Scott Locklin on Nanotechnology and Drexler
Some of you may have been following Scott Locklin's "reality check" on nanotechnology, which was linked by CrunchGear and Hacker News.
My opinion of the post is that is confuses Drexlerian nanotech with nanotechnology "in general", and makes many major errors, including denying the existence of micromachines and nano-sized elements that drive larger systems.
The article is also wrong because it claims that, in his book, Eric Drexler is just porting macroscale designs to the nano-world, but the entire work (Nanosystems) takes great pains to analyze the differences between the nanoscale and macroscale and introduce engineering innovations that could be a good starting point for true molecular manufacturing. Another error the article makes is suggesting that Drexler dismisses using biology as tools for nanomachines, which is ironic considering that Drexler advocates "molecular and biomolecular design and self-assembly" approaches to molecular nanotechnology, and often discusses the protein folding path on his blog.
Drexler posted a response to Locklin in the comments section:
Hi Scott,
In my view, molecular and biomolecular design and self-assembly are the most promising directions for lab research in atomically precise nanotechnology. There’s been enormous progress — complex, million-atom atomically-precise frameworks, etc. — but much of the work isn’t called “nanotechnologyâ€, and this leaves many observers of the field confused about where it stands. I follow this topic in my blog, Metamodern.com.
Regarding the longer-term prospects for this branch of nanotechnology, there’s a publication that offers good starting point for serious discussion.
The technical analysis that I presented in my book Nanosystems: Molecular Machinery, Manufacturing, and Computation, (it’s based on my MIT dissertation) was examined in a report issued by the National Academy of Sciences, on “The Technical Feasibility of Site-Specific Chemistry for Large-Scale Manufacturingâ€. The report finds no show-stoppers. It notes uncertainties regarding potential system performance and “optimum research pathsâ€, however, and closes with a call for funding experimental research.
This report was prepared by a scientific committee convened by the U.S. National Research Council in response to a request from Congress. It is based on the scientific literature, and on an NRC committee workshop with a range of invited experts and extensive follow-on discussion and evaluation.
I think that this report (and the Battelle/National Labs technology roadmap) deserves more attention from serious thinkers. It deflates a lot of mythology about a topic that just might be real and important.
If either of these publications has been mentioned above, I missed it.
In general, I think Locklin's post is a very well-designed piece of flamebait, and I commend him for drawing attention to his post. Some group of people really love talking about nanotechnology, and they need some outlet, and this is a fine outlet of the week. Locklin is right that a lot of nanotechnology is just chemistry or materials science with a cool name slapped onto it, but certainly not all of it.
Funny quote from the comments thread: "any sufficiently advanced technology is indistinguishable from a rigged demo".
Assorted Items July 12th, 2010
Here is a roundup of recent interesting items.
Stratfor: The Caucasus Cauldron
"Of all the regions of the world, this one is among the most potentially explosive. It is the most likely to draw in major powers and the most likely to involve the United States. It is quiet now — but like the Balkans in 1990, quiet does not necessarily reassure any of the players."
Bina48 was in the New York Times, as you may have heard. I previously attended a mock trial where the fictional AI Bina48 was seeking asylum as a sovereign individual from the company that created her, Exabit. This new robotic Bina48 was created by David Hanson of Hanson Robotics. Hanson will be speaking at the upcoming Singularity Summit 2010 in San Francisco. Politics Daily also has a post reacting to Bina48. A local Vermont news station has more quotes.
Discover has coverage of a recent breakthrough in tooth regeneration gel.
Beverly Nuckols at the Texas GOP blog has somewhat of an odd response to Ron Bailey. She is responding to Bailey's quote where he said:
I ended by explaining that as a minority preference (at least for now) transhumanists must argue for liberty and not be seduced by democratic happy-talk. When people of good will deeply disagree on moral issues that don't involve the prevention of force or fraud, it is a fraught exercise to submit their disagreement to a panel of political appointees or a democratic vote. That way leads to intolerance, repression, and social conflict.
I definitely agree with this on a certain level. I feel we are living in a nanny state that facilitates increasing self-domestication of the human species. The book The Ten Thousand Year Explosion also has more great material on this hypothesis. When social conformism becomes such a powerful selection pressure in the cultural development of the species, we have to step back and reevaluate what we are becoming. My experience in school in a suburb of San Francisco (Burlingame) led me to believe that I was being conditioned to be an mindlessly obedient white-collar wage slave. I'm sure it is worse in many places in the US and around the world.
