system
Given the following post, respond as if you're commenting on Reddit:
human
What are the biggest obstacles in 3D printing human tissue or organs?
gpt
Complexity of structure. An organ consists of multiple different tissues arranged in very specific ways. Tissues themselves have their constituent fibres and/ or cells arranged in specific ways too. It’s not just this homogenous thing. 

Because in physiology structure= function, even if we can figure out how to tell stem cells how to differentiate, we then need to tell them how to organise.