Design considerations for an assembler

by

Ralph C. Merkle
Xerox PARC
3333 Coyote Hill Road
Palo Alto, CA 94304
merkle@xerox.com
www.merkle.com

Copyright 1995 by Xerox Corporation. All Rights Reserved.

This paper was presented at the Fourth Foresight Conference on Molecular Nanotechnology. The final version was published in Nanotechnology 7 (1996) pages 210-215. This web version differ in some respects from the published version.

This paper is available on the web at http://www.zyvex.com/nanotech/nano4/merklePaper.html

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Abstract

Assemblers have been proposed as general purpose manufacturing devices, able to build a wide range of useful products as well as copies of themselves. If such systems are to be built they must first be designed; and before they can be designed in detail we need to know at the system level what major subsystems are needed, what functions they perform, and how they interact with each other. This paper attempts to fill this need and describes the subsystems and components required for a relatively "simple" assembler.

Introduction

It is now widely believed that nanotechnology (see http://www.zyvex.com/nano) is feasible and will likely be developed in the coming decades. However, there is still significant uncertainty about what molecular manufacturing systems might look like and even greater uncertainty about how best to proceed in developing such systems. This uncertainty is not aided by the fact that many different development pathways are likely to work, and that to a significant extent the selection of a specific pathway is arbitrary. Further, as our present vantage point suggests that all pathways will require at least a few major stages, the analysis of the advantages and disadvantages of alternative paths is rendered complex. We try to reduce this complexity by outlining a "simple" molecular manufacturing system.

The direct manufacture of this "simple" system with current technology seems unlikely to be feasible. As a consequence, it will probably be necessary to develop other systems that are easier to synthesize using today's methods, but which are sufficiently powerful that we can use them to synthesize more sophisticated systems. This might be likened to the ascent of a tall mountain in stages, with base camps established at intermediate elevations. The precise nature of these intermediate stages depends on the design of the final stage. The present proposal might be likened to a final base camp, close enough to the peak that it's clear that a final assault from this final base camp would reach the peak, but far enough removed that it's significantly easier to reach the base camp than the peak.

A present guess about what a mature molecular manufacturing system might look like is some form of convergent assembly system (Merkle, 1997a). It should be possible to manufacture such a system given the capabilities of the simple diamondoid assembler described here. We do not propose the construction of a convergent assembly system directly because it would be significantly more complex and would need significantly more resources to complete.

What is nanotechnology?

The term nanotechnology is often used to refer to almost any technology where some characteristic dimension is smaller than a micron. By contrast, the usage in this paper refers specifically to molecular nanotechnology, a.k.a. molecular manufacturing. The basic thesis of this field is that most structures consistent with physical and chemical law can be manufactured inexpensively. This possibility was first mentioned by Feynman in a visionary talk in 1959 (Feynman,1960). More recently, it has been the subject of considerably more detailed analysis in Nanosystems (Drexler, 1992).

Current proposals for molecular manufacturing systems revolve around two central concepts: positional control (Merkle, 1993) and self replication (Merkle, 1992). At the macroscopic scale, the idea of using positional control as part of the manufacturing process is not only quite natural it is hard to imagine a modern factory without it. Parts are moved from place to place by conveyor belts, robotic arms, human arms, and a variety of other contrivances. After being positioned appropriately with respect to each other they are then bonded, glued, soldered, screwed, clamped, hooked or otherwise connected to each other. The resulting assemblage of parts is then moved along and usually connected to other parts.

At the molecular scale, the most common method of assembling parts is some form of self assembly. Molecular parts (in test tubes) are stirred together and allowed to gyrate, bump, jostle and bang into each other under the influence of thermal noise. By exercising great ingenuity, chemists are routinely able to synthesize a remarkably wide range of small molecular structures without the molecular equivalent of hands: the parts spontaneously arrange themselves in the desired pattern. It seems intuitively plausible that adding positional control at the molecular level should enable the synthesis of a much broader range of structures than is now feasible.

Positional control at the molecular scale is, however, a new concept that has yet to be fully accepted. While experimental successes (Eigler and Schweizer, 1990) clearly demonstrate that positioning molecular parts does not violate any fundamental laws of nature, such successes are today limited to specific systems. Perhaps the clearest support for the idea that more general capabilities are feasible comes from ab initio and molecular mechanics modeling of specific reactions that should prove useful in the synthesis of desired structures. See, for example, Theoretical studies of a hydrogen abstraction tool for nanotechnology (Musgrave et al., 1991) or Surface patterning by atomically-controlled chemical forces: molecular dynamics simulations (Sinnott et al., 1994), as well as the discussion of mechanosynthesis in Nanosystems (Drexler, 1992).

