-0.5Sr exhibited good mechanical properties,biocompatibility and corrosion resistance.Furthermore,the influence mechanism of alloying elements on the properties of magnesium alloys was discussed.The extruded Mg-3Zn-0.5Sr alloy was prepared and the influence mechanism of extrusion treatment on the microstructure,mechanical properties and corrosion resistance of the magnesium alloy was discussed.The results showed that the addition of appropriate Zn and Ca elements could refine the grains of magnesium alloys to a certain extent.Sr presented better effect on grain refinement of magnesium alloys,but the tensile strength and elongation decreased with the increase of Sr content.Appropriate amount of second phase and uniform distribution helped to prevent corrosion,but too much second phase would aggravate galvanic corrosion.The extruded alloy grains were fine equiaxed crystals.The hot extrusion process could greatly refine the grains,and the grain diameter was reduced to one-tenth of the as-cast state,which caused the fine grain strengthening effect.The tensile strength of the extruded alloy was doubled,reaching 250.4±12.53MPa;the compressive strength had also been doubled,reaching 145.9±8.31 MPa;the hardness and elongation of the as-cast Mg alloy was greatly improved.However,the extrusion process increased the grain boundaries in the alloy and accelerated the intergranular corrosion.Therefore,the as-cast Mg-2Zn-0.5Ca and Mg-3Zn-0.5Sr alloys were selected as the substrate for further research.Alloying has a limited effect on improving the corrosion resistance of magnesium alloys.The surface treatment method of micro-arc oxidation was adopted in this paper to modify the alloy.Mg-2Zn-0.5Ca alloy was employed as the substrate,four additives(Na2WO4,nano-hydroxyapatite,K2TiF6 and NaF)were added into the 10 g/L Na5P3O10+2 g/LNaOH+10 mL/L C3H8O3 base electrolyte according to the orthogonal design of four factors three levels(L9(34)).Self sealing bioactive micro-arc oxidation(MAO)coatings were fabricated on Mg-2Zn-0.5Ca alloys through orthogonal experiments.The effects of four additives on the microstructure,mechanical properties,phase composition,corrosion resistance and biocompatibility of MAO coatings were investigated.The results showed that the addition of Na2WO4 and n-HA could improve the bonding strength between the film and the substrate.The addition of K2TiF6 promoted the formation of the MgF2 phase and increased the thickness of the film.The addition of nano-hydroxyapatite and K2TiF6 showed self-sealing effects and contributed to the corrosion resistance of the samples significantly.The optimal combination of factors and levels considering both mechanical properties and corrosion resistance was:0 g/L Na2WO4,2.5 g/L NaF,5 g/L n-HA,5 g/L K2TiF6.The growth mechanism of MAO coatings combining with the visual phenomenon was discussed as well.Based on orthogonal experiments,a self-healing antibacterial micro-arc oxide film containing silver was prepared on Mg-3Zn-0.5Sr alloy.Different concentrations of CH3COOAg(0,1,2,3 g/L)were added into the electrolyte to investigate the effects of the additives on properties of MAO coatings.The MTT test and fluorescence assay was employed to investigate the in vitro cytocompatibility of Ag-containing MAO coatings.The results show that the addition of CH3COOAg in the electrolyte led to the formation and deposition of Ag2O and Ag2CO3 phases,which sealed the micro-pores,restored the MAO coatings and improved the corrosion resistance of the samples significantly.The nano-scale silver-containing particles in the coating and the gradually released Ag+during the degradation process killed the bacterias by contact and release methods respectively,showing strong antibacterial ability against E.coli.It has been verified that the sample prepared by adding 2g/L CH3COOAg possessed the highest bonding strength,compact and uniform structure,excellent corrosion resistance,cytocompatibility and antibacterial properties.In addition,the samples would not cause acute toxicity in mice.This paper used the layer-by-layer assembled method to prepare chitosan(containing A/W)/heparin sodium and chitosan(containing TiO2)/heparin sodium composite film on the silver-containing micro-arc oxidation film.The interface combination,in vitro and in vivo biodegradation and other properties of the composite samples were studied.By controlling the number of cycles to prepare LbL layer and the content of TiO2 in the LbL layer,the performance of the sample could be flexibly controlled.The influence mechanism of layer numbers and concentration of TiO2 on the mechanical properties,corrosion resistance,and biocompatibility of the composite film was revealed.In addition,the in vitro degradation mechanism of composite samples in simulated body fluids was discussed.The research results showed that the LbL sample prepared after 3 cycles effectively sealed the micropores in MAO film,presented the most uniform and compact morphology,combined well with the MAO film,and had the best corrosion resistance.The graft of heparin sodium layer slowed down the swelling effect of chitosan,improved the corrosion resistance of the composite sample,and effectively inhibited the adhesion of platelets,reduced the coagulation rate,and improved blood compatibility.The addition of TiO2 would enhance the wettability of the LbL film and promote cell adhesion.The sample with the addition of 4g/L TiO2 showed the best cell compatibility.And the corrosion rate(5.03×10-6 g/(cm2·h))of CS-4 was the slowest during 15 days of immersion in SBF.The pH was stable during the degradation process,whilst the samples exhibited good biocompatibility and bioactivity.All composite samples would not cause acute toxicity in vivo.Micro-CT data showed that the Mg alloy group degraded rapidly in vivo.There were still many cavities in the femoral bone marrow after implantation for 28 weeks.The CS-4 group degraded relatively slowly,and the bone tissue healed well.After AgAC-2 and CS-4 rods were implanted into the femur of rats,bone absorption was less than the group of Mg alloy,and the number of new bone formation was much more.The surface treatment played an effective protective effect on the magnesium alloy substrate,slowed down the degradation rate,and adjusted the release rate of Mg2+,while enhanced the activity of osteoblasts,thus,accelerated new bone formation and tissue healing.There were no obvious signs of inflammation in the bone tissue.The research has greatly improved the comprehensive performance of magnesium-based alloys as bone implants and laid a solid foundation for the further clinical application of magnesium alloys. |
