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3-D Printing Flexible, Biodegradable Stents That Fit Individual Patients’ Bodies

11 October 2016

Example of a stent used in an endovascular aneurysm repair.Example of a stent used in an endovascular aneurysm repair.

An engineering duo from Northwestern University’s Engineering Department are using 3-D printing techniques to create a new kind of vascular stent that can be customized to suit a specific patients’ body type.

Currently, majority of the stents used are composed of metal and are available off-the-shelf in different sizes.

“The physician has to guess which stent size is a good fit to keep the blood vessel open. But we’re all different and results are highly dependent on physician experience, so that’s not an optimal solution,” said Guillermo Ameer, professor of biomedical engineering in Northwestern’s McCormick School of Engineering and professor of surgery in the Feinberg School of Medicine.

The problem with this method is that if a stent does not fit correctly, they can move into the artery and fail. If this is the case, doctors have to re-open the blocked stent or bypass it with a vascular graft – an expensive and risky process.

“There are cases where a physician tries to stent a patient's blood vessel, and the fit is not good,” said Ameer. “There might be geometric constraints in the patient’s vessel, such as a significant curvature that can disturb blood flow, causing traditional stents to fail. This is especially a problem for patients who have conditions that prevent the use of blood thinners, which are commonly given to patients who have stents. By printing a stent that has the exact geometric and biologic requirements of the patient’s blood vessel, we expect to minimize the probability of these complications.”

Ameer and associate professor of mechanical engineering, Cheng Sun, came up with the 3-D printing method as a solution to this.

To create these customized stents, the engineers adapted a 3-D printing technique, called projection micro-stereo-lithography, and used a polymer previously developed in Ameer’s lab. The technique uses a liquid photo-curable resin or polymer to print objects with light.

When a pattern of light hits the polymer, it converts it into a solid that is then slowly displaced to cure the next layer of liquid polymer. The printing method allows the team to engineer a stent that precisely matches specific design characteristics.

Some advantages of Sun’s 3-D printing technique, known as micro continuous liquid interface production (microCLIP), include high resolution, the ability to print up to 100 stents at a time, and speed—printing a 4-cm stent in just minutes.

While traditional stents are made with metal wire mesh, Ameer used a citric-acid based polymer which proved to be flexible, biodegradable, and even has inherent antioxidant properties.

As an added bonus, the material allows drugs to be loaded on and slowly released at the implantation site to improve the healing process in the blood vessel wall.

(Image via Northwestern University)(Image via Northwestern University)Ameer has previously shown that the polymer can be engineered to inhibit clot formation when applied to vascular grafts. The stent is strong and biodegradable, allowing it to exercise its mechanical function during the vessel’s initial dilation and slowly dissolve as the re-opened blood vessel recovers.

“In theory, it’s safer because the patient doesn’t have permanent foreign metal devices in the body,” said Ameer. “If, for any reason in the future, the surgeon needs to go back in to that location in the vessel, they can. There’s not a metal stent in the way.”

Biodegradable stents in existence today are made from plastics like those used for sutures. The material is not as strong as wire mesh and can take longer than metal stents to fully expand when deployed. The 3-D printed stent, on the other hand, can be fabricated with the thinner profile of traditional metal wire stents, so it is more compatible with the body.

Ameer and Sun envision the future where doctors can use the dimensions of a patient’s vessel, obtained using standard imaging techniques available at hospitals, to print a stent on site to fit the vessel’s dimensions exactly. The stent would be packaged and given to the surgeon for implantation.

Next, Ameer plans to investigate how long it takes for his biodegradable stent to break down and absorb into the body. His team also aims to investigate new stent designs to improve their long-term performance.

“Not only can we customize the stent for a patient’s blood vessels,” he said, “but we can create all new types of patient-specific medical devices that could make the outcomes of surgical procedures better than what they are today.”

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