Effect of Rare-Earth Element Microdoping on Ti–6Al–7Nb Alloys for Biomedical Applications: Materials Characterization and In Vivo Biocompatibility Tests

The paper focuses on materials characterization and in vivo biocompatibility tests of Ti–6Al–7Nb–0.3REE wt.% alloys (REEs—Y, Ce, La) for use as a promising material to produce personalized medical implants and shed light on possible toxicity effects of REE alloy microdoping. All alloys were produced by the electric arc melting method and characterized by scanning electron microscopy (SEM), optical microscopy (OM), energy-dispersive X-ray spectroscopy analysis (EDX), X-ray diffraction (XRD), true density analysis, micro- and nanoindentation methods, and reducing/oxidation melting techniques. True density of alloys increased in the following order: Ti−6Al−7Nb−0.3Y (4.4563 ± 0.1075 g/cm3) < Ti−6Al−7Nb−0.3Ce (4.7255 ± 0.2853 g/cm3) < Ti−6Al−7Nb−0.3La (4.8019 ± 0.0111 g/cm3). XRD analysis indicated that Ti–6Al–7Nb–0.3Y alloy consisted of single α–Ti phase in comparison with Ti–6Al–7Nb–0.3La (α–Ti to β–Ti = 82 to 18) and Ti–6Al–7Nb–0.3Ce (α–Ti to β–Ti = 90.5 to 9.5). The single-phase Ti–6Al–7Nb–0.3Y alloy had the finest α–Ti phase crystallites (22.32 nm); the larger α–Ti crystallites in the dual-phase Ti–6Al–7Nb–0.3Ce and Ti–6Al–7Nb–0.3La (30.77 nm and 29.83 nm, respectively) suggested the presence of the β–Ti phase (23.34 nm and 25.61 nm, respectively). REE microdoping of alloys changed the lattice volume (∆V): α–Ti phase—0.269% for Ti–6Al–7Nb–0.3Y, 1.799% for Ti–6Al–7Nb–0.3Ce, 0.595% for Ti–6Al–7Nb–0.3La; and β–Ti phase—0.334% for Ti–6Al–7Nb–0.3Ce, 0.670% for Ti–6Al–7Nb–0.3La. Nanohardness (H) and elastic modulus (E) increased in the following order: Ti−6Al−7Nb−0.3La (4.01 GPa and 135 GPa, respectively) < Ti−6Al−7Nb−0.3Y (4.39 GPa and 137 GPa, respectively) < Ti−6Al−7Nb−0.3Ce (4.67 GPa and 146 GPa, respectively). In vivo tests were conducted using 46 sexually mature male Wistar rats by means of skin implantation of samples with d = 11 mm and h = 1 mm. Our research shows that Ti–6Al–7Nb–0.3La alloy (Group 2) and Ti–6Al–7Nb–0.3Ce alloy (Group 3) induced sustained hepatotoxic and nephrotoxic effects. Ti–6Al–7Nb–0.3Y alloy induced a slight local inflammatory response; however, serum biochemical analysis suggested this effect was compensated.