Link says they are using human embryonic stem cells as the “ink”.
Aside from the moral questions, embryonic stem cells outside of their natural environment, a human in very early development, grow wild. They don’t necessarily grow in to the target tissue, in this case bone, because of their pluripotency. And they tend to grow wild due to their natural proliferating capabilities.
All the mechanicals work, they can easily change the ink to adult stem cells derived from the patient.
Everything I have heard indicates that adult stem cells are actually better suited for this kind of application. Not sure why they would be using embryonic ones here.