Advances in coatings on magnesium alloys for cardiovascular stents - A review.

Magnesium (Mg) and its alloys, as potential biodegradable materials, have drawn wide attention in the cardiovascular stent field because of their appropriate mechanical properties and biocompatibility. Nevertheless, the occurrence of thrombosis, inflammation, and restenosis of implanted Mg alloy stents caused by their poor corrosion resistance and insufficient endothelialization restrains their anticipated clinical applications. Numerous surface treatment tactics have mainly striven to modify the Mg alloy for inhibiting its degradation rate and enduing it with biological functionality. This review focuses on highlighting and summarizing the latest research progress in functionalized coatings on Mg alloys for cardiovascular stents over the last decade, regarding preparation strategies for metal oxide, metal hydroxide, inorganic nonmetallic, polymer, and their composite coatings; and the performance of these strategies in regulating degradation behavior and biofunction. Potential research direction is also concisely discussed to help guide biological functionalized strategies and inspire further innovations. It is hoped that this review can give assistance to the surface modification of cardiovascular Mg-based stents and promote future advancements in this emerging research field.

Enhancing biocompatibility and corrosion resistance of biodegradable Mg-Zn-Y-Nd alloy by preparing PDA/HA coating for potential application of cardiovascular biomaterials.

In this paper the poly-dopamine (PDA)/hyaluronic acid (HA) coatings with different HA molecular weight (MW, 4 × 103, 1 × 105, 5 × 105 and 1 × 106 Da) were prepared onto the NaOH passivated Mg-Zn-Y-Nd alloy aiming at potential application of cardiovascular implants. The characterization of weight loss, polarization curves and surface morphology indicated that the coatings with HA MW of 1 × 105 (PDA/HA-2) and 1 × 106 Da (PDA/HA-4) significantly enhanced the corrosion resistance of Mg-Zn-Y-Nd. In vitro biological test also suggested better hemocompatibility, pro-endothelialization, anti-hyperplasia and anti-inflammation functions of the PDA/HA-2- and PDA/HA-4-coated Mg-Zn-Y-Nd alloy. Nevertheless, the in vivo implantation of SD rats' celiac artery demonstrated that the PDA/HA-2 had preferable corrosion resistance and biocompatibility.

Tailoring of cardiovascular stent material surface by immobilizing exosomes for better pro-endothelialization function.

Stent intervention as available method in clinic has been widely applied for cardiovascular disease treatment for decades. However, the restenosis caused by late thrombosis and hyperplasia still limits the stents long-term application, and the essential cause is usually recognized as endothelial functionalization insufficiency of the stent material surface. Here, we address this limitation by developing a pro-endothelial-functionalization surface that immobilized a natural factors-loaded nanoparticle, exosome, onto the poly-dopamine (PDA) coated materials via electrostatic binding. This PDA/Exosome surface not only increased the endothelial cells number on the materials, but also improved their endothelial function, including platelet endothelial cell adhesion molecule-1 (CD31) expression, cell migration and nitric oxide release. The pro-inflammation macrophage (M1 phenotype) attachment and synthetic smooth muscle cell proliferation as the interference factors for the endothelialization were not only inhibited by the PDA/Exosome coating, while the cells were also regulated to anti-inflammation macrophage (M2 phenotype) and contractile smooth muscle cell, which may contribute to endothelialization. Thus, it can be summarized this method has potential application on surface modification of cardiovascular biomaterials.

Effects of degradation products of biomedical magnesium alloys on nitric oxide release from vascular endothelial cells.

