From Self-Assembly to Mechanosynthesis

In an ongoing series, I’ve been discussing paths forward from today’s atomically precise fabrication methods to advanced molecular manufacturing. The posts that address broad topics are:

Modular Molecular Composite Nanosystems
Toward Advanced Nanotechnology: Nanomaterials (1)
Toward Advanced Nanotechnology: Nanomaterials (2)
Self-Assembly for Nanotechnology

In some areas I’ve discussed, lab research is active today; in other areas, research (at least, of the implementation-directed kind) must await a series of capability-building advances in atomically precise fabrication. The central question today is, What are good aim-points for productive research?

This post continues my discussion in “Self-Assembly for Nanotechnology” by outlining a spectrum of assembly methods that spans the range between self-assembly and strong mechanical guidance. This is an excerpt from a work in progress, and may suffer somewhat from being pulled out of context:

Assembly by Soft Machines

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.

In pure self-assembly, Brownian motion is unconstrained, components encounter one another randomly, and only distinct and specific binding interactions can bring them together to form a specific, complex structure.

In strongly guided mechanical assembly, by contrast, binding between components is nonspecific, hence the pattern of assembly must be directed entirely by mechanical constraints on motion (in which elastic restoring forces limit the amplitude of thermal fluctuations).

In the intermediate case of weakly guided-self assembly, soft mechanical constraints (for example, polymeric tethers) direct components to particular regions, and within the allowed range of motion, binding interactions between components determine the exact (atomically precise) location and orientation of the bound component.

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.

The above discussion will provide some background for a coming post on strongly guided assembly, in which I 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.)

Link text updated 10 Feb 09

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