Development of hafnium metal and titanium‐hafnium alloys having apatite‐forming ability by chemical surface modification

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Abstract
Hafnium (Hf) has attracted considerable attention as a component of biomedical titanium (Ti) alloys with low Young's moduli and/or shape‐memory functionalities, because its cytotoxicity is as low as that of Ti. The drawback of metals is that their bone‐bonding ability is generally low. It is known that apatite formation in the body is a prerequisite for bone‐bonding. Although several chemical treatments have been proposed for preparing Ti for bone‐bonding, there have been no similar investigations for Hf. In the present study, NaOH‐ and heat‐treatments were applied to pure Hf and Ti‐Hf alloys and their bone‐bonding ability was assessed in vitro with the use of simulated body fluid (SBF). After NaOH‐ and heat‐treatments, anatase formed on alloys with low Hf content (20–40% (atom%) Hf); mixtures of sodium titanate and hafnium titanate formed on alloys with similar Ti and Hf content (60% Hf); and hafnium oxide formed on alloys with high Hf content (80% Hf and pure Hf). Precipitates of apatite were observed on all the metals in SBF, except for the alloy with 60% Hf. We speculated that the hafnium titanate formed on this alloy had a low apatite‐forming ability owing to its high negative surface charge, which inhibited P adsorption. The apatite‐forming abilities of the Ti‐Hf alloys strongly depended on their Hf content. The present results indicate that Hf‐based materials have good potential for bone‐bonding. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 106B: 2519–2523, 2018.

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