Lining medical stents with hairlike fuzz could fend off infections

Covering stents with hairlike structures called cilia may help keep the implanted medical tubes clean, preventing infection, researchers report April 28 in the Proceedings of the National Academy of Sciences. When exposed to ultrasound waves — which would be generated by equipment outside the body — the cilia wiggle and flush out substances stuck to the devices.

Crystallized compounds and a sticky biofilm made by microbes often cause urinary stents and catheters to stop working, says biomedical engineer Francesco Clavica of the University of Bern in Switzerland. The tubes drain urine in the body’s “wet and warm environment, so it’s the perfect environment for bacteria to grow.”

Those microbes, along with salts that leave crusty deposits, can create blockages inside the tubes that promote infection, and sharp spots outside that hamper a device’s removal, Clavica says. Because of this, experts say a stent connecting a kidney to the bladder should be replaced every three to six months. Worldwide, more than 1.5 million of these stents are inserted annually.

So Clavica and colleagues designed a cleanable stent made of a silicon-based polymer. The prototype rectangular stents, about 150 micrometers tall, 4 millimeters wide and 25 millimeters long, have slanted cilia covering their shorter inner walls.

A high-speed camera revealed that cilia 100 micrometers tall — about the thickness of a sheet of paper — move fluid at speeds of up to 10 millimeters per second when ultrasound is applied. The sound waves make the cilia vibrate, which generates vortices of fluid that rotate in opposite directions on either side of each cilium tip. That motion would push urine and gunk in the direction the tips are pointing, towards the bladder. “If you create a carpet of cilia, they [add] to each other, so that’s why you get this jet,” Clavica says.

Carpets of cilia are frequently found in nature. Baby sea stars use cilia to create whorls in the water that pull food in, which inspired the new stent design, says roboticist Daniel Ahmed of ETH Zurich.

The cilia-lined stents cleared crystal clusters left by artificial urine, as well as bacteria and biofilm. Moreover, the ultrasound-based system didn’t harm human-derived cells.

Still, these specialized tubes, which would have cilia along their inner and outer walls, may take almost a decade to reach patients, Clavica says. Clavica, Ahmed and their colleagues plan to test the stents in pigs next. But one day, the technology may allow patients to clean their devices at home, Ahmed says. “That’s our big dream.”