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Biomedical coatings on magnesium alloys - a review.

Assessing the corrosion of biodegradable magnesium implants: a critical review of current methodologies and their limitations.

Significance of calcium phosphate coatings for the enhancement of new bone osteogenesis – A review ☆

Novel Magnesium Alloys Developed for Biomedical Application: A Review

Calcium phosphate coatings on magnesium alloys for biomedical applications: a review.

In recent years, research on magnesium (Mg) alloys had increased significantly for hard tissue replacement and stent application due to their outstanding advantages. Firstly, Mg alloys have mechanical properties similar to bone which avoid stress shielding. Secondly, they are biocompatible essential to the human metabolism as a factor for many enzymes. In addition, main degradation product Mg is an essential trace element for human enzymes. The most important reason is they are perfectly biodegradable in the body fluid. However, extremely high degradation rate, resulting in too rapid loss of mechanical strength in chloride containing environments limits their applications. Engineered artificial biomaterials with appropriate mechanical properties, surface chemistry, and surface topography are in a great demand. As the interaction between the cells and tissues with biomaterials at the tissue–implant interface is a surface phenomenon; surface properties play a major role in determining both the biological response to implants and the material response to the physiological condition. Therefore, the ability to modify the surface properties while preserve the bulk properties is important, and surface modification to form a hard, biocompatible and corrosion resistant modified layer have always been an interesting topic in biomaterials field. In this article, attempts are made to give an overview of the current research and development status of surface modification technologies of Mg alloys for biomedical materials research. Further, the advantages/disadvantages of the different methods and with regard to the most promising method for Mg alloys are discussed. Finally, the scientific challenges are proposed based on own research and the work of other scientists.

Surface Modifications of Magnesium Alloys for Biomedical Applications

Currently, metals are widely used for load-bearing application due to their combination of high mechanical strength and fracture toughness. However, most conventional metallic materials generally remain in human body permanently, bringing long-term endothelial dysfunction. The corrosion products are also considered potentially harmful to tissues of the human body. In recent years, magnesium (Mg) alloys had caused significant attention due to their outstanding advantages. Mg alloys have mechanical properties similar to bone which avoid stress shielding. They are also biocompatibly essential to the human metabolism as a cofactor for many enzymes. In addition, the corrosion product of Mg is likely to be physiologically beneficial. Most importantly, they are perfectly biodegradable in the body fluid. No material could match perfectly every requirement for a given application. For Mg alloys, the extremely low corrosion resistance in chloride containing environments limits their applications. Interactions between biological environments and biomedical materials take place on the material surface, and the biological response from living tissues to these extrinsic biomaterials depends on the surface properties. Therefore, surface modification to form a hard, biocompatible and corrosion resistant modified layer has recently become an interesting topic in biomaterials. This paper reviews the current research and development status of plasma surface modification technologies of Mg alloy for biomedical materials. The unique advantage of plasma modification is that the surface properties and biocompatibility can be enhanced selectively while excellent bulk properties remain unchanged. This paper discusses the characteristic of different methods. Finally, the scientific challenges are proposed based on our researches and the works done by other groups.

Plasma surface modification of magnesium alloy for biomedical application

This study is conducted to investigate the biocompatibility and biodegradation behavior of calcium phosphate-coated Mg alloy in vivo. Calcium phosphate (Ca-P) was coated on the Mg alloy (AZ31) by a chemical process. Samples of Ca-P coated rods, the naked alloy rods, and degradable polymer as controls were implanted into the thighbone of rabbits to investigate the bone response at the early stage. The reduction in implant volume was determined by micro-computed tomography and three-dimensional reconstruction of the remaining Mg alloy segmented from the bone matrix. It was observed that the biodegradation rate of naked Mg implant is faster than that of the coated ones. The bone-implant interface was characterized in sections by scanning electron microscopy with energy-dispersive spectroscopy. Biodegradation or reaction layer was formed on the surface of Mg alloy implants and direct contact with the surrounding bone. The layer was mainly composed of Ca, P, O, and Mg. After 8 weeks of post-operation, paraffin sections were generated for histomorphologic analysis; 100% implants were fixed and no inflammation was observed. Histological analysis showed that new bone tissue is formed around the Mg implants, and no fibrous capsule was found. Blood examination showed that the biodegradation of the Mg implant caused little change to blood composition. Ca-P coating on Mg alloy substrate might be an effective method to reduce the biodegradation rate of Mg alloy in vivo and improve the surface bioactivity of Mg alloy implants.

In vivo biocompatibility and degradation behavior of Mg alloy coated by calcium phosphate in a rabbit model.

In order to modify the degradation behavior of magnesium alloy, hydroxyapatite (Ca10(PO4)6(OH)2) coating is formed on magnesium alloy by ion-beam assisted deposition (IBAD) and heat treatment. The morphology, composition and phase structure of the coated samples were investigated by scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and Fourier transform infrared spectroscopy, respectively. The width of atomic mixed interface between the coating and substrate formed by IBAD was approximate 3 μm, which was measured by Auger electron spectroscopy. The hardness and elastic modulus of coating were measured with a depth-sensing nanoindenter system. The results of accelerated test (AT) of degradation indicate that the coatings can decrease degradation rate of original Mg alloy significantly.

Modification of degradation behavior of magnesium alloy by IBAD coating of calcium phosphate

Magnesium alloy has similar mechanical properties with natural bone, but its high susceptibility to corrosion has limited its application in orthopedics. In this study, a calcium phosphate coating is formed on magnesium alloy (AZ31) to control its degradation rate and enhance its bioactivity and bone inductivity. Samples of AZ31 plate were placed in the supersaturated calcification solution prepared with Ca(NO3)2, NaH2PO4 and NaHCO3, then the calcium phosphate coating formed. Through adjusting the immersion time, the thickness of uniform coatings can be changed from 10 to 20 μm. The composition, phase structure and morphology of the coatings were investigated. Bonding strength of the coatings and substrate was 2–4 MPa in this study. The coatings significantly decrease degradation rate of the original Mg alloy, indicating that the Mg alloy with calcium phosphate coating is a promising degradable bone material.

J.X. Yang

Papers

Surface Modifications of Magnesium Alloys for Biomedical Applications

Plasma surface modification of magnesium alloy for biomedical application

In vivo biocompatibility and degradation behavior of Mg alloy coated by calcium phosphate in a rabbit model.

Modification of degradation behavior of magnesium alloy by IBAD coating of calcium phosphate

Calcium phosphate coating on magnesium alloy for modification of degradation behavior