The development of Nickel titanium shape memory alloys (NiTi-SMA) coincides with the idea of biological osteosynthesis (BO) and is an ideal material for orthopedic internal fixation. Does elemental nickel produce cytotoxicity? What is the biocompatibility of the surface modified NiTi-SMA? This paper reviews the recent progress, problems and development directions of NiTi-SMA biocompatibility. Nickel titanium shape memory alloys (NiTi-SMA) have the advantages of light weight, high strength, fatigue resistance, corrosion resistance, and high resilience, making them a promising orthopedic metal biomaterial [1]. Research applications of NiTi-SMA are increasing year by year, but there is not enough evidence to demonstrate its biocompatibility for long-term implantation into the muscle [2]. Based on years of application of metallic biomaterials such as swan-like memorably-compressive connector (SMC), we summarize the histocompatibility and cytotoxicity of NiTi alloys to provide a basis for the selection of NiTi-SMA materials for further development and research. 1. Biocompatibility and cytotoxicity of Ni and Ti materials The effect of biomaterials on the host after implantation into the organism is a very complex process, and three main biological reactions occur, namely tissue reaction, blood reaction and immune reaction, which are essential for the evaluation of biocompatibility. Regarding the determination of biocompatibility, several conditions should be included: (1) the type and number of inflammatory cells; (2) the degree of vascular distribution; (3) the presence or absence of encapsulation membrane formation; (4) the presence or absence of steatosis; (5) histochemical and biochemical analyses of tissues adjacent to the implant; (6) biochemical analyses of tissues distant from the implant; and (7) changes in the appearance and structure of the metal implant. Among the potentially toxic metals for humans, nickel is second only to silver in terms of toxicity. If nickel material alone is implanted in an organism, the leached element nickel will be very cytotoxic. After entering the organism, nickel is deposited mainly in the skin, central nervous system, kidneys, and liver [3]. Nickel is able to bind to ribonucleic acid (RNA) and proteins and depolymerize RNA and proteins, nickel also hinders muscle contraction and disrupts enzymes. Low concentrations of nickel (15-30ug/ml) can inhibit the growth of fibroblasts cultured in vitro. When nickel is absorbed into the bloodstream, it can complex with alpha-macroglobulin to form nickel fibrinolytic enzymes. Pure elemental nickel and nickel salts have been shown to have carcinogenic effects, with nickel sulfide (Ni3S2) and nickel sulfide (NiS) being carcinogenic. In addition, nickel is one of the most common metal allergens. Modern β-titanium alloys, on the other hand, tend to choose elements with better biocompatibility and try to discard the cytotoxic ones. For example, the vanadium in titanium alloys stimulates the production of more bone resorption factors by megalophils, and these cytokines play an important role in implant loosening, so today titanium alloys are free of toxic elements such as vanadium and aluminum. Compared with Ti-6Al-4V, fixation with surface-hardened Ti-Nb-Zr splints is stronger and the chance of postoperative infection is reduced. 2. Biocompatibility and cytotoxicity of NiTi alloy NiTi alloy contains about 50% nickel, how is its biocompatibility when used as human implants? Does elemental nickel produce cytotoxicity? Long-term experimental and clinical studies are needed to understand an in vivo implant material before a conclusion can be reached. There are some changes in the research methods regarding the biocompatibility of NiTi alloy: (1) more osteoblasts, fibroblasts and endothelial cells are used in in vitro experiments; (2) stainless steel, Ti-6Al-4V and other metallic materials are used as controls with NiTi alloy; (3) the number of in vivo experiments has increased; (4) a combination of various analysis methods: such as ultramicroscopic properties of cell-material interface, cell-material interface reaction, histological reaction of soft tissues around the implant, and whether there is any adverse effect on new bone production. Nowadays, most scholars believe that NiTi alloy is a safe material for in vivo implantation, summarizing the reasons as follows: (1) the passivation film on the surface of NiTi alloy is mainly composed of titanium oxide and contains only a very small amount of nickel, which is the fundamental reason for its good tissue response; (2) the nickel in NiTi alloy exists in the chemotactic state, and even if there is dissociation in the human body, it is still a very small amount; (3) the cytotoxicity observed in the in vitro experiments reactions are due to the gradual concentration of nickel, a condition that is impossible to develop in the in vivo environment. Bogdanski D et al [4] examined the biocompatibility of NiTi alloys using osteoblast-like osteosarcoma cells (SAOS-2,MG-63), human primary fibroblasts (HOB) and murine fibroblasts (3T3).Kapanen A et al [2, 5] used osteoblast-like ROS-17 in co-culture with stainless steel, pure titanium and pure nickel, respectively The NiTi alloy was confirmed to have less surface corrosion and to be well accepted by the cells; human osteoblasts and fibroblasts were cytocompatible when co-cultured with NiTi alloy, and the release of nickel had reached a level similar to that of stainless steel after 2 d of culture. armitage DA et al [6] conducted cytotoxicity and compatibility studies with fibroblasts and endothelial cells, and the NiTi alloy surface in the test There was no difference between the two cytocompatibilities; the hemolytic reaction caused by NiTi alloy did not differ from that of 316L stainless steel and polished titanium; platelet tests illustrated that polished NiTi alloy significantly contributed to thrombosis compared to 316L stainless steel and polished titanium, while heat-treated NiTi alloy significantly reduced thrombosis. In an in vivo experimental study, Kujala S et al [7] used NiTi intramedullary nails placed in murine bone and found that it could promote bone healing and the formation of new bone was mainly woven bone. -Kapanen A et al [8] took NiTi-SMA, stainless steel and Ti-6Al-4V (titanium-6%aluminium-4%vanadium) and implanted them in mice for 8 weeks. was detected close to normal, and more cartilage and bone tissues were induced; the BMD was reduced in the stainless steel and Ti-6Al-4V groups, indicating that NiTi-SMA has good biocompatibility. 