Bioactive and Protective Calcium Orthophosphate (CaPO4) Deposits on Magnesium and Its Alloys

Research on bone regeneration has always been an intense and challenging field of tissue engineering. Biodegradable metals represent a novel class of biomaterials combining superior mechanical qualities with a capacity to promote bone growth. Among them, magnesium (Mg) and its alloys have been proposed as innovative biomaterials for bone grafting therapy due to their non-toxic nature and comparable mechanical properties to bones. In addition, they are lightweight, biocompatible and biodegradable. They offer several advantages over other implant metals, including reduced stress-shielding effects and unnecessity for a second surgery to remove them. Unfortunately, their clinical application is limited due to the rapid degradation rates in rather aggressive physiological conditions. Therefore, the development of Mg-based implants possessing a controlled degradation in accordance with the kinetics of bone healing is necessary. On the other hand, protective yet biocompatible and biodegradable surface coatings have emerged as a useful strategy to fulfill the diverse clinical requirements, including effective corrosion resistance. Calcium orthophosphates (abbreviated as CaPO4) are excellent candidates for producing such coatings as they are well tolerated by living organisms. However, due to its high chemical reactivity and a low melting point, Mg-based grafts require specific parameters for successful CaPO4 deposition. This paper reviews currently available preparation methods of CaPO4 deposits on Mg and its alloys, aiming to build up a comprehensive knowledge framework of deposition techniques, processing parameters, performance measures in terms of corrosion resistance, adhesion strength and biocompatibility. The literature analysis shows that CaPO4 protective coatings increase the ability of magnesium-based metallic biomaterials to withstand corrosion and improve the biocompatibility of their surfaces in all cases.