It is a tool that has a feeding mechanism which supplies the material. The material you use all depends upon the type of feeding mechanism that is on each machine and how the process then works. A hamburger "printer" uses a large tube that has raw meat in it that is squeezed out into a shape.

The human tissue style printer is much more advanced. I do not know a whole lot about it. From what I understand is that the "printer" makes a mesh and then tissue is grown over it.