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.