3-D printing is merely the method of construction. What about the materials themselves ?
How does one make muscle and kidney or liver tissue ?
They are doing a lot of work with what they refer to as "scaffolds". They form the skeleton, as it were, for the printer to put living cells on. Material copasetic to the real thing. They used syringes to lay down living cells in a gel. Cells probably grown in dishes. And other syringes to lay down proteins to help feed the living cells.
For an organ transplant, they would start with your own cells to grow the new cells for the transplant organ from.