Seeking to achieve the software control of matter seems like an excellent idea. The self-assembly of macroscale objects comprised of materials both inorganic (e.g., diamond crystals) and organic (e.g., DNA/protein-based life) is already demonstrated in nature. But self-assembly processes will probably not be sufficient to make all of the things we would like to build. As noted in the final report of the recently completed Congressionally-mandated review of the U.S. National Nanotechnology Initiative by the National Research Council (NRC) of the National Academies and the National Materials Advisory Board (NMAB): “For the manufacture of more sophisticated materials and devices, including complex objects produced in large quantities, it is unlikely that simple self-assembly processes will yield the desired results. The reason is that the probability of an error occurring at some point in the process will increase with the complexity of the system and the number of parts that must interoperate.”

The opposite of self-assembly processes is positionally controlled processes, in which the positions and trajectories of all components of intermediate and final product objects are controlled at every moment during assembly. Positional processes should allow more complex products to be built with high quality, and should enable more rapid prototyping. Positional assembly is the norm in conventional macroscale manufacturing (e.g., cars, appliances, houses) but has not yet been seriously investigated experimentally for nanoscale manufacturing. Of course, we already know that positional fabrication will work in the nanoscale realm. This is demonstrated in the biological world by ribosomes, which positionally assemble proteins in living cells by following a sequence of digitally encoded instructions (even though ribosomes themselves are self-assembled). Lacking this positional fabrication of proteins controlled by DNA-based software, large, complex, digitally-specified organisms would probably not be possible and biology as we know it might cease to exist.

Today, vast sums of money are already being invested in self-assembly-based biotechnology approaches to manufacturing. By contrast, only small sums are currently directed towards exploring positionally controlled molecular manufacturing using organic materials, and almost no resources are being devoted to positionally controlled molecular manufacturing using inorganic materials. Thus even a fairly large investment in the former area would probably have negligible incremental impact, while even a small investment in the latter area could have significant incremental impact, on progress in molecular manufacturing technology.

The most important inorganic materials may be the rigid covalent or “diamondoid” solids, since these could potentially be used to build the most reliable and complex nanoscale machinery using positional assembly. Preliminary theoretical studies have suggested great promise for these materials in molecular manufacturing. The NMAB/NRC Review Committee recommended that experimental work aimed at establishing the feasibility (or lack thereof) of positional molecular manufacturing should be pursued and supported: “Experimentation leading to demonstrations supplying ground truth for abstract models is appropriate to better characterize the potential for use of bottom-up or molecular manufacturing systems that utilize processes more complex than self-assembly.” One possible rough outline for a combined experimental and theoretical program to explore the feasibility of nanoscale positional manufacturing techniques, starting with the positionally controlled mechanosynthesis of diamondoid structures using simple molecular feedstock and progressing to the ultimate goal of a desktop nanofactory appliance able to manufacture macroscale quantities of molecularly precise product objects according to digitally-defined blueprints, is available at the Nanofactory Collaboration website (http://www.MolecularAssembler.com/Nanofactory/Challenges.htm).

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