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Molecular nanotechnology (MNT) is the engineering of functional systems at the molecular scale. An equivalent definition would be "machines at the molecular scale designed and built atom-by-atom". This is distinct from nanoscale materials. Based on Richard Feynman's vision of miniature factories using nanomachines to build complex products (including additional nanomachines), this advanced form of nanotechnology (or molecular manufacturing) will make use of positionally-controlled mechanosynthesis guided by molecular machine systems. MNT would involve combining physical principles demonstrated by chemistry, other nanotechnologies, and the molecular machinery of life with the systems engineering principles found in modern macroscale factories. Its most well-known exposition is in the books of K. Eric Drexler.
Formulating a roadmap for the development of MNT is now an objective of a broadly based technology roadmap project led by Battelle (the manager of several U.S. National Laboratories) and the Foresight Institute. The roadmap should be completed by early 2007. In August 2005, a task force consisting of 50+ international experts from various fields was organized by the Center for Responsible Nanotechnology to study the societal implications of molecular nanotechnology.
While conventional chemistry employs stochastic processes driven toward some equilibrium to obtain stochastic results, and biology exploits stochastic processes to obtain deterministic results based on complex enzyme-catalyzed reaction chains optimized through billions of years of evolutionary feedback, molecular nanotechnology would employ novel (and as yet unspecified) deterministic nanoscale processes to obtain deterministic results. The desire in molecular nanotechnology would be to place molecular moieties in deterministic locations with deterministic orientation to obtain desired chemical reactions, and then to build systems by further assembling the products of these reactions.
Ralph Merkle has compared today's manufacturing methods (in contrast to mechanosynthesis) to an attempt to build interesting Lego brick constructions while wearing boxing gloves: "Casting, grinding, milling and even lithography move atoms in great thundering statistical herds. It's like trying to make things out of LEGO blocks with boxing gloves on your hands. Yes, you can push the LEGO blocks into great heaps and pile them up, but you can't really snap them together the way you'd like." It has been posited that molecular nanotechnology could offer much cleaner manufacturing processes than today's bulk technology.
Background and implications
Projected Applications and Capabilities
Smart Materials and Nanosensors
One proposed application of MNT is the development of so-called smart materials. This term refers to any sort of material designed and engineered at the nanometer scale to perform a specific task, and encompasses a wide variety of possible commercial applications. One example is materials designed to respond differently to various molecules; such a capability could lead, for example, to artificial drugs which would recognize and render inert specific viruses. Another is the idea of self-healing structures, which would repair small tears in a surface naturally in the same way as self-sealing tires or human skin; and while this technology is relatively new, it is already seeing commercial application in various engineering plastics.
A nanosensor created by MNT would resemble a smart material, involving a small component within a larger machine that would react to its environment and change in some fundamental, intentional way. As a very simple example: a photosensor could passively measure the incident light and discharge its absorbed energy as electricity when the light passes above or below a specified threshold, sending a signal to a larger machine. Such a sensor would cost less and use less power than a conventional sensor, and yet function usefully in all the same applications — for example, turning on parking lot lights when it gets dark.
While smart materials and nanosensors both exemplify useful applications of MNT, they pale in comparison with the complexity of the technology most popularly associated with the term: the replicating nanorobot.
MNT nanofacturing is popularly linked with the idea of swarms of coordinated nanoscale robots working together, as proposed by Drexler in his 1986 popular discussions of the subject. It is proposed that sufficiently capable nanobots could construct more nanobots.
However, critics doubt both the feasibility of self-replicating nanobots and the feasibilty of control if self-replicating nanobots could be achieved: they cite the possibility of mutations removing any control and favoring reproduction of mutant pathogenic variations. Advocates address the second doubt by arguing that bacteria are (of necessity) evolved to evolve, while nanobot mutation can be actively prevented by common error-correcting techniques. Similar ideas are advocated in the Foresight Guidelines on Molecular Nanotechnology.
Recent technical proposals for MNT nanofactories do not include self-replicating nanobots, and recent ethical guidelines prohibit self-replication. 
