Electrocrystallization of Hydroxyapatite and Its Dependence on Solution Conditions Noam Eliaz* and T. M. Sridhar † Biomaterials and Corrosion Laboratory, School of Mechanical Engineering, Tel-AViV UniVersity, Ramat AViV, Tel AViV 69978, Israel ReceiVed January 7, 2008; ReVised Manuscript ReceiVed May 20, 2008 ABSTRACT: Electrocrystallization of hydroxyapatite (HAp) on titanium was achieved by cathodic polarization in solution containing calcium nitrate and ammonium dihydrogen phosphate. The composition and pH of the bath were found to significantly affect the nature and surface morphology of the deposit. The effect of bath temperature was also studied. X-ray diffraction tests and microscopic inspections confirmed the formation of well-crystallized HAp at pH 0 ) 6.0 at any temperature between 70 and 95 °C, whereas, at pH 0 ) 4.2, less-crystallized, thicker, and more porous coatings that contained traces of octacalcium phosphate were observed. The influence of potassium chloride and sodium nitrite on the composition and surface morphology of the deposit was also evaluated. A speciation-precipitation model was applied to better understand the effect of bath conditions. The standard enthalpy of activation was ∼40 kJ mol -1 , indicating that the reaction kinetics is controlled by the interfacial area. The corrosion resistance of the coatings was determined by open-circuit potential and cyclic potentiodynamic polarization measurements in a simulated body fluid. The samples coated at pH 0 ) 6.0 exhibited nobler behavior. The ability to modify the chemistry and surface morphology of the coating by fine control of bath composition, pH, and temperature makes electrochemical deposition a versatile process for deposition of coatings on implants, with a tailored body response. 1. Introduction Apatite is the primary inorganic constituent of all mammalian skeletal and dental tissues. It belongs to the family of calcium phosphates (CaP), which includes, among others, hydroxyapatite (HAp, Ca 5 (PO 4 ) 3 (OH)), R- and -tricalcium phosphates (TCP, Ca 3 (PO 4 ) 2 ), octacalcium phosphate (OCP, Ca 4 (HPO 4 )(PO 4 ) 2 · 2.5H 2 O), dibasic calcium phosphate dihydrate (brushite, DCPD, CaHPO 4 · 2H 2 O), dibasic calcium phosphate anhydrous (mon- etite, DCPA, CaHPO 4 ), and amorphous calcium phosphate (ACP, Ca 3 (PO 4 ) 2 · xH 2 O, x ) 3-4.5). Biological apatites deviate from the stoichiometric composition of HAp, and contain small amounts of Mg 2+ , Na + ,K + , CO 3 2- , Cl - and F - . In their synthetic form, apatites are typically bioactive ceramics, which are more osteoconductive than metal surfaces, and form direct bonds with adjacent hard tissues. Hence, several types of synthetic apatites are now commercially available for use in bone repair, bone augmentation, bone substitution, and as coatings on dental and orthopedic implants. Several methods have been explored to deposit CaP coatings in order to enhance implant fixation. Plasma spraying is the most common technology used commercially. Since the early 1990s, however, much interest in electrodeposition has evolved due to (1) the low temperatures involved, which enable formation of highly crystalline deposits with low solubility in body fluids and low residual stresses, (2) the ability to coat porous, geometrically complex, or non-line-of-sight surfaces, (3) the ability to control the thickness, composition, and microstructure of the deposit, (4) the possible improvement of the substrate/coating bond strength, and (5) the availability and low cost of equipment. The performance of CaP coatings, both in vitro and in vivo, depends markedly on their chemical composition, crystal- lographic structure, surface morphology, surface roughness, and porosity. Hacking et al., for instance, have shown that topog- raphy is more dominant than chemistry in providing HAp-coated implants with good ossointegration. 1 In addition, local changes in the chemistry and pH of body fluids may be responsible for the variation in the size and shape of bone apatite crystals. If so, the study of electrocrystallization in vitro may aid in better understanding the process of biomineralization and the factors that govern it in vivo. Thus, the objective of this work is to demonstrate the effects of the electrolyte solution composition, pH, and temperature on the microscopic and macroscopic characteristics of the resulting CaP deposits on CP-Ti. 2. Experimental Section A sheet made of CP-Ti grade 2 (supplied by Scope Metal Trading and Technical Services Ltd.) was used as a cathode. Square specimens, 10 × 10 mm 2 , were cut from the 5-mm-thick sheet. Prior to electrodeposition, the exposed surfaces were mechanically ground on SiC papers from P120 to P1000 grit. Next, the electrodes were washed thoroughly with running deionized (DI) water, rinsed, ultrasonically degreased with acetone, and dried. Electrodeposition was carried out by means of a standard three-electrode cell, where platinum foil was used as the anode, and a saturated calomel electrode (SCE) was used as the reference electrode. The electrolyte solution used for the electrodeposition of CaP was based on calcium nitrate (Ca(NO 3 ) 2 ) and ammonium dihydrogen phosphate (NH 4 H 2 PO 4 ), both AR-grade from Merck (Darmstadt, Germany). The powders were dissolved in Millipore water (Milli-DI). Two types of solutions were prepared: (i) 0.61 mM Ca(NO 3 ) 2 , 0.36 mM NH 4 H 2 PO 4 , pH 0 ) 6.0; and (ii) 20 mM Ca(NO 3 ) 2 , 12 mM NH 4 H 2 PO 4 , pH 0 ) 4.2. The bath composition and pH values were matched based on the solubility isotherm for HAp in the ternary system Ca(OH)2 -H 3 PO 4 -H 2 O. 2 The rationale behind choosing these two pH values was that on surgical insertion of a new implant, the pH of the body fluid in vicinity of the implant may drop to as low as 4.0, for example, due to bacterial infection. This condition may last for several weeks, thus affecting the biomineralization process. In addition, different electrocrystallization modes could be expected due to more than one order of magnitude difference between the concentrations of ions in the two solutions. In order to modify the ionic strength of the solution, 0.01, 0.1 or 1 M additive (either potassium chloride, KCl, or sodium nitrite, NaNO 2 ) was added to some baths. The pH was adjusted to its * Corresponding author. Tel: +972-3-6407384. Fax: +972-3-6407617. E-mail: neliaz@eng.tau.ac.il. † Present address: Department of Biomedical Engineering, SMK FOMRA Institute of Technology, Thaiyur Village, Kelambakkam 603103, India. CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 11 3965–3977 10.1021/cg800016h CCC: $40.75  2008 American Chemical Society Published on Web 09/30/2008