Athanasiou, the past president of the Biomedical Engineering Society, is working on engineering cartilage that does not require scaffolding for use in the knee and the jaw. "We have discovered that when it comes to cartilage tissue engineering, we can make tissue that looks like real tissue and has all the characteristics of real tissue without using scaffolding," he says.

Using cartilage cells from donors, he has grown scaffoldless cartilage of various shapes and contours in vitro, which means that osteoarthritis could eventually be treated by resurfacing the entire joint with newly grown cartilage.

That doesn't mean there aren't obstacles. The issue of rejection - because the cells are not compatible with the patient - is looming on the horizon, but the risk may not be as severe with cartilage as it is with other tissues. "We can tissue-engineer the structures," Athanasiou says. "The problem that we are faced with - between what we're making now and turning this into a patient-specific product - is the sources of cells we have to use."

They have been using bone marrow cells and human embryonic stem cells, which are limited and controversial. One of the recent research interests is to begin using skin cells. "Clearly," Athanasiou says, "that would be a boon."

ANSETH AND ATHANASIOU are reluctant to predict when their technologies will become commonplace. "If we stay with cartilage, I believe we are within five years of seeing the applicability of that work," Athanasiou says.

Cartilage, bone, and skin (which Anseth's group has created) are just the beginning. Anseth is also working on engineering tissue heart valves that have the ability to grow, which would replace current methods and offer a striking possibility for children born with heart-valve defects. That is the ultimate, of course - regenerating organs such as hearts, livers, and kidneys. There are more than 40,000 people in need of heart transplants in the United States annually, but only 2,000 to 3,000 donor hearts are available each year for such transplants.