Enough that it is not considered a viable approach for general purpose BCIs in adult subjects. You can get away with injecting one to study or affect a small localized region, but that's about it [source: chat with a Lieber lab collaborator].

Unless of course you get Cultured and put it in place at the embryonic stage, so the brain grows into it.

'cultured' as in 'cell culture' or as 'embraced by the Culture' (where the 'neural lace' comes from)?

have an upvote.

/dontexplainthejoke