The breakthrough not only allows an object made up of many different materials to be printed, but also lets the user change the look and feel of a single material used to print an object. It's possible to print an object with hard and compressible sections out of a single material, even if the raw material isn't flexible in itself. — Gizmag
Now you can 3D print a single object with multiple materials and varying densities, thanks to MIT's Computer Science and Artificial Intelligence Lab (CSAIL). Through an adapted software called Spec2Fab, the designer can specify precisely which materials are to be used in each part of the printed object.
As if MIT isn't impressive enough, check out their work with 4D printing.