Abstract: the purpose of titanium alloy coating is to improve its biocompatibility and biological induction in organisms. Hydroxyapatite coating is the most mature biological coating of titanium alloy. There are many methods to prepare hydroxyapatite, and the coating prepared by each method has its own characteristics.

　　Key words: medical titanium alloy; Surface treatment; Hydroxyapatite; Research progress

　　introduction

　　In biomedicine, titanium alloy is mainly used for bone replacement and repair. Its surface coating or surface modification can improve the stability, biocompatibility and biological induction of titanium alloy. [1、2]

　　Titanium alloy metal material is a kind of bio inert material. Although there is no problem in biocompatibility, it does not have biological induction. Without surface treatment, there will be inflammatory reaction between new bone and its surface. Hulshoff [3] et al. Pointed out that tissue fibrosis occurs on the surface of titanium implanted in human body. It takes up to six months to connect with the bone.

　　The composition and structure of hydroxyapatite (HA) [CA10 (PO4) 6 (OH) 2] are similar to those of natural bone. The composition and structure of hydroxyapatite synthesized by different methods can also be adjusted to different states, so that when hydroxyapatite is used as an implant, its surface is well combined with new bone. Hydroxyapatite (HA), fluoroapatite (FA), tricalcium phosphate（ β- TCP) and so on.

　　1. Plasma spraying method

　　Plasma spraying method is to melt hydroxyapatite into plasma at high temperature and accelerate the plasma to spray onto titanium alloy materials.

　　It is found that after heat treatment in the temperature range of 6OO ℃ ~ 800 ℃, near nano-sized particles appear on the surface crack or edge of HA coating, which is HA crystal after amorphous crystallization in the coating. [4]

　　This method was applied earlier. The main problem is that the combination of hydroxyapatite and titanium mainly depends on physical methods, and the combination of hydroxyapatite and titanium is not strong enough due to the residual stress [5,6] caused by temperature gradient during spraying.

　　2. Preparation of biomimetic solution

　　The so-called biomimetic solution method is to put titanium alloy into simulated body fluid. Titanium alloy forms hydroxyapatite coating on the surface of titanium alloy in simulated body fluid like hydroxyapatite in nature.

　　Lenka Jon et al. [7] studied that: first treat with alkali solution and then put it into simulated body fluid. The coating of titanium alloy is hydroxyapatite containing carbonate. And the coating is not uniform after 20 days of immersion. If the titanium alloy is acid etched first, then treated with alkaline solution, and then put into simulated body fluid, uniform hydroxyapatite can be obtained.

　　The titanium alloy can also be treated by micro arc oxidation, and then put into the simulated body fluid. For example, won Hoon Song [8] and others did this. They were immersed in simulated body fluid at concentrations of 1 and 1.5 times to obtain hydroxyapatite coating.

　　Researchers [9] found that some groups have the induction effect of deposition process, such as - PO4H2, - COOH, - CONH2, - oh and - NH2.

　　3. sol gel (Sol-gel) method

　　Sol-gel method is a relatively mature method in the preparation of coatings, such as the preparation of semiconductor thin films in the electronics industry. Therefore, researchers also use this method to prepare hydroxyapatite on titanium alloy matrix. The advantage of this method is that some other elements can be added when preparing the coating, and the content of various elements can be effectively controlled. For example, Weng Wenjian et al. [10] used ethanol system to add fluorine-containing components to obtain fluorine-containing hydroxyapatite. Hydroxyapatite containing fluorine has smaller solubility and thermal expansion coefficient, but its biological activity is reduced. Kim [11] and others did similar work, but the difference is that the components of the ethanol system they adopted are different.

　　Although sol-gel method has the advantages of adjustable coating components, this method is not suitable for different titanium alloy surface shapes. And the adhesion of the coating is not strong enough.

　　4. Preparation by other methods

　　In addition to the methods mentioned above, the methods for preparing hydroxyapatite include chemical vapor deposition, electron beam deposition [12], ion beam sputtering deposition [13], pulsed laser deposition [14], etc. These methods have their own shortcomings, some preparation processes are complex, and some adhesion is not good enough.

　　5. Conclusion

　　Hydroxyapatite, as an inorganic substance, has good biocompatibility, but it does not have biological induction. In order to improve its biological induction, researchers have developed a variety of preparation methods to prepare composite coatings and other coatings, but the stability of these coatings has not been confirmed.