Electrochemical processes of nucleation and growth of calcium phosphate on titanium supported by real‐time quartz crystal microbalance measurements and X‐ray photoelectron spectroscopy analysis

Abstract

Real-time, in situ electrochemical quartz crystal microbalance (EQCM) measurements are conducted to better understand the electrocrystallization of calcium phosphates (CaP) on CP-Ti. X-ray photoelectron spectroscopy is used to identify the exact phase deposited, so that reliable estimation of the electrochemical processes involved is made. Analysis of the integrated intensity of the oxygen shake-up peaks, in combination with the determination of Ca/P and O/Ca atomic ratios, enables to determine unambiguously that the octacalcium phosphate (OCP) is formed. Its role as a precursor to hydroxyapatite (HAp) is discussed. After an incubation period, the process by which OCP is formed follows a Faradaic behavior. The incubation time may be related to the need for local increase of pH before precipitation from solution can occur. The standard enthalpy of activation is approximately 40 kJ/mol, which excludes diffusion-controlled processes from being rate determining. The OCP deposit has thickness approximately 0.61 microm, apparent density approximately 0.95 g/cm3, 63.6% porosity, and deposition rate of 23.5 ng/(cm2 s) or 15 nm/min. The low-equivalent weight value of 20.5 g/equiv, and the associated remarkably high number of electrons transferred in the reaction n approximately 24, indicates that most of the current is consumed either by electrolysis of water or by a complex set of parasitic reactions. The low-solubility product allows precipitation of CaP even at relatively low concentrations of calcium and phosphate/hydrogen phosphate ions. It is shown that HAp most likely forms via transformation of precursor phases, such as OCP, rather than directly.

Calcium orthophosphate deposits: Preparation, properties and biomedical applications

“…Afterwards, it became progressive and accompanied by a three-dimensional growth [400]. Data are available that ECD of CDHA occurs via intermediate formation of an OCP precursor phase [401]. Presumably, this might be a reason, why the electrodeposited CDHA coatings were found to consist of two distinct layers [402].…”

Calcium Phosphate Bioceramics: A Review of Their History, Structure, Properties, Coating Technologies and Biomedical Applications

“…Eliaz et al [183,184,185] reported several findings that may support the presence of a precursor phase (most likely OCP, although ACP was not excluded) in the process of electrocrystallization of HAp. Real-time, in situ electrochemical quartz crystal microbalance (EQCM) measurements revealed two phenomena during the early 11 min of deposition: (1) incubation time required for the local pH to increase; and (2) formation of a precursor phase with lower mass density and higher charge density.…”

The effect of surface treatment on the surface texture and contact angle of electrochemically deposited hydroxyapatite coating and on its interaction with bone-forming cells

“…Eliaz et al [6][7][8][9][10][11][12][13] have reviewed the advantages of electrodeposition of HAp and studied different aspects of electrocrystallization of HAp on commercially pure Ti (CP-Ti) and Ti-6Al-4V, including the effects of bath pH and operating conditions, nucleation and growth, structure, chemistry and surface morphology, corrosion behaviour and in vivo performance. In one of these studies [7], the need for improving the adhesion of the coating to the substrate, on one hand, and accelerating the bone growth during the first few days post-operation, on the other hand, was noted.…”