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What Is Molecular Manufacturing?

 

 

The term “molecular manufacturing” has been associated with all sorts of futuristic stuff, from bloodstream robots to gray goo to tabletop factories that can make a new factory in a few hours. This can make it hard for people who want to understand the field to know exactly what's being claimed and studied. This essay explains what the term originally meant, why the approach is thought to be powerful enough to create a field around, why so many futuristic ideas are associated with it, and why some of those ideas are more plausible than they may seem.

Original Definition

Eric Drexler defined the term “molecular manufacturing” in his 1992 technical work Nanosystems. His definition used some other terms that need to be considered first.

Mechanochemistry In this volume, the chemistry of processes in which mechanical systems operating with atomic-scale precision either guide, drive, or are driven by chemical transformations.

In other words, mechanochemistry is the direct, mechanical control of molecular structure formation and manipulationto form atomically precise products. (It can also mean the use of reactions to directly drive mechanical systems—a process that can be nearly 100% efficient, since the energy is never thermalized.) Mechanochemistry has already been
demonstrated: [Oyabu] has used atomic force microscopes, acting purely mechanically, to remove single silicon atoms from a covalent lattice and put them back in the same spot.
http://focus.aps.org/story/v11/st19

Mechanosynthesis Chemical synthesis controlled by mechanical systems operating with atomic-scale precision, enabling direct positional selection of reaction sites; synthetic applications of mechanochemistry.
Suitable mechanical systems include AFM mechanisms, molecular manipulators, and molecular mill systems.

In other words, mechanosynthesis is the use of mechanically guided molecular reactions to build stuff. This does not require that every reaction be directly controlled. Molecular building blocks might be produced by ordinary chemistry; products might be strengthened after manufacture by crosslinking; molecular manufactured components might be joined into products by self-assembly; and building blocks similar to those used in self-assembly might be guided into chosen locations and away from alternate possibilities. Drexler’s definition continues:

Processes that fall outside the intended scope of this definition include reactions guided by the incorporation of reactive moieties into a shared covalent framework (i.e., conventional intramolecular reactions), or by the binding of reagents to enzymes or enzymelike catalysts.

The point of this is to exclude chemistry that happens by pure self-assembly and cannot be controlled from outside. As we will see, external control of the reactions is the key to successful molecular manufacturing. It is also the main thing that distinguishes molecular manufacturing from other kinds of nanotechnology.

The principle of mechanosynthesis—direct positional control—can be useful with or without covalent bonding. Building blocks like those used in self assembly, held together by hydrogen bonding or other non-covalent interactions, could also be joined under mechanical control. This would give direct control of the patterns formed by assembly, rather than requiring that the building blocks themselves encode the final structure and implement the assembly process.

Molecular manufacturing The production of complex structures via
nonbiological mechanosynthesis (and subsequent assembly operations).

There is some wiggle room here, because “complex structures” is not defined. Joining two molecules to make one probably doesn't count. But joining selected monomers to make a polymer chain that folds into a predetermined shape probably does.

Machine-phase chemistry The chemistry of systems in which all potentially reactive moieties follow controlled trajectories (e.g., guided by molecular machines working in vacuum).

This definition reinforces the point that machine-phase chemistry is a narrow subset of mechanochemistry. Mechanochemistry does not require that all molecules be controlled; it only requires that reactions between the molecules must be controlled. Mechanochemistry is quite compatible with “wet” chemistry, as long as the reactants are chosen so that they will only react in the desired locations. A ribosome appears to fit the requirement; Drexler specified that molecular manufacturing be done by nonbiological mechanosynthesis, because otherwise biology would be covered by the definition.

Although it has not been well explored, machine-phase chemistry has some theoretical advantages that make it worth further study. But molecular manufacturing does not depend on a workable machine-phase chemistry being developed. Controversies about whether diamond can be built in vacuum do not need to be settled in order to assess the usefulness of molecular manufacturing.

Extending molecular manufacturing

As explained in the first section, the core of molecular manufacturing is the mechanical control of reactions so as to build complex structures. This simple idea opens up a lot of possibilities at the nanoscale. Perhaps the three most important capabilities are engineering, blueprint delivery, and the manufacture of manufacturing tools. These capabilities reinforce each other, each facilitating the others.

It is often thought that the nanoscale is intractably complex, impossible to analyze. Nearly intractable complexity certainly can be found at the nanoscale, for example in the prediction of protein folding. But not everything at the nanoscale is complex. DNA folding, for example, is much simpler, and the engineering of folded structures is now pretty straightforward. Crystals and self-assembled monolayers also have simple aspects: they are more or less identical at a wide range of positions. The mechanical properties of nanoscale structures change as they get extremely small, but even single-nanometer covalent solids (diamond, alumina, etc) can be said to have a well-defined shape.

The ability to carry out predictable synthesis reactions at chosen sites or in chosen sequences should allow the construction of structures that are intricate and functional, but not intractably complex. This kind of approach is a good fit for engineering. If a structure is the wrong shape or stiffness, simply changing the sequence of reactions used to build it will change its structure—and at least some of its properties—in a predictable way.

