What biology does and doesn’t prove about nanotechnology

The recent comments by Alec Broers in his Reith Lecture about the feasibility or otherwise of the Drexlerian flavour of molecular nanotechnology have sparked off a debate that seems to have picked up some of the character of the British general election campaign (Liar! Vampire!! Drunkard!!! ). See here for Howard Lovy’s take, here for TNTlogs view. All of this prompted an intervention by Drexler himself (channeled through Howard Lovy), which was treated with less than total respect by TNTlog. Meanwhile, Howard Lovy visited Soft Machines to tell us that “when it comes to being blatantly political, you scientists are just as clumsy about it as any corrupt city politician I’ve covered in my career. The only difference is that you (I don’t mean you, personally) can sound incredibly smart while you lie and distort to get your way.” Time, I think (as a politician would say), to return to the issues.

Philip Moriarty, in his comment on Drexler’s letter, makes, as usual, some very important points about the practicalities of mechanosynthesis. Here I want to look at what I think is the strongest argument that supporters of radical nanotechnologies have, the argument that the very existence of the amazing contrivances of cell biology shows us that radical nanotechnology must be possible. I’ve written on this theme often before (for example here), but it’s so important it’s worth returning to.

In Drexler’s own words, in this essay for the AAAS, “Biology shows that molecular machines can exist, can be programmed with genetic data, and can build more molecular machines”. This argument is clearly absolutely correct, and Drexler deserves credit for highlighting this important idea in his book Engines of Creation. But we need to pursue the argument a little bit further than the proponents of molecular manufacturing generally take it.

Cell biology shows us that it is possible to make sophisticated molecular machines that can operate, in some circumstances, with atomic precision, and which can replicate themselves. What it does not show is that the approach to making molecular machines outlined in Drexler’s book Nanosystems, an approach that Drexler describes in that book as “the principles of mechanical engineering applied to chemistry”, will work. The crucial point is that the molecular machines of biology work on very different principles to those used by our macroscopic products of mechanical engineering. This is much clearer now than it was when Engines of Creation was written, because in the ensuing 20 years there’s been spectacular progress in structural biology and single molecule biophysics; this progress has unravelled the operating details of many biological molecular machines and has allowed us to understand much more deeply the design philosophy that underlies them. I’ve tried to explain this design philosophy in my book Soft Machines; for a much more technical account, with full mathematical and physical details, the excellent textbook by Phil Nelson, Biological Physics: Energy, Information, Life, is the place to go.

Where Drexler takes the argument next is to say that, if nature can achieve such marvelous devices using materials whose properties, constrained by the accidents of evolution, are far from optimal, and using essentially random design principles, then how much more effective will our synthetic nano-machines be. We can use hard, stiff materials like diamond, rather than the soft, wet and jelly-like components of biology, and we can use the rationally designed products of a mechanical engineering approach rather than the ramshackle and jury-rigged contrivances of biology. In Drexler’s own words, we can expect “molecular machine systems that are as far from the biological model as a jet aircraft is from a bird, or a telescope is from an eye”.

There’s something wrong with this argument, though. The shortcomings of biological design are very obvious at the macroscopic scale – 747s are more effective at flying than crows, and, like many over-40 year olds, I can personally testify to the inadequacy of the tendon arrangements in the knee-joint. But the smaller we go in biology, the better things seem to work. My favourite example of this is ATP-synthase. This remarkable nanoscale machine is an energy conversion device that is shared by living creatures as different as bacteria and elephants (and indeed, ourselves). It converts the chemical energy of a hydrogen ion gradient, first into mechanical energy of rotation, and then into chemical energy again, in the form of the energy molecular ATP, and it does this with an efficiency approaching 100%.

Why does biology work so well at the nanoscale? I think the reason is related to the by now well-known fact that physics looks very different on the nanoscale than it does at the macroscale. In the environment we live in – with temperatures around 300 K and a lot of water around – what dominates the physics of the nanoscale is ubiquitous Brownian motion (the continuous jostling of everything by thermal motion), strong surface forces (which tend to make most things stick together), and, in water, the complete dominance of viscosity over inertia, making water behave at the nanoscale in the way molasses behaves on human scales. The kind of nanotechnology biology uses exploits these peculiarly nanoscale phenomena. It uses design principles which are completely unknown in the macroscopic world of mechanical engineering. These principles include self-assembly, in which strong surface forces and Brownian motion combine to allow complex structures to form spontaneously from their component parts. The lack of stiffness of biological molecules, and the importance of Brownian motion in continuously buffeting them, is exploited in the principle of molecular shape change as a mechanism for doing mechanical work in the molecular motors that make our muscles function. These biological nanomachines are exquisitely optimised for the nanoscale world in which they operate.

It’s important to be clear that I’m not accusing Drexler of failing to appreciate the importance of nanoscale phenomena like Brownian motion; they’re treated in some detail in Nanosystems. But the mechanical engineering approach to nanotechnology – the Nanosystems approach – treats these phenomena as problems to be engineered around. Biology doesn’t engineer around them, though, it’s found ways of exploiting them.

My view, then, is that the mechanical engineering approach to nanotechnology that underlies MNT is less likely to succeed than an approach that seeks to emulate the design principles of nature. MNT is working against the grain of nanoscale physics, while the biological approach – the soft, wet, and flexible approach, works with the grain of the way the nanoscale works. Appealing to biology to prove the possibility of radical nanotechnology of some kind is absolutely legitimate, but the logic of this argument doesn’t lead to MNT.

