[extropy-chat] Research question: power supply forsilicon-based neural prostheses

Anders Sandberg asa at nada.kth.se
Tue Mar 13 00:40:36 UTC 2007

Robert Bradbury wrote:
> It largely depends on whether you anticipate driving it from the body or
> some self-contained or "plug-in" power source.  Assume the brain is a 10W
> machine.  So adding another 10-30W Si "drain" on the glucose energy the
> body
> can supply probably isn't going to stretch things too much.  But if you
> want
> to add 100-1000W then you have some real problems.

Yes. Normal human wattage is on the order of 100 Watts, and the brain
seems to be around 20 Watt. If energy is extracted from blood sugar there
will be a bit of competition with the brain; we don't want to cause
glucose dips since they negatively affect thinking and mood. An even
consumption of a few watts would likely not be much of a problem for the
regulatory systems, since the brain requirements can go up and down quite
quickly and we can fairly easily replenish ourselves in a modern food rich

The main limit of a multiwatt device is going to be heat dissipation. This
paper http://www.media.mit.edu/physics/publications/papers/99.01.MONET.pdf
shows how to dissipate 30 Watts of heat from a forearm computer, but it
takes up plenty of area. Brain implants will have to be in direct contact
with venous blood to dissipate heat, and they must not become too hot or
they will cause tissue damage. Hmm, maybe install a Zalman heat sink crest
linked to my sagittal sinus?

But the flow speed doesn't seem to be that great. Based on this
http://www.springerlink.com/content/m0548374p4487453/ the speed is about
15.2 cm/s, and if we assume a diameter of 37 mm^2
(based on ref 1 in
http://www.aans.org/education/journal/neurosurgical/jun02/12-6-cp1.pdf )
we get a volume of 5.55 cm^3/s. Putting in the heat capacity of water,
that gives a heat removal capacity of 23.31 W if we accept a one degree
heating (which is probably *far* too much!) In fact, that number is
suspiciously close to the brain's dissipation; I wouldn't be surprised if
there was a heat limitation aspect here on our biology.

My own intuition would be to put as much processing and infrastructure
outside the body, only keeping the communications link and some necessary
processing inside. Distributing it outside the rather closed environment
of the skull is probably a good idea too.

Anders Sandberg,
Oxford Uehiro Centre for Practical Ethics
Philosophy Faculty of Oxford University

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