Surface nano- and micro-texturing of β-type titanium alloys through hydrothermal treatments for biomedical applications

β-Titanium (β-Ti) alloys are promising materials for biomedical implants due to their low Young's modulus, high strength, and avoidance of potentially toxic alloying elements. Furthermore, β-Ti alloys interactions at the implant-tissue interface can be modulated by controlling micro- and nanoscale features of the surface. In this work, we investigated the surface modification of two β-Ti alloys through hydrothermal treatments (HTs) using alkaline and acidic aqueous solutions, with the aim of identifying a simple, low-cost hydrothermal strategy to construct biofunctional micro- and nano-textured coatings. Ti-45Nb (wt.%) and Ti-24Nb-4Zr-8Sn (wt.%) alloys were cast by arc melting and underwent β-solution treatment to stabilise the bcc crystal structure. The metal substrates were hydrothermally treated using either an acidic H2O2/H3PO4 solution or alkaline NaOH and KOH solutions. Surface characterisation using electron microscopies revealed distinct micro- and nano-textured coatings. Microstructure characterisation by GI-XRD and micro-Raman spectroscopy showed that the coatings obtained after acidic treatment consisted of a mixture of titanium and niobium phosphates and titanium-niobium oxide, whereas those produced by NaOH treatment consisted of sodium titanates. Wettability tests showed that NaOH treatment produced highly hydrophilic surfaces, indicating more favourable surfaces for biological interactions. Haemolysis assays demonstrated the haemocompatibility of the annealed alloys and most of the hydrothermally treated samples. Overall, these findings demonstrate that single-step HTs can generate micro- and nano-textured coatings on low-modulus β-Ti alloys, with their morphology, phase composition, and wettability governed by the interplay between alloy composition and solution chemistry, supporting the potential of this approach to tailor β-Ti surfaces for biomedical applications.