One of the most important applications of MNT would be medical nanorobotics or nanomedicine, an area pioneered by Robert Freitas in numerous books  and papers . The ability to design, build, and deploy large numbers of medical nanorobots would, at an optimum, make possible the rapid elimination of disease and the reliable and relatively painless recovery from physical trauma. Medical nanorobots might also make possible the convenient correction of genetic defects, and help to ensure a greatly expanded healthspan. More controversially, medical nanorobots might be used to augment natural human capabilities. However, mechanical medical nanodevices would not be allowed (or designed) to self-replicate inside the human body, nor would medical nanorobots have any need for self-replication themselves  since they would be manufactured exclusively in carefully regulated nanofactories.
Another proposed application of nanotechnology involves utility fog  — in which a cloud of networked microscopic robots (simpler than assemblers) changes its shape and properties to form macroscopic objects and tools in accordance with software commands. Rather than modify the current practices of consuming material goods in different forms, utility fog would simply replace most physical objects.
Yet another proposed application would be phased-array optics (PAO). PAO would used the principle of phased-array millimeter technology but at optical wavelengths. This would permit the duplication of any sort of optical effect but virtually. Users could request holograms, sunrises and sunsets, or floating lasers as the mood strikes. PAO systems were described in BC Crandall's Nanotechnology: Molecular Speculations on Global Abundance in the Brian Wowk article "Phased-Array Optics".
Potential Social Impacts
Despite the current early developmental status of nanotechnology and molecular nanotechnology, much concern surrounds MNT's anticipated impact on economics and on law. Some conjecture that MNT would elicit a strong public-opinion backlash, as has occurred recently around genetically modified plants and the prospect of human cloning. Whatever the exact effects, MNT, if achieved, would tend to upset existing economic structures by reducing the scarcity of manufactured goods and making many more goods (such as food and health aids) manufacturable.
It is generally considered that future citizens of a molecular-nanotechnological society would still need money, in the form of unforgeable digital cash or physical specie (in special circumstances). They might use such money to buy goods and services that are unique, or limited within the solar system. These might include: matter, energy, information, real estate, design services, entertainment services, legal services, fame, political power, or the attention of other people to your political/religious/philosophical message. Furthermore, futurists must consider war, even between prosperous states, and non-economic goals.
If MNT were realized, some resources would remain limited, because unique physical objects are limited (a plot of land in the real Jerusalem, mining rights to the larger near-earth asteroids) or because they depend on the goodwill of a particular person (the love of a famous person, a painting from a famous artist). Demand will always exceed supply for some things, and a political economy may continue to exist in any case. Whether the interest in these limited resources would diminish with the advent of virtual reality, where they could be easily substituted, is yet unclear; one reason why it might not is a hypothetical preference for "the real thing".
Molecular nanotechnology is one of the technologies that some analysts believe could lead to a Technological Singularity. Some feel that molecular nanotechnology would have daunting risks. It conceivably could enable cheaper and more destructive conventional weapons. Also, molecular nanotechnology might permit weapons of mass destruction that could self-replicate, as viruses and cancer cells do when attacking the human body. Commentators generally agree that, in the event molecular nanotechnology were developed, humankind should permit self-replication only under very controlled or "inherently safe" conditions.
A fear exists that nanomechanical robots, if achieved, and if designed to self-replicate using naturally occurring materials (a difficult task), could consume the entire planet in their hunger for raw materials, or simply crowd out natural life, out-competing it for energy (as happened historically when blue-green algae appeared and outcompeted earlier life forms). Some commentators have referred to this situation as the "grey goo" or "ecophagy" scenario. K. Eric Drexler considers an accidental "grey goo" scenario extremely unlikely and says so in later editions of Engines of Creation. The "grey goo" scenario begs the Tree Sap Answer: what chances exist that one's car could spontaneously mutate into a wild car, run off-road and live in the forest off tree sap?