The Guardian has a new article out on Edward Cope, the UC Santa Cruz professor who is creating an Artificial Intelligence that writes moving pieces of classic music. A student at UC Santa Cruz told me that he is the only professor he knows of who can elicit a standing ovation from his students after a lecture.
ZDNet has coverage of Wendell Wallach's recent keynote at the World Futurist Society conference, "Navigating the Future: Moral Machines, Techno Sapiens, and the Singularity". You may recognize the image from his title slide as from a blog post of mine on cybernetic upgrades. I found that image unattributed on an image board.
Here's an article on using narrow AI in improving team sports.
Vote on these proposals to the President's Council of Advisors on Science and Technology, please. I will provide more info in my next post.
There's been a variety of exciting materials science and nanotechnology-related news from Nanowerk in the last week or two. There's been a breakthrough in printable conductive ink that requires no secondary curing (good news for personal fabrication), asymmetric nanostructures to diagnose the early signs of cancer (the key to curing cancer will be detecting it early), self-assembling nanodevices that move and change shape on demand, and coverage of a recent nanoscale photography competition. The most exciting piece of news from there recently, however, have to do with the development of a new superhard, superconducting material, BC5, a type of boron-doped diamond. Here's the first paragraph of the press release:
What could be better than diamond when it comes to a superhard material for electronics under extreme thermal and pressure conditions? Quite possibly BC5, a diamond-like material with an extremely high boron content that offers exceptional hardness and resistance to fracture, but unlike diamond, it is a superconductor rather than an insulator. A research team in China studying BC5 describes its potential in the Journal of Applied Physics, which is published by the American Institute of Physics (AIP).
An intertwined matrix of diamond and BC5 could provide the ultimate raw materials for rugged, miniaturized electronics. All of our electronic devices today are extremely fragile relative to what they could be. In the not-too-distant future, it may be possible to gently dunk our smartphones and laptops into salt water, lemon juice, whatever, and pull them right out and keep using them as if nothing happened. I want a laptop I can throw across the room without breaking it.
There was also recent news on using boron nanowire in body armor, including armor as thin as a T-shirt. Does anyone know if a boron carbide-reinforced T-shirt would stop, say, pistol bullets? I'm sure all the relevant numbers to make that calculation are out there. Carbon and boron -- a match made in material science Heaven?
In a recent post responding to the NYT article about him and his wife, Robin Hanson said, in referring to the opposition to cryonics in the comments of that article:
It seems clear to me that opposition is driven by the possibility that it might actually work. If people were sure it wouldn’t work there’d be no point in talking about selfishness, immortality, etc. If the main issue were a waste of money we’d see an entirely different reaction.
Most of the public appears to see radical life extension and cryonics as potentially workable, just morally troubling. It seems to me that fear over life extension tends to diminish when one's own life and health are put at risk, for all but the most dedicated paleo-conservatives.
Singularity Hub has good coverage of the recent lung-on-a-chip news.
Also: recently I've been kicking around the idea of doing a shared transhumanist/futurist blog, let me know if you'd be interested in contributing or could help with the IT side of things. Something that focuses on the same wide range of issues as Accelerating Future, and includes a mix of news and opinion, but has more people than just me. Especially get in touch with me if you are in the San Francisco area and want to help. My email can be found by clicking on my portrait in the lower left section of this site's sidebar. If you're interested, respond with an email, not in the comments. Thanks.
Nadrian Seeman Shares $1M Nanotech Prize
Congratulations to Ned Seeman, who is sharing the $1 million Kavli Prize in nanoscience with IBM's Don Eigler, who was behind the team that made the IBM logo in atoms. Seeman was awarded the prize for the discovery of structural DNA nanotechnology, in 1979 according to the Kavli website. Seeman has given presentations on DNA nanotechnology at the Foresight Institute conferences and at last year's Singularity Summit, and recently made a major breakthrough in nanotechnology with a nanoscale assembly line.
I had the opportunity to meet Dr. Seeman at a Center for Responsible Nanotechnology conference in Tuscon in 2007. He was skeptical about the idea of achieving molecular manufacturing within the next couple decades.
Will macroscale molecular manufacturing be achieved by a structural DNA route, the "Tattoo Needle" architecture, the foldamer route, the Waldo route, the diamondoid route, or something else? That is the question all the cool kids are asking.
