Right now, tissue engineering relies on bioreactors to grow sheets of cells. These machines often look like large fish tanks, filled with a rich soup of nutrients and chemicals that cells need to grow on a specialized trellis. The problem, explain Mouthuy and Carr, is that bioreactors currently "fail to mimic the real mechanical environment for cells." In other words, human cells in muscles and tendons grow while being stretched and moved around on our skeletons. Without experiencing these natural stresses, the tissue grafts produced by researchers often have a broad range of structural problems and low cell counts.