External links and references
- Molecular Self-Assembly papers (http://www.esi-topics.com/msa/)
- Beyond molecules: Self-assembly of mesoscopic and macroscopic components (http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=122665)
- Whitesides, G. M. & Grzyboski, B. (2002) Science 295, 2418-2421.
Molecular Nanotechnology (MNT) is nanotechnology using "molecular manufacturing", an anticipated technology based on positionally-controlled mechanosynthesis guided by molecular machine systems. It involves 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.
Ralph Merkle has compared today's chemistry (in contrast to mechanosynthesis) to an attempt to build interesting Lego brick constructions while wearing boxing gloves. Because conventional chemistry has no tools that allow us to place a particular molecule in a particular place (so that it bonds in a predictable way), we must work with randomly moving molecules. As a result, when we cause a particular chemical reaction, we frequently get a mix of several different product species. We must often follow up after the reaction with a physical filtering process to extract the species we actually wanted, with the other species discarded as waste. Nanotechnology could therefore offer much cleaner manufacturing processes than today's bulk technology offers.
Drexler has noted that some British writers have applied the term "nanotechnology" to microscale technologies like MEMS. This prompted Drexler to use the term "molecular nanotechnology".
Recently, the term "nanotechnology" is applied to the currently available fine-scale chemistry or materials science or molecular engineering. That would include "nanotech" suntan lotion and "nanotech" stain-resistant pants. This has prompted Drexler to use the term "zettatechnology" to refer to molecular manufacturing and its products. The "zetta" prefix is a reference to number of atoms in macro-sized product, unlike the Nano prefix for the number of subdivisions of a meter. Such a product is likely to have around a sextillion (1021) distinct atomic parts. (http://nanodot.org/article.pl?sid=03/10/29/2017242) (http://www.azonano.com/details.asp?ArticleID=56)
Hypothetical applications and capabilities
Smart Materials and Nanosensors
One application of nanotechnology is the development of so-called smart materials. This term refers to any sort of material designed and engineered at the nanometre 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 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 nanotechnology, they pale in comparison with the complexity of the technology most popularly associated with the term: the replicating nanorobot.
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. In theory, nanobots could construct more nanobots.
However, critics doubt the feasibility of controllable self-replicating nanobots: they cite the possibility of mutations removing any control and favoring reproduction of mutant pathogenic variations. Advocates counter 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 (http://www.foresight.org/guidelines/index.html).
Recent technical proposals for nanofactories do not include self-replicating nanobots, and recent ethical guidelines prohibit self-replication.
One of the most important applications of molecular nanotechnology will be medical nanorobotics or nanomedicine. The ability to design, build, and deploy large numbers of medical nanorobots will make possible the rapid elimination of disease and the reliable and relatively painless recovery from physical trauma. Medical nanorobots will also make possible the convenient correction of genetic defects, and can help to ensure a greatly expanded healthspan. More controversially, medical nanorobots could be used to augment natural human capabilities. However, mechanical medical nanodevices will not be allowed (or designed) to self-replicate inside the human body, nor will medical nanorobots have any need for self-replication themselves  (http://www.nanomedicine.com/NMI/2.4.2.htm#p19) since they will be manufactured exclusively in carefully regulated nanofactories.
Another proposed application of nanotechnology involves utility fog  (http://discuss.foresight.org/~josh/Ufog.html) — 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".
Hypothetical social impacts
Despite the current early developmental status of nanotechnology, much concern surrounds its anticipated impact on economics and on law. Some conjecture that nanotechnology 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, nanotechnology would probably upset existing economic structures, as it should reduce the scarcity of manufactured goods and make many more goods (such as food and health aids) manufacturable.
Most futurists and all economists believe that future citizens of a nanotechnological society would still need money, in the form of unforgeable digital cash or physical specie (http://www.rfreitas.com/Nano/TangibleNanomoney.htm) (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.
Some resources will 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 will diminish with the advent of virtual reality, where they can be easily substituted, is yet unclear, although the only reason why it might not is hypothetical irrational preference towards "real thing".
Beyond the fantasy scenarios, nanotechnology has daunting risks. It enables cheaper and more destructive conventional weapons. Also, nanotechnology permits weapons of mass destruction that self-replicate, as viruses and cancer cells do when attacking the human body. Commentators generally agree that humankind should permit self-replication only under very controlled or "inherently safe" conditions.
A fear exists that nanomechanical robots, 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 sometimes refer to this situation as the "grey goo" or "ecophagy" scenario. K. Eric Drexler considers an accidental "grey goo" scenario extremely unlikely. 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 these dangers, the Foresight Institute (founded by K. Eric Drexler to prepare for the arrival of future technologies) has drafted a set of guidelines  (http://www.foresight.org/guidelines/current.html) 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. Diamond Nanofactories
In Engines of Creation, Drexler mentioned (http://www.foresight.org/EOC/EOC_Chapter_1.html#section06of10) universal molecular assemblers, which could hypothetically "build almost anything that the laws of nature allow to exist." Drexler's colleague Merkle has since argued (http://www.foresight.org/impact/impossible.html#Fear), however, that Drexler never claimed such "universal" systems could actually build arbitrary molecules.
