Drexler is discussing stages of increasing control over self assembly. This will set up a later post where he will discuss strong control over self assembly. He will show a quantitative measure which will rank the selection of suitability of material for strongly controlled self assembly. The general ranking of suitability of material for strongly controlled self assembly will be
1. Various macromolecular structures
2. Fools’ gold
Along the spectrum of possible assembly methods, those that exploit weak motion constraints occupy a strategic position between pure self-assembly and strongly directed mechanosynthesis. Weak motion constraints is that you use something external to the molecules being self assembled to shift the odds that what you want to have happen does happen. An unaltered self assembly process could have 50% folding right and 50% folding left, but then you somehow bias what is happening so that there is a 95% chance it will fold right and 5% is folding left. You could shove the molecules so that there is some wall of the left.
This changes the nature of the challenge of providing uniquely binding interfaces to direct assembly. Roughly speaking:
* Pure self-assembly requires that each pair of component interfaces be unique.
* Sequential self-assembly [discussed outside this excerpt, but the idea should be clear] requires only that each of the simultaneously exposed interfaces be unique
* Weakly guided self-assembly requires only that each pair of simultaneously exposed interfaces within a limited range of motion be unique.
* Strongly guided mechanical assembly directs components to unique sites without relying on the properties of component interfaces at all.
Self-assembly is sometimes described as “supramolecular synthesis”, and weakly guided assembly that relies on weak bonds can similarly be described as “supramolecular mechanosynthesis”.
Note that strongly guided mechanical assembly methods are applicable to components that are as different as nanoscale bricks and molecular fragments, and that bind to one another by forces as different as hydrophobic interactions and covalent bonds.
Drexler is providing some background for a coming post on strongly guided assembly, in which he will present a quantitative metric that is highly relevant to molecular manufacturing and places fool’s gold above diamond. It places various macromolecular structures above both.