New Paper: Optimal Tooltip Trajectories in a Hydrogen Abstraction Tool Recharge Reaction Sequence for Positionally Controlled Diamond Mechanosynthesis
Robert Freitas alerts me to a new mechanosynthesis paper published by the Nanofactory Collaboration group in the Journal of Computational and Theoretical Nanoscience. Here's some info:
Denis Tarasov, Natalia Akberova, Ekaterina Izotova, Diana Alisheva, Maksim Astafiev, Robert A. Freitas Jr., “Optimal Tooltip Trajectories in a Hydrogen Abstraction Tool Recharge Reaction Sequence for Positionally Controlled Diamond Mechanosynthesis,†J. Comput. Theor. Nanosci. 7(February 2010):325-353 [29 pages]
It is our first published paper with our Russian collaborators and is now available online. This paper represents the first extensive DMS (Diamond Mechno-Synthesis) tooltip trajectory analysis, examining a wide range of viable multiple degrees-of-freedom tooltip motions in 3D space that could be employed to recharge the hydrogen abstraction tool, a key reaction set in DMS.
Abstract:
The use of precisely applied mechanical forces to induce site-specific chemical transformations is called positional mechanosynthesis, and diamond is an important early target for achieving mechanosynthesis experimentally. A key step in diamond mechanosynthesis (DMS) employs an ethynyl-based hydrogen abstraction tool (HAbst) for the site-specific mechanical dehydrogenation of H-passivated diamond surfaces, creating an isolated radical site that can accept adatoms via radical-radical coupling in a subsequent positionally controlled reaction step. The abstraction tool, once used (HAbstH), must be recharged by removing the abstracted hydrogen atom from the tooltip, before the tool can be used again. This paper presents the first theoretical study of DMS tool-workpiece operating envelopes and optimal tooltip trajectories for any positionally controlled reaction sequence – and more specifically, one that may be used to recharge a spent hydrogen abstraction tool – during scanning-probe based ultrahigh-vacuum diamond mechanosynthesis. Trajectories were analyzed using Density Functional Theory (DFT) in PC-GAMESS at the B3LYP/6-311G(d,p) // B3LYP/3-21G(2d,p) level of theory. The results of this study help to define equipment and tooltip motion requirements that may be needed to execute the proposed reaction sequence experimentally and provide support for early developmental targets as part of a comprehensive near-term DMS implementation program.
So, what does it mean? Well, in the Freitas-Merkle mechanosynthetic tooltip design, there are three primary tasks for three primary tools -- 1) abstracting (removing) hydrogen from a carbon surface (carbon surfaces tend to have a monoatomic layer of hydrogen), 2) placing a carbon dimer (C=C) on a hydrogen-free carbon surface, then 3) putting down a hydrogen to cover up the surface and prevent it from spontaneously rearranging itself or otherwise engaging in unwanted reactions. This paper zooms in on a specific part of tool #1, the "recharge sequence" portion, where the abstraction tool gets rid of the hydrogen it just grabbed from a surface and gets ready to grab again.
What Does a Buckyball Undergoing Unimolecular Disassociation by Use of Extremely High Levels of Vibrational Excitation Look Like?
How about a C60/C240 collision at 300eV?
H/t Machine Phase.
Dangers of Molecular Nanotechnology, Again
Over at IEET, Jamais Cascio and Mike Treder essentially argue that the future will be slow/boring, or rather, seem slow and boring because people will get used to advances as quickly as they occur. I heartily disagree. There are at least three probable events which could make the future seem traumatic, broken, out-of-control, and not slow by anyone's standards. These three events include 1) a Third World War or atmospheric EMP detonation event, 2) an MNT revolution with accompanying arms races, and 3) superintelligence. In response to Jamais' post, I commented:
I disagree. I don't think that Jamais understands how abrupt an MNT revolution could be once the first nanofactory is built, or how abrupt a hard takeoff could be once a human-equivalent artificial intelligence is created.
Read Nanosystems, then "Design of a Primitive Nanofactory", and look where nanotechnology is today.
For AI, you can do simple math that shows once an AI can earn enough money to pay for its own upkeep and then some, it would quickly gain the ability to take over most of the world economy.
Have Giulio or Jamais read "Design of a Primitive Nanofactory" or Nanosystems?
