Creating a nano-cortex is really pretty easy. Frighteningly easy. All you have to do is have some powdered computronium in a sterile solution, a syringe that can go through bone, and one poor bastard. The hard part is getting the needle into the brain without killing that poor bastard in the process. Seeing as it is already automatically fatal, most of those poor bastards aren't volunteers. Straps, a good stout table, sedation, and a power drill are handy.
Computronium is a barely understood material, but one that is vital to the creation of AISCs, and machine cortices. It is also almost universally present in the form of quantum computer chips, so can be found almost anywhere. This material only works in a concealed state, if it is laid bare for observation, it completely and utterly fails. It is either considered to be a superstate material that is affected by observation (Heisenberg's Uncertainty Principle in physical form) or a dimensional material that has to be sealed to operate.
Some people are depraved, they try to take the stuff as a drug. It is toxic as hell, and it kills fairly painfully. It also causes some wicked hallucinations. People with cybernetics have seen their augmentations boosted, but they are posioned by the stuff just the same. Just slower.
The injection of computronium into the brain has a different affect. The material will begin to bond with the brain cells, and start replacing them. This starts a process where any raw materials in the body that can be scavenged to turn into more computronium is scavenged. Over the course of several days, the victim will see severe neurological problems, decrease in motor function, severe anemia, and vary rapid signs of malnutrition. Within a week of the injection, the victim will have the body with a lifetime of malnutrition and osteoporosis. Some survive longer by being pumped full of metal slurry, which their body both consumes and is poisoned by. Eventually the technomophosis is complete when the entire brain has been replaced by computronium, its shape and structure all replaced with metal and metal salts. The victim never survives this point.
If I Only Had a Brain
A nano-cortex can be used in place of a machine cortex to control autons, or in a genome computer interface. These nano-cortices are much cheaper to make than normal machine cortices, but there are several reasons that even the jaded pragmatism of the CE hasn't followed this course. The nano-cortex is a fundamentally static object. Unlike machine cortices or a human brain, the nano-cortex is not able to learn or adapt. It has a very set series of actions it can follow, and a very limited ability to interact with the world around it. It is in more than on context, fossilized. Also, unlike a machine cortex they cannot be wiped, reprogrammed, debugged, or otherwise modified or adapted.
When used in conjunction with a limiter, something to control how far the technomorphosis goes, partial lobes of the brain can be turned into simulacra of an artificial brain. This is limited, and the main use is to technoformat just the cerebellum of the brain, facilitating superior neuroconductivity between the host and CogNet interface equipment. This is generally limited to oddball and freakshow hackers who are working with homemade or scrap built gear, or with people who are motivated enough to risk a painful death for faster cognitive reaction time.