In a recent pilot experiment in pigs, researchers sutured the cell-laden scaffolds over damaged cartilage in the animals' knees. Six months later, new tissue had formed, with a smooth surface and mechanical properties similar to those of native cartilage. The tissue also expressed molecular markers characteristic of normal cartilage. "Ultimately, it's important for this new tissue to have an extracellular matrix made of native cartilage molecules so that, in the long term, the properties of new tissue will emulate that of real cartilage," says Alan Grodzinsky, director of the Center for Biomedical Engineering, at MIT, who was not involved in the work.

The stem-cell-seeded scaffolds repaired the damage better than scaffolds with no cells or those seeded with ordinary cartilage cells, although scientists don't yet know why. It may be because the stem cells proliferate better than cartilage cells, or because they are more receptive to molecular signals coming from the wounded tissues.

A number of other tissue-engineering approaches using cell transplants, scaffold materials, or a combination of the two are currently under way, including some in clinical trials. (The most advanced of them use one or the other, largely because it is easier to gain approval from the Food and Drug Administration this way.) Tuan says that he aims to begin human tests in the next two years. First, his team must do additional studies in large animals, such as goats or sheep, over a longer period of time, to make sure that the treatment is safe and effective. The polymer that Tuan uses is already approved for medical use, and the cells would come from the patients themselves, eliminating risk of immune rejection.

Tuan's group is also working on making the scaffolds bioactive, tagging them with biological molecules that encourage growth of the appropriate cells. Ultimately, he would like to design a system in which stem cells can be collected from the patient and immediately delivered to the scaffold without culturing, which would then be transplanted into the patient.