10 thoughts on “What biology does and doesn’t prove about nanotechnology”

  1. Well said, Richard.

    You might also be aware that Drexler himself believes that the “bio-memetic” approach is the most likely near-term workable solution to nanotech. However, he still believes that “mechanosynthesis” will be the “ultimate” form of nanotech.

  2. I don’t understand your reasoning. You ask, why does biology work so well at the nanoscale? And presumably this also implies, why does it work SO MUCH BETTER at the nanoscale than at larger scales? Then you talk about brownian motion and such. Fine, of course biology uses the principles of physics relevant to the scale at which it acts. At the microscale this will involve the phenomena you describe: self-assembly, surface forces, etc.

    But that doesn’t explain why biology works SO WELL at the microscale! Why doesn’t biology suck at the microscale, just like it sucks at the macroscale? Or, turning it around, if biology can do so well at the microscale, using the appropriate physical phenomena with this supposed near-100% efficiency, why does it do so badly at the macroscale?

    I don’t see that you have really explained anything. You have observed that biology appears to do a good job at the microscale, and anyone can see that it does a lousy job at the macroscale. What is the reason for this? To actually explain it, you need to point to something that differentiates the microscale from other scales, and to show why that differentiating factor would cause biology to be much better able to adapt at the microscale.

    You should also recognize that Drexler’s designs, if they work (and of course we don’t know this yet), would actually exceed biological performance in some respects even at the microscale. For example, characteristic transport speeds in Drexler’s designs are similar to those we are familiar with, perhaps the mm/sec to m/sec range. Intracellular biological transport mechanisms operate more typically at microns/sec.

  3. I have personally chosen to follow the ‘soft wet flexible’ approach for pursuing nanotechnology. And I believe that it has been worthwhile to critically challenge Drexler’s industrial nanotech proposals, because legitimate criticism can often serve to improve and strengthen one’s work.

    But I would caution against too much optimism that the current biological paradigm is the ultimate path to nanotech nirvana. It’s unlikely to expect that our existing methods of macroscopic industrial engineering will cleanly transfer to the microscopic level. But there is obviously something quite powerful and distinct about such methods in comparison to natural biological systems. It may just be a matter of learning how to apply industrial design in just the right way at the microscopic scale in order to produce new and very powerful methods of molecular assembly. There is precedent in nature for a radical overthrow of the ‘old order’. This happened when the soft, flexible and baroque RNA molecules where largely displaced by the lean, mean protein machines! I suspect that similar molecular assembly revolutions are pending. Now that we have entered the ‘cognitive-based’ phase of evolution, I also think that they will happen a lot more quickly than the ribosomal revolution.

  4. Kurt, you’re quite right. The soft and wet approach is what Drexler himself has been arguing in favour of for some time. It’s ironic given all the sound and fury between MNT supporters and nanoscientists, that operationally, there is very little between what Whitesides and Drexler would recommend for funding policy for nanotechnology, even though their views of the ultimate goal might be very different.

    Howard, thank you! But … “not bad, for a scientist”! You’re just trying to provoke a reaction. Well, you won’t get one from me. I’ll just mention an opinion poll I saw recently, about what proportions of the general population would trust different professional groups to tell the truth. Professors came in at a respectable 76%, but journalists were at the bottom, with a trust rating of only 16%. Astonishingly, this is actually below the level of trust people have in politicians, who score 20%. Of course, this is Britain, and maybe things are different in the USA.

    Hal, the key point that perhaps I haven’t made clear is that biology is optimised by evolution, and the substrate that evolution directly operates on fundamentally belongs to the nanoscale – protein synthesis and protein folding. A gene makes a protein, and a protein makes a molecular machine. Phenomena at a bigger scales are shaped very much more indirectly by the operation of highly complex networks of many molecular machines. So we might expect that the process of evolutionary optimisation is very much more direct at the nanoscale level at which molecular machines operate. So it isn’t just that “biology uses the principles of physics relevant to the scale at which it acts” – biology is most directly optimised for the nanoscale.

    William, I’m not at all certain that we’ll achieve Nirvana by any route, nanotechnological or otherwise. In fact in my more saturnine moments I think we’ll be doing pretty well if we stop things getting worse. But I don’t necessarily disagree with you that there might not be some revolution in nanoscale engineering, substantially surpassing what nature has delivered, in the future. What does seem implausible to me, though, is that this revolution will take the shape of an approach to industrial engineering that, fundamentally, wouldn’t look very strange to the 18th century Derbyshire millwrights who got the English industrial revolution started.

  5. Hi Richard — It seems to me that you are once again making the case that hard, dry MNT would be more difficult to do than soft, wet nanobio. This point has been made many times, but I don’t know anyone who disagrees with it. To make an interesting case against MNT, you need to show why it physically can’t be done, or why it wouldn’t be useful. Is there something in the laws of science that will prevent hard, dry MNT from being achieved? If so, I haven’t heard it, and I’ve been looking for it since the late 1970s. At this point, I’m quite doubtful the case can be made.–CP

  6. Well, I guess any correct argument is obvious if you think about it with enough clarity. As for the case against hard, dry, MNT, there is not, of course, an easy proof that it is contrary to the