A functional coating for implantable Ti alloy

Titanium (Ti) and its alloys are materials commonly used in biomedical devices due to their excellent compatibility with body tissues and excellent mechanical properties. When used as orthopedic implant devices, the passivation oxide on the surface protects the surface exposed to the body environment to enhance born adherence on the implant surface. However, upon the destruction of the passive layer by wear corrosion, these materials release metal ions into the human tissues, resulting in wear particles with TiO₂ and oxides of Ti alloys that are extremely toxic and dangerous for the human body. Besides, reports by several studies show that the presence of toxic nano-sized TiO₂ particles in the human body is generally more cytotoxic and its toxicity is large in anatase phase TiO₂ as compared to rutile phase TiO₂. As a result, there is an urgent need for more effective and reliable biocompatible surface coatings for implant biomaterials with no or minimal dangers to the human body under varying loadings.

Among the available methods for bioactive coating on Ti alloys, laser application by surface remelting, surface alloying, and cladding with dissimilar materials has attracted significant research attention. Unfortunately, the high-power laser beam degrades the mechanical properties of Ti alloys. Recently, anodic oxidation have been proposed as alternative techniques. Previous findings revealed that anodized titanium materials could produce porous surfaces favorable for osseointegration. Specifically, elastic Ti alloys having low Young’s modulus are widely accepted as orthopedic implants capable of suppressing stress disproportion. TiNbSn is a potential candidate because it is non-cytotoxic and has low Young’s modulus, and recently the alloy was approved as a medical device from the Ministry of Health, Labour and Welfare, Japan. Although anodized TiNbSn alloys is expected to suppress the release of metal ions and the generation of wear debris, a thorough understanding of their mechanical properties is necessary for their practical applications.

To this end, PhD candidate. Miki Hatakeyama, Professor Naoya Masahashi and Professor Shuji Hanada from Tohoku University in collaboration with Dr. Yasuhiro Michiyama from Osaka Research Institute of Industrial Science and Technology and Dr. Hiroyuki Inoue from Osaka Prefecture University investigated the mechanical properties such as wear resistance of anodized TiNbSn alloy for biomaterials. The TiNbSn alloy was prepared in a sodium tartrate acid electrolyte with hydrogen peroxide under high voltage. The results were compared to those of pure Ti and Ti6A1l4V alloy, with a close focus on the microstructural evolution of the anodic oxide. The work is currently published in the journal, Materials Science & Engineering A.

Results showed that anodized TiNbSn alloy contained randomly distributed pores and the oxide was rutile TiO₂. In contrast, the anodized Ti and Ti6A1l4V alloy was anatase TiO­₂ characterized by mille-feuille structure comprising of alternating oxide layers with low- and high-density pores in cross section. Among the investigated samples, anodized TiNbSn exhibited the highest surface roughness, hardness, wear-resistance and exfoliation strength properties which were mainly attributed to the porous, highly adhesive, and hard rutile TiO₂. Whereas neither debris nor galling were observed on the rough and hard surface of the anodized TiNbSn alloy, anodized Ti and TiNbSn alloy generated debris combined with galling alongside plastic deformation. The coefficient of friction of the anodized TiNbSn alloy was reduced under wet conditions due to the lubrication effect associated with the hydrophilic surface provided by the well crystallized TiO₂ and the rough surface.

In summary, the research team revealed superior wear resistance of anodized TiNbSn alloys to those of anodized Ti and Ti6Al4V under both wet friction and dry conditions. The findings suggested that anodization is an effective approach for overcoming the wear resistance of TiNbSn alloy and suppressing the generation of debris due to long-term exposure to the biological environment. In a statement to Advances in Engineering, the authors explained the anodized TiNbSn alloy is a potential candidate for implant biomaterials.