The purpose of the titanium alloy coating is to improve its biocompatibility and bio-inducibility. Hydroxyapatite coating is currently the most mature biological coating of titanium alloy. There are many methods for preparing hydroxyapatite, and the coating prepared by each method has its own characteristics.
In biomedicine, titanium alloy is mainly used for bone replacement and repair. Its surface coating or surface modification improves the stability, biocompatibility and bioinducibility of titanium alloy.
Titanium alloy metal material is a bio-inert material. Although there is no problem with biocompatibility, it is not bioinducible. Without surface treatment, there will be an inflammatory reaction between the new bone and its surface. Hulshoff et al pointed out that tissue fibrosis will appear on the titanium surface of the implanted human body. It takes up to 6 months to connect it to the bone.
The composition and structure of hydroxyapatite (HA) [Ca10 (PO4) 6 (OH) 2] are similar to the inorganic components 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), etc., all belong to this category of materials.

1. Plasma spraying method
Plasma spraying method is to melt hydroxyapatite into plasma at high temperature, and then spray this plasma onto titanium alloy material.
Experiment found that after heat treatment in the temperature range of 6OO ℃ ~ 800 ℃, near nano-sized particles appear on the surface cracks or edges of the HA coating, which is the amorphous HA crystal in the coating.
This method was applied earlier, and the main problem is that the combination of hydroxyapatite and titanium mainly depends on the physical method, and due to the residual stress caused by the temperature gradient in the spray, the combination of hydroxyapatite and titanium is not strong enough.

2. Bionic solution preparation
The so-called bionic solution method is to put the titanium alloy into the simulated body fluid. In the simulated body fluid, the titanium alloy forms the hydroxyapatite coating on the surface of the titanium alloy like the formation of hydroxyapatite in nature.
Lenka Jon et al. Researched that: first, the alkaline solution was processed, and then placed in the simulated body fluid, the coating of the titanium alloy was hydroxyapatite containing carbonate. And the coating is not uniform after 20 days of soaking. If the titanium alloy is first etched by acid, then processed by lye, and then placed in the simulated body fluid to obtain uniform hydroxyapatite.
You can also use a micro-arc oxidation method to treat the titanium alloy first, and then put it into the simulated body fluid. Such as Won-Hoon Song and so on. They were soaked in simulated body fluids at 1 and 1.5 times the concentration to obtain a hydroxyapatite coating.
Researchers [9] found that some groups have an induction effect on the deposition process, such as -PO4H2, -COOH, -CONH2, -OH and -NH2.

3. 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. So the researchers also used this method to prepare hydroxyapatite on the titanium alloy substrate. 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. Added fluorine-containing components with ethanol system to obtain fluorine-containing hydroxyapatite. Fluorine-containing hydroxyapatite has smaller properties in terms of solubility and thermal expansion coefficient, but the biological activity is reduced. Kim et al. Did similar work, except that the components of the ethanol system they used were different.
Although the sol-gel method has the advantage of adjustable coating composition, this method is not suitable for various titanium alloy surface shapes. And the binding force of the coating is not strong enough.

4. Preparation by other methods
In addition to the methods mentioned above, there are chemical vapor deposition, electron beam deposition, ion beam sputtering deposition, pulsed laser deposition, etc. for preparing hydroxyapatite. These methods have their own shortcomings, some of which are complicated in preparation process, and others are not good enough.

5 Conclusion
As an inorganic substance, hydroxyapatite has good biocompatibility, but it is not bioinducible. In order to improve its bioinducibility, 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.