The scientists 3D printed a heart, but it seems that the potential is there to make virtually any 'replacement part' desired. As Al pointed out, the tissue comes from the patient so it shouldn't be rejected. Note that all of this is possible only because they are able to reprogram fat cells to be stem cells, which could then become heart tissue, or presumably any other organ or body part.
I've been waiting for scientists to use stem cells to "grow" replacement tissue. It hadn't occurred to me that they would 3D print a complete organ.