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3-D printing offers promise of custom stents with perfect fits

A 3-D printer constructed a model of a human trachea.
Jon Christian
A 3-D printer constructed a model of a human trachea.

Dr. George Cheng holds in his hand a highly unusual piece of medical equipment: an airway stent meant to hold open the trachea of a patient who is having trouble breathing. But unlike mass-produced stents, this stent was made using a 3-D printer, so it perfectly fits the interior contours of a particular patient’s airway.

“We’ll make a device that’s just for you,” said Cheng, a research fellow at Beth Israel Deaconess Medical Center. “I think this is going to be the next revolution.”

The finger-size prototype, the shape of a lumpy, hollow “Y” and made of medical-grade silicone, could prove to be a more effective tool for patients who suffer from stenosis, cancer-related breathing problems, and other airway issues.

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“We think that by 3-D printing, we can make the perfect-size stent for the patient,” said Adnan Majid, an assistant professor of medicine at Harvard Medical School and a pulmonary specialist at Beth Israel who has served as Cheng’s adviser on the research. “It may be faster, cheaper, and ultimately translate into better patient care.”

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The process of 3-D printing creates solid objects by slowly extruding layers of plastic or other materials, based on a pattern designed in a computer program. Once confined to sophisticated industrial processes, 3-D printing has become popular with tinkerers and hobbyists and is spreading quickly to many industries as the technology becomes more affordable and easier to use. Companies are using 3-D printers to make highly customized products, from running shoes to toys to jewelry.

And the medical world is exploring many uses for the technology, including making replacement body parts and implants used to stimulate growth in damaged tissue.

A traditional stent (left) and two customized stents made with a 3-D printer.
Beth Israel Deaconess Medical Center
A traditional stent (left) and two customized stents made with a 3-D printer.

Cheng first became interested when he read a 2013 article in the New England Journal of Medicine that detailed how researchers had implanted a tracheal splint, made with a 3-D printer, in a boy with a collapsed airway. He wondered whether the data from the CT scan of a trachea could be used to produce stents or other airway prostheses to help patients breathe.

He received a $50,000 grant from the Center for Integration of Medicine and Innovative Technology to fund his research and consulated with Majid, who would go on to become his adviser on the project.

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Airway stents seem an obvious medical device that would benefit from customization. Commercial versions look like they are fused from sections of clear garden hose with small studs to hold them in place, or similar segments of metallic mesh. Because they only approximate the shape of a patient’s airway, airway stents may not fit snugly and can slide around in the trachea; the tube can also become blocked by mucus, or new tissue may form in reaction to the foreign object.

“Our current stents aren’t perfect,” said Stephen Yang, a professor of surgery at Johns Hopkins who is not involved in the Beth Israel research. “We have to pull them off the shelf, and we never know if we have the right size available. If you have a well-conformed stent to the patient’s airway, that sounds like a great solution.”

It is possible for doctors to order an airway stent with certain customized dimensions, but only by sending the measurements to a manufacturer. The resulting stent is still made of tubular segments rather than curved to fit a particular patient, though, and the process can take months. With 3-D printing, Cheng believes, that turnaround could be cut to just three or four days.

Added Mahid: “With this stent, we can achieve strength and flexibility similar to the patient’s airway.”

Because he had only a rudimentary knowledge of the technology, Cheng sought to tap other medical professionals, as well as Boston’s growing community of 3-D printing specialists and hobbyists.

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One key addition to the team was Adam Wilson, a 3-D printing enthusiast whom Cheng met through a nurse practitioner at Beth Israel. Wilson helped Cheng overcome a technological limitation: The silicone used for the stent could not be used in a 3-D printer. So Wilson had the idea to create a mold that could be printed in plastic. Researchers could then pour silicone into the mold to create the finished stent.

“My part of the process is fairly tedious, or at least it was at first,” said Wilson, who then wrote a program that automates the process of creating a mold based on the CT scan of a patient’s trachea.

Another contributor was Sebastian Ochoa, a Beth Israel colleague who with Cheng toyed with the idea of making the exterior surface out of a permeable mesh, which would prevent the buildup of bacteria against the walls of a patient’s airway.

Neither had the expertise to perform those complex manipulations of the 3-D files, so Ochoa reached out to a friend, Noah Garcia, an architect at Gertler and Wente Architects in New York City. Garcia’s experience of using computers to design facades for buildings turned out to be strangely well suited to manipulating the surface of the stent.

“At first I was skeptical myself,” Garcia said. “But what I discovered was that there is a direct relationship between biomechanical engineering and architecture.”

George Cheng (right) and Sebastian Ochoa displayed a prototype stent made with a 3-D printer. A clinical trial on such devices is planned at Beth Israel Deaconess next year.
George Cheng (right) and Sebastian Ochoa displayed a prototype stent made with a 3-D printer. A clinical trial on such devices is planned at Beth Israel Deaconess next year.

Garcia eventually created variations on the mesh design that included a coil, a double helix, and a diagonal grid that held the shape of the airway while making contact with only portions of it. Wilson is investigating ways to create a mold for making these more complex designs out of silicone.

Cheng and Majid hope to hold a clinical trial on the custom stents at Beth Israel next year.

Looking ahead, they have approached a stent manufacturer to discuss how custom 3-D printing can be used on a larger scale. Cheng and Ochoa also hope their work on airway stents will help fine-tune research being done elsewhere in the medical world, including 3-D printing of complex body parts such as knee and hip joints.