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Preface

Manufactured products are made from atoms, and their properties depend on how those atoms are arranged. This volume summarizes 15 years of research in molecular manufacturing, the use of nanoscale mechanical systems to guide the placement of reactive molecules, building complex structures with atom-byatom control. This degree of control is a natural goal for technology: Microtechnology strives to build smaller devices; materials science strives to make more useful solids; chemistry strives to synthesize more complex molecules; manufacturing strives to make better products. Each of these fields requires precise, molecular control of complex structures to reach its natural limit, a goal that has been termed molecular nanotechnology.

It has become clear that this degree of control can be achieved. The present volume assembles the conceptual and analytical tools needed to understand molecular machinery and manufacturing, presents an analysis of their core capabilities, and explores how present laboratory techniques can be extended, stage by stage, to implement molecular manufacturing systems. It says little about applications other than computation (describing 10910^{9}-instruction-per-second submicron scale CPUs executing 1016\sim 10^{16} instructions per second per watt) and manufacturing (describing desktop devices able to produce precisely structured, kilogram-scale products from simple chemical feedstocks). Surveys of broader implications appear elsewhere (Drexler, 1986a; 1989c; Drexler et al., 1991).

The intended readership

Molecular manufacturing is linked to many areas of science and technology. In writing this volume, I have been guided by an imaginary committee of readers with differing demands.

One is a reader with a general science background, interested in the basic principles, capabilities, and nature of molecular nanotechnology, but not in the mathematical derivations. Accordingly, I have attempted to summarize the chief results in descriptions, diagrams, and example calculations, and have included comparisons of this field to others that are more familiar. Such a reader can skip many sections without becoming lost.

Another is a student considering a career in the field. This reader demands an introduction to the foundations of molecular nanotechnology presented in terms of the basic physics, calculus, and chemistry taught to students in other fields. Accordingly, I have grounded most derivations in basic principles, developing intermediate results as needed.

The rest of the committee includes a physicist, a chemist, a molecular biologist, a materials scientist, a mechanical engineer, and a computer scientist. Each has deep professional knowledge of a particular field. Each demands answers to special questions that presuppose specialized knowledge. Each knows the exceptions that hide behind most generalizations, and the approximations that hide behind most textbook formulas. Accordingly, the discussion sometimes dives into a topic that readers outside the relevant discipline may find opaque. Skipping past these topics will seldom impair comprehension of what follows.

Each of these specialists also represents a community of researchers able to advance the development of molecular nanotechnology. Accordingly, many of the discussions implicitly or explicitly highlight open problems, inviting work in theoretical analysis, in computer-aided design and modeling, and in laboratory experimentation. I hope that this volume will be seen both as a guide and as an invitation to a promising new field.

The nature of the subject

Our ability to model molecular machines-of specific kinds, designed in part for ease of modeling - has far outrun our ability to make them. Design calculations and computational experiments enable the theoretical study of these devices, independent of the technologies needed to implement them. Work in this field is thus (for now) a branch of theoretical applied science (Appendix A).

Molecular manufacturing applies the principles of mechanical engineering to chemistry (or should one say the principles of chemistry to mechanical engineering?) and uses results drawn from materials science, computer science, and elsewhere. But interdisciplinary studies can foster misunderstandings. From every disciplinary perspective, a superficial glance suggests that something is wrongapplying chemical principles leads to odd-looking machines, applying mechanical principles leads to odd-looking chemistry, and so forth. The following chapters offer a deeper view of how these principles interact.

Criticism of criticism

Research in molecular nanotechnology requires a design perspective because it aims to describe workable systems. It is easy to describe unworkable systems, and criticisms of a critic's own bad design have on occasion been presented as if they were criticisms of molecular nanotechnology as a whole. Some examples: assuming the use of flexible molecules, then warning that they will have no stable shape; assuming the manipulation of unbound reactive atoms, then warning that they will react and bond to the manipulator; assuming the use of materials with unstable surfaces, then warning that the surfaces will change; assuming that reactive gases permeate nanosystems, then warning that reactions will occur; assuming that nanomachines must "see," then warning that light waves are too long and xx-rays too energetic; assuming that nanomachines swim from point to point, then warning that Brownian motion makes such navigation impossible; assuming that nanomachines dissipate enormous power in a small volume, then warning of overheating; and so on, and so forth. These observations constitute not criticisms, but rediscoveries of elementary engineering constraints.

Use of tenses

In ordinary discourse, "will be" suggests a prediction, while "would be" suggests a conditional prediction. Using these future-tense expressions is inappropriate when discussing the time-independent possibilities inherent in physical law.

In speaking of spacecraft trajectories to Pluto, for example, to say that they "will be" is to predict the future of spaceflight; to say that they "would be" is to remind readers of the uncertainties of budgets and life. Both phrases distract from the analysis of celestial mechanics and engineering trade-offs. The present tense is more serviceable: One can say that as-yet unrealized spacecraft trajectories to Pluto "are of two kinds, direct and gravity assisted," and then analyze their properties without distraction. Similarly, one can say that as-yet unrealized nanomachines of diamondoid structure "are typically stiffer and more stable than folded proteins." Much of the discussion in this volume is cast in this timeless present tense; this is not intended to imply that devices like those discussed in Parts I and II presently exist.

Citations and apologies

It is much easier to grasp and apply the main results of a field than it is to provide a balanced guide to the recent work, omitting no useful citations. I am sure that my discussions of chemistry and protein engineering, for example, omit papers fully as valuable as the best included. I apologize to authors I have slighted.

Less forgivable are those instances (which I cannot yet identify) in which I may have rederived some result that should be attributed to an earlier author, perhaps well known in some specialty. In interdisciplinary research, one cannot spend a professional lifetime immersed in a single literature, and such failures of attribution become likely-mathematics often yields results more easily than does a library. Any such lapses brought to my attention will be corrected in future editions; their most likely locations are Chapters 5, 6, and 7.

Aside from these lapses, material presented without citation falls into two categories that I trust are distinct. First, well-known principles and results from established fields-physics, statistical mechanics, chemistry-are used without citing Newton, Boltzmann, Pauling, or their kin. Second, the designs, concepts, and analytical results that are both specific to nanomechanical systems and not attributed to someone else are to the best of my knowledge original contributions, many presented for the first time in this volume.

It also seems necessary to apologize for doing theoretical work in a world where experimental gains are often so hard-won. If this theoretician's description of possibilities seems to make light of experimental difficulties, I can only plead that it would soon become tedious to say, at every turn, that laboratory work is difficult, and that the hard work is yet to be done.

Acknowledgments

The research behind this volume began in 1977 , stimulated by the growing literature on biological molecular machines. Basic results appeared in a refereed paper (Drexler, 1981). The present work began as notes for a course taught at Stanford in 1988 at the invitation of Nils Nilsson; early versions of some chapters did service as a doctoral thesis at MIT in 1991. During this long gestation, many people have contributed through discussion, criticism, and detailed review.

I thank the participants of the monthly series of nanotechnology seminars (some centered on draft chapters of this volume) organized by Ralph Merkle at the Xerox Palo Alto Research Center for wide-ranging discussion and criticism. These have included Lakshmikantan Balasubramaniam, David Biegelsen, Ross Bringans, David Fork, Babur Hadimioglu, Stig Hagstrom, Conyers Herring, Tad Hogg, Warren Jackson, Noble Johnson, Martin Lim, Jim Mikkelsen, John Northrup, K. V. Ravi, Paolo Santos, Mathias Schnabel, Bob Street, Lars-Erik Swartz, Eugen Tarnower, Dean Taylor, Rob Tow, and Chris Van der Walle.

For reviews, suggestions, and specific pieces of help, I thank Jeff Bottaro, Randall Burns, Jamie Dinkelacker, Greg Fahy, Jonathan Goodman, Josh Hall, Robin Hanson, Norm Hardy, Ted Kaehler, Markus Krummenacker, Arel Lucas, Tim May, John McCarthy, Mark Samuel Miller, Chip Morningstar, Russell Parker, Marc Stiegler, Eric Dean Tribble, John Walker, and Leonard Zubkoff.

For discussion and suggestions that helped in preparing a 1989 draft paper that became Section 15.3, I thank Joe Bonaventura, Jeff Bottaro, William DeGrado, Bruce Erickson, Barbara Imperiali, Jim Lewis, Danute Nitecki, Chris Peterson, Fredric Richards, Jane Richardson, and Kevin Ulmer. For similar help in developing ideas in Section 15.4, I thank Tom Albrecht, John Foster, Paul Hansma, Jan Hoh, Ted Kaehler, Ralph Merkle, Klaus Mosbach, and Craig Prater. For sponsoring the initial 1981 publication, I thank Arthur Kantrowitz.

For remarkable efforts while this work was on its way to fulfilling the thesis requirement of an interdepartmental doctoral program hosted by the Media Arts and Sciences Section at MIT, I thank the committee's chair, Marvin Minsky (Department of Electrical Engineering and Computer Science; Media Arts and Sciences Section), as well as committee members Alexander Rich (Department of Biology), Gerald Sussman (Department of Electrical Engineering and Computer Science), Rick Danheiser (Department of Chemistry), and Steven Kim (Department of Mechanical Engineering), with special thanks to Steve Benton and Nicholas Negroponte (Media Arts and Sciences Section) for making the program possible in an environment hospitable to new research directions.

Ralph Merkle has helped greatly by providing steady encouragement and extensive opportunities for discussion during the writing of this volume, by reviewing it (and helping to obtain other reviews), and by collaborating on several of the design studies described. Special thanks also go to Jeffrey Soreff, whose checking of mathematical results and physical reasoning has been just barely incomplete enough for him to escape blame for the remaining errors: with his other help, this places his contribution in a class by itself. Barry Silverstein, John Walker, and the Institute for Molecular Manufacturing each provided essential support for a major portion of this work. Diane Cerra and Bob Ipsen of John Wiley & Sons made publication a pleasure. Chris Peterson, my spouse and partner, provided essential support of kinds too numerous to list.

At the other pole of involvement, the most general thanks go to members of the hundred or more audiences at universities and industrial laboratories in the U.S., Europe, and Japan who have listened to presentations of these ideas and aided in their development by intelligent questioning. I hope that this volume provides many of the answers they sought.