What are the plausible scientific limits of molecular nanotechnology?
Richard Jones, author of Soft Machines has written an interesting critique of the room-temperature molecular nanomachinery propounded by Drexler:
If biology can produce a sophisticated nanotechnology based on soft materials like proteins and lipids, singularitarian thinking goes, then how much more powerful our synthetic nanotechnology would be if we could use strong, stiff materials, like diamond. And if biology can produce working motors and assemblers using just the random selections of Darwinian evolution, how much more powerful the devices could be if they were rationally designed using all the insights we've learned from macroscopic engineering.
But that reasoning fails to take into account the physical environment in which cell biology takes place, which has nothing in common with the macroscopic world of bridges, engines, and transmissions. In the domain of the cell, water behaves like thick molasses, not the free-flowing liquid that we are familiar with. This is a world dominated by the fluctuations of constant Brownian motion, in which components are ceaselessly bombarded by fast-moving water molecules and flex and stretch randomly. The van der Waals force, which attracts molecules to one another, dominates, causing things in close proximity to stick together. Clingiest of all are protein molecules, whose stickiness underlies a number of undesirable phenomena, such as the rejection of medical implants. What's to protect a nanobot assailed by particles glomming onto its surface and clogging up its gears?
The watery nanoscale environment of cell biology seems so hostile to engineering that the fact that biology works at all is almost hard to believe. But biology does work--and very well at that. The lack of rigidity, excessive stickiness, and constant random motion may seem like huge obstacles to be worked around, but biology is aided by its own design principles, which have evolved over billions of years to exploit those characteristics. That brutal combination of strong surface forces and random Brownian motion in fact propels the self-assembly of sophisticated structures, such as the sculpting of intricately folded protein molecules. The cellular environment that at first seems annoying--filled with squishy objects and the chaotic banging around of particles--is essential in the operation of molecular motors, where a change in a protein molecule's shape provides the power stroke to convert chemical energy to mechanical energy.
In the end, rather than ratifying the ”hard” nanomachine paradigm, cellular biology casts doubt on it. But even if that mechanical-engineering approach were to work in the body, there are several issues that, in my view, have been seriously underestimated by its proponents.
Put all these complications together and what they suggest, to me, is that the range of environments in which rigid nanomachines could operate, if they operate at all, would be quite limited. If, for example, such devices can function only at low temperatures and in a vacuum, their impact and economic importance would be virtually nil.
The entire article is definitely worth a read. Jones advocates more attention to "soft" nanotech, which is nanomachinery with similar design principles to biology -- the biomimetic approach -- as the most plausible means of making progress in nanotech.
As far as near-term room-temperature innovations, he seems to make a compelling case. However the claim that "If ... such devices can function only at low temperatures and in a vacuum, their impact and economic importance would be virtually nil" strikes me as questionable. It seems to me that atomic-precision nanotech could be used to create hard vacuums and more perfectly reflective surfaces, and hence bring the costs of cryogenics down considerably. Desktop factories using these conditions could still be feasible.
Furthermore, it bears mentioning that cryonics patients could still benefit from molecular machinery subject to such limitations, even if the machinery is not functional at anything remotely close to human body temperature. The necessity of a complete cellular-level rebuild is not a good excuse not to cryopreserve. As long as this kind of rebuild technology is physically plausible, there arguably remains an ethical imperative to cryopreserve patients facing the imminent prospect of decay.
In fact, this proposed limitation could hint at an alternative use for cryosuspension that is entirely separate from its present role as an ambulance to the future. It could perhaps turn out that there are forms of cellular surgery and repair which are only feasible at those temperatures, which are nonetheless necessary to combat aging and its complications. The people of the future might actually need to undergo routine periods of cryogenic nanosurgery in order to achieve robust rejuvenation. This would be a more pleasant prospect than cryonics in that it would be a proven technology at that point; and most likely the vitrification process could be improved sufficiently via soft nanotech to reduce the damage from cooling itself significantly.