Ultrasound-activated cilia for biofilm control in indwelling medical devices

Abstract

Biofilm formation and encrustation are major issues in indwelling medical devices, such as urinary stents and catheters, as they lead to blockages and infections. Currently, to limit these effects, frequent replacements of these devices are necessary, resulting in a significant reduction in patients' quality of life and an increase in healthcare costs. To address these challenges, by leveraging recent advancements in robotics and microfluidic technologies, we envision a self-cleaning system for indwelling medical devices equipped with bioinspired ultrasound-activated cilia. These cilia could be regularly activated transcutaneously by ultrasound, generating steady streaming, which can be used to remove encrusted deposits. In this study, we tested the hypothesis that the generated streaming can efficiently remove encrustations and biofilm from surfaces. To this end, we developed a microfluidic model featuring ultrasound-activated cilia on its wall. We showed that upon ultrasound activation, the cilia generated intense, steady streaming, reaching fluid velocity up to 10 mm/s. In all our experiments, this mechanism was able to efficiently clean typical encrustation (calcium carbonate and oxalate) and biofilm found in urological devices. The generated shear forces released, broke apart, and flushed away encrusted deposits. These findings suggest a broad potential for ultrasound-activated cilia in the maintenance of various medical devices. Compared to existing methods, our approach could reduce the need for invasive procedures, potentially lowering infection risks and enhancing patient comfort.

Keywords: biofilm and encrustation; microfabrication; ultrasound; ureteral stent; urinary catheter.

Fig. 1.

Applications of ultrasound-activated ciliary bands (UACBs) for biofilm and encrustation control. (A) Urinary tract indwelling medical devices (ureteral stent and urinary catheter) suffering from encrustation and biofilm growth. The ureteral stent features two pigtails which curl in the renal pelvis and bladder to prevent migration. (B) Schematic of bacterial attachment and crystal deposition on ureteral stent and urinary catheter surfaces. The inset shows an example of a Scanning Electron Microscopy (SEM) image of an encrusted ureteral stent retrieved from a patient, highlighting a side hole that facilitates urine exchange from the ureter to the stent lumen. (C) Schematics (Top) and image (Bottom) of a ciliated polydimethylsiloxane Stent-on-a-Chip (SoC) model with UACBs (~175 μm) placed on its wall.

Fig. 2.

Flow field generated by the UACBs. Flow profile visualization (Top) using an image stack of 500 consecutive frames captured at a frame rate of 2,800 fps. The respective flow velocity (Bottom) was computed using Particle Tracking Velocimetry (PTV) (Materials and Methods) using 2 µm particle tracers. The UACBs (~100 μm) were activated with a square wave signal of frequency $f=17.0\mathrm{kHz}$ and peak-to-peak voltage $V_{\mathit{PP}}=45.0\mathrm{V}$.

Fig. 3.

SoC surface cleaning via UACBs. (A) Schematics of the expected effect of ultrasound actuation on the UACBs (~100 μm), leading to surface cleaning. Time sequence of microparticles (6 µm) (B) and encrustation (C) removed after UACB actuation with acoustic fields with a frequency $f=13.6\mathrm{kHz}$ and peak-to-peak voltage amplitude of $V_{\mathit{PP}}=52.5\mathrm{V}$, and $f=99.6\mathrm{kHz}$ and $V_{\mathit{PP}}=58.5 \mathrm{V}$, respectively. The insets in each Top-Right corner show a zoomed-in view of the cleaning event.

Fig. 4.

Cleaning of Escherichia coli JM83 biofilm with UACBs. (A) Image sequence illustrating removal of E. coli JM83 biofilm (black area) in a SoC. The UACBs (~100 μm) were activated four times with a square wave signal with a frequency $f=99.6 \mathrm{kHz}$ and peak-to-peak voltage ${\mathrm{V}}_{\mathrm{PP}}=52.5 \mathrm{V}$ for ${\mathrm{t}}_{\mathrm{acoustic}}=30 \mathrm{s}$, with 1-min resting periods between two consecutive bursts. (B) The boxplot (Top) illustrates the percentage of the cleaned area (N = 7 experiments), compared to full image minus UACBs, over activation bursts, highlighting the variability and median effectiveness of UACBs in reducing biofilm mass. For statistical comparisons, the one-sided Mann–Whitney U test was used, showing P value below 0.05 between bursts 1 and 2. The bursts are shown as a step signal (Bottom), indicating when the ultrasound was activated.

Fig. 5.

UACBs can release, trap, and disintegrate crystal clusters. (A) The image sequence depicts an example of encrusted wall with UACBs (~100 μm), releasing crystal clusters. The clusters are trapped, attracted, and disintegrated using a square signal at a frequency $f=99.6 \mathrm{kHz}$ and peak-to-peak voltage amplitude $V_{\mathit{PP}}=58.5 \mathrm{V}$. For reference, the image sequence also shows a crystal cluster in the bottom wall, without UACBs, remaining fully intact postultrasound activation. (B) Analysis of the largest crystal cluster size over time after ultrasound actuation, providing insights into the crystal disintegration timeline.

Fig. 6.

Impact of ultrasound activation on cell health and function. (A) Bright-field microscopy of TEU-2 cells under control (no ultrasound activation) and ultrasound-activated conditions. The cells were exposed to ultrasound four times using a square wave signal with a frequency $f=99.6 \mathrm{kHz}$ and peak-to-peak voltage ${\mathrm{V}}_{\mathrm{PP}}=52.5 \mathrm{V}$ for ${\mathrm{t}}_{\mathrm{acoustic}}=30 \mathrm{s}$ each time, with 1-min resting periods between two consecutive bursts. Representative bright-field images of TEU-2 cells. (B) Dot plots represent cell populations categorized as healthy, early apoptotic, late apoptotic, or necrotic, based on Annexin V (x-axis) and PI (y-axis) staining. (C) The Upper Right panel shows representative histograms of propidium iodide (PI) staining, highlighting the distribution of cells across G0/G1, S, and G2/M phases in control (green), while the Lower Right panel represents ultrasound-activated (red) conditions. Merged histograms (Left) illustrate the overlap between the two conditions. (D) Representative comet assay images visualize DNA integrity in control (C) and ultrasound-activated cells (UA). Media-deprived TEU-2 cells served as a positive control (P) to induce DNA damage. The boxplot shows the tail moment of the different samples.