Ureteral stents and urinary catheters are routinely used for urinary drainage in ureteral obstruction (e.g., stones, tumours, strictures) and chronic urinary retention due to impaired bladder emptying. Long-term implantation is frequently complicated by biofilm formation and crystalline encrustation, leading to device failure, repeated interventions, and increased device morbidity (1,2). Current strategies primarily focus on material, coating, and design modifications, with limited clinical impact in long-term interventions. This in vitro study evaluates whether ultrasound-triggered streaming, generated by the interaction of ultrasound with microstructured stent and catheter surfaces, can generate sufficient wall shear stress to remove encrustations. In the context of prior 2D microfluidic stent- and catheter-on-chip models (3), this investigation advances to actual 3D-microstructured surfaces in a tissue-mimicking environment. The final aim is to develop stents and catheters that enable on-demand, non-invasive, transcutaneous ultrasound-driven cleaning to improve long-term performance and reduce repeated device replacements.

Inspired by the fluid transport and cleaning functions of ciliary arrays, soft “ciliated” surfaces were developed. The ciliated architecture is designed to harness acoustic sharp-edge streaming, a phenomenon that arises when ultrasound interacts with sharp microstructures (3). Our ciliated surfaces (hereafter “ciliated carpets”) were fabricated via a two-photon polymerization process (Microscale 3D-printer: Nanoscribe, Photonic-Professional-GT, IP-S resin) and attached to polymeric substrates mimicking stent and catheter surfaces. Encrustation was induced by placing ciliated carpets in mineral-rich artificial urine on a nutating mixer (40 rpm), generating continuous flow over the surface and promoting encrustation over 3 days. The encrusted ciliated carpets were immersed in water within a half-tube model of the ureter or urethra (width = 4 mm, height = 2 mm), embedded in a tissue phantom (depth T = 2–4 cm) with acoustic properties of human soft tissue (Fig. 1a). Ultrasound actuation was applied using an ultrasound probe consisting of a 1 MHz-piezoelectric transducer (driven with 80 volts peak-to-peak), positioned on the phantom and coupled via ultrasound gel. In this setup, we also characterized various microstructures using an inverted epi-fluorescence microscope equipped with a high-speed camera. Specifically, we assessed the ultrasound responsiveness for different microstructure heights H (see Fig. 1a), ranging from 10 to 50 μm by analysing the streaming velocity via flow tracer particle tracking (PTVlab software).

The performance of ultrasound-driven streaming along the ciliated carpets in tissue depth T = 4 cm showed a clear dependence on microstructure height H. Higher microstructures exhibited a stronger response to ultrasound, with H = 10 µm cilia showing the lowest performance, whereas H = 40 – 50 µm structures generated streaming velocities exceeding v = 5 mm/s, suggesting higher wall shear stress (Fig. 1b). Based on these findings, encrustation removal experiments were conducted using 50 µm microstructures, demonstrating effective cleaning of encrusted surfaces for the first time at a tissue depth of T = 2 cm within 5 s of continuous ultrasound exposure (Fig. 1c).

These findings suggest that non-invasive, ultrasound-driven encrustation disruption enables the rapid cleaning of stent and catheter surfaces through a transcutaneous approach. Intracorporeal cleaning appears feasible, although greater tissue depths (up to ~12 cm for ureteral applications) and the potential cleaning of softer biofilm deposits require further investigation. Despite the increased surface area of the microstructured surfaces, which may favour biofilm formation, effective and repeatable ultrasound-driven cleaning could enable a viable long-term placement of stents and catheters.

In this study, we demonstrated for the first time that non-invasive cleaning efficacy is maintained in 3D microstructures when activated by ultrasound, thereby extending beyond 2D microfluidic experiments and advancing toward a realistic stented ureter scenario. On-demand ultrasound-driven cleaning for long-term stenting and catheterization may offer an effective alternative to passive coatings and avoid repeated, invasive device replacement.

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Amado, P., Dillinger, C., Bahou, C., Hashemi Gheinani, A., Obrist, D., Burkhard, F., ... & Clavica, F. (2025). Ultrasound-activated cilia for biofilm control in indwelling medical devices. Proceedings of the National Academy of Sciences, 122(18), e2418938122.