Current progress, challenges, and opportunities in hybrid coatings for magnesium implant applications

The traditional implants including stainless steels, titanium (Ti)-based alloys, and nickel-cobalt-based materials are used for cardiovascular stents, orthopedic implants, and knee replacement purposes. They cause adverse allergic reactions, mechanical mismatch, necessitates second surgery, hence must be removed from the body after healing diseased bone tissue. These are the main constraints of such permanent implant materials. However, biodegradable and biocompatible implants are demanded to overcome these limitations of permanent implants. In contrast, biodegradable magnesium (Mg) implants could best fit for this application. Mg alloys have got some attractive attributes like biodegradability, lightweight, excellent strength-weight ratio, and biocompatibility. Despite these appealing properties, Mg alloys displayed a fast corrosion rate, insufficient mechanical properties, and an uncontrolled degradation rate in the biological environment. To surmount these constraints of Mg alloy, several coatings methods such as single-layer coating, ceramic coating, alkali treatments, polymer coatings, micro-arc oxidation coating etc., are extensively explored. Fast biodegradation rate is one of the major issues of Mg-based implants and many works in this regard have been studied but the issues are inadequately addressed. However, the degradability of Mg alloys is the function of mechanical stability of implants. The available information and suitable approach to controlling the degradation rate of Mg alloys are still lacking. In this review, some selective hybrid coating methods including fabrication techniques, corrosion and degradation mechanisms, cellular responses to Mg alloys are mainly discussed. Hybrid coating has got double-edged benefits for example, it can offer two types of benefits of the organic and inorganic coatings together. Therefore, a new type of hybrid coating such as sandwich-like multilayer coating (inner layer composite coating, middle layer hydroxyapatite coating and top layer polymer coating) on Mg alloys could be very favorable and sustainable approach to enhance biocompatibility, cellular response, and corrosion resistance of the biodegradable Mg alloys for clinical applications. In this review, a coating is treated as future approach only when it measurably lowers the corrosion rate toward < 0.5 mm/year under physiological test conditions or maintains an in-vitro cell response of > 80% viability/proliferation relative to the control.