In 1992, Drexler published Nanosystems: molecular machinery, manufacturing, and computation (http://www.zyvex.com/nanotech/nanosystems.html), 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 universal assembler, it would still be enormously capable. Diamondoid structures and other stiff hydrocarbons have a wide range of possible applications, going far beyond current MEMS technology.
The Smalley-Drexler Debate
Several researchers, including Dr. Smalley, have attacked (http://www.sciamdigital.com/browse.cfm?ITEMIDCHAR=F90C4210-C153-4B2F-83A1-28F2012B637&methodnameCHAR=&interfacenameCHAR=browse.cfm&ISSUEID_CHAR=6A628AB3-17A5-4374-B100-3185A0CCC86&ArticleTypeSubInclude_BIT=0&sequencenameCHAR=itemP) the notion of univeral assemblers, leading to a rebuttal (http://www.imm.org/SciAmDebate2/smalley.html) from Drexler and colleagues, and eventually to an exchange of letters (http://pubs.acs.org/cen/coverstory/8148/8148counterpoint.html). Smalley argues that chemistry is extremely complicated, reactions are hard to control, and that a universal assembler is science fiction. Drexler and colleagues, however, challenge the relevance of Smalley's arguments to the limited and rigidly-controlled chemistry proposed in Nanosystems.
The Feasibility of the Proposals in Nanosystems
The feasibility of Drexler's proposals largely depends, therefore, on whether the designs in Nanosystems will work. Supporters of molecular nanotechnology frequently claim that no significant errors have been discovered in Nanosystems since 1992. And even some critics concede  (http://www.softmachines.org/wordpress/index.php?p=50#comment-523) 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.
Critics claim, however, that Nanosystems omits important chemical details about the low-level 'machine language' of molecular nanotechnology (Smalley (http://www.wired.com/wired/archive/12.10/drexler.html), Moriarty (http://www.softmachines.org/wordpress/index.php?p=70), Atkinson (http://www.nanotech-now.com/Atkinson-Phoenix-Nanotech-Debate.htm)). 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 extremely speculative foundations.
Drexler argues  (http://www.softmachines.org/PDFs/Moriarty_Phoenix_1.pdf) 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 Sociey 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."
Existing Work on Diamond Mechanosynthesis
There is some peer-reviewed research 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 (http://www.rfreitas.com/Nano/JNNDimerTool.pdf). 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 (http://www.molecularassembler.com/JCTNPengMar04.pdf). 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 (http://www.molecularassembler.com/JCTNMannMar04.pdf). Mann, Peng, Freitas and Merkle. J. Comput. Theor. Nanosci. Vol. 1, No. 1 2004.
For the paper by Mann, et al., the researchers used over 5 years of CPU time to simulate a series of "tool tips" which could be used to place a pair of carbon atoms (a dimer) onto a diamond surface. The most promising tip succeeded in placing the carbon dimer onto the diamond surface once in five simulations, and it had to be positioned with great accuracy to avoid bonding the dimer incorrectly. Furthermore, the tips were difficult to recharge with a second carbon dimer, and were only stable in carefully controlled environments.
Further research to consider alternate tips 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 nanotechnology with two more books, Unbounding the Future: the Nanotechnology Revolution  (http://www.foresight.org/UTF/Unbound_LBW/) and Nanosystems: Molecular Machinery, Manufacturing, and Computation  (http://www.zyvex.com/nanotech/nanosystems.html). Unbounding the Future, an easy-to-read book, introduces the ideas of nanotechnology in a not-too-technical way; and Nanosystems provides an in-depth analysis of nanomachines and molecular manufacturing, with thorough scientific analyses of their feasibility and performance. Other notable works in the same vein are Nanomedicine Vol. I (http://www.nanomedicine.com/NMI.htm) and Vol. IIA (http://www.nanomedicine.com/NMIIA.htm) by Robert Freitas and Kinematic Self-Replicating Machines  (http://www.MolecularAssembler.com/KSRM.htm) by Robert Freitas and Ralph Merkle.
- Nanotechnology: Molecular Speculations on Global Abundance Edited by BC Crandall (ISBN 0262531372) offers interesting ideas for MNT applications.
- Foresight Institute (http://www.foresight.org)
- Responsible Nanotechnology (http://crnano.typepad.com/) A blog from the Center for Responsible Nanotechnology
- Center for Responsible Nanotechnology (http://www.crnano.org/overview.htm)
- Howard Lovy's NanoBot (http://nanobot.blogspot.com/) a blog
- Main Page - Wise-Nano (http://wise-nano.org/w/Main_Page) A wiki for MNT
- Wired article on Drexler (http://www.wired.com/wired/archive/12.10/drexler.html)
- Dr. Freitas's bibliography on mechanosynthesis (http://foresight.org/stage2/mechsynthbib.html) (also includes related techniques based on scanning probe microscopy)
- The Molecular Assembler website of Robert A. Freitas Jr. (http://www.molecularassembler.com/)
- Nanotechnology Now (http://www.nanotech-now.com/) Nanotechnology basics, news, and general information
- Zyvex Nanotechnology (http://www.zyvex.com/nano)
- National Nanotechnology Initiative (http://www.nano.gov)
- Center for Responsible Nanotechnology (http://www.crnano.org)
- Institute for Molecular Manufacturing (http://www.imm.org/)
- The NanoAging Institute (http://www.nanoaging.com)