Mechanical Properties And Biocompatibility Of Ti-Mo-O Alloys For Spinal Fixation Devices

Due to high specific strength,low elastic modulus,excellent corrosion resistance and good biocompatibility,titanium alloys have obtained wide application and attention in the field of biomedical application.Compared to the current widely used（α+β）type Ti-6Al-4V alloy,theβ-type titanium alloys exhibit lower modulus and better biocompatibility.However,the relatively low yield strength due to the deformation mode of stress-induced?″-martensitic transformation or{332}<113>twinning limits their application as orthopedic implants.In addition to the above-mentioned properties,springback behavior should also be considered for the materials of implant rod in spinal fixation devices.The implant rod with low springback is more conducive for surgeons to complete the operation successfully.β-type titanium alloys with diversities of phase transformation and deformation modes provide the possibility to control the springback behavior in a wide range,but related studies were not insufficient.Taking into account the two factors of“Mo equivalency”and solid solution strengthening,Ti-7.5Mo-x O,Ti-10Mo-x O and Ti-15Mo-x O（x=0.1%-0.5%,mass%）were used as the experimental materials in this paper.The effect of Mo/O elements on the evolution of phase transformation and plastic deformation mode was analyzed.The modulus,strength and springback behavior of the alloys were analyzed by three-point bending tests.The effect of phase transformation and deformation mode on springback ratio was studied on the basis of their effect on modulus and strength.The biocompatibility of Ti-Mo-O alloys was investigated by the adhesion,proliferation,alkaline phosphatase（ALP）activity and extracellular matrix mineralization of mouse osteoblast-like cells（MC3T3-E1）.Based on the results of springback behavior and biocompatibility,the applicability of the alloys for the spinal fixation devices was evaluated.The results showed that under the constant O content,the microstructure transformed from acicular?″-martensite to equiaxedβ-phase with the increasing Mo content,and the athermalω-phase first increased and subsequently decreased.The plastic deformation mode changed from stress-induced?″-martensitic transformation to{332}<113>twinning and dislocation slip.Under the constant Mo content,the phase stability also gradually increased with an increase in O content,and the stress-induced?″-martensitic transformation and{332}<113>twinning were suppressed.Compared to Ti-7.5Mo-x O and Ti-15Mo-x O alloys,more athermalω-phase was found in Ti-10Mo-x O alloys,and these alloys exhibited the highest bending modulus and bending yield strength.Due to the decrease in athermalω-phase and enhancement of the solid solution strengthening with the increasing O content,the bending modulus was basically constant.Whereas the bending yield strength gradually increased under the coupling effect of the change of deformation mode and enhancement of solid solution strengthening.Springback ratio first increased and subsequently decreased as the Mo content increased,exhibiting opposite tendency with the bending modulus,thus,it was mainly dependent on modulus.Whereas springback ratio gradually increased as the O content increased,exhibiting similar tendency with the bending yield strength,thus,it was mainly determined by strength.With the extension of culture time,the cell adhesion and spreading area on the surface of Ti-Mo-O alloys remarkably increased,and the cell amount continuously increased.The aforementioned results indicated that these alloys exhibited the high ability of cell adhesion and proliferation.These alloys maintained a high level of ALP activity,and no obvious calcium deposition was detected after osteogenic culture for 7 and 14days.Ti-Mo-O alloys exhibited the good biocompatibility due to the formation of stable oxide films of TiO₂ and MoO₂ on their surface.