The downside is time. Since it takes a few weeks to grow enough cells in the lab, they're not available to help repair immediate damage at the time of a heart attack (unless patients want to store cells on the chance that they will need them one day.) Also, most cells have been implanted during open heart surgery. Doctors are looking for simpler, less invasive ways of delivering the cells. Limited evidence suggests that cell transplants are generally safe. But effectiveness is still unproven and no one knows how long the transplanted cells may survive in the body.