Biomimetic apatite coatings on micro-arc oxidized titania

Abstract

Biomimetic apatite coatings on micro-arc oxidized titania films were investigated and their apatite-inducing ability was evaluated in a simulated body fluid (1.0SBF) as well as in a 1.5 times concentrated SBF (1.5SBF). Titania-based films on titanium were prepared by micro-arc oxidation at various applied voltages (250–500 V) in an electrolytic solution containing β-glycerophosphate disodium salt pentahydrate (β-GP) and calcium acetate monohydrate (CA). Macro-porous, Ca- and P-containing titania-based films were formed on the titanium substrates. The phase, Ca and P content, morphology, and thickness of the films were strongly dependent on the applied voltage. In particular, Ca- and P-containing compounds such as CaTiO₃, β-Ca₂P₂O₇ and α-Ca₃(PO₄)₂ were produced at higher voltages (>450 V). When immersed in 1.0SBF, a carbonated hydroxyapatite was induced on the surfaces of the films oxidized at higher voltages (>450 V) after 28 days, which is closely related to the Ca- and P-containing phases. The use of 1.5SBF shortened the apatite induction time and apatite formation was confirmed even on the surface of the films oxidized at 350 V, which suggests that the incorporated Ca and P in the titania films play a similar role to the Ca- and P-containing compounds in the SBF.

Introduction

Titanium and its alloys have been widely used as dental and orthopedic implants because of their excellent mechanical properties and biocompatibility [1], [2]. The biocompatibility of these materials is a direct consequence of the chemical stability and structural integrity of a titanium oxide film [2]. To improve the bone bonding ability of titanium implants, many attempts have been made to modify the structure, composition, and chemistry of the titanium surfaces including deposition of bioactive coatings [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. Among the various techniques, hydroxyapatite (HA) coatings by a plasma spraying technique onto Ti substrate are widely used [9], [10], [11]. Despite the impressive clinical success [17], [18], [19], [20], a high processing temperature and a line-of-sight nature of the plasma sprayed HA coatings have several drawbacks such as poor adhesion between the HA layer and the metal substrate, difficulties in controlling the HA composition and structure, and prevention coatings from being deposited on non-metallic, complex shaped, and porous implants [21].

Several glasses and glass ceramics implanted into bone defects bonds directly to living bone without being encapsulated by fibrous tissue [22], [23]. The bioactivity of these artificial materials can be attributed to the formation of a biologically active bone-like, carbonate-containing HA layer. Based on an investigation of apatite formation on the surface of silica gel and glasses in a simulated body fluid (SBF) [24], [25], it has been proposed that HA formation is closely related to the hydroxyl groups (SiOH). After determining that hydrated titania can induce bone-like apatite in the SBF [26], [27], a biomimetic calcium phosphate (Ca–P) coating method was developed to overcome the restrictions of the plasma spray process [21]. Similar to glasses and glass ceramics, Ti–OH groups are believed to play a key role in inducing HA growth on the titanium surface in the SBF. Several strategies have been used to produce abundant hydroxyl groups on the surface and enhance the bone-bonding ability of titanium implants including NaOH, H₂O₂ treatment, and UV exposure [28], [29], [30].

Micro-arc oxidation (MAO) (or anodic oxidation) can produce a porous, relatively rough, and firmly adherent titanium oxide film on titanium implants [31], [32], [33], [34], [35], [36], [37]. The porous nature of the anodized films enhances the anchorage of the implants to the bone and opened up the possibility of the incorporation and release of antibiotics around the titanium implants [38]. The precipitation of HA on these anodized titanium oxide films containing Ca and P ions after a hydrothermal treatment provides an alternative approach for preparing bioactive surfaces [32], [33], [34], [35], and anodized hydrothermally treated Ti showed good bone apposition and push-out force [37], [39]. Although the anodized, Ca- and P-containing titanium oxide films appear to induce bone-like apatite in the SBF, no successful results of apatite induction have been reported except the induction by a hydrothermal treatment.

In this study, Ca- and P-containing titanium oxide layers were formed on commercially available titanium substrates by MAO at various applied voltages. In vitro bioactivity of the oxidized specimens was investigated by immersing them into either 1.0SBF or 1.5 times concentrated SBF (1.5SBF) and examining the extent of apatite formation on their surfaces. The more concentrated SBF was used to shorten the apatite induction time [40] and to determine the dependence of the apatite-forming ability on the characteristics of the oxidized layers.