Blood vessels for bypass
Engineer organs, of course, are not the only ones in need of blood vessels. Too often a person's own organs could do with an off-the-shelf replacement artery. In fact, diseases of the blood vessels--small- and medium-bore arteries, in particular--account for most deaths in the United States. Usually when these are replaced, as in coronary artery bypass surgery, the substitutes are taken from elsewhere in the patient's body. For instance, in bypass surgery, a leg vein may be removed and installed as an artery in the heart, a procedure that's painful for the patient and time-consuming for the surgeon.
At Duke University Medical Center (Durham, N.C.), Laura Niklason fabricates blood vessels from pig and cow cells. Similar to real arteries, these conduits comprise a layer of so-called endothelial cells, which make up the inner lining of veins and arteries, surrounded by a layer of smooth-muscle cells, the kind that are not under conscious control. Both layers are embedded in a biodegradable tube-shaped scaffold hooked up to a small pump. The pump beats like a heart, pulsing nutrient fluid through the blood vessels-to-be.
The pulsing seems to help strengthen and differentiate the growing tissue, in keeping with the widely held belief that exposing tissue to the kind of environment it will find in the body helps it to grow properly. So far, results have been encouraging, reports Niklason, an assistant professor of biomedical engineering at Duke. Vessels grown from cow cells "have excellent mechanical properties," she says. "They're fully as strong as native arteries."
Niklason is also looking into the effects of electrical stimulation on the growth and development of blood vessel cells. In living creatures, cardiovascular tissues receive electrical stimulation from the heart. "We don't know yet, but mimicking these electrical environments may improve the growth and differentiation of tissues," she says.
Ultimately, Niklason would like to develop a system for mass-producing human blood vessels, and that is clearly on the agenda at tissue engineering companies. ATS was granted a patent on the formation of "tubular structures," including blood vessels, in 1999. But Niklason thinks it will be a decade or two before she--or anyone else--makes human vessels that are ready for the market. "If tissue-engineered blood vessels rupture or tear, the patient could die," she says. "So they have to clear a much higher bar than some other types of tissue."
Tissues to come
What will future tissue engineering firms be capable of making? Don't expect too much too soon, experts caution. Even before manufacturing and automation come into play, scientists need to work out a number of critical issues, including finding good sources of cells for the engineered organs, dealing with the immune system's response to them, building better scaffolds, and learning how to better preserve the organs once they're constructed.
"It's very difficult to replace what biology has taken millions of years to develop. So we have to be realistic," ATS's Ratcliffe says. "We should be able to return people to normal lives. But can we make things as good as or better than biology has made them? That's a very long-term goal."
So, in all likelihood, is the goal of growing vital organs for implantation in humans. Niklason thinks it could take tissue engineers as long as 40 years to clear all the scientific and regulatory hurdles on the road to products like tissue-engineered hearts and livers.
All the same, it may be only a matter of time until tissue engineers can replicate virtually every part of the human body. "It's not outlandish to think it could be done at some point," says Robert Langer, a professor of chemical and biomedical engineering at MIT and a pioneer in the field. "The issue is how long it takes to get there. I don't know if it'll be 20 years or 500 years."
