Ti-Nb-Zr-Ta medium entropy alloys (MEAs) are promising for implantable devices due to their excellent biocompatibility, corrosion resistance, and low metal-ion release. Yet a persistent limitation of this alloy family is the lack of intrinsic antibacterial activity, which leaves implanted devices vulnerable to biofilm formation and infection. The micro/nanostructured Ti-Nb-Zr-Ta MEA films were developed by oblique-angle (glancing-angle) magnetron sputtering (OAD/GLAD) and the resulting coatings exhibited light-activated photothermal antibacterial activity. The examination was carried out on how deposition geometry controlled composition, crystal structure, and morphology. These features were further linked to optical absorptance, photothermal conversion, and antibacterial efficacy under near-infrared (NIR) irradiation. A series of Ti-Nb-Zr-Ta films were deposited at incident angles of 0°, 45°, and 80° without external substrate heating. Comprehensive characterization was conducted, including chemical composition, elemental oxidation states, phase constitution, and multiscale morphology, together with measurements of optical absorptance, photothermal response, and biological performance (cytocompatibility and antibacterial activity).
Guided by an average valence electron concentration (VEC) of 4.5 and an atomic size mismatch of 9.4%, the alloy system preferentially stabilized a body-centered cubic (BCC) phase. Consistently, all sputtered films crystallized in BCC. The shadowing effect inherent to oblique-angle deposition strongly modulated the diffracting microstructure and the resulting optical-thermal behavior. As the incident angle increased, X-ray diffraction peaks shifted to lower 2θ, broadened in full width at half maximum (FWHM), and diminished in intensity. Concomitantly, compositional analysis revealed a monotonic rise in Ti and Nb contents with a corresponding reduction in Zr and Ta, alongside higher oxygen incorporation. Microstructurally, the columns became increasingly tilted and porous with the increasing incident angle. This trend arose from flux self-shadowing during OAD. Incoming species preferentially deposited on target-facing facets, while opposite facets remained shadowed. Limited adatom diffusion at ambient substrate temperature precluded efficient backfilling of these shadowed regions, thereby promoting inclined columnar growth, a looser film network, and increased surface roughness. At an incident angle of 80°, the mean column-substrate angle decreased to 43°, and the root-mean-square roughness increased to 4.97 nm, confirming the emergence of an open, high-surface-area columnar topology.
These OAD-induced structural features translated into markedly enhanced photothermal behavior through a synergistic set of mechanisms. Firstly, hybridization among Ti, Nb, Zr, and Ta increased the electronic density of states. Secondly, the porous, tilted-column morphology acted as an efficient optical-trap scaffold that drove multiple internal reflections and prolonged optical path lengths. Thirdly, increased oxidation at higher deposition angles suppressed specular reflectance and transmittance, further raising effective absorptance. Fourthly, the loose nanoscale architecture strengthened localized plasmonic resonances and dissipative losses, converting more photon energy into heat. As a result, the films showed fast NIR-I photothermal heating. Under 808 nm irradiation (2 W/cm2), the 80° film reached 110 ℃ within 3 min. Importantly, the intrinsic biological inertness of Ti, Nb, Zr, and Ta underpinned excellent biocompatibility across all samples. Leveraging the light-trapping architecture and higher oxidation degree, the high-angle films displayed potent photothermal antibacterial activity. After 10 min of NIR exposure, surface-adherent bacterial colonies on the 80° film were completely eradicated, demonstrating effective on-demand sterilization while maintaining the benign baseline of a bioinert metallic system.
Collectively, these findings establish oblique-angle magnetron sputtering as a facile and scalable route to architected Ti-Nb-Zr-Ta MEA coatings in which deposition geometry serves as a powerful handle to co-tune composition, BCC microstructure, and porous columnar morphology, thereby optimizing broadband optical absorption and heat generation for light-activated antibacterial action. Beyond clarifying the links between flux shadowing, oxygen uptake, and photothermal conversion, this work positions Ti-Nb-Zr-Ta MEA films as compelling candidates for infection-resistant implants and also suggests a general strategy for leveraging geometry-assisted photothermal effects in other bioinert medium-entropy alloys.
Key words
magnetron sputtering /
oblique-angle deposition /
Ti-Nb-Zr-Ta /
medium entropy alloy /
photothermal properties /
antibacterial properties
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Funding
National Natural Science Foundation of China (NSFC)（52571273）
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