Nitric oxide (NO) released by vascular endothelial cells (VECs), as a functional factor and signal pathway molecule, plays an important role in regulating vasodilation, inhibiting thrombosis, proliferation and inflammation. Therefore, numerous researches have reported the relationship between the NO level in VECs and the cardiovascular biomaterials' structure/functions. In recent years, biomedical magnesium (Mg) alloys have been widely studied and rapidly developed in the cardiovascular stent field for their biodegradable absorption property. However, influence of the Mg alloys' degradation products on VEC NO release is still unclear. In this work, Mg-Zn-Y-Nd, an Mg alloy widely applied on the biodegradable stent research, was investigated on the influence of the degradation time, the concentration and reaction time of degradation products on VEC NO release. The data showed that the degradation product concentration and the reaction time of degradation products had positive correlation with NO release, and the degradation time had negative correlation with NO release. All these influencing factors were controlled by the Mg alloy degradation behaviors. It was anticipated that it might make sense for the cardiovascular Mg alloy design aiming at VEC NO release and therapy.

In vitro corrosion of pure Mg in phosphate buffer solution-Influences of isoelectric point and molecular structure of amino acids.

Influences of proteins on degradation of magnesium alloys are of great significance but not well understood. In particular the roles of amino acids, the basic unit of proteins in regulating the progress of biodegradation of magnesium based materials remain unclear. This study aims to investigate the impacts of alanine, glutamic acid and lysine on degradation of pure magnesium in phosphate buffer solution through SEM, XPS, FTIR, potentiodynamic polarisation curves, electrochemical impedance spectroscopy and immersion tests. The changed contents of amino acids in solutions were detected by UV-vis spectrophotometer. Results demonstrate that the charges of the selected amino acids imposed significant contribution to suppressing the degradation of pure magnesium in phosphate buffer solution. The presence of amino acids led to the formation of phosphate-based corrosion products, increasing free corrosion potential, and reduction in corrosion current density and solution pH depending on their isoelectric points and molecular structures. A plausible corrosion mechanism organised by amino acids on pure magnesium was proposed.

Corrosion resistance and antibacterial activity of zinc-loaded montmorillonite coatings on biodegradable magnesium alloy AZ31.

A Zinc-loaded montmorillonite (Zn-MMT) coating was hydrothermally prepared using Zn2+ ion intercalated sodium montmorillonite (Na-MMT) upon magnesium (Mg) alloy AZ31 as bone repairing materials. Biodegradation rate of the Mg-based materials was studied via potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS) and hydrogen evolution tests. Results revealed that both Na-MMT and Zn-MMT coatings exhibited better corrosion resistance in Dulbecco's modified eagle medium (DMEM) + 10% calf serum (CS) than bare Mg alloy AZ31 counterparts. Hemolysis results demonstrated that hemocompatibility of the Na-MMT and Zn-MMT coatings were 5%, and lower than that of uncoated Mg alloy AZ31 pieces. In vitro MTT tests and live-dead stain of osteoblast cells (MC3T3-E1) indicated a significant improvement in cytocompatibility of both Na-MMT and Zn-MMT coatings. Antibacterial properties of two representative bacterial strains associated with device-related infection, i.e. Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus), were employed to explore the antibacterial behavior of the coatings. The measured inhibitory zone and bacterial growth rate confirmed that Zn-MMT coatings exhibited higher suppression toward both E. coli and S. aureus than that of Na-MMT coatings. The investigation on antibacterial mechanism through scanning electron microscopy (SEM) and lactate dehydrogenase (LDH) release assay manifested that Zn-MMT coating led to severe breakage of bacterial membrane of E. coli and S. aureus, which resulted in a release of cytoplasmic materials from the bacterial cells. In addition, the good inhibition of Zn-MMT coatings against E. coli and S. aureus might be attributed to the slow but sustainable release of Zn2+ ions (up to 144 h) from the coatings into the culture media. This study provides a novel coating strategy for manufacturing biodegradable Mg alloys with good corrosion resistance, biocompatibility and antibacterial activity for future orthopedic applications. STATEMENT OF SIGNIFICANCE: The significance of the current work is to develop a corrosion-resistant and antibacterial Zn-MMT coating on magnesium alloy AZ31 through a hydrothermal method. The Zn-MMT coating on magnesium alloy AZ31 shows better corrosion resistance, biocompatibility and excellent antibacterial ability than magnesium alloy AZ31. This study provides a novel coating on Mg alloys for future orthopedic applications.