Finally, research has shown that titanium dioxide nanoparticles do not pass the first layer of the skin — the stratum corneum — and are not carcinogenic (7Trusted Source, 15Trusted Source).

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 [2–8]. The oxide film obtained from oxygen ion implantation is thicker, about several microns [9–14]. Hydrogen peroxide treatment of titanium alloy surface is a process of chemical dissolution and oxidation [15, 16]. The dense part of the oxide film is less than 5 nm [17–21]. The oxide film generated from the thermal oxidation method has a porous structure, and its thickness is commonly about 10-20 μm [22–25]. 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 [26–28]. Applied with the anodic oxidation method, the surface can generate a porous oxide film of 10 μm to 20 μm thickness [29–31]. Similarly, the oxide film generated from the microarc oxidation method is also porous and has a thickness of 10 μm to 20 μm [32, 33].