Chapter 1

1.3. Comparisons

Molecular nanotechnology resembles and overlaps with other fields, yet differs substantially. A discussion of resemblances can illustrate the applicability of existing knowledge and emerging developments; a discussion of differences can warn of false analogies and consequent misunderstandings.

Table 1.3 compares several existing production processes — conventional fabrication, microfabrication, solution-phase chemistry, and biochemistry — with molecular manufacturing. The following sections provide a more detailed comparison of molecular manufacturing and molecular nanotechnology with these other processes and their products. Appendix B focuses on areas of these fields having special relevance molecular manufacturing; the present section makes broad comparisons to the mainstream.

1.3.1. Conventional fabrication and mechanical engineering

a. Similarities: components, systems, controlled motion, manufacturing. Many mechanical engineering concepts apply directly to nanomechanical systems. As shown in Chapters 3 to 6, methods based on classical mechanics suffice for much of the required analysis. As shown in Chapters 9 to 11, beams, shafts, bearings, gears, motors, and the like can all be constructed on a nanometer scale to serve familiar mechanical functions. As a consequence, macromechanical engineering and nanomechanical engineering share many design issues and analytical techniques.

Both molecular and conventional manufacturing systems use machines to perform planned patterns of motion: they shape, move, and join components to build complex three-dimensional structures. Systems of both kinds can manufacture machines, including machines used for manufacturing (Chapter 14).

**Table 1.3**. Typical characteristics of conventional machining, micromachining, solution-phase chemistry, biochemistry, and molecular manufacturing.^a
Characteristic	Conventional fabrication	Micro- fabrication	Solution chemistry	Bio- chemistry	Molecular manufacturing
^aProduct scale, defect rates, and cycle times vary widely from process to process within most of these families; feature scale varies widely within the first two. ^bThe defect rate listed for biochemistry corresponds to high-reliability DNA replication processes that include kinetic proofreading (Watson et al., 1987); most biochemical defect rates are higher. All rates are on a per-component basis.
Molecular precision	no	no	yes	yes	yes
Positional control	yes	yes	no	partial	yes
Typical feature scale	1 mm	1µ	0.3 nm	0.3 nm	0.3 nm
Typical product scale	1 m	10 mm	1 nm	10 nm	100 nm+
Typical defect rate^b	10^–4	10^–7	10^–2	10^–11	10^–15
Typical cycle times	1 s	100 s	1000 s	10^–3s	10^–6 s
Products described by	materials and shapes	materials and shapes	atoms and bonds	monomer sequences	atoms and bonds

b. Differences: scale, molecular phenomena. Despite these similarities, nanomechanical engineering forms a distinct field. The familiar model of objects as made of homogeneous materials, though still useful (Chapter 9), often must be replaced by models that treat objects as sets of bonded atoms ( Chapters 2 and 3). Thermally excited vibrations are of major importance, and quantum effects are sometimes significant (Chapters 5, 6, and 7). Further, nanomachines suffer from molecular damage mechanisms (Chapter 6); molecular phenomena permit (and demand) novel bearings (Chapter 10); scaling laws favor electrostatic over electromagnetic motors (Chapters 2 and 11); and the basic operations of manufacturing on a molecular scale are chemical transformations (Chapters 8 and 13). These transformations typically move the system between discrete states, leading to structures that are either exactly right or clearly wrong; this resembles digital logic more closely than it does metalworking.

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