3. surface modification and cytotoxicity of NiTi-SMA Scholars at home and abroad have made a lot of work on the processing and manufacturing of NiTi-SMA and surface optimization treatment, and there are various methods to surface treat NiTi-SMA [9]. However, can surface treatment improve its biocompatibility? Which surface treatment method is more effective to use? How to improve the surface properties of the material? All are current challenges that need to be solved. The good biocompatibility and corrosion resistance of NiTi-SMA are closely related to its surface oxide film, which helps to keep NiTi-SMA relatively inert under physiological environment. The surface oxidation treatments include heat treatment, mechanical polishing, and electropolishing; electron microscopy, X-ray diffraction, and X-ray point photon spectroscopy are required for the analytical study of the interface and surface.Thierry et al [10] concluded that the biocompatibility and good corrosion resistance of NiTi-SMA come from the uniform distribution of the oxide layer on its surface, which is mainly composed of titanium oxide, with minimal distribution of nickel. which has a minimal distribution of nickel.Armitage DA et al [6] by cytotoxicity studies, heat treatment of NiTi alloys can significantly reduce thrombosis; and the oxidation of nickel elements on the surface of the material reduces the concentration of nickel elements on the surface.Firstov GS et al [11] used the thermogravimetric method to analyze the oxidation kinetics, showing different oxidation habits at about 500°C, 500°C – 600°C with a nickel-free region in the oxide layer; 500°C oxidation produces a protective nickel-free oxide layer of which contains a relatively small amount of nickel elements in the air-oxide interface. Electropolishing technology, that is, the use of electropolishing plus chemical passivation method to treat the surface of NiTi alloy, preliminary research shows that: this method can form a very thin oxide layer on the metal surface, so that the material finish and corrosion resistance significantly improved; can make the release of nickel elements to non-toxic level in vivo. Other common surface treatment methods are ion injection, plasma spraying hydroxyapatite, etc. Chemical surface treatment is a simple process compared to other surface treatments, and can form a uniform layer of modification on the surface of complex-shaped materials. Among them, plasma treatment is one of the important means of surface modification, which requires the use of OSCE AES and scanning electron microscopy SEM for the analytical study of the material interface.A study by Tan L et al [12] found that plasma treatment has a great effect on the Ti/ Ni ratio of NiTi alloy surface, which can cause the bias or enrichment of Ni and Ti elements on the surface, where the Ti enrichment of elements increases the activity of the material surface and facilitates the bonding of NiTi alloys with polymer films. When DC plasma treats the surface of the sample, the components of its electrode will be deposited onto the surface of the sample. Aluminum elements are harmful to human body, therefore, from the perspective of safety of medical materials, DC plasma treatment device should not use aluminum electrode, and it is recommended to use RF plasma treatment method. Coating treatment on the surface of NiTi alloy, such as polymerized tetrafluorcethlene (PPFTE) spraying, can improve the corrosion resistance, reduce the release of nickel ions, and reduce the cytotoxicity of NiTi alloy.Choi J et al [13] immersed the alloy in supersaturated calcium phosphate solution. The thickness of the calcium phosphate coating was controlled by time. The porous nature of the surface microcrystals could tolerate changes in temperature and bending of the alloy, producing a physiologically consistent surface, reducing the release of nickel elements, improving the biocompatibility of NiTi-SMA, and increasing the adhesion of leukocytes and platelets to the alloy. 4. Problems and development trend of NiTi-SMA During the development of NiTi biomaterials, domestic scholars have also made a lot of research work. However, it should also be seen that we have many problems to be solved in terms of material improvement and surface property optimization. The use of surface treatment methods such as coating can reduce the release of nickel elements in NiTi alloys. But: what is the effect of surface modified alloy materials on cytotoxicity? How to maintain the integrity of various passivation films in body fluids and in vivo stressed environments? Porous NiTi alloy material (porous nickel-titanium alloy) has attracted much attention [14], because this material has a large contact area with bone, and bone can grow into the material pores and form a good attachment. However, due to the large surface area of the material, it also faces a greater test in terms of surface corrosion resistance and nickel ion release.Es-Souni M et al [15] tried to add copper to NiTi alloy to improve the material properties, and the preliminary conclusion was that copper can improve the mechanical properties and corrosion resistance of the material, but copper is cytotoxic and can make the biocompatibility reduced. Poon RW et al [3] applied carbon plasma cultivation or precipitation to make the surface layer of NiTi-SMA material mixed with ions of amorphous carbon, which could significantly improve the corrosion resistance of the material and reduce the release of nickel elements; cytotoxicity tests illustrated that both treatments could contribute to the adhesion and proliferation of osteoblasts.Starosvetsky D et al [16] used a powder immersion coating method to (original powder immersion reaction assisted coating, PIRAC) improved the surface properties and enhanced the corrosion resistance of NiTi-SMA. In conclusion, NiTi-SMA has a unique shape memory effect and good biocompatibility. Since the 1980s, the Department of Orthopedics of Changhai Hospital has developed a series of memory alloy bone joiners using the ommaginous phase reversibility of NiTi-SMA, among which SMC is specifically used for the treatment of long bone stem fractures of the upper limbs [17, 18]. The used SMC was treated by the relevant production process to make a uniform and dense distribution of passivation film on its surface, which is firmly bonded to the metal surface, and combined with the coating technology, it can effectively improve the corrosion resistance and cytocompatibility of NiTi-SMA. We believe that with the in-depth research on the biocompatibility of NiTi-SMA and the improvement and development of new alloy materials, NiTi-SMA will be more in line with the physiological conditions of the organism and show a broader application prospect.