Scientists are on the brink of being able to grow replacement heart valves in a lab from human tissue, which could help 100,000 people a year.
MINNEAPOLIS — Robert Tranquillo came to the University of Minnesota in 1987 with a doctorate in chemical engineering and a budding interest in the mechanical forces that help wounds heal.
Today, he and his research team stand at the brink of a medical breakthrough — engineering replacement parts for the human circulatory system — and he can just about see the culmination of his life's work: an assembly line of arteries and heart valves manufactured from human tissue.
Their novel manufacturing process, described in the latest issue of the Annals of Biomedical Engineering, could create promising new alternatives for the 90,000 American adults who need replacement heart valves every year. And, for the 10,000 children who need a similar procedure, it may lead to valves that grow along with their young bodies.
“Our approach is very simple, and I'd say elegant,” Tranquillo said in a recent interview. “That's why I stuck with it for 20 years, convinced that it's going to work someday.”
The research journey that carried Tranquillo from wound mechanics to engineered heart valves illustrates the importance of plodding, basic science — and the way that serendipity sometimes rewards diligence.
Tranquillo, 56, says it was his high school calculus teacher who awakened his interest in science and mathematical models that describe the movement of cells. After earning his doctorate at the University of Pennsylvania, Tranquillo spent a year at Oxford University in England, where a group of scientists was working on a mathematical theory to describe how embryos take shape based on mechanical interactions between cells and their surroundings.
That led him to research on how and why wounds close, and eventually to the pioneering work he and his research team have conducted: a novel process that uses human cells to convert a Jell-O-like substance into body parts strong enough and flexible enough to implant into the human circulatory system.
Their work, which has attracted more than $13 million in federal grants over the years, has focused primarily on developing a pediatric heart valve that will grow along with the child's body. But along the way, Tranquillo said, they realized they had developed a viable alternative for adult implants that currently rely on pig valves or mechanical devices.
Tranquillo, who is now head of biomedical engineering at the University of Minnesota, began his work there by infusing skin cells, called fibroblasts, into gels containing collagen or fibrin, a protein involved in the formation of blood clots. That same year, the term “tissue engineering” was first used at the National Science Foundation to describe methods for regenerating faulty biological processes.
“I thought, well ... if tissue engineering is really going to evolve and become important, maybe we should redirect some of our efforts for entrapping cells in collagen and fibrin to making a tissue,” Tranquillo said.
One of the first things his team aimed at, he said, was making an artery.
As a first step, Tranquillo's lab team had to figure out how to convert the natural fibrin gel they were using, which had the consistency of Jell-O, into something strong enough to withstand the pressures of the circulatory system. They initially used magnets to align the spaghetti-like network of “fibril” strands, which made the gel stiffer. But other, parallel experiments showed that forces exerted by cells themselves did an even better job of aligning the fibrils.
Researchers created tubes about the size of a human artery by forming the cellularized gel into a tubular mold with a glass mandrel at the center. Tranquillo said cell forces acted on the fibrin as he hoped — producing more collagen, the main component in connective tissue found in mammals. The resulting material had denser fibers aligned around the circumference of the tube, just like a native artery.
Even so, the material wasn't quite strong enough to do the work of an artery.
Enter Zeeshan Syedain, a former graduate student whom Tranquillo credits with several game-changing discoveries in their decadelong association.
Syedain, 31, created several “bioreactors” to condition the tissue-engineered tubes. In the latest version, the tubes are placed over a flexible inner sleeve, then the bioreactor uses suction and expansion to stretch and relax the tubes. After a few weeks, this “exercise” causes the cells to produce more even collagen, strengthening the tissue to the point that it exceeds the body's own characteristics, Tranquillo said.
But a tube is not a valve.
The site of most valve disorders in adults is the heart's left ventricle, and Tranquillo and his students struggled to come up with a way to make a tricuspid valve similar to the one that fails most often in that high-pressure chamber.
Syedain, now a research associate and consultant, came through again. He found a 1996 patent for a “tubular heart valve” that is made from cow tissue. The tissue is stitched into a tube and placed around a frame that resembles a three-pronged crown.
Tranquillo's researchers applied the idea to their tissue-engineered tubes. They expanded them to an internal diameter of 22 mm — about the size of a typical adult heart valve — and placed them around a frame they'd built. When they applied suction to the tube, the sides collapsed inward the way a balloon does when someone sucks out the air. When the flow reversed, the tube reopened. In other words, it functioned like a tricuspid valve. The university has patented the process.
“The metrics for this engineered heart valve are as good as any commercial valve,” Tranquillo said. “If it wasn't for Zeeshan, I don't know that we'd be having this conversation.”
Replacement heart valves, whether taken from pigs or constructed from cow tissue, work reasonably well for adults. But they have some drawbacks: They require anti-clotting medications and they wear out after 15 to 20 years. And because they cannot grow with the patient, they're not well-suited for juveniles.
Tranquillo says his team's tissue-engineered valves shouldn't have those problems. But if they're produced using skin cells from a donor, the patient would have to take immunosuppressants, which have undesirable side effects. That leaves two options. They could use a patient's own cells to produce a valve — a process being pursued by the Mayo Clinic in Rochester, Minn., which is considering a collaboration with Tranquillo. Or they could kill the foreign cells, leaving a collagenous scaffold. In the latter case, a patient's own cells would then attach to the valves, either in a bioreactor or after they're implanted. Either way, the end result would be a living valve.
Tranquillo said the engineered arteries have become extensively “recellularized” just by being implanted in a patient.
“So it's not something that's like a piece of scar tissue,” Tranquillo said. “It seems to be regenerating.”