It is not always easy to control things at the nanoscale. Most of our tools are orders of magnitude larger, and more or less clumsy; it's like trying to handle toothpicks with telephone poles. Despite this, a few techniques and approaches have been developed that can handle individual molecules and atoms, and move larger objects by fractions of nanometers.
A separate approach is to handle huge numbers of molecules at once, and set up the conditions just right so that they all do the same thing, something predictable and useful. Chemistry is an example of this; the formation of self-assembled monolayers is another example. The trouble with all of these approaches is that they are limited in the amount of information that can be delivered to the nanoscale. After a technique is used to produce an intermediate product, a new technique must be applied to perform the next step. Each of these steps is hard to develop. They also tend to be slow to use, for two reasons: big tools move slowly, and switching between techniques and tools can take a lot of time.

Molecular manufacturing has a big advantage over other nanoscale construction techniques: it can usefully apply the same step over and over again. This is because each step takes place at a selected location and with selected building blocks. Moving to a different location, or selecting a different building block from a predefined set, need not insert enough variation into the process to count as a new step that must be developed and characterized separately.

A set of molecular manufacturing operations, once worked out, could be recombined like letters of an alphabet to make a wide variety of predictable products. (This benefit is enhanced because mechanically guided chemistry can play useful games with reaction barriers to speed up reactions by many orders of magnitude; this allows a wider range of reactants to be used, and can reduce the probability of unwanted side
reactions.) The use of computer-controlled tools and computer-aided translation from structure to operation sequence should allow blueprints to be delivered directly to the nanoscale.

Although it is not part of the original definition of molecular manufacturing, the ability to build a class of product structures that includes the manufacturing tools used to build them may be very useful.
If the tools can be engineered by the same skill set that produces useful products, then research and development may be accelerated. If new versions of tools can be constructed and put into service within the nanoscale workspace, that may be more efficient than building new macro-scale tools each time a new design is to be tested. Finally, if a set of tools can be used to build a second equivalent set of tools, then scaleup becomes possible.

The idea of a tool that can build an improved copy of itself may seem
counterintuitive: how can something build something else that's more complex than itself? But the inputs to the process include not just the structure of the first tool, but the information used to control it.
Because of the sequential, repetitive nature of molecular manufacturing, the amount of information that can be fed to the process is essentially unlimited. A tool of finite complexity, controlled from the outside, can build things far more physically complex than itself; the complexity is limited by the quality of the design. If engineering can be applied, then the design can be quite complex indeed; computer chips are being designed with a billion transistors.

From the mechanical engineering side, the idea of tools building tools may be suspect because it seems like precision will be lost at each step. However, the use of covalent chemistry restores precision.
Covalent reactions are inherently digital: in general, either a bond is formed which holds the atoms together, or the bond is missing and the atoms repel each other. This means that as long as the molecules can be manipulated with enough precision to form bonds in the desired places, the product will be exactly as it was designed, with no loss of precision whatsoever. The precision required to form bonds reliably is a significant engineering requirement that will require careful design of tools, but is far from being a showstopper.

Scaleup

The main limitation of molecular manufacturing is that molecules are so small. Controlling one reaction at a time with a single tool will produce astonishingly small masses of product. At first sight, it may appear that there is no way to build anything useful with this approach.
However, there is a way around this problem, and it’s the same way used by ribosomes to build an elephant: use a lot of them in parallel. Of course, this requires that the tools must be very small, and it must be possible to build a lot of them and then control them all. Engineering, direct blueprint injection, and the use of molecular manufacturing tools to build more tools can be combined to achieve this.

The key question is: How rapidly can a molecular manufacturing tool create its own mass of product? This value, which I'll call “relative productivity,” depends on the mass of the tool; roughly speaking, its mass will be about the cube of its size. For each factor of ten shrinkage, the mass of the tool will decrease by 1,000. In addition, small things move faster than large things, and the relationship is roughly linear. This means that each factor of ten shrinkage of the tool will increase its relative productivity by 10,000 times; relative productivity increases as the inverse fourth power of the size.

A typical scanning probe microscope might weigh two kilograms, have a size of about 10 cm, and carry out ten automated operations per second.
If each operation deposits one carbon atom, which masses about 2x10^-26 kg, then it would take 10^26 seconds or six billion billion years for that scanning probe microscope to fabricate its own mass. But if the tool could be shrunk by a factor of a million, to 100 nm, then its relative throughput would increase by 10^24, and it would take only 100 seconds to fabricate its own mass. This assumes an operation speed of 10 million per second, which is about ten times faster than the fastest known enzymes (carbonic anhydrase and superoxide dismutase). But a relative productivity of 1,000 or even 10,000 seconds would be sufficient for a very worthwhile manufacturing technology. (An inkjet printer takes about 10,000 seconds to print its weight in ink.) Also, there is no requirement that a fabrication operation deposit only one atom at a time; a variety of molecular fragments may be suitable.

