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1.2.1. Example: a nanomechanical bearing

As discussed in Section 1.3, the mechanical branch of molecular nanotechnology forms a field related to, yet distinct from, mechanical engineering, microtechnology, chemistry, and molecular biology. An example may serve as a better introduction than would an attempt at a written definition.

Figure 1.1 shows several views of one design for a nanomechanical bearing like those discussed in greater depth in Chapter 10. ( Figure 1.2 describes some conventions used in illustrations like Figure 1.1) In a functional context, many of the bonds shown as hydrogen terminated would instead link to other moving parts or to a structural matrix. Several characteristics are worthy of note:

  • The components are °polycyclic, more nearly resembling the fused-ring structures of diamond than the open-chain structures of biomolecules such as °proteins.

  • Accordingly, each component is relatively stiff, lacking the numerous opportunities for internal rotation about bonds that make °conformational analysis difficult in typical biomolecules.

  • Repulsive, nonbonded interactions strongly resist both rotations of the shaft away from its axial alignment with the ring, and displacement along that axis or perpendicular to it.

  • Rotation of the shaft about its axis within the ring encounters negligible energy barriers, indicating a nearly complete absence of static friction.

  • The combination of °stiffness in five degrees of freedom with facile rotation in the sixth makes the structure act as a good bearing, in the conventional mechanical engineering sense of the term.

  • The absence of significant static friction in a system that places bumpy surfaces in firm contact with no intervening lubricant is surprising by conventional mechanical engineering standards.

  • No solvent is illustrated, and there is no reason to think that the bearing structure shown would in fact be soluble.

  • Neither of the components of the bearing is a plausible target for synthesis using reagents diffusing in solution; barring unprecedented chemical cleverness, their construction requires mechanosynthetic control.

How typical are these characteristics? Stiff, polycyclic structures are ubiquitous in the designs presented in Part II. Many components are designed to combine stiff constraints in some degrees of freedom with nearly free motion in others, thereby fulfilling roles familiar in mechanical engineering; nonetheless, a detailed understanding of how those roles are fulfilled requires analyses based on uniquely molecular phenomena. The operating environment assumed for the nanomechanical and mechanosynthetic systems discussed in Part II is high vacuum, rather than a solvent. Finally, the designs in Part II (unlike those described in Part III) are consistently of a scale and complexity that precludes synthesis using present techniques.

Figure 1.1.End views and exploded views of a sample overlap-repulsion bearing design (shown in both ball-and-stick and space-filling representations). Geometries represent energy minima determined by the MM2/CSC molecular mechanics software. Note the six-fold symmetry of the shaft and fourteen-fold symmetry of the surrounding ring; with a least common multiple of 42, this combination yields low energy barriers to rotation of the shaft within the ring. Bearing structures are discussed further in Chapter 10. (MM2/CSC denotes the Chem3D Plus implementation of the MM2 molecular mechanics force field. The MM2 model is discussed in Section 3.3.2; Chem3D Plus is a product of Cambridge Scientific Computing, Cambridge, Massachusetts.

Figure 1.2. Conventions for atom representation using shading and relative sizes. The “H (fixed)” atom represents a hydrogen atom held at fixed spatial coordinates; these are used to model some mechanical constraints applied by a larger structure (e.g., in stiffness calculations). All radii are set equal to the values for 0.1nN compressive contacts given by Eq.(3.20).

The bearing shown in Figure 1.1 suggests the nature of other systems described in Part II. For example, the combination of a bearing and shaft suggests the possibility of extended systems of power-driven machinery. The outer surface of the bearing suggests the possibility of a molecular-scale gear. The controlled rotary motion of the shaft within the ring, together with the concept of extended systems of machinery, suggests the possibility of controlled molecular transport and positioning, which is necessary for advanced mechanosynthesis.

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