| Ti and its alloys are widely used for dental devices due to their high specific strength,low elastic modulus,excellent corrosion resistance and biocompatibility.However,clinically commonly used Ti-6A1-4V has the risk of releasing toxic Al and V ions,which may cause harm to the human body.Additionally,the higher elastic modulus of Ti-6Al-4V(120 GPa)in comparison to cortical bone(15 GPa)leads to a stress-shielding effect,resulting in peri-implant inflammation.Hence,a gradient structure is proposed in this study.Gradient porous Ti scaffolds with different porosity were fabricated by selective laser melting(SLM)using modified hydride-dehydride(HDH)Ti powders.The effects of porosity on pore characterization,microstructure,mechanical properties,corrosion resistance,cell and blood compatibility were investigated systematically.The main contents and results are as follows:To solve the problem of insufficient matching between the high strength and low modulus of the commonly used porous structures,such as bone-like trabecular structure and lattice gradient structure,four different structures of Primitive,I-WP,Diamond,and Gyroid were designed by the triply periodic minimal surface method.Ansys simulation results showed that the Gyroid structure exhibited the highest permeability and smaller maximum equivalent stress at the same porosity compared with other structures,which means that the Gyroid structure is more suitable to be selected as the basic unit for the gradient designing.Based on the simulation results,7 different gradient structures with an average porosity of 20-80%were then designed.The average pore size was in the range of 300-775?m.The results of static simulation in the oral environment showed that with porosity increased both the maximum equivalent stress of the implants and the maximum equivalent stress and strain of surrounding cortical bone and cancellous bone increased gradually.When the porosity was in the range of 20%-50%,the average pore size was between 300-550?m,which meets the requirement for cell adhesion,proliferation,and differentiation;the maximum equivalent stress of Gyroid gradient structure in the oral environment was between 75-280 MPa,which is less than 50%of the yield strength(400 MPa);the maximum equivalent stress and maximum equivalent strain of cortical bone and cancellous bone were 2-45 MPa and 800-2800 ??,respectively,which is benefit for bone reconstruction.The effects of ball milling on powder characteristics of HDH Ti powders and SLM printing parameters on the mechanical properties of the parts were studied.The results showed that ball milling could improve the particle morphology of the irregular HDH Ti powder.The flowability of HDH Ti powder was significantly improved and the Carr flowability index was 77.74g/cm3 under a ball-to-battery ratio of 5:1,a rotation speed of 200r/min,and ball milling for 4h,which was 19.5%higher than the original powder.The microstructure of the pure Ti parts fabricated using the modified powders under different processing parameters composed of martensite ?' and the tensile strength and elongation were in the range of 950-980 MPa and 9.5-17.5%,respectively.When the laser power was 175W,scanning speed was 600mm/s and scanning distance was 0.09 mm,the part exhibited the highest mechanical properties with a tensile strength of 985.4 MPa and an elongation of 17.3%,which exceeded the ASTM B381 forged Ti-6A1-4V alloy level.Gradient porous implants with porosity of 20-50%were fabricated based on the optimized gradient structure and SLM process.As porosity increased,the yield strength decreased from 477.5 MPa to 206.1 MPa;the elastic modulus decreased from 20.7 GPa to 8.6 GPa;the yield-to-elastic ratio of scaffold reached 0.022-0.024.In comparison to the highest values reported by currently published literature,the yield-to-elastic ratio of gradient scaffolds fabricated in this study enhanced by 18-33%,which significantly improves the mechanical adaptability of porous Ti implants.When the average porosity was 30%,the elastic modulus of the gradient porous part was 14.6 GPa,which is equivalent to the elastic modulus of human cortical bone,and the yield strength was 351.5 MPa,which is about 1.7 times than that of human cortical bone.The corrosion resistance,permeability,and biocompatibility of the parts with different porosity gradient structures were further studied.With the increase in porosity from 20%to 50%,permeability increased from 0.143 × 10-9 m2 to 1.945×10-9m2;corrosion current density increased from 4.3×10-9 A/cm2 to 7.2×10-8 A/cm2;impedance decreases from 560.9 k?/cm2 to 210.7 k?/cm2.The extracts of porous scaffolds with different porosity showed no cytotoxic on MG-63 cells after 1-5 days culturing,and the cell proliferation rate was between 99-103%.According to ISO standards,the toxicity level is 0-1.The cytoskeleton was not significantly deformed after 1-5 days of co-culturing of the porous scaffolds with different porosities and MG-63 cells,and the number of cells increased significantly with the increase of porosity.The hemolysis rate of scaffolds with different porosity was between 1.44%and 2.76%,which is lower than the requirements of the medical device(<5%).The mice exhibited no toxic symptoms,such as dyspnea,reduced exercise,after injecting extracts of porous scaffolds with different porosity after 1-3 days.The comprehensive results of mechanical properties,corrosion resistance,and biocompatibility in vivo and in vitro indicate that the Gyroid graded porous pure Ti parts with an average porosity of 30%not only has an elastic modulus equivalent to that of the human cortical bone and has a 1.7 times yield strength than that of the human cortical bone,but also exhibits excellent corrosion resistance,permeability,cell,and blood compatibility,which make it an attractive new dental implant alloy. |
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