Preparation Of Ordered Nanopore Coatings On NiTi Alloy And Their Corrosion Behavior And Biological Performance

Nearly equiatomic nickel titanium(NiTi)alloy has been widely used as orthopedic implants due to its unique shape memory effect,superelasticity,and low elastic modulus.However,the major concern of the NiTi implant is bacterial infection,lack of ability to promote fracture repair and poor osseointegration because it is a biologically inert material.Another problem of NiTi-based implant in clinical application is its poor corrosion resistance.Surface modification of NiTi alloy can confer it ideal comprehensive performance,thus has been regarded as an effective and promising approach to solve the above problems.Anodization is a low-cost surface modification method for valve metal and its alloy,especially suitable for human implant materials with complex shape.Coatings with different compositions and structures can be prepared on the surface of implant materials by anodization via adjusting the technological parameters.In this paper,the Ni-Ti-O nanopore(NP)coating was prepared on the surface of the NiTi alloy by anodization.Firstly,the effect of electropolishing(EP)pretreatment on the order of Ni-Ti-O NP coating was studied.Then,the long-term stability of the Ni-Ti-O NP coating and the effect of NP length on its properties were explored.Finally,the feasibility of Ni-Ti-O NP coatings as drug carrier was studied with Zn as the representative.The main research results are as follows:(1)EP pretreatment of the substrate before anodization can effectively remove the irregular layer on the surface of Ni-Ti-O NP coating,and make the ordered Ni-Ti-O nanopore adequately exposed.After EP pretreatment,the thickness of Ni-Ti-O NP coating increases,and the corrosion resistance improves.At the same time,the antibacterial activity of the coating is significantly improved due to the increase of Ni²⁺release amount.Exposure of ordered NPs can change the morphology of bone marrow mesenchymal stem cells(BMSCs)and induce their differentiation towards osteogenesis.In addition,the pitting pits on the surface of Ni-Ti-O nanotubes disappeared after EP pretreatment,and regular nanotubes were formed at both macroscopic and microscopic levels.(2)The Ni-Ti-O NP coating remained its porous morphology and amorphous microstructure after immersing in phosphate buffer solution(PBS)and deionized water for 60days,and its chemical composition did not change significantly.Corrosion resistance,antibacterial properties and biocompatibility were not significantly degraded.An interesting phenomenon is that the concentration of Ni²⁺in the immersion solution decreases with increases immersion time,which is caused by the crystallization of Ni(OH)₂on the coating surface due to the supersaturation of Ni²⁺concentration in the solution.(3)The Ni-Ti-O NP with different lengths(1.30-18.5?m)can be prepared by changing the anodization time,and the corrosion resistance and antibacterial properties of the samples are improved with the increase of the NP length.The NP length was positively correlated with the proliferation,type I collagen secretion and extracellular matrix mineralization capacity of BMSCs.(4)Zn element in the form of ZnTiO₃can be uniformly doped into the NP coating by hydrothermal treatment of Ni-Ti-O NP coating in Zn(CH₃COO)₂solution.The corrosion current density of the samples after hydrothermal treatment decreased by 1 order of magnitude,and the release of Ni²⁺was significantly reduced compared with that of the untreated NP.However,it has excellent long-term antibacterial properties due to the continuous release of Zn ions.In addition,Zn-incorporated Ni-Ti-O NP coating can promote osteogenic differentiation of BMSCs.