Knowledge of where we are today in nanotechnology, plus Nanosystems, plus "Design of a Primitive Nanofactory", equals scary.
Where we are today: basic molecular assembly lines
The most important breakthrough: a reprogrammable universal assembler
Shortly thereafter: a basic nanofactory
Shortly thereafter: every nation with nanofactory technology magnifies its manufacturing potential by a factor of hundreds or more.
Chris Phoenix gets it. Jurgen Altmann gets it. Mark Gubrud gets it. Thomas Vandermolen gets it. Eric Drexler seems to have gotten it a long time ago. Michio Kaku, Annalee Newitz, and many others have called molecular nanotechnology "the next Industrial Revolution".
When will others get it? Here's a quote from the CRN page on the dangers of molecular nanotechnology:
Molecular manufacturing raises the possibility of horrifically effective weapons. As an example, the smallest insect is about 200 microns; this creates a plausible size estimate for a nanotech-built antipersonnel weapon capable of seeking and injecting toxin into unprotected humans. The human lethal dose of botulism toxin is about 100 nanograms, or about 1/100 the volume of the weapon. As many as 50 billion toxin-carrying devices—theoretically enough to kill every human on earth—could be packed into a single suitcase. Guns of all sizes would be far more powerful, and their bullets could be self-guided. Aerospace hardware would be far lighter and higher performance; built with minimal or no metal, it would be much harder to spot on radar. Embedded computers would allow remote activation of any weapon, and more compact power handling would allow greatly improved robotics. These ideas barely scratch the surface of what's possible.
Will weapons like these in the hands of every backwater terrorist and militia lead to a future that is "slow" or "boring? It could lead to a future where numerous major cities become essentially uninhabitable.
Here's a potentially illuminating quote:
“Revolutions are cruel precisely because they move too fast for those whom they strike.â€
Jacob Bronowski
Nature: “A proximity-based programmable DNA nanoscale assembly line”
io9 has coverage of Nadrian Seeman's latest work in nanotechnology: the first nanoscale assembly line! This is big news. If you were at Singularity Summit 2009 back in October and listening very carefully, you might have heard Seeman mention this device seven months in advance of its formal announcement! Now that's foresight.
The full Nature article describing the device is here.
Survey: Hiding Risks Can Hurt Public Support for Nanotechnology
Here's an interesting news item from Eurekalert:
A new national survey on public attitudes toward medical applications and physical enhancements that rely on nanotechnology shows that support for the technology increases when the public is informed of the technology's risks as well as its benefits – at least among those people who have heard of nanotechnology. The survey, which was conducted by researchers at North Carolina State University and Arizona State University (ASU), also found that discussing risks decreased support among those people who had never previously heard of nanotechnology – but not by much.
"The survey suggests that researchers, industries and policymakers should not be afraid to display the risks as well as the benefits of nanotechnology," says Dr. Michael Cobb, an associate professor of political science at NC State who conducted the survey. "We found that when people know something about nanotechnologies for human enhancement, they are more supportive of it when they are presented with balanced information about its risks and benefits."
The survey was conducted by Cobb in collaboration with Drs. Clark Miller and Sean Hays of ASU, and was funded by the Center for Nanotechnology in Society at ASU.
However, talking about risks did not boost support among all segments of the population. Those who had never heard of nanotechnology prior to the survey were slightly less supportive when told of its potential risks.
In addition to asking participants how much they supported the use of nanotechnology for human enhancements, they were also asked how beneficial and risky they thought these technologies would be, whether they were worried about not getting access to them, and who should pay for them – health insurance companies or individuals paying out-of-pocket. The potential enhancements addressed in the survey run the gamut from advanced cancer treatments to bionic limbs designed to impart greater physical strength.
If you are someone who writes or speaks on the topic of nanotechnology, this means that you shouldn't be afraid to discuss the risks. In fact, mentioning the risks should be part of your default spiel. Engines of Creation was not afraid to discuss some of the risks. The Center for Responsible Nanotechnology, when it was more active, had a crucial role in making the risks of nanotechnology more widely known, but the vast majority of contemporary organizations and publications that discuss nanotechnology shy away from the immense risks.
I've previously written at length about the dangers of advanced nanotechnology, and frequently recommend the book Military Nanotechnology as a guide to some of these risks. Essential essays or pages include "Molecular Nanotechnology and the World System" by Tom McCarthy, "Nanotechnology and International Security" by Mark Gubrud, "Military, Arms Control, and Security Aspects of Nanotechnology" by Altmann and Gubrud, CRN's dangers page, and my page enumerating additional dangers.
