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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.

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A typical EEG would measure signals in the range of about 10 µV to 100 µV in amplitude when measured from the scalp[31] and is about 10–20 mV when measured from subdural electrodes and the frequency is well below 20 KHz, and frequently below 70 Hz.

This paper by Linear Tech talks about harvesting energy from sources as low as 20 mV. So, in theory, the technology is barely able to extract energy from electrodes placed under the skin. And by "just barely", I mean that it might work in a lab but that's about it. It won't work from electrodes placed on the skin.

What we don't know is just how much power (watts) could be extracted. It it probable that there isn't enough to power a very efficient LED or MCU.

Keep in mind that if you are efficient at extracting energy then you are in effect preventing the energy in that "brain signal" from going to where it is needed-- in essence, preventing your brain from doing its function. I do not see this as a viable source of energy.

There are other ways to harvest energy from vibrations and shock-- and theses should be more useful and practical than what you are proposing here. A vibration sensor placed on the hands/feet or even over an artery on your neck/arm/leg would give you more energy (educated guess) than inserting an electrode under your skin. And it wouldn't turn the world into a real-life version of The Matrix.

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  • \$\begingroup\$ This is how the borg got started (sorry) \$\endgroup\$ – Andy aka Jun 17 '13 at 20:49
  • \$\begingroup\$ Actually the device I have in mind will be resting on the surface of the cortex and will be very very small, so those numbers are just about right for what I have in mind. But no, Andy, no Borg ;-) David, thank you so much for your help! \$\endgroup\$ – Jeremy Slater Jun 18 '13 at 12:30
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No, it doesn't produce detectable alternating current.

Brain waves are alternating voltage signals. This is usually abbreviated as "AC," where AC is taken to mean alternating voltage. Yes, that's jargon. Anyone in the biz is expected to know that AC isn't referring to a current. There's no such term as "AV," although sometimes you'll see it expressed as "AC voltage."

Go look up info on DIY EEG circuits: "open eeg" and instructables.

Or put it this way: a 1mV signal with a 10K source impedance might provide you with a fraction of a nanowatt of energy flow. But skin electrode signals are typically smaller than 1mV!

There is one situation where brain currents are an issue. In a field-free chamber, where magnetic fields are as completely shielded as we know how, brain currents can be detected with SQUID magnetometers and similar devices. But "detectable with a few million bucks of equipment" is different than "detectable."

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  • \$\begingroup\$ Sorry, just saw that. Yes, I'm aware of magnetoencephalography - I work with two of those units on a regular basis for clinical use. To address the objection - I'm not going for drawing power from the scalp surface, but rather from the cortical surface, and again on a very small scale where those energy flows may be sufficient. My knowledge gap (which is considerable) lies in the area of how one designs the receiver in the first place. \$\endgroup\$ – Jeremy Slater Jun 18 '13 at 12:35
  • \$\begingroup\$ DOH! Yeah, D. Kessner links that lin tech reference: switching a charged capacitor through a stepup transformer. If low power isn't your limit, then the real challenge is to make them self-starting. Needs 2V before the converter can make two volts. Easier to bootstrap with an ext supply the first time, then just never let it fall to zero. Of course much easier to use RFID techniques instead: powered external xmitter via RF/antenna or a local coil. Heh, subcutaneous PV cell, and hat full of LEDs? Array of peltier junctions and hat full of ice cubes? \$\endgroup\$ – wbeaty Jun 19 '13 at 7:23

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