The Molecular Machine Path
to Molecular Manufacturing (1):
Foldamers and Brownian Assembly

by Eric Drexler on 2009/12/25

Part 3 of a series prompted by the upcoming 50th anniversary
of Feynman’s historic talk, “There’s Plenty of Room at the Bottom”.

A machine tool used to make machines
Lathe, 1911
A machine tool,
used to make machines

In my view, the most attractive way forward in developing advanced molecular machine systems is by exploiting the molecular machine systems that are available today. Historically, the human race developed advanced manufacturing systems by using machines to make better machines that made still better machines (eventually automated), and making increasingly advanced products along the way. This development process advanced the material basis of civilization from blacksmithing with hammer and anvil, to the machine technologies of the Industrial Revolution, to the ongoing manufacturing revolution described by Moore’s Law. We can do likewise with molecular machines, climbing a ladder of technologies that, step by step, leads to atomically precise manufacturing systems and products of increasing capability and performance.

Here, I’d like to outline an engineering perspective on current molecular machine systems, the new technologies they support, and how these provide a basis for developing a molecular machine systems with extended capabilities.

Atomically precise molecular fabrication today

As you probably know, programmable molecular machines that use mechanosynthesis to make complex, atomically precise products are in use today. These productive nanosystems are applied by industry to make nanoscale products in gram, kilogram, and multi-ton quantities. They are also used to make molecular machine systems, including productive nanosystems of the same kind.

Design methodologies have been developed that exploit these productive nanosystems to make novel products that include diverse, atomically precise, nanometer-scale structures; these consist of peptide foldamers with properties that extend into the range of high-performance engineering polymers. Paths forward from this technology base could enable an attractive scenario, from an industrial point of view, in which incremental advances in productive nanosystem technology enable large-scale manufacturing of improved products.

These products can be more complex than one might guess, because nanometer-scale components can bind to one another in specific geometries to form larger structures. Components with appropriate, solvophilic surface structures can move freely and spontaneously through a liquid medium, driven by Brownian motion, and bind strongly when they encounter complementary surfaces on other nanoscale components. This process of selective, atomically precise binding — sometimes termed “Brownian assembly” — can join components to form larger and more complex systems, including the productive nanosystems in current use.

The ability to make large, complex, atomically precise structures is growing rapidly. Using methods that exploit Brownian assembly of smaller components, made both by productive nanosystems and chemical synthesis, researchers can now routinely design and fabricate complex, three-dimensional, atomically precise structures at a 100-nm, million-atom scale. These can be extended by incorporating components that provide structural interfaces to the products of other, more specialized nanotechnologies (nanolithography, quantum dots, carbon nanotubes…), and this step promises to yield systematic methodologies for designing and fabricating atomically precise composite nanosystems with a wide range of electronic, chemical, and mechanical functions. Researchers working at Caltech and IBM have taken the first steps toward combining nanosystems of this kind with nanoscale circuitry to produce a new class of digital devices.

The productive nanosystems I refer to above are, of course, ribosomes and nucleic acid polymerases, the programmable molecular machines that assemble polypeptide and polynucleotide chains. In making these polymers, productive nanosystems assemble monomers of different kinds in sequences specified by information encoded in the sequences of (other) polynucleotides, and these sequences determine how, for example, a foldamer product will fold and the functions that the resulting component can perform.

(In writing this section, I’ve minimized the use of terms with strong genetic or nutritional associations, which seem, as I’ve discussed, more distracting than useful when discussing molecular engineering.)

Toward new productive nanosystems and expanded capabilities

Advances along these lines can support the development of artificial productive nanosystems that are specialized to produce complex, atomically precise components of new kinds. The most accessible advances in this direction would be devices that expand the range of available foldamers. Clever exploitation of existing productive nanosystems has already expanded the range of products by enabling the use of a wider range of monomeric building blocks; new productive nanosystems could add the ability to build foldamers of wholly new kinds that offer (for example) stiffer backbones and greater chemical and physical stability.

The productive nanosystems in use today can operate only in aqueous environments, and their products are usually (but not always) used under the same conditions. I expect that next-generation productive nanosystems built from components of this sort will also be constrained to operate in aqueous environments. For chemical reasons, the presence of water limits the range of fabrication operations that these devices can perform, but these constraints allow more than one might suppose. Within the scope are not only novel foldamers and highly cross-linked 2D and 3D polymeric nanostructures, but also high-modulus inorganic solids, such as metal oxides and pyrite. Even metals and semiconductors are within the scope of aqueous synthesis.

The ability to make better and more robust components will, of course, enable the fabrication of better and more robust products, including better and more robust productive nanosystems that are not constrained to operate in aqueous environments. And these, of course, will provide means for working with a wider range of materials, enabling the production of components and systems that are even better. The expanded scope of component fabrication can be applied to improve Brownian assembly, but constrained Brownian assembly will become more practical and desirable as fabrication technologies advance.

Note that graphenes, carbon nanotubes, and related structures are locally 2D, and can be regarded as extreme cases of “highly cross-linked polymeric nanostructures”. The useful electronic and mechanical properties of these materials are legendary, and they also work well as low-friction nanoscale moving parts. With the aid of nanoscale arrays of catalytic particles, materials of this class have been synthesized at room temperature.

Beyond ribosome-level complexity

A productive nanosystem can build chains by controlling positions in just one dimension, extending a 1D chain by adding monomers to the end. A mechanism of a similar kind, with essentially 1D control could extend a 2D sheet by stepping along an edge, adding monomers to the end of a row. In some implementations, devices of this sort could be simpler than a ribosome.

To extend a complex structure with a 3D bond network, however, will typically require adding building blocks of specific kinds at specific locations across a 2D surface. This will typically require a mechanism that can step through a series of positions with two degrees of freedom — a step toward greater complexity.

See also:

Studies of advanced atomically precise fabrication:

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