To produce a gram of product will take on the order of a gram of nanoscale tools. This means that huge numbers of the tools must be controlled in parallel: information and power must be fed to each one.
There are several possible ways to do this, including light and pressure. If the tools can be fastened to a framework, it may be easier to control them, especially if they can build the framework and include nanoscale structures in it. This is the basic concept of a nanofactory.

Nanofactories and their products

A nanofactory is (will be) an integrated manufacturing system containing large numbers of nanoscale molecular manufacturing workstations (tool systems). This appears to be the most efficient and engineerable way to make nanoscale productive systems produce large products. With the workstations fastened down in known positions, their nanoscale products can more easily be joined. Also, power and control signals can be delivered through hardwired connections.

The only way to build a nanofactory is with another nanofactory.
However, the product of a nanofactory may be larger than itself; it does not appear conceptually or practically difficult to build a small nanofactory with a single molecular manufacturing tool, and build from there to a kilogram-scale nanofactory. The architecture of a nanofactory must take several problems into account, in addition to the design of the individual fabrication workstations. The mass and organization of the mounting structure must be included in the construction plans. A small fraction (but large number) of the nanoscale equipment in the nanofactory will be damaged by background radiation, and the control algorithms will have to compensate for this in making functional products. To make heterogeneous products, the workstations and/or the nanoproduct assembly apparatus must be individually controlled; this probably requires control logic to be integrated into the nanofactory.

It may seem premature to be thinking about nanofactory design before the first nanoscale molecular manufacturing system has been built. But it is important to know what will be possible, and how difficult it will be, in order to estimate the ultimate payoff of a technology and the time and effort required to achieve it. If nanofactories were impossible, then molecular manufacturing would be significantly less useful; it would be very difficult to make large products. But preliminary studies seem to show that nanofactories are actually not very difficult to design, at least in broad outline. I have written an [80-page paper] that covers error handling, mass and layout, transport of feedstock, control of fabricators, and assembly and design of products for a very primitive nanofactory design. My best estimate is that this design could produce a duplicate nanofactory in less than a day. Nanofactory designs have been proposed that appear to be much more flexible in how the products are formed, but they have not yet been worked out in as much detail.
http://www.jetpress.org/volume13/Nanofactory.htm

If there is a straightforward path from molecular manufacturing to nanofactories, then useful products will not be far behind. The ability to specify every cubic nanometer of an integrated kilogram product, filling the product with engineered machinery, will at least allow the construction of extremely powerful computers. If the construction material is strong, then mechanical performance may also be extremely good; scaling laws predict that power density increases as the inverse of machine size, and nanostructured materials may be able to take advantage of almost the full theoretical strength of covalent bonds rather than being limited by propagating defects.

Many products have been imagined for this technology. A few have been designed in sufficient detail that they might work as claimed. Robert Freitas's [Nanomedicine Vol. I] contains analyses of many kinds of nanoscale machinery. However, this only scratches the surface. In the absence of more detailed analysis identifying quantitative limits, there has been a tendency for futurists to assume that nano-built products will achieve performance close to the limits of physical law. Motors three to six orders of magnitude more powerful than today's; computers six to nine orders of magnitude more compact and efficient; materials at least two orders of magnitude stronger—all built by manufacturing systems many orders of magnitude cheaper—it's not hard to see why futurists would fall in love with this field, and skeptics would dismiss it. The solution is threefold: open-minded but quantitative investigation of the theories and proposals that have already been made; constructive attempts to fill in missing details; and critical efforts to identify unidentified problems with the application of the theories.
http://nanomedicine.com/NMI.htm

Based on a decade and a half of study, I am satisfied that some kind of nanofactory can be made to work efficiently enough to be more than competitive with today's manufacturing systems, at least for some products. In addition, I am satisfied that molecular manufacturing can be used to build simple, high-performance nanoscale devices that can be combined into useful, gram-scale, high-performance products via straightforward engineering design. This is enough to make molecular manufacturing seem very interesting, well worth further study; and in the absence of evidence to the contrary, worth a measure of preliminary concern over how some of its possible products might be used.

 

Chris has studied nanotechnology and molecular manufacturing for more than a decade. After successful careers in software engineering and dyslexia correction, Chris co-founded the Center for Responsible Nanotechnology in 2002, where he is Director of Research. Chris holds an MS in Computer Science from Stanford University.

Copyright © 2004-2005 Chris Phoenix

 


The Center for Responsible Nanotechnology(TM) (CRN) is an affiliate of World Care(R), an international, non-profit, 501(c)3 organization. All donations to CRN are handled through World Care. The opinions expressed by CRN do not necessarily reflect those of World Care

 

 

The contents of this page, including the views expressed above, are the responsibility of the author. They do not represent the views or policies of Nano Tsunami Dot Com, except where explicitly stated.

 

 

Chris Phoenix

CRN Director of Research

 

 
 


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