Remote biofilm dislodgment using focused acoustic vortex.

Abstract

Biofilms constitute a major challenge in treating implant-associated and chronic infections due to their structural resilience and drug resistance, particularly as implant demand rises due to aging populations. Conventional methods are often invasive, complex, and costly, while focused ultrasound (FUS) poses risks related to biocompatibility and tissue damage. Distinguished by its helical phase structure and rotational energy distribution, focused acoustic vortex (FAV) theoretically generates stronger rotational forces and acoustic streaming than FUS under identical acoustic conditions. This study investigates the feasibility of FAV technique for biofilm removal. Biofilms in vitro model were established using Escherichia coli, and a 2-MHz custom-built ultrasound transducer was employed to generate either FAV or FUS. Results indicated that FAV activation generated a centripetal vortical flow with rapid rotation, which was adjustable via acoustic pressure and duty cycle. Conversely, FUS generated solely outward acoustic streaming, exhibiting a flow velocity 43.6 % lower than that of FAV. At 1.75 MPa, implementing a 10 % duty cycle and a 180 s treatment, FAV removed 97 % of the biofilm, whereas FUS removed only 7 %. To achieve a comparable removal rate (95.8 %), FUS required 4 MPa for 10 minutes. Streaming velocity (R² = 0.99) exhibited a strong correlation with biofilm removal, while inertial cavitation (R² = 0.19) exhibited a weak correlation; thus, the former was identified as the primary contributing mechanism. Importantly, FAV treatment resulted in minimal thermal elevation (<5 °C) and no significant reduction in cell viability, demonstrating its biosafety under the applied acoustic parameters. Synergistic tests with antibiotics further suppressed biofilm regrowth for up to 72 h, reducing bacterial concentration by 91 %. Future work will focus on in vivo biofilm models and assessing the safety and efficacy of combined treatments to advance clinical applications.