Vanadium-free Ti-based high- and medium-entropy alloys as next-generation biomedical implant materials: a critical review

Abstract

Load-bearing biomedical implants in severe physiological environments must compromise mechanical compatibility, corrosion resistance, and long-term biocompatibility. The fear of the long-term liberation of vanadium and aluminum has provoked the creation of vanadium-free options, despite Ti-6Al-4V remaining clinically viable. This has proved promising, with high- and medium-entropy alloys (HEAs/MEAs) of titanium containing non-toxic two stabilizing elements such as Nb, Ta, Mo, and Zr. Concentrating on the interdependent nature of alloy composition, microstructural development, phase stability, corrosion characteristics, ion release, and biological response, the present critical review article discusses vanadium-free Ti-based HEAs/MEAs and their potential use in biomedical implants. Ti-based HEAs/MEAs have been found to possess low elastic modulus values, corrosion resistance (owing to the presence of multicomponent passive oxide films), and good in vitro cytocompatibility compared to conventional Ti-6Al-4V alloys. This review highlights that microstructural heterogeneity, particularly in additively produced alloys, may have a significant influence on corrosion and biological performance, and that high configurational entropy is not sufficient to ensure improved performance. The challenges that are left to overcome include the lack of standardized testing procedures and long-term in vivo validation. This review presents research priorities so that the clinical translation of vanadium-free Ti-based HEAs/MEAs to be used in load-bearing applications is carried out safely.

Keywords

Vanadium-free alloys / high-entropy alloys / medium-entropy alloys / biomedical implants / corrosion resistance / biocompatibility / complex concentrated alloys / multi-principal element alloys

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