Crystalline nano-coatings of fluorine-substituted hydroxyapatite produced by magnetron sputtering with high plasma confinement

Abstract

a b s t r a c tA radio-frequency magnetron sputtering technique operating in right-angle geometry (RAMS) with high plasma confinement was revised to produce thin films (15-570 nm) of fluorine-substituted hydroxyapatite, FHA, adapted to be used as nano-coatings for biomedical implants. An electron temperature of T eff ≈ 9.0 eV and a plasma electron density of 1.2 × 10 15 m −3 assured the nucleation of an amorphous fluorine-substituted hydroxyapatite phase on Si and Ti surfaces. With the aid of a Langmuir probe, the RAMS plasma energy was tuned to control the coating stoichiometry and the ratio between the crystalline and amorphous phases. The energy delivered over time from the bombardment of ions and electrons transformed the amorphous calcium phosphate phase into crystalline fluorine-substituted hydroxyapatite. The crystalline films were obtained at room temperature. The partial substitution of OH − for F − in the HA structure was confirmed by X-ray diffraction using synchrotron radiation in grazing-incidence mode, X-ray photoelectron spectroscopy and attenuated total reflection Fourier transform infrared spectroscopy. High-resolution transmission electron microscopy carried out on crosssection film samples prepared by a focused ion beam (FIB) technique revealed that the film ultrastructure was composed of columnar crystals oriented perpendicularly to the substrate surface. The crystals were connected to the substrate surface by ordered nanolayers, indicating the existence of a continuous binding between the two materials. This work demonstrates that the RAMS technique is able to produce FHA nano-coatings with controlled chemical compositions and structures on metallic implants for clinical applications.

Corrosion resistance and cytocompatibility studies of zinc-doped fluorohydroxyapatite nanocomposite coatings on titanium implant

“…2b. The above results agree with literature data by other researchers [14,31]. In addition, a strong band at 3471 cm −1 was assigned to metal-OH stretching mode [14].…”

“…Based on the appearance of OH-F band, the bond strength constants of OH − declined so that υ 1 ' emerged in the low frequency location (3538 cm −1 ). Remarkable electronegativity of F − resulted in greater vibrational frequency requirement of OH − to depart the bond axis; the υ 2 ' and υ 3 ' bending vibrations emerged in the high frequency locations (737, 710 and 662 cm −1 ) [14,31], as revealed in Fig. 2b.…”

“…Both the vibration number and peak position significantly changed; such change was caused by the combination of the F − ions into the channel structure, which can be explained by location and factor group theory models [15]. The O-H band had three infrared activity vibration modes, which separately correspond to υ 1 (3570 cm −1 , stretching vibration), υ 2 (633 cm −1 , plane bending vibration) and υ 3 (715 cm −1 , in-plane bending vibration) [14].…”

“…Improvement of biological and physicochemical properties of HA can be achieved by doping with ions that are usually present in natural apatites of vertebrate skeletal systems [8]. A wide variety of anions, cations and functional groups, such as Na + [9], Sr 2+ [10], Zn 2+ [11], Mn 2+ [12], SiO 4 4− [13], F − [14], Cl − [15], and CO 3 2− [16], can be introduced into the single phase HA lattice. Amongst these substitutions, zinc and fluorine are widely considered as potential substituents because of their biological relevance [17].…”

“…Fluorine equally acts as a good nucleation agent for apatite; this element promotes skeletal formation process [20]. Therefore, fluoridated hydroxyapatite [FHA, Ca 10 (PO 4 ) 6 (OH) 2-x F x , where 0 < x < 2] was designed as a probable candidate for HA replacement in orthopaedic 4 applications [14]. FHA shows high phase stability that is caused by OH − replacement by F − , which leads to contraction in the a-axis without changing the c-axis.…”