As this website explains: "Multiple Micro Electrode Array, this is the actual Chip. This chip will be surgically implanted directly into a human nerve or into specific area of the brain. The Chip at the moment is a little smaller than a Tic Tac breath mint. The chip implanted into the pioneer, a Professor Kevin Warwick, had 100 Electrodes; each electrode looks like a small needle and is capable of detecting electric signals traveling through the nervous system and also sending electrical signals to the nervous system, this was all performed via a RF (Radio Frequency) device connected to the MMEA."
How classified is DARPA? This article, which highlights the secrecy behind a DARPA facility, will give you some idea.
