My man, let’s call him Bob, is an engineered organism with a mix of both machinery and biology working in harmony. His skeleton and half of his major organs are machine. However, cells engineered specifically to work alongside the machinery encase his skeleton, giving him a human shape. The outer layer of the skin is like basic human skin and tears and scrapes easily - it's just there to blend in - underneath that, however, I would like a super tough fiber or leather.  

Biology isn’t my forte, and my character’s biology is frequently very different due to the machinery being involved.

* The material has to be able to be made by a cell, although this doesn’t have to be limited to what human cells can do.
* Having it being able to protect against handheld firearms would be nice, but at least against getting cut by a sword.
* It would be nice to be able to produce it from the compounds of normal food, however it’s fine if my character needs to eat mud or something to get the right materials to make this super skin.
* It needs to be flexible like skin - no plate armor or crystal armor. This material needs to be hidden underneath normal skin and not hinder the movement of the character in any way.

If no material fits this description then I’ll just have to get another order of unobtainium from Amazon, but thanks for the help.