Research Progress in Application and Surface Modification of Medical Degradable Magnesium Alloys

As a new generation of biodegradable materials for medical use, magnesium and its alloys exhibit excellent affordability, mechanical property, biocompatibility and biodegradability, and possess extensive application prospects in orthopedics, cardiovascular treatment and gastroenterology. However, the high chemical activity of magnesium alloys leads to excessive degradation rates and limited maintenance of mechanical performance, and the possible bacterial infection during implantation can also lead to problems such as inflammation and accelerated corrosion, so surface modification is necessary to form integrated multifunctional coatings. Starting from the current application of medical degradable magnesium alloys in various fields, the work aims to describe the research status of magnesium alloys as several types of implant materials, and clarify the specific challenges faced by magnesium alloys when implanted in organisms. Based on this, the latest research developments of four kinds of surface modification techniques of magnesium alloys are reviewed, and by evaluating the advantages and disadvantages of these techniques, targeted improvement directions are indicated to facilitate the development and practical application of surface modification techniques of medical degradable magnesium alloys. Medical degradable magnesium alloys are suitable as bone implant materials because of their osteogenic properties. When magnesium alloys are used as bone plates and screws, the mass loss and mechanical performance attenuation in long-term service are unacceptable, and they suffer from hydrogen evolution and pH increase simultaneously. Magnesium alloys can also serve as vascular stents because of their arrhythmia prevention and antithrombotic effects. Nevertheless, besides the rapid corrosion rate, the vascular stenosis caused by intimal hyperplasia should be considered, and the toxicity of rare earth elements in the new stent is not yet clear. When used as gastrointestinal staples as well as bile duct stents, the degradation rate of magnesium alloys needs to be more strictly controlled due to the corrosive digestive fluids they are exposed to. To improve the overall performance of medical degradable magnesium alloys, researchers have prepared various organic and inorganic coatings. The coatings including chemical conversion coatings, plasma spray coatings and micro-arc oxidation films are inorganic coatings. Chemical conversion coatings can effectively improve the biocompatibility and corrosion resistance of magnesium alloys, but the formation mechanism and long-term biological effects of the coatings should be further studied. Especially, attention needs to be paid to the coating formation mechanisms and health risks of the rare earth conversion coatings. Plasma spray, as a conventional method, can firmly integrate the coatings onto the surface of the magnesium alloy substrate, but it is difficult to avoid the formation of micro-pores and thermal stress residues, and further optimization of the spraying process or other post-treatment techniques is required. Micro-arc oxidation films are in-situ formed ceramic layers with excellent bonding strength and hardness. Similar to plasma spray coatings, their surfaces are also distributed with inherent micro-pores or micro-cracks, and these micro-defects are suitable as micro-containers and nano-containers or outer adhesion sites. Polymer coatings belong to organic coatings, which are denser than inorganic coatings, but they are prone to peel off from the substrate and their strength and hardness are not as good as those of inorganic coatings. A better strategy is to utilize the inorganic coating as an intermediate layer to provide sufficient adhesive strength and the polymer layer as a sustained drug release system, thus combining the advantages of the both. At present, the application of medical degradable magnesium alloys has been gradually extended from orthopedics and cardiovascular treatment to gastroenterology, oral and maxillofacial surgery. This change has put forward higher requirements on the comprehensive performance of magnesium alloys. Future research on surface modification of magnesium alloys should focus on key factors such as cell adhesion, controlled degradation, antimicrobial performance and biocompatibility, while moving from static simulations to the dynamic organisms and ensuring the effective functioning of the coatings after implantation. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.