@Assalya as a point of note, he's been preparing this for months...

And sorry (my mistake); the knife would be monomolecular. I assumed that this was possible, given this settings' science-fiction status...

If that's not possible, I have another idea in any case: I'd just kill all the guards in preparation of escaping (I'd be able to do it without getting out of my cell).

And if that seems too unlikely, I'll try and think of some other way to show that Jacob is very determined to get out of prison.