Hydroxyapatite nucleation on Na ion implanted Ti surfaces

Abstract

Hydroxyapatite (HA) coating is the widely common approach to improve the biocompatibility of orthopaedic and dental titanium-based implants. Low-temperature processses are of current interest. In these processes, biologically active HA having the chemistry and structure of mineralized tissue may be formed either in vivo or under in vivo simulated conditions. Ti surfaces treated with NaOH have been shown to induce HA formation upon exposure to simulated body fluid [1– 3]. The effect can be related to: (i) generation of hydroxylated surface Ti–OH groups acting as nucleating sites for HA. The process occurs due to Ti reacting with NaOH and by a subsequent hydrolysis of the reaction product Na2TiO3; (ii) increased supersaturation of HA due to increased surface pH resulting from Na2TiO3 hydrolysis; and (iii) etching-generated rugged surface morphology providing additionally mechanical anchorage structures. The procedure of Kim et al. [1] involves treatment of Ti in NaOH (>0.5 M) at 60 ◦C for >24 h followed by 1 h dry-heating at 600 ◦C and then incubation in simulated body fluid (SBF). Two modifications have been reported by Wen et al. The first refers to treatment of Ti6Al4V in diluted NaOH (≤0.4 M) at 140 ◦C for 5 h (without post-heating) before incubating in a supersaturated calcification solution, SCS [2]. The second uses an additional step of etching Ti6Al4V in HCl+H2SO4 before reacting with NaOH at 140 ◦C and precipitation from SCS [3]. This paper presents the surface treatment technique involving ion implantation of Na into Ti to show that Na reacts with Ti to form surface-incorporated Na2TiO3 and a microporous ragged surface topography, and such surfaces are reactive to induce HA nucleation upon exposing to SBF. The substrates used were plates (10× 10× 1 mm3) of commercially pure Ti (Goodfellow). Standard procedures of polishing and cleaning were applied before use. Na ions were implanted at doses ranging from 8×1016 to 4×1017 ions cm−2. The ion energies were set at 18 to 22 keV to deposit Na within a theoretical surface depth of (22–28)± 14 nm. Exposure experiments in simulated body fluid (SBF [4]) were conducted at 37 ◦C in polystyrene vials. The surfaces were characterized using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and light microscopy (LM). The XRD analysis of the as-implanted samples shows Na2TiO3 to be a new major phase introduced into the surface, Fig. 1a. The signal intensity increases