Next time you're in the audience at a talk or see a blog post extolling the benefits of nanotechnology (especially molecular nanotechnology), consider making a comment that you'd like to see more thought on the risks. I believe that some of the purveyors of molecular nanotechnology are actively avoiding discussing its grave potential risks.
Atomistic Small Bearing: Dynamics Performed in NanoEngineer-1, Visualized in Blender
From Machine Phase. This is a movie of the atomistic bearing described by Eric Drexler in Nanosystems. Remember to read Drexler's article or watch this video to understand, "the rotation-induced speed of the shaft surface is substantially lower than the (apparent) vibrational speeds of the atoms". The thermal vibrations in the bearing actually take place much faster than the shaft motion. What you see in the video is only maybe 1/1000 of the actual thermal vibration motions. Because these sorts of videos have a limited frame rate, we get a "strobe light effect" where we only selectively see the vibration. If the video were portraying the thermal vibration on a timescale where you could actually see each part of the action, then the actual shaft surface would be moving at a glacial pace. The upshot of all of that is that the friction and heating in this device would not be nearly as high as it appears at a casual glance.
In another post, Tom Moore points out new software by Miron Cuperman which partially automates the process of using csv data from NanoEngineer-1 to render animations in Blender.
Diamond Trees (Tropostats): A Molecular Manufacturing Based System for Compositional Atmospheric Homeostasis
Robert Freitas has a new idea for a product that could be built using molecular manufacturing -- diamond trees designed to sequester carbon dioxide. The concept is fleshed out in technical detail at a paper now available at the Institute for Molecular Manufacturing website. Let's bring up that abstract!
The future technology of molecular manufacturing will enable long-term sequestration of atmospheric carbon in solid diamond products, along with sequestration of lesser masses of numerous air pollutants, yielding pristine air worldwide ~30 years after implementation. A global population of 143 x 109 20-kg “diamond trees†or tropostats, generating 28.6 TW of thermally non-polluting solar power and covering ~0.1% of the planetary surface, can create and actively maintain compositional atmospheric homeostasis as a key step toward achieving comprehensive human control of Earth’s climate.
On the topic of MNT, I also wonder what it will take for the skeptics to become convinced that the technology is plausible. Positional atomic placement has already been demonstrated, including at room temperature. Will complex rotating 3D nanosystems convince them? I doubt those are far off.
Our Friend Gadolinium
Brian Wang directs our attention to one important part of Rob Freitas' radionuclide page:
The mass of the alpha-particle is ~7000 times greater than that of an electron, so the velocity and hence the range of a-particles in matter is considerably less than for beta-particles of equal energy. Consequently the optimum radionuclide for medical nanorobots is predominantly an alpha emitter.
Among all gamma-free alpha-only emitters with t1/2 > 106 sec, the highest volumetric power density is available using Gd148 (gadolinium) which a-decays directly to Sm144 (samarium), a stable rare-earth isotope. A solid sphere of pure Gd148 (~7900 kg/m3) of radius r = 95 microns surrounded by a 5-micron thick platinum shield (total device radius R = 100 microns) and a thin polished silver coating of emissivity er = 0.02 suspended in vacuo would initially maintain a constant temperature T2 (far from a surface held at T1 = 310 K)
75-year half-life, initially generating 17 microwatts of thermal power which can be converted to 8 microwatts of mechanical power by a Stirling engine operating at ~50% efficiency. (Smaller spheres of Gd148 run cooler.) While probably too large for most individual nanorobot designs, such spheres could be an ideal long-term energy source for a swallowable or implantable "power pill" (Chapter 26) or dedicated energy organ (Section 6.4.4). A ~0.2 kg block of pure Gd148 (~1 inch3) initially yields ~120 watts, sufficient in theory to meet the complete basal power needs of an entire human body for ~1 century (given suitable nucleochemical energy conversion and load buffering mechanisms, and a sufficiently well-divided structure).
