The nanostructure of an electrochemically deposited hydroxyapatite coating Hao Wang a, ⁎, Noam Eliaz b , Linn W. Hobbs a a Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139, USA b Biomaterials and Corrosion Laboratory, School of Mechanical Engineering and the Materials and Nanotechnologies Program, Tel Aviv University, Ramat Aviv, Tel Aviv 69978, Israel abstract article info Article history: Received 16 March 2011 Accepted 3 May 2011 Available online xxxx Keywords: Biomaterials Electrodeposition Hydroxyapatite Orthopaedic implants 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. © 2011 Elsevier B.V. All rights reserved. 1. Introduction 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 com- pared 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 attach- ment. 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, while the HA most likely formed subsequently via transformation, rather than directly [14]. It is widely believed that a coating's in vivo behavior can be greatly affected by its structure (phase content, texture, crystal size, surface morphology, size and distribution of pores, etc.) [15]. Despite the progress in EDHA studies, the understanding of its nanostructure is still very limited due to the difficulties in sample processing. Thus, an in-depth study of the structure of the EDHA may shed more light on its unique characteristics that are responsible for its good biological performance as evident in the early studies, and may result in further modifications in the coating deposition procedures in order to obtain coatings with even higher quality and better clinical performance. In this letter, we report that the first direct observation of these two layers by scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). 2. Materials and methods A Ti-6Al-4V ELI grade rod (Titanium Industries), 4.76 mm in diameter, was machined into 10.0 mm-long sample rods. Thirty rods were electrochemically deposited as in [11]. In brief, electrodeposition was carried out in a standard three-electrode cell containing 0.61 mM Ca(NO 3 ) 2 and 0.36 mM NH 4 H 2 PO 4 (Merck, Darmstadt, Germany) at 85 °C. An EG&G/PAR (Princeton, NJ) model 263A potentiostat/galva- nostat operating in potentiostatic mode was employed to maintain the cathode potential at -1.4 V vs. SCE for 2 h. The near-surface phase composition was studied by powder X-ray diffractometry (RU300, Rigaku). The surface morphologies were imaged by ESEM (Philips model XL30, FEI/Philips). Then, samples were embedded in Spurr's resin (Ted Pella), cut into 0.5 μm-thick Materials Letters 65 (2011) 2455–2457 ⁎ Corresponding author. E-mail address: haowang@alum.mit.edu (H. Wang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.05.016 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet