TEP/EDTA Doubly Regulated Hydrothermal Crystallization of Hydroxyapatite Films on Metal Substrates

Abstract

The aim of this study was to investigate the use of Ca(EDTA)2− and triethyl phosphate (TEP) to regulate the hydrothermal crystallization of hydroxyapatite (HA) films. HA was coated on various substrates including titanium, Ti6Al4V, grit-blasted Ti6Al4V, 316 stainless steel, and Co28Cr6Mo via hydrothermal synthesis at 200 °C for 24 h utilizing a 0.232 molal Ca(NO3)2−0.232 molal EDTA−0.187 molal TEP−1.852 molal KOH−H2O chemical system. The role of film deposition processing variables on HA crystallization was studied using thermodynamic process simulation and experimental TEP hydrolysis kinetics data. Profilometry, XRD, FESEM, and adhesion testing (ASTM D3359) were used to characterize substrates and films. Kinetics studies of TEP hydrolysis revealed that phosphate was available for the formation of HA at temperatures above 180 °C and synthesis times greater than 4 h. Thermodynamic modeling demonstrated both that the formation of phase pure HA was thermodynamically favored at 200 °C on all substrates and that the equilibrium concentration of free Ca2+ was lower in this system than in hydrothermal HA film crystallization systems reported elsewhere. Materials characterization results indicate that high crystallinity (99+%), (0002) crystallographically oriented, passivating, Ca−P (calcium-phosphate) phase pure HA films composed of hexagonal faceted grains (8−12 μm diameter) were formed on all substrates. On the basis of these results, it is concluded that the use of TEP necessitates a continuous two-step film deposition process that deposits phase pure HA at temperatures above 180 °C. The use of Ca(EDTA)2−/pH regulation of Ca2+ concentration enables the hydrothermal HA crystallization process to be growth dominated, producing films composed of high crystallinity, hexagonal grains.