The last part is the punchline, of course. Freitas acknowledges future design challenges such as energy conversion, load buffering, and division of structure. If these challenges are overcome, a large block of Gd148 (or simply gadolinite ready to be processed into pure gadolinium) could supply nutrition to millions of people for millennia. Gadolinium has a half-life of 75 years, so you'd need double as much for each 75-year period you wish to avoiding refueling for, but storing gadolinium in its stable gadolinite form seems avoid this problem. Unfortunately, gadolinite is fairly rare and gadolinium itself is only found in the Earth's crust at a 6.2 ppm level. By comparison, the abundance of gold in the Earth's crust is only 0.0011 ppm. According to this page, annual production of gadolinium is 200 tons.
Just to throw some numbers out there, if one cubic inch is enough per person per century, a million people would require a million cubic inches. That can fit in a cube 9 x 9 x 9 ft large. According to Freitas' numbers, this would weigh about 200,000 kg, or 200 metric tonnes, which is on par with today's annual production. If demand for gadolinium grew, it seems plausible that its cost would fall greatly -- after all, gold is about 6,000 times rarer and our annual production is 2,800 tons. Feeding ten billion people with gadolinium, if that were possible, would require about 2,000,000 metric tonnes for the first century. At an extraction rate of 200,000 metric tonnes per year, it could be done in a decade. This would require increasing current production by a factor of 1,000. According to this book, gadolinite can contain 40% rare earth oxides, 5% of which consists of gadolinium itself. That means that gadolinium makes up about 2% of the total. (Wrong: see comments.) Processing ten million metric tonnes of the ore annually would yield the required amount. For comparison, we extract 1.2 billion tons of iron from the Earth's crust annually.
Update: all of the above is wrong for one reason or another, as pointed out in the comments, but at least I had fun. I was confusing chemical stability with nuclear stability and made the mistake that I thought gadolinium-148 would be nuclear-stable in its gadolinite form, which is wrong. The atomic number of gadolinium is 64 meaning that gadolinium-148 contains 20 extra neutrons above neutron-proton parity. It seems to me that we'd eventually have to find a less safe and cheaper isotope to make this work on a large level if it's suitable in practice and we ever want to.
Radioisotopic Food Nanobots: Freitas Response
I recently wrote to Rob Freitas about his radioisotope-powered food nanorobot idea that, if it works, could allow people to eat at severely reduced levels for as long as a century or more. As far as I can tell, food would still be needed due to cell loss from shedding skin cells and the like, but this would likely be relatively little. As Roko pointed out, the gadolinium-powered nanobots could reconstitute ATP from waste products like urea. The gadolinium would just provide the energy for running the chemical reactions needed to produce fresh ATP.
Here is the email I wrote to Rob Freitas:
Hi Robert, I saw an idea of yours posted at the World Future Society, and blogged it. Me and my readers weren't clear on some of the details, and a few google searches turned up nothing. All of us would appreciate if you would weigh in on the thread and answer our burning questions.
Thanks, and I'm always impressed by all the ideas you come up with.
Best,
Michael
Here is the response (posted with permission):
Hi, Michael.
The 148Gd power source proposal was described in NMI (1999) at http://www.nanomedicine.com/NMI/6.3.7.1.htm. The semiconductor shell structure crudely illustrated in Fig. 6.7 is intended to be an atomically precise structure. The radioactive 148Gd is kept permanently encapsulated while inside the body. The minimum radius for this powerplant is on the order of ~11 microns, so it is clearly intended for fixed-site multi-nodal (not bloodborne) use.
I haven't yet published any detailed scaling studies specifically describing dietary-related nanorobotic systems. These proposals now exist only in rough form in my long (across 2 decades!) accumulated notes for Chapter 26 in Vol. III of my Nanomedicine book series. I hope to find time to publish NMIII sometime in this decade.Best wishes,
Rob Freitas
I read the page that Freitas linked. Here's one of the core specs:
A (1 micron)3 cube of Gd148 produces ~5 a-particles/sec, yielding an output current of ~1 picoampere at ~3 volts (e.g., ~3 pW).
Interesting! The page also points out that the cost of Gd148 must be brought down significantly before it becomes a feasible power source, because in 1998 it cost about a dollar per two cubic microns(!) This is expensive stuff. The number of nanobots that might be used would need to be on the order of a hundred trillion (not a billion, as I wrote previously), each with a cubic micron-sized power core, though 11 microns across due to shielding. Given the 1998 cost of Gd148, a full system would cost about $50 trillion for the fuel alone! Near the top of the page it says, "Selection of an optimum radioactive fuel is guided primarily by safety criteria".
An interesting idea, and food for nanotechnological thought.