Yes, that is a vague question. Here are the specifics: the normal operation of human cerebral cortex generates a detectable alternating current at the cortical surface. The existence of this current allows for the recording of electroencephalography and electrocorticography. In theory, one should be able to construct some kind of receiver that could sit on the cortical surface and when properly oriented, extract electrical power from the field being generated. Yes, the amount of energy obtained would likely be very small - but for the uses I have in mind the power requirements are minimal.
So my specific question is: what would the circuit design of such a receiver look like? Thank you for your attention.