Magnesium metal and its alloys have good mechanical compatibility and biocompatibility, and are known as a revolutionary new generation biomaterial or third generation biomaterial. However, the higher activity and lower corrosion resistance of magnesium alloys make the degradation rate and bone formation rate not suitable. The uncontrollable degradation of magnesium alloy limits its application in clinical medicine. Surface functional modification is the main way to improve the corrosion resistance and biocompatibility of biomedical magnesium alloys.

Recently, Professor Zeng Rongchang and his collaborators from Shandong University of Science and Technology summarized the research progress of biodegradable magnesium alloy surface coatings in the past 20 years and established a knowledge framework system for the surface modification coatings of degradable magnesium alloys. According to the characteristics of the discipline, the coating includes mechanical, chemical, physical and biological coatings; according to the function, the coating includes degradable, biologically active, antibacterial, self-cleaning, self-healing, self-sacrifice, drug release, and biological adaptation With biocompatible coating. In this paper, the coatings are classified from the perspective of energy and physics, and the correlation system between the coatings is established. The phosphate conversion coating coating system is also constructed. The properties, preparation methods and problems of conversion film, deposited film, mechanical coating and functional coating are systematically introduced. Highlighted the introduction of chemical conversion coatings (phosphate conversion coatings, fluoride conversion coatings, phytic acid conversion coatings), ionic liquid conversion coatings, bionic and biomineralization coatings, alkaline heat treatment conversion coatings, micro-arc oxidation (anodic oxidation) coatings And the preparation method, performance and problems of hydrothermal coatings.

Classification and correlation of biomedical magnesium alloy surface coating methods

The magnesium alloy chemical conversion film is formed by a chemical reaction between the magnesium substrate and the film-forming liquid, and is usually carried out in an aqueous solution. The process includes metal dissolution and precipitation of precipitates. Because the substrate directly participates in the film formation process, the bonding force between the coating and the substrate is relatively strong, and it can be used as a primer layer to improve the bonding force between the outer layer and the magnesium alloy substrate;
Magnesium alloy deposited coating refers to a non-in-situ coating whose substrate does not participate in the formation of the coating. The matrix and the coating are connected by intermolecular forces (such as electrostatic forces, hydrogen bonding, etc.) and mechanical forces. Due to the weak adhesion between the deposited layer and the substrate, it is not suitable as an intermediate layer, and is generally used as the outermost layer or functional layer. The composition of the deposited coating is flexible and diverse, and has good biological activity. It can be used as the surface layer of the composite coating to achieve multi-functional coating. This article describes co-deposition coatings, physical vapor deposition coatings, atomic layer deposition coatings, electrodeposition coatings and simple dip coatings.
The magnesium alloy mechanical coating refers to the deformed surface and subsurface produced during the severe plastic deformation process, and no chemical reaction is involved in the preparation process. Without introducing any substances (such as deposited minerals and attached organic matter), a layer of fine structure different from the original surface is obtained. The two mechanical coating methods of friction stir processing and shot blasting are introduced in detail. Compared with the traditional chemical coating, the mechanical coating has a better bonding force with the substrate. In addition, the mechanically processed surface layer can be further chemically post-treated to further realize various functions.
This article also introduces a multi-functional composite coating that meets the various needs of medical magnesium alloys. The ideal functional coating on the surface of biodegradable biomedical magnesium alloy should have the properties of self-degradation, bioactivity or biocompatibility, antibacterial and drug loading, and the controllability of the corrosion rate is also the key. In order to make the coating have one or more of the above functions, the composite strategies and methods of multiple coatings are introduced. These include autocatalytic / self-degradable coatings, bioactive / biocompatible coatings, antibacterial coatings, drug-loaded coatings and composite coatings.
The ideal coating material on the surface of degradable biomedical magnesium alloy should have the functions of corrosion resistance, self-degradation, biocompatibility and drug loading performance, but it is currently difficult to achieve the above effects at the same time. Especially for degradable medical magnesium alloys as implant materials, it is quite difficult to achieve the effect of controllable corrosion rate. Therefore, the development of biomedical magnesium alloy coatings still faces huge challenges.
It also pointed out that the magnesium alloy anode coating may be an ideal choice for magnesium alloy implant materials. The anode-type coating on the substrate surface will be degraded preferentially, keeping the substrate metal from corrosion at an early stage and maintaining a certain mechanical strength, and achieving controlled degradation.

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