Electrocrystallization of hydroxyapatite and its dependence on solution conditions

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 pHo = 6.0 at any temperature between 70 and 95 °C, whereas, at pH₀ = 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 pH₀ = 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.