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TiO2 comes in many different forms. However, only a few of these forms are considered food-grade (acceptable to be added to food). Many studies that raised concern about the safety of TiO2, including the concern for genotoxicity, used forms of TiO2 that are not considered acceptable for use in food and have different properties than food-grade TiO2. Other studies did use food-grade TiO2, but took steps to break the material down into smaller particles than what would normally be found in food.

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The conventional surface treatment methods of titanium alloy include glow discharge plasma deposition, oxygen ion implantation, hydrogen peroxide treatment, thermal oxidation, sol-gel method, anodic oxidation, microarc oxidation, laser alloying, and pulsed laser deposition. These methods have different characteristics and are applied in different fields. Glow discharge plasma deposition can get a clean surface, and the thickness of the oxide film obtained is 2 nm to 150 nm [28]. The oxide film obtained from oxygen ion implantation is thicker, about several microns [914]. Hydrogen peroxide treatment of titanium alloy surface is a process of chemical dissolution and oxidation [1516]. The dense part of the oxide film is less than 5 nm [1721]. The oxide film generated from the thermal oxidation method has a porous structure, and its thickness is commonly about 10-20 μm [2225]. The oxide film from the sol-gel method is rich in Ti-OH, a composition that could induce apatite nucleation and improve the combining of implants and bone. It has a thickness of less than 10 μm [2628]. Applied with the anodic oxidation method, the surface can generate a porous oxide film of 10 μm to 20 μm thickness [2931]. Similarly, the oxide film generated from the microarc oxidation method is also porous and has a thickness of 10 μm to 20 μm [3233].

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