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41544_Ward's World+MGH Nanotechnology

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2 even in nanosystems of microscopic scale. For example, a cubic micrometer of a typical material contains roughly 1011 atoms; with generalized atomic control on that scale, a cubic microm- eter could contain roughly 109 distinct functional components. Systems based on mechanical (rather than electronic) degrees of freedom are particularly tractable. These are of special interest, because programmable nanoscale mechanical systems could be used to produce atomically precise structures of arbitrary complexity. The development of productive nano- systems is a key strategic objective. Productive nanosystems Nature demonstrates a class of productive nanosystems based on polymeric components operating in a liquid medium. Ribosomes, for example, work as digitally controlled machine tools that read genetic information (six bits per codon) and use it to direct the assembly of sequences of amino acids. The resulting polymers (proteins) fold to make nanometer-scale objects with precise arrangements of atoms. Together with nucleic acids (made by other programmable machines), these molecular objects form the working parts of a wide range of bi- ological molecular machines, including ribosomes themselves. These larger structures are made through a self-assembly process in which Brownian motion brings components to- gether and complementary molecular surfaces cause selected components to bind in a precise manner. Biological systems provide not only examples of productive nanosystems but also tools to use and models for the develop- ment of next-generation systems. Developments in the design of novel protein and nucleic acid structures, including simple molecular machines, can be combined with a broad spectrum of current nanotechnologies in the construction of early-gener- ation productive nanosystems. Self-assembly can join engi- neered biomolecular objects with atomically precise nanopar- ticles, fibers, and other products to make composite structures with properties beyond those found in biological systems. As this path moves closer to fundamental physical limits, one direction of advance is toward better materials. Biomolecules have large monomers, low bond density, and low stiffness; ad- vanced materials with higher bond densities can be stronger, stiffer, more fine-grained, and more regular. As with macro- scopic machinery, the use of superior materials will enable better performance. Although precisely structured devices with high bond densi- ties cannot be made yet, they are amenable to computational design and modeling. Among the devices that have been ana- lyzed are gears, shafts, bearings, belts, electric motors, comput- ers, and programmable positioning mechanisms. These could be used to transform matter, energy, and information. Their components resemble those of conventional machine systems. For example, a design for a gearbox for transforming shaft power from higher to lower angular speeds would consist of a rigid framework supporting input and output shafts mounted on bearings. The gears would have teeth and obey the usual rules regarding gear ratios; however, in detail, the differences would be substantial. Each gear tooth typically would consist of a single row of atoms, with the smoothness of interatomic force fields enabling the gears and bearings to operate without an added lubricant (Fig. 1). Systems containing enough parts to serve the function of a programmable molecular assembly mechanism or a computer require several million to several billion atoms, and their volumes occupy a significant fraction of a cubic micrometer. Basic principles Nanotechnology based on productive nanosystems requires a combination of familiar molecular and mechanical prin- ciples in unfamiliar applications. In solution-phase reactions, molecules move by diffusion and encounter each other in all possible positions and orientations. The resulting molecular transformations are accordingly difficult to direct. Productive nanosystems, in contrast, could exploit mechanosynthesis, the use of mechanical devices to guide the motions of reac- tive molecules. By applying positional control to conventional molecular reactions, mechanosynthesis could cause structural changes to occur at precise locations in a precise sequence. Nanotechnology (continued) + ward ' s science Fig. 1: Simulation of a carbon nanotube–based gear. (Credit: NASA Ames Research Center)

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