In light of this perception of potential danger, the Foresight Institute (founded by K. Eric Drexler to prepare for the arrival of future technologies) has drafted a set of guidelines  for the ethical development of nanotechnology. These include the banning of free-foraging self-replicating pseudo-organisms on the Earth's surface, at least, and possibly in other places.
Technical Issues and Criticism
Universal Assemblers vs. Diamondoid Nanofactories
A section heading in Drexler's Engines of Creation reads  "Universal Assemblers", and the following text speaks of molecular assemblers which could hypothetically "build almost anything that the laws of nature allow to exist." Drexler's colleague Ralph Merkle has noted that, contrary to widespread legend, , Drexler never claimed that assembler systems could build absolutely any molecular structure. The endnotes in Drexler's book explain the qualification "almost": "For example, a delicate structure might be designed that, like a stone arch, would self-destruct unless all its pieces were already in place. If there were no room in the design for the placement and removal of a scaffolding, then the structure might be impossible to build. Few structures of practical interest seem likely to exhibit such a problem, however."
In 1992, Drexler published Nanosystems: molecular machinery, manufacturing, and computation, a detailed proposal for synthesizing stiff, diamond-based structures using a table-top factory. Although such a nanofactory would be far less powerful than a protean universal assembler, it would still be enormously capable. Diamondoid structures and other stiff covalent structures, if achieved, would have a wide range of possible applications, going far beyond current MEMS technology. However, no proposal was put forward for building the table-top factory in the absence of a near-universal assembler.
The Smalley-Drexler Debate
Several researchers, including Dr. Smalley, have attacked the notion of universal assemblers, leading to a rebuttal from Drexler and colleagues, and eventually to an exchange of letters. Smalley argues that chemistry is extremely complicated, reactions are hard to control, and that a universal assembler is science fiction. Drexler and colleagues, however, note that Drexler never proposed universal assemblers able to make absolutely anything, but had instead proposed more limited assemblers able to make a very wide variety of things. They challenge the relevance of Smalley's arguments to the more specific proposals advanced in Nanosystems.
The Feasibility of the Proposals in Nanosystems
The feasibility of Drexler's proposals largely depends, therefore, on whether designs like those in Nanosystems could be built in the absence of a universal assembler to build them and would work as described. Supporters of molecular nanotechnology frequently claim that no significant errors have been discovered in Nanosystems since 1992. Even some critics concede  that "Drexler has carefully considered a number of physical principles underlying the 'high level' aspects of the nanosystems he proposes and, indeed, has thought in some detail" about some issues.
Other critics claim, however, that Nanosystems omits important chemical details about the low-level 'machine language' of molecular nanotechnology (Smalley, Atkinson, Moriarty, Jones). They also claim that much of the other low-level chemistry in Nanosystems requires extensive further work, and that Drexler's higher-level designs therefore rest on speculative foundations.
Drexler argues  that we may need to wait until our conventional nanotechnology improves before solving these issues: "Molecular manufacturing will result from a series of advances in molecular machine systems, much as the first Moon landing resulted from a series of advances in liquid-fuel rocket systems. We are now in a position like that of the British Interplanetary Society of the 1930s which described how multistage liquid-fuelled rockets could reach the Moon and pointed to early rockets as illustrations of the basic principle."
In any case, as Richard Feynman once said, "It is scientific only to say what's more likely or less likely, and not to be proving all the time what's possible or impossible."
The fundamental question to address is whether or not most structures consistent with physical law can in fact be manufactured. Note that this framing of the question avoids endless digressions about what Drexler did or did not propose, which are ultimately beside the point. Either (a) such a manufacturing capabiliity is feasible, in which case the focus should be on what it looks like and how to develop it, or (b) such a manufacturing capability is for some reason infeasible, in which case the focus should be on understanding why it can not be done.
Existing Work on Diamond Mechanosynthesis
There is some peer-reviewed theoretical work on synthesizing diamond by mechanically depositing carbon atoms (a process known as mechanosynthesis).
