How does the signal cross from the ions of the brain to the electrons of the chip? · Lobeworks/17

Through a field, by capacitive coupling.

How does the signal cross from the ions of the brain to the electrons of the chip?

Through a field, by capacitive coupling. No ion enters the metal: at the boundary the ions of the fluid line up against the surface and the metal's electrons rearrange to face them, a few nanometres apart, forming the electrical double layer, which behaves as a capacitor (Q=CVQ = CVQ=CV). When a spike shifts the charge on the fluid side, the electrons on the metal side follow, and the voltage of the metal track moves with them. In one phrase: ions outside, electrons inside, a field in between.