Researchers at the USC Viterbi School of Engineering led by Professor Yong Chen have demonstrated a new method of 3D printing multiple materials. By "multiple" we mean two materials that can be combined in different ratios. For example, the process can print an object that has both hard and soft portions - in different degrees - and do this in a single print operation.
The only other process known to accomplish this type of multi-material 3D printing is Stratasys' PolyJet process. However, the new USC process, a modification of their previously developed mask-image-projection-based stereolithography (MIP-SL) process, is dramatically faster than the Stratasys process.
The new process seems destined for commercial use eventually, but as of this writing, it's not known when products using the MIP-SL multi-material approach. We hope it's soon.