Ureteral stents and urinary catheters are medical devices commonly used to drain urine from renal pelvis to the urinary bladder and from the bladder to the outside the body, respectively. However, their long-term usage — such as ureteral stents used for ureteral stricture/tumor—often results in blockages due to encrustation and biofilms formation. (1) Contemporary research on encrustation-repellent ureteral stents and catheters primarily focuses on novel materials, designs, and coatings. Alternatively, recent discoveries indicate a notable interaction between local fluid dynamics, i.e., low fluid flow regions, and encrusted stent sites. (2) In earlier experiments, we showed that ultrasound-activated cilia can generate localized fluid flow (so-called acoustic streaming) of up to 10 mm/s. (3) This microfluidic study aims at proving the hypothesis that the high wall shear (WSS, associated with the acoustic streaming), generated by the activated cilia, can clean efficiently encrusted surfaces. Our final goal is to produce  stents and catheters with good long-term performance which can be cleaned transcutaneously.

The fabrication of microfluidic channels (height = 150 µm) with ciliated side walls was accomplished through conventional soft lithography and the replica molding method using polydimethylsiloxane (PDMS, Sylgard 184 Silicone Elastomer). To build a microfluidic model of a Stent/Catheter (Fig. 1a), a PDMS microchannel and a piezoelectric transducer were attached onto a conventional microscope glass slide enabling ultrasonic actuation of the ciliated channel side walls. This glass slide complex was then mounted onto an inverted optical microscope and the piezoelectric transducer was driven by an electronic function generator and a signal amplifier. Throughout all experimental sessions we employed stimulating (ultra-) sound frequencies of f = 15 - 105 kHz with peak-to-peak voltage (Vpp) amplitudes of Vpp = 30.0 - 60.0 V. We performed two different experiments focusing on the ciliated microchannel wall during ultrasound activation: i) Acoustic-streaming evaluation: we filled the microchannels with a solution containing 2 µm-diameter polystyrene passive flow tracers and ii) Encrustation-cleaning effect: we perfused the microchannels with super saturated artificial urine containing carbonate crystals for 30 mins. The carbonate crystals, contained in the artificial urine, sedimented and encrusted the wall of a ciliated microchannel. Eventually, ultrasonic actuation of the Stent or Catheter-on-Chip models for ~15s was performed and analyzed.

We investigated the acoustic streaming flow profile generated in the ciliated microchannel side wall when actuated with a frequency of f = 17.0 kHz and a voltage amplitude of Vpp = 45.0 V. The acoustic cilia generated an intense acoustic streaming with velocities ranging up to v = 10.0 mm/s (Fig. 1b). More importantly, streaming of the passive 2 µm flow tracer particles was observed in close vicinity to the ciliated microchannel wall. As a result, we concluded that the induced acoustic streaming can generate high WSS. To assess the potential of this WSS in cleaning encrusted surfaces, Fig. 1c presents a detailed image sequence summarizing the experimental outcome. The sequence demonstrates the rapid release, subsequent manipulation, and rapid disintegration of a clustered carbonate crystal at the ciliated site within the microchannel. The exposure to an ultrasonic field of frequency f = 99.6 kHz and Vpp = 58.5 V resulted in a remarkable encrustation-cleaning effect.

In our microfluidic proof-of-concept study, we demonstrated the capability of ultrasound-activated cilia to eradicate encrustation from microchannel side walls thanks to the induce acoustic streaming. The on-demand generation of near-to-the-wall microstreaming, reaching velocities up to v = 10 mm/s, results in significant WSS. As a result, clustered carbonate crystals encrusting the channel side wall can be released, disintegrated, and flushed away by shear forces originating from (ultra-) sonically induced steady streaming enhanced by the artificial cilia.

Our results unveil the potential of ultrasound-activated cilia for non-invasive transcutaneous cleaning applications in urinary stents and catheters. Ultimately, this ultrasonic solution can prolong significantly the longevity of these medical devices, thereby  improving patients’ quality of life.

Kawahara, Takashi, et al. "Ureteral stent encrustation, incrustation, and coloring: morbidity related to indwelling times." Journal of endourology 26.2 (2012): 178-182.
Mosayyebi, Ali, et al. "Particle accumulation in ureteral stents is governed by fluid dynamics: in vitro study using a “stent-on-chip” model." Journal of endourology 32.7 (2018): 639-646.
Dillinger, Cornel, Nitesh Nama, and Daniel Ahmed. "Ultrasound-activated ciliary bands for microrobotic systems inspired by starfish." Nature communications 12.1 (2021): 6455.

Continence 12S (2024) 101594
DOI: 10.1016/j.cont.2024.101594