Drexler responds

This morning brought a somewhat tetchy email from K. Eric Drexler, not entirely happy about my article in Physics World, The future of nanotechnology. There were three main complaints:

1. That he, Drexler, could not be held responsible for the “ridiculous artist’s concepts” that have become associated with his work. Thus my criticism of the nanosubmarine illustration isn’t a fair criticism of MNT. Actually, I have some sympathy with his predicament on this, in that I’m sure that the elementary errors that show up in the particularly silly image I chose wouldn’t be there if Drexler had had anything to do with it. Nonetheless, my criticism of these images does make one important point very clear – you shouldn’t expect macroscopic engineering design concepts to apply to directly to the nanoworld. Is this a fair criticism of MNT? I think it is – to quote from the preface of Nanosystems; “Molecular manufacturing applies the principles of mechanical engineering to chemistry”.

2. Next he argues that my statement that “Strong surface forces may make the moving parts of a NEMS device stick together and seize up” reflects a lack of study of the appropriate section of Nanosystems, chapter 10, which argues that very low friction is to be expected between atomically smooth diamond surfaces. It’s worth noting first of all that this statement in my article isn’t actually directed at MNT at all, but at top-down NEMS. Nonetheless, I do believe that the discussion in Nanosystems does substantially underestimate the problems of friction and dissipation at the nanoscale. This is a rather technical discussion, which I will enlarge on at a later time.

3. Finally, he objects that I have not proved my central contention, that biology is highly optimised for the nanoscale, pointing out that biology hasn’t been able to explore the space of non-aqueous molecular machine systems. This gets to the heart of the argument of Soft Machines. A crucial, though obvious, point, is that it only makes sense to talk about optimisation in the context of a particular environment, and what is optimised for ambient operation at 300 K in the presence of water is not the same as what is optimised for ultra-high vacuum at a temperature of 3 K. I wouldn’t exclude the possibility that MNT would work at 3 K in UHV, but I think that what works in ambient conditions is much more interesting, if only because medicine is likely to be such an important application of nanotechnology.

4 thoughts on “Drexler responds”

  1. You left off the most interesting case, which is a device whose internals are free of water, but which has a surface with a well-defined interface to the aqueous environment. And it runs at 300K. See Freitas’ respirocyte for example. This design space has clearly not been well explored by Nature.

  2. Hal, I think in this kind of device the interface between the device and the aqueous environment will be problematic. Protein adsorption is a huge problem for most surfaces in bodily fluids, and I don’t think diamondoid surfaces will be any exception. For a more detailed discussion of this point, together with a response from Freitas, see the comments on this item on the CRN blog. Adsorption of proteins and other macromolecules will cause problems both by triggering defensive responses from the host organism, and by gumming up the valves, filters and so on that connect the inside of the device to the environment.

  3. That is a good point about the unknowns regarding biocompatibility of the diamond surfaces. Freitas does discuss this in the respirocyte paper in Section 5 and especially in Section 5.7, where he mentions PEG coatings and even mechanical scrapers among possible countermeasures if they prove necessary.

    Respirocytes really only need to accept O2, CO2 and glucose, all much smaller than proteins, so this should permit a wide variety of strategies for surrounding the device with biocompatible structures or coatings.

    More generally, this points to a design strategy of giving a hard diamondoid device a “soft surface” coating, with the area between the soft surface and the hard interior being a controlled environment. This gives you biocompatibility while benefiting from an enormous increase in the design space due to diamondoid mechanisms.

    I want to also add that the mention of 3K diamondoid is a red herring; Drexler’s and Freitas’ analyses in Nanosystems and Nanomedicine are generally directed at room temperature operation.

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