Important progress on that front came two years ago, when MIT biomedical engineers Robert Langer and Laura Niklason (now at Duke University Medical Center) grew entire blood vessels from a few cells collected from pigs. Niklason, who led the effort and did much of the work during a stint in Langer’s lab, started by taking small biopsies from the carotid arteries of six-month-old miniature swine. She isolated smooth muscle cells from each tissue sample and used those cells to seed the outer surface of a tubular scaffold built of a biodegradable polymer used in sutures. Next, Niklason cultured each new vessel in its own special growth chamber called a bioreactor. Bioreactors are standard in tissue engineering, but in this case there was a twist.As Langer explains, “What we did is we set up these little pumps that beat like a heart and hooked them up to the artificial blood vessels.” The researchers found that the pulsation encouraged the muscle cells to migrate inward, enveloping microscopic fragments of the polymer, and ultimately made the blood vessels much stronger. After growing the vessels in the pulsing environment for several weeks, they added endothelial cells-the thin, flat cells that line the inside of many tissues, including blood vessels-to their inner surfaces, and grew them for a few more days.
“That single change totally changed everything,” says Langer. “We were actually able to make blood vessels that looked like real vessels.” They functioned like real blood vessels too, staying open and clot-free for several weeks when the researchers grafted them into large arteries in the pigs’ legs. “The key to getting this to work was to mimic what the body did” by growing the vessels in an environment that pulsed just as a real circulatory system does, says Langer.