Dr. Martin Bocks Seeks to Solve Big Cardiology Problems in Small Children
Bare metal stents are nothing new. Physicians have been implanting them in patients for decades to open blocked blood vessels and repair weak ones, preventing the harmful effects that might otherwise occur. And where these are quite effective in adults, there’s one simple but potentially life-threatening problem with using stents in children—they don’t grow with the patient.
“Right now, we use stents that are FDA approved for adult indications such as coronary or peripheral vascular disease, or stents for the biliary tract,” said Dr. Martin Bocks, a pediatric cardiology specialist at the Cleveland Ohio’s UH Rainbow Babies & Children’s Hospital. “We use them off label, and try to make them fit children suffering from pulmonary artery stenosis or aortic coarctation. These small stents might work when they're six months old, but by the time the patient is a teenager, the 6 mm stent that was once just fine should now be a 16 mm stent. It's a known downside of endovascular stenting in infants and toddlers.”
The only options at that point are a surgery or a high risk interventional procedure to intentionally fracture or split the old stent. Surgery is effective most of the time at removing the old stent material and repairing the vessel, but this requires a bypass operation, which has its own risks. In the cathlab cardiologists may try to “unzip” the stent, by blowing up a balloon with the stent and intentionally splitting it lengthwise and allowing it to expand. There are significant risks associated with the approach, as well, and only certain stents can be treated in this manner. Bocks says several approaches exist but none is ideal, leading to repeated procedures and surgeries that cumulatively lead to both increased morbidity and mortality in these populations.
“We do all these things that sound really crazy, because that's our only option,” he explained. “What would be ideal is a stent you can put in now and that will go away in about a year, one that will be absorbed by the body after the patient has healed or remodeled the treated area, or can easily be re-stented with a larger stent once most of the initial stent material has degraded.”
For the last several years, this has been Bocks’ mission. Working with biomedical engineers from Michigan Tech University and Case Western Reserve University, he has developed a new type of stent made of a special zinc alloy that safely degrades over time and is gradually taken up by the body. It is not permanent like those made of Nitinol, stainless steel, or other bare metals, but shares many of the same properties.
Dr. Bocks explains that the manufacturing process is much the same as any other metal stent, except that the zinc is much harder to work with. “We start with the raw material provided by the team at Michigan Tech, have it cast into rods, and extruded into tubes or a cannulae, which are then laser cut according to our stent design. It’s electro-polished, crimped onto a balloon, and delivered. So far, it looks like an excellent alternative for pediatric patients.”
He also fixes holes in hearts. Here, the problems with children are similar to those of artery repair. Dr. Bocks’ proposed solution is to design and 3D print patient-specific septal occluders that can be delivered via transcatheter approach into the human heart. The occluders will be manufactured from another bioresorbable material, this one called Poly-Glycerol-Dodecanoate (PGD), which has the unique property of shape memory, which happens at around body temperature.
The compressed device can be delivered through a long sheath into the heart, where the warmth of the surrounding blood expands the device into its final, double-disc shape, which both anchors the device in place and serves as the scaffold for heart tissue to grow over it. And since the occluder is made of a bioabsorbable material, it will disappear over time once the body has fully encapsulated the device and sealed off the original defect.
“What we’re working towards is a way to manufacture these based on the patient’s anatomy,” Dr. Bocks said. “If there’s a hole that's 9 millimeters by 13 millimeters, somewhat oval-shaped, we can make a device that would fit that shape according to the image we find on the patient’s echogram. We 3D print the device, sterilize it, and ship it out to the clinician to put in. That's our goal, to have customized 3D printing available for these occluders, and to continue our work with the stents. Both would be the first of their kind. PGD is the only material that we know of with shape memory properties that is also completely bioabsorbable, and the zinc alloy we’re working with also shows great promise. We’re very hopeful that in a few more years, these conditions will be much easier to treat.”