Toward Advanced Nanotechnology: Nanomaterials (5)
My previous post in this series, Nanostructures, Nanomaterials, and Lattice-Scaled Stiffness, explains why the lattice-scaled modulus, Klm, is an important figure of merit: For a set of machines made of different materials, but with similar structures (similar numbers and arrangements of lattice cells), the Klm parameter determines the energy required for a thermal fluctuation to cause an error when the critical distances are scaled by the lattice size. Error rates decline exponentially with increasing energy, and hence with increasing Klm.
Lattice-based scaling is appropriate both for internal machine motions (parts slipping, etc.) and for operations performed on external structures made of the same material — for example, when using a machine to direct the fabrication of machine parts. One might think that stiffer materials are always better, but this is mistaken: The size of the lattice cells matters, and more than one might expect. To review,
Klm = Ea3r2err,
where E = Young’s modulus, a = the lattice parameter (this is a simplification, since not all crystals have cubic symmetry), and r2err accounts for the ratio of the minimum error displacement to the lattice parameter; rerr = (1/2)1/2 is a common value.
The following graphic compares several materials by Young’s modulus and by the lattice-scaled modulus:
All the materials shown above are found in nature (ignoring the thorium in cerianite, which is mostly cerium dioxide). All but one, diamond, can be synthesized in water, at atmospheric pressure, near room temperature. Pyrite (“fool’s gold”) is often a product of biomineralization, and bacteria can synthesize magnetite as nanocrystals of controlled size and shape. Polymeric blocks with mechanical properties comparable to keratin could consist of any of a wide range of engineered proteins or other foldamers, and as can be seen, the value of Klm shifts from worst to best with a factor of 3 increase in block size.
As we explore implementation pathways that lead toward advanced nanotechologies, it’s important to keep in mind that conditions for forming pyrite (and a range of other hard, inorganic materials) are compatible with soft-material technologies; these include macromolecular templates, crystal-growth promoters and inhibitors, and surface-binding molecules with diverse functions. Continued progress in engineering interfaces between macromolecules and inorganic crystals will be of critical importance.
Are soft and hard machines at odds with each other? Surely not. Soft biomolecules and hard inorganic solids have worked together since a bacterium first succeeded in gluing itself to a mineral grain, and perhaps long before, at the origin of life itself. There is no gap between soft and hard nanomachines: The technologies form a continuum, and working together, they can form a bridge.
Updated 25 Aug 2009: Corrected graph label and some ambiguous text.