Not impossible to replicate, just unstable when replicated. Why that would be the case, beyond dramatic needs, is unclear. Something I've never disputed. Some things may be more difficult to replicate than other things, like the vaccine samples in "Code of Honor", but I've not seen anything explicitly described as "impossible to replicate". Strictly speaking, it was replicated, as in a copy of Worf's spinal cord was made and used to replace his severed one. The fact that it took hours instead of seconds doesn't seem like anything worth quibbling over. And maybe that's all that's ever encountered - problems with replication, not impossiblities. Maybe if Crusher took more time to work on the unstable replicated vaccine, she would have solved it. A deadline, with thousands of lives in peril, prevented her from bothering with the replicator when a natural stable source was nearby. It's funny though that Neelix managed to live with holographic lungs for some time, but the replicator couldn't make one. I might guess that tissue compatibility might be a problem, so just replicating one from the database and installing it in Neelix might kill him. But I can't see why taking more time to clone some of Neelix's tissue and figuring out how to make a compatible lung couldn't be done. I wonder if the genetronic replicator was still in beta when Voyager was launched. Thanks for the tip! I'll let my physics students know that little known fact about compounds and their relationships to elements. You know, just in case their chemistry prof forgot to tell them. I think you missed my point, which was that if the finished product isn't available for replication, why wouldn't they replicate the elements and/or compounds that compose the "beta-matrix compositor" and build it by hand from scratch?