From: Robert J. Bradbury (bradbury@www.aeiveos.com)
Date: Sun Oct 03 1999 - 11:11:29 MDT
I previously sent this to Spike, but is probably of interest to the
group...
On Fri, 1 Oct 1999, Spike Jones wrote:
> ... a group of coworkers today, I noticed they didnt seem to understand
> my comment that we already *have* nanomachines running around in
> our bodies, repairing damage, taking out harmful...foreign nanomachines.
Actually, you have ~40 trillion foreign self-replicating nanomachines
in your body. So many in fact that there are more copies of foreign
operating programs (genomes) than your own genome! The foreign programs
are 2-3 orders of magnitude smaller though.
> They were uniformly under the impression that nanomachines had to
> be made of silicon, like 7 of 9's.
Well 7 of 9, has a lot more mass as wet-[bio]nanotech than dry-nanotech.
> They thought there is something different about carbon based
> microrganisms and a replicating assembler. Am I missing something,
> or is there some fundamental reason why a replicating assembler must
> be made of something other than carbon?
Replicating assemblers can probably be made out of almost anything.
Bacteria and eukarotic cells that make up your body are wet-nanotech.
They constitute a complete proof of principle that molecular nanoscale
self-replicating machines are feasible. The work by TIGR & Ventner seems
to indicate that you can get working self-replicating machines that
consist of ~250-300 "parts" (proteins). With that few parts, they only
operate if they are in a very rich & protected environment (with lots of
small molecule building materials & little competition from neighbors)
such as within another cell.
Now you can contrast that with the Drexlerian approach where you would
like to build assembler that has perhaps dozens of parts, e.g.
the picture of the nanoassembler arm [Nanosystems, pg 401] and
and the various rods & guides from the rod-logic computer [pgs 344-360].
Ralph did a paper a couple of years ago on the small-molecule feedstock
that is required for this approach (in contrast to a feedstock of
DNA bases, amino acids and lipid molecules). I suspect though, when
you talk about a complete self-replicating system (from small molecule
feedstock) you are talking hundreds of parts just like a bacteria.
You just have to keep in mind that the "ecology" required to
supply the self-replicating machines is going to be quite different
depending on the materials that they are built from.
> Seems like carbon is an ideal material for that purpose.
Carbon is great for wet-nanotech because of the various bond types
(esp. single & double) and compounds (esp. hydrophilic & hydrophobic)
it forms. Carbon is great for dry-nanotech because it forms the
strongest known material (diamond).
However, there is *no* requirement for diamond in dry nanotech.
Sapphire (Al2O3) works quite well, Boron Nitride is also very
strong. They would of course require different chemistries
(and molecular feedstocks) for construction. There is also
no reason that you could not use silicon, or aluminium or
even iron oxide. Of course, these materials do not have the
strength of the stronger materials and so you can't operate
at the same limits. Since things like the Young's modulus
(elasticity), Shear modulus, Ultimate Tensile Strength, etc.
of these materials are different, you have to make adjustments
in the physical dimensions of your nanomachinery. You have
to build pressure/vacuum containers with thicker walls, can't
stress the materials as much with rapid movements, need bigger
beams, trusses, etc., can't rotate your motors, gears or flywheels
as fast, etc.
You could explain that almost all of the MEMS or Microfluidics work
being done now is based on Si, silicon dioxide and silicon nitride.
They will keep making those scales smaller and smaller and will
continue to build increasingly sophisticated machines. At the
same time the organic chemists and to a lesser degree some
inorganic chemists are getting very clever about constructing
quite complex molecules. (The rotating motors recently constructed
from a few dozen atoms comes to mind. These tend to be primarily
carbon based, but the have to throw in things like O, N, S and
some metal atoms to get the shapes that they need to do something
useful.)
It may also be interesting to note that things like tooth enamel,
bones, sea shells and wood are all examples of relatively precise
atomic-scale structures (effectively dry nanotech) that are
manufactured by wet-nanotech. All of these materials are
excellent nanotech building materials (I could see a city
built out of them...).
The transition we have to make is to bridge the gap between
small simple chemical molecules and larger macro-scale
"machined" [lithographed?] structures while maintaining "relatively"
perfect atomic placement on those scales (as is done in your body when
assembling things). Whether bottom-up or top-down or a mixture
will come out of the race first remains unclear at this time.
Spike, if you use Excel, I can send you a spreadsheet that has a huge
collection of materials properties. It that makes for interesting
study on the best materials from which different things can be built.
For example -- you probably want to build the heat-reflecting layers
of the walls of your nano-house out of gold leaf a few microns thick.
If you want the highest energy density flywheel you might build
it out of silicon carbide or even tungsten carbide. And in
your aircar, you probably want the non-structural surfaces to
be either boron nitride or beryllium carbide.
Robert
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