- Theoretical Analysis of a Carbon-Carbon Dimer Placement Tool for Diamond Mechanosynthesis. Merkle and Freitas, J. Nanosci. Nanotech. 2003, Vol. 3, No. 03.
- Theoretical Analysis of Diamond Mechanosynthesis. Part I. Stability of C2 Mediated Growth of Nanocrystalline Diamond C(110) Surface. Peng, Freitas and Merkle. J. Comput. Theor. Nanosci. Vol. 1, No. 1 2004.
- Theoretical Analysis of Diamond Mechanosynthesis. Part II. C2 Mediated Growth of Diamond C(110) Surface via Si/Ge-Triadamantane Dimer Placement Tools. Mann, Peng, Freitas and Merkle. J. Comput. Theor. Nanosci. Vol. 1, No. 1 2004.
- Design and Analysis of a Molecular Tool for Carbon Transfer in Mechanosynthesis. Allis and Drexler. J. Comput. Theor. Nanosci. Vol. 2, 45–55, 2005
- Theoretical Analysis of Diamond Mechanosynthesis. Part III. Positional C2 Deposition on Diamond C(110) Surface using Si/Ge/Sn-based Dimer Placement Tools. Peng, Freitas, Merkle, Von Ehr, Randall and Skidmore. J. Comput. Theor. Nanosci. Vol. 3, pages 28-41, 2006.
The most recent paper in this continuing research effort by Freitas, Merkle and their collaborators reports that the most-studied mechanosynthesis tooltip motif (DCB6Ge) successfully places a C2 carbon dimer on a C(110) diamond surface at both 300K (room temperature) and 80K (liquid nitrogen temperature), and that the silicon variant (DCB6Si) also works at 80K but not at 300K. These tooltips are intended to be used only in carefully controlled environments (e.g., vacuum). Maximum acceptable limits for tooltip translational and rotational misplacement errors are reported in paper III -- tooltips must be positioned with great accuracy to avoid bonding the dimer incorrectly. Over 100,000 CPU hours were invested in this latest study. The DCB6 tooltip motif, initially described at a Foresight Conference in 2002, was the first complete tooltip ever proposed for diamond mechanosynthesis and remains the only tooltip motif that has been successfully simulated for its intended function on a full 200-atom diamond surface.
Further research  to consider additional tooltips will require time-consuming computational chemistry and difficult laboratory work.
A working nanofactory would require a variety of well-designed tips for different reactions, and detailed analyses of placing atoms on more complicated surfaces. Although this appears a challenging problem given current resources, many tools will be available to help future researchers: Moore's Law predicts further increases in computer power, semiconductor fabrication techniques continue to approach the nanoscale, and researchers grow ever more skilled at using proteins, ribosomes and DNA to perform novel chemistry.
Current useful reference works
- Drexler and others have extended the ideas of molecular nanotechnology with two more books, Unbounding the Future: the Nanotechnology Revolution  and Nanosystems: Molecular Machinery, Manufacturing, and Computation . Unbounding the Future, an easy-to-read book, introduces the ideas of molecular nanotechnology in a not-too-technical way; and Nanosystems provides an in-depth, physics-based analysis of hypothetical nanomachines and molecular manufacturing, with extensive analyses arguing in favor of their feasibility and performance. Other notable works in the same vein are Nanomedicine Vol. I and Vol. IIA by Robert Freitas and Kinematic Self-Replicating Machines  by Robert Freitas and Ralph Merkle.
- Nanotechnology: Molecular Speculations on Global Abundance Edited by BC Crandall (ISBN 0-262-53137-2) offers interesting ideas for MNT applications.
- Foresight Institute
- Main Page - Wise-Nano A wiki for MNT
- Dr. Freitas's bibliography on mechanosynthesis (also includes related techniques based on scanning probe microscopy)
- The Molecular Assembler website of Robert A. Freitas Jr.
- Nanotechnology Now Nanotechnology basics, news, and general information
- Eric Drexler's personal website and digital archive
- National Nanotechnology Initiative
- Institute for Molecular Manufacturing