Why Haven't There Been Nerve Regrowth Experiments In Humans?

I can't find it, but Back In The Day (late 1990s at the latest),
I read some popular science articles about (and I could be getting
any part of this wrong) a silicon wafer with micro-holes in it. The
holes were lined with copper, IIRC. A lab animal's nerve was cut (I
thought it was the spinal column, but who knows?), the wafer was
placed between the two severed ends, and it regrew just fine.

This was, as I say, back in the late 90s at the latest, and I think
it may have been as early as the late 80s. I can't find that exact
experiment (if someone else can, I'd love a pointer; my email is
at the bottom of this essay), but there's been a lot of similar
research I have been able to find. In no particular order:

So, given all that, here's my question: why hasn't any of this been
tried on humans? As someone with relatively minor spinal cord
injuries, I assure you that if I was a parapelegic I'd jump at the
chance to try this stuff. I mean, it's not like it can get
worse then a severed spinal cord.

Seriously, I don't get why I haven't heard about miracle cures yet.
The closest thing I can find is
people in mexico using stem cells,
which really isn't the same sort of thing.

Feel free to drop me a line if
you have any insight.