Defect-Induced Local Piezoresponse of Anodic Nanotube Arrays on Ti-Rich Medium-Entropy Alloy for Stimulus-Responsive Bioactive and Antibacterial Surfaces.

Developing implant surfaces with electromechanically active properties offers the potential to mimic the natural electromechanical cues of bone, thereby enhancing cell-material interactions and improving implant biofunctionality. This study reports that anodically grown TiO2(B)-containing nanotube arrays on a Ti-rich medium-entropy alloy (Ti65-Zr18-Nb16-Mo1) exhibit a measurable electromechanical response arising from defect- and structure-related local symmetry disruption. Nanotubes were fabricated via anodic oxidation at systematically varied voltages and durations, followed by microwave-assisted thermal treatment. Structural analyses confirmed the formation of TiO2(B)-rich nanotubes, with a maximum fraction of 70.6% (30 V, 30 min). EPR spectroscopy verified abundant oxygen-vacancy-related defect states, supporting the proposed defect-induced local symmetry-breaking mechanism. The optimized nanotube architecture exhibited lower interfacial charge-transfer resistance and the highest longitudinal piezoelectric coefficient (d33 = 1.40 pC/N). Ultrasonic stimulation enhanced apatite deposition, MG-63 cell adhesion and proliferation, and antibacterial activity, increasing bactericidal rates from 76.1% to 91.3% for S. aureus and from 69.3% to 90.2% for E. coli. These results establish a structure-defect-electromechanical response-bioactivity relationship, highlighting TiO2(B)-rich nanotube arrays on Ti-rich medium-entropy alloys as promising multifunctional implant coatings.