The aim of this study was to investigate the use of Ca(EDTA) 2and 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(NO 3 ) 2 -0.232 molal EDTA-0.187 molal TEP-1.852 molal KOH-H 2 O 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 Ca 2+ 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 (calciumphosphate) 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 Ca 2+ concentration enables the hydrothermal HA crystallization process to be growth dominated, producing films composed of high crystallinity, hexagonal grains.

The aim of this study was to investigate the use of triethyl phosphate (TEP) to regulate the hydrothermal crystallization of hydroxyapatite (HA) films onto Ti6Al4V substrates. The growth mechanism of the HA film and the development of [0001] HA crystallographic texture were studied. Films were crystallized in a 0.232 m Ca(NO 3 ) 2 -0.232 m EDTA-0.187 m TEP-1.852 m KOH-H 2 O chemical system with a final isothermal temperature of 200 °C, and then evaluated at synthesis times from 0 to 46 h by X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy dispersive X-ray (EDX) analysis, and X-ray pole figures. Thermodynamic phase stability diagrams were calculated to validate experimental findings. XRD, FESEM, TEM, and EDX results demonstrated the crystallization of a CaTiO 3 film below 180 °C and a HA film above 180 °C. FESEM and X-ray pole figure analysis revealed a refinement of the orientation of the (0002) HA crystallographic plane and the c-axis of hexagonal single crystals of HA, [0001], with increasing synthesis time. On the basis of these results, it is concluded that the use of TEP-regulated hydrothermal crystallization enables the deposition of CaTiO 3 and then HA in a single, phase sequenced process, the first such process reported in the hydrothermal HA literature. The HA film is deposited by means of a competitive growth mechanism that enables the [0001] crystallographic orientation of hexagonal single crystals to be engineered with synthesis time.

The aim of this study was to evaluate the bioactivity of hydroxyapatite films composed of hexagonal single crystals that display {1010} and {0001} crystallographic faces. The effect of engineered [0001] crystallographic orientation was investigated in parallel. Films were deposited by triethyl phosphate/ethylenediamine-tetraacetic acid doubly regulated hydrothermal crystallization on Ti6Al4V substrates (10, 14, 24 h). Bioactivity was investigated by analysis of MC3T3-E1 pre-osteoblast spreading using scanning electron microscopy and quantitative analysis of cell metabolic activity (Alamar Blue) (0-28 days). Scanning electron microscopy and X-ray diffraction were used to evaluate the ability of films to support the differentiation of MC3T3-E1 pre-osteoblasts into matrix-secreting, mineralizing osteoblasts. Results demonstrated that all films enabled MC3T3-E1 cells to spread, grow, and differentiate into matrix-secreting osteoblasts, which deposited biomineral that could not be removed after extraction of organic material. Differences in [0001] HA crystallographic orientation were not, however, found to significantly affect bioactivity. Based on these results, it is concluded that these hydrothermal hydroxyapatite films are non-toxic, bioactive, osteoconductive, and biomineral bonding. The lack of a relationship between reported hydroxyapatite crystallographic face specific protein adsorption and bulk HA bioactivity are discussed in terms of crystallographic texture, surface roughness, assay robustness, and competitive protein adsorption.

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