In vitro long-term antibacterial performance and mechanism of Zn-doped micro-arc oxidation coatings

Micro-arc oxidation (MAO) coatings containing 2.86 wt%, 5.83 wt% and 8.81 wt% Zn (Zn-2.86 wt%, Zn-5.83 wt% and Zn-8.81 wt%) were separately fabricated on Ti6Al4V alloys using EDTA-ZnNa₂ electrolytes. In vitro antibacterial examination exhibits that the antibacterial rates of Zn-2.86 wt%, Zn-5.83 wt% and Zn-8.81 wt% against Staphylococcus aureus (S. aureus) are 76.0 %, 100.0 % and 99.2 %, respectively. Reactive oxygen species (ROS) level of MAO samples is significantly higher than that of the untreated Ti6Al4V. Zn-containing coatings especially Zn-5.83 wt% induces the strongest oxidative stress on S. aureus due to relatively high released Zn²⁺ concentration. Moreover, qPCR analysis shows that MAO samples inhibit the icaADBC transcription and result in the down-regulation of PIA production, thereby mitigating biofilm formation. After immersion in simulated body fluid (SBF) for 3, 8 and 14 d, the antibacterial rate of Zn-5.83 wt% is 84.7 %, 63.2 % and 12.5 % respectively, and ROS level of MAO samples is also significantly higher than that of the untreated Ti6Al4V even after 14 d of immersion, suggesting that the antibacterial performance of MAO samples can last a relatively long immersion period and exhibit large application potential in orthopedic clinic.