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.
Here is the response (posted with permission):
The 148Gd power source proposal was described in NMI (1999) at http://www.nanomedicine.com/NMI/184.108.40.206.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.
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.