Printing living tissues using cells has started n the last decade and it is now in normal medical practice for skin grafts. For skin the approach it quite straightforward: you place a few cells in a beaker with nutrients and an enzyme that stops cells from sticking with one another. In this way they keep multiplying and once you have got a sufficient number of them (the process is reasonably quick, 2 days would normally do) you can print them using a 3D printer that is actually printing on a 2 dimensional surface mapping the shape of the printed tissue to the surface where the graft needs to be done. A different enzyme is used to neutralise the one stopping cells from sticking with one another so that once printed they actually stick together creating a layer of skin (actually it is a bit more complicated but you get the idea).
Cells getting “printed” on a surface are living things and they tend to move around. This is not an issue with skin since the skin is made, basically, by the same type of cells and it is irrelevant where a specific cell will end up if it moves after printing.
For the creation of homogeneous 3D structure, like a windpipe (a trachea) or a jaw, the cells are printed onto a scaffold that provides the desired 3D shape and again since a single type of cells is required it works pretty well.
However, printing an organ consisting of different types of cells is tricky. If the cells move around after they have been layered by the 3D printer, which they do, you will never get the desired 3D structure that is essential for the organ to be functional.
Here comes the result of researchers at the Oxford University. They have found a way to constrain the cells to stay where they are layered by the 3D printer by encapsulating them inside tiny spheres made by a lipid monolayer. Over time this encapsulating monolayer fades away but at that point the organ structure is complete and cells no longer move around.
The first experiments have shown a good survival rate of cells managed in this way. Researchers hope to move into animal experimentation soon.