Interview with Prof. Stefan Jockenhövel, Department of Tissue Engineering & Textile Implants, RWTH Aachen
Braided stents are nothing new in medicine, but their manufacturing process is still time-consuming. This is why Professor Stefan Jockenhövel from the RWTH Aachen University and his team want to make machine production possible.
Professor Jockenhövel, laser-cutting machines are often used to manufacture stents. You, on the other hand, braid implants. Wherein lies the difference?
Stefan Jockenhövel: It depends on the diameter a stent is intended to have. For minimal luminal diameters, for instance for stents intended for use in coronary arteries, laser cutting is the best option. A laser-cutting machine is able to cut very small pieces and is very precise. With braided stents, you always have individual fibers that interlace. The material is thicker at these points. Yet, if you need stents for the carotid artery, lungs or peripheral arteries you generate an enormous amount of scrap with laser-cut stents. Up to 98 percent of the expensive material is cut out and discarded. This is where braided stents have a clear advantage.
How are braided stents manufactured?
Jockenhövel: You can do a traditional braid, as we know it from a hair braid for instance. However, you then have open ends at both ends. When you insert these stents into tissue, it causes permanent irritation thanks to the sharp edges. However, you can also make a stent from one single fiber by producing it manually. We didn’t invent this technique, but we created individual designs that can now be used to produce airway stents. One person needs approximately one day to make such a stent. This is why the industry solves this cost problem by outsourcing the production to Asia. Now we want to automate this process by collaborating with the industry. Our goal is for the production of one stent to only take one hour at most thanks to automation. We are currently in the conceptual design phase of the machine to accomplish this goal.
How and why does the material receive shape-memory?
Jockenhövel: We use nitinol for the stents, a shape-memory alloy. Many are familiar with a paper clip you put into hot water when it is bend and that subsequently assumes its original form again. You can imprint this type of temperature-sensitive shape memory. Actually, we don’t need the shape-memory property quite as much for our stents. We use nitinol to produce our stents because of its superelastic properties. This gives the stent self-expanding properties and exerts permanent force on the arterial wall. What makes shape-memory materials so special is that the blood vessel can be kept open with a stent made from this material, even with different diameters in the arterial wall. That means, in case of changes, for instance if the vessels are pulsating, force can always be exerted on the arterial wall. This results in far less tissue irritation.
What other medical implants and products could be manufactured this way?
Jockenhövel: Other products are also conceivable, but they will always have a beehive structure. Frequently, the stent is only a platform to build something else on. Airway stents, for example, are coated with a plastic material that contains active ingredients to fight lung cancer. This prevents the cancer from growing and closing off the airways. That is to say, the stent is primarily intended to keep the airways open, but it can also serve as the carrier of a substance. You can also take this a step further and apply the patient’s own cells, so-called respiratory epithelium on the stent so it keeps itself clean. This prevents mucus from restricting the airways. We are pursuing this process at the RWTH Aachen University in the so-called PulmoStent Project. We have also developed this coating application as a platform technology for other vascular stents, for the esophagus or the gastrointestinal tract for example. It can be used wherever lumina need to be coated with a patient’s own substance.