We shall assume, for the remainder of this article, that molecular positional control is an essential prerequisite for molecular manufacturing.

Self replication

The second central concept on which current proposals for molecular manufacturing are based is that of self replication. The utility of this is fairly obvious: it lets us make things inexpensively. While the addition of positional control to our manufacturing methods should let us build a much wider range of structures than could otherwise be considered, the manufacture of a few molecular sized components will not, by itself, greatly alter the economics of the world's manufacturing base. If we are to build macroscopic structures which are molecular in precision using positional control to position and assemble molecular size components, we will need mole quantities of positional devices. This has two implications: the devices themselves will have to be small, and their manufacture will have to be completely automated (human intervention in the manufacture of each of a mole of devices seems implausibly labor intensive given the present population).

A conceptually simple way of meeting these requirements is to embody the needed capabilities in a self replicating device called an assembler, which has one or more positional devices controlled by a small general purpose computer. As the assembler can make copies of itself (the author does work at Xerox) a single assembler can manufacture a second, those two can make two more, those four can make four more, etc. Exponential growth allows the rapid manufacture of as many assemblers as might be desired, while the fact that they are under computer control permits them to be reprogrammed to build other structures that are deemed useful (molecular computers, for example).

Although we have elevated the concept of self replication to the status of a central principle, it would be incorrect to assume that self replication, in and of itself, is valuable. A device able to make copies of itself but which was unable to make anything else (and was otherwise of no particular value) would be of no great value. The value of the assembler rests fundamentally on its ability to make a great many different things under programmatic control. Self replication is used as a method of achieving economy in the manufacturing process, not as an end in itself.

Design objectives

As there are a great many possible assemblers, and as their characterisics vary considerably, it is necessary to provide some context for the particular design approach selected here.

First, we seek a design which fairly obviously will let us build a reasonably broad range of useful products, including more sophisticated assemblers. We do not demand that the proposed design be able to directly manufacture the full range of products which might be desirable, but will instead restrict ourselves to the class of "diamondoid" structures, defined in Nanosystems (Drexler, 1992) as including structures made from hydrogen; first row elements such as boron, carbon, nitrogen, oxygen and fluorine; and perhaps some second row elements such as silicon, phosphorous, sulfur and chlorine. Metals and other elements will generally (though not always) be excluded from consideration. We will frequently confine ourselves to hydrogen and carbon, as hydrocarbon structures are relatively easy to analyze and can often provide remarkable materials properties (e.g., diamond, graphite, and related structures). Potential energy functions which provide a good description of the behavior of hydrocarbons are available. Brenner's potential (Brenner, 1990) is an example. It is able to model not only relatively stable structures, but also transition states and less stable structures. The utility of this potential for modeling a wide variety of hydrocarbon-based molecular machines and synthetic reactions has been noted by Brenner (Brenner et al., 1996). One example available on the web is the work of Robertson (Robertson et al., 1994), who did molecular dynamics simulations of molecular gears (available at http://chem.iupui.edu/Chem/Research/Robertson/Roberston.html).

The restriction to diamodoid structures has two major advantages. First, these structures have some of the most desirable materials properties known. They involve strong covalent bonds, and hence include the strongest and stiffest materials. Their other materials properties -- chemical, thermal, electrical, optical etc. -- are also remarkable. Second, their behavior (particularly when we confine ourselves to relatively simple structures) is easier to understand and analyze.

We will further restrict ourselves to the class of "small" diamondoid structures, e.g., structures which are about the same size as the proposed assembler.

We choose the broadcast architecture. In this architecture, the assembler does not have an on-board computer, but is instead directed by simple broadcast instructions. This architecture is similar to the "SIMD" architecture used in some massively parallel computers. A central computational element (macroscopic) broadcasts simple instructions to a large number of assemblers. The assemblers are barely able to decode and execute these simple instructions, thus reducing their complexity and size.

Several things must be described if we are to specify an assembler, as discussed in (Merkle, 1992). For this proposal, we will use:

Conclusions

We have briefly outlined an architecture for a self-replicating assembler able to synthesize the class of small diamondoid structures. While there is no obvious synthetic route to manufacturing this system with today's technology, it is significantly simpler than previous proposals. Further work aimed at defining more accessible proposals appears worthwhile. A series of proposals ranging from simple low performance systems that might be directly synthesized with today's technology to high performance systems able to make a wide range of diamondoid structures is needed, along with a clear description of how each proposed system would manufacture systems in the next stage of complexity and performance.

References



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