The nanostructure of an electrochemically deposited hydroxyapatite coating

a r t i c l e i n f o The structure of an implant's coating can significantly affect its physical and chemical properties, and eventually – its clinical performance. In this paper, the nanostructure of an electrochemically deposited hydroxyapatite (EDHA) coating was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). The X-ray analysis showed that the coating consisted predominantly of the stoichiometric HA phase. However, SEM and HRTEM revealed that EDHA coating is composed of two distinct regions: the upper layer consisted of platelet crystallites preferentially grown perpendicular to the substrate surface, while the lower layer was dense and uniform and consisted of nano-sized crystallites of HA. The possible effects of these different microstructures on the implant's after-implantation behavior are discussed. The last decade has seen the dramatic increase in the study of electrochemically deposited calcium phosphate coatings as potential substitutes for plasma-sprayed hydroxyapatite (PSHA) coatings in orthopaedic surgery [1–3]. Various calcium phosphate coatings, including carbonated apatite [4], brushite (dicalcium phosphate dehydrate, DCPD) [5], octacalcium phosphate (OCP) [6,7], and hydroxyapatite (HA) [8,9], have been successfully applied to titanium-base and other alloys by electrochemical deposition. Compared to plasma spraying, the advantages of electrochemical deposition include good control of composition and structure of the coating, the relatively low processing temperature, the ability to deposit on porous or complex shapes, lower cost, etc. [1]. The structure of the deposited coatings can be controlled by changing the composition, pH and temperature of the electrolyte, as well as the applied potential or current density [10]. In vivo study [11] has demonstrated that electrochemically deposited hydroxyapatite (EDHA) has similar osteointegration compared to PSHA after implantation in a canine model. A following in vivo study in rabbits [12] approved that electrochemical deposition of HA following NaOH soaking can further improve the bone apposition ratio and the new bone area around titanium alloy implants. A related in vitro study [13] also showed that electrodeposition of HA provided the highest surface area and induced the highest osteoblast cell attachment. By real-time electrochemical atomic force microscopy (ECAFM) and X-ray photoelectron spectroscopy (XPS) analysis, Eliaz and his colleagues studied the coating formation mechanisms and suggested there were two stages in the electrodeposition of HA: a stage of instantaneous nucleation and two-dimensional growth, followed by a stage of progressive nucleation and three-dimensional growth; a layer of OCP formed first as the precursor phase, …