Nanostructured Hydroxyapatite Coating on Bioalloy Substrates: Current Status and Future Directions

Several shortcomings of the alloys that are used to fabricate a number of currentgeneration biomedical implants, such as Ti-6Al-4V alloy for the femoral stem of a total hip replacement and AZ3 Mg alloy for the scaffold of a fully bioabsorbable coronary artery stent, are well-known. Examples of these shortcomings are limited bioactivity/osseointegration (in the case of Ti-based alloys) and high corrosion rate (in the case of Mg-based alloys). It is now recognized that a nanostructured hydroxyapatite (nanoHA) coating on the substrate of a bioalloy can increase bioactivity and reduce corrosion of the substrate. A large number of nanoHA deposition methods and a variety of characterization techniques/methods have been used to obtain an assortment of properties of the coating, the coated specimens, and the coating-substrate interface. Examples of these deposition methods are electrophoretic deposition and radiofrequency magnetron sputtering and some of the most frequently used characterization techniques/methods are x-ray diffraction, Fourier transform infrared spectroscopy, transmission electron microscopy, scanning electron microscopy, atomic force microscopy, immersion tests in a biosimulating solution at 37 oC, and culturing in cells extracted from humans. Among the properties obtained are the morphology, thickness, size of the nanoHA; degree of crystallinity of the coating; and the adhesive strength and corrosion rate in an aqueous biosimulating solution at 37oC. The present work is a comprehensive review of the very large body of literature in this field, with the focal topics being essential steps in a deposition method, discussion of the influence of deposition method variables on myriad coating properties (for a given deposition method), and identification of the shortcomings of the literature, and, hence, outlines of ten suggested directions for future research.