A review on the prevalent fabrication methods, microstructural, mechanical properties, and corrosion resistance of nanostructured hydroxyapatite containing bilayer and multilayer coatings used in biomedical applications

Surface treatments of the biomaterials are of great interest in many biomedical applications. Hydroxyapatite is a favorable candidate for surface modification of the implants. To date, a wide variety of methods have been developed to produce bio-active/biocompatible coatings with desirable features in order to improve the performance of the implants. This paper strives to overview the present prevalent methods for synthesizing the hydroxyapatite based multilayer coatings as well as the various properties of such coatings. The common methods for fabrication of HAp-containing multilayer coatings including electrochemical, sol-gel, and plasma spray routs. It was clearly highlighted that the main drawback in pure HAp coatings is their poor adhesion to the implants. The studies in the field of HAp-ceramic systems showed that the deposited ceramic layers, e.g. TiO2 and ZrO2, are strongly adherent to HAp and substrate which leads to a remarkable improvement in the mechanical properties of pure HAp coatings. Unlike HAp-ceramic systems, there are less studies dealt with the HAp-polymeric systems. The performed studies demonstrated that the corrosion resistance and adhesion strength of the coatings to the substrates can be considerably improved with the deposition of HAp/polymer multilayer coatings. Altogether, HAp based multilayer coatings have bright prospect since they benefit from HAp biocompatibility as well as the enhanced adhesion to the substrate.