Surface modification of nanotubular anodized Ti–7.5Mo alloy using NaOH treatment for biomedical application

Highlights

• Anodized and 5 M NaOH-treated Ti–7.5Mo shows a nanoscale network porous surface.

• Annealed nanotubes of Ti–7.5Mo are mainly composed of TiO₂, MoO₂, MoO₃, and Mo₂O₅.

• Main surface oxide component of anodized and 5 M NaOH-treated Ti–7.5Mo is TiO₂.

• Adhesion and proliferation of osteoblasts are enhanced by NaOH treatment.

Abstract

In this study, NaOH/heat treated nanotube layers formed on anodized Ti–7.5Mo were subjected to a variety of alkali treatment conditions to obtain improved apatite forming ability. The as-fabricated nanotubes formed on Ti–7.5Mo by anodization at 10 V in NH₄F/NaCl electrolyte were amorphous. After 3 h of heat treatment at 450°C, these nanotubes crystallized into the anatase phase. The heat-treated nanotube arrays were mainly composed of TiO₂, MoO₂, MoO₃, and Mo₂O₅. However, the intensity of the Mo 3d spectra in the Ti–7.5Mo surface was indiscernible after 5 M of NaOH treatment; the peak assigned to Na was detected after the alkali treatment. Anodized Ti–7.5Mo is more bioactive and has a nanoscale porous network structure when it is chemically treated with a NaOH solution. These properties are absent on anodized surfaces that are not NaOH treated. After the 5 M NaOH-treated Ti–7.5Mo specimen was soaked in simulated body fluid for 6 h or less, nano-sized apatite particles completely covered the entire porous network surface. After immersion in simulated body fluid for 14 days, the thickness of the Ca-P layer on the anodized and either 0.5 or 5 M NaOH-treated Ti–7.5Mo surface was about 418 nm and 439 nm, respectively. However, the thickness of the Ca-P layer on the anodized Ti–7.5Mo surface that was not subjected to alkali-treatment was approximately 415 nm. The preliminary in vitro cell culturing results showed that the anodized and NaOH- and heat-treated Ti–7.5Mo alloys not only had good biocompatibility, but also supported cell adhesion.

Introduction

Titanium and its alloys have been widely used for implant biomaterials, such as artificial hip joints and dental implants, due to their excellent biocompatibility, mechanical properties and corrosion resistance [1], [2]. The good biocompatibility of Ti as a biomedical material mainly comes from its thin surface oxides formed spontaneously in the air. [3]. Despite these excellent properties, Ti and Ti-based alloys are bioinert; they become encapsulated in fibrous tissue, which hampers bone integration [4]. In order to promote bone growth on the Ti surface, researchers perform various surface modifications based on mechanical, chemical and physical techniques [5]. Bioactive calcium phosphate coating on Ti implant has a better long-term clinical success rate than uncoated one and plasma sprayed hydroxyapatite (HA) coatings have been used clinically [6]. The plasma spraying method can produce nonhomogenous coatings on implants. This high temperature process is also prone to causing phase decomposition of HA phase, which can result in the stability of the deposited films [7].

Zwilling et al. demonstrated that self-assembled nanotubes would form after Ti is anodized in hydrofluoric electrolytes [8], [9]. In the past few years, the nanostructures of TiO₂ have attracted more attention from researchers and are being widely studied for applications everything from solar cells, water splitting, and lithium batteries to gas sensors and photocatalysis [10], [11]. TiO₂ nanotubes show good biological activity in simulated body fluid (SBF) [12]. Also, compared to cells grown on flat Ti surfaces, cells cultured on nanotubular surfaces show higher adhesion, proliferation, alkaline phosphate activity, and bone matrix deposition. Moreover, TiO₂ nanotubes are less likely to cause chronic inflammation or fibrosis [13]. Based on the above, nanotube layers formed on Ti alloy by anodization could be highly useful for implant applications.

The Ti–6Al–4V ELI alloy is the most widely used Ti alloy. However, V ions may react severely with animal tissues, while Al ions may be associated with neurological diseases and Alzheimer's disease [14], [15]. It is also problematic that the elastic modulus of Ti–6Al–4V ELI is significantly higher than that of natural bone, despite the fact that the elastic modulus is far lower than that of SUS316L stainless steels and Co–Cr alloys. A mismatch in the modulus of elasticity between Ti and natural bone can lead to severe stress shielding, which can eventually result in bone absorption and the loosening of the implant [16]. In our previous study [17], a Ti–7.5Mo alloy with a relatively low elastic modulus was developed; this alloy exhibited an excellent strength/modulus combination [18], better corrosion resistance [19], and good biocompatibility [20]. Like other biomedical Ti alloys, it was bioinert [21].

Because of the advantages of Ti–7.5Mo, nanotubular oxides grown on this alloy are expected to have a bright future. In this study, self-organized nanotubes were synthesized on Ti–7.5Mo alloy that was anodized in NH₄F/NaCl electrolytes. In our previous study [21], [22], [23], [24], we found that the apatite formation ability (bioactivity) of the NaOH-treated Ti–7.5Mo alloy was excellent; significantly, it was even better than that of commercially pure Ti. In this study, nanotube layers formed on anodized Ti–7.5Mo were subjected to a variety of alkali/heat treatment conditions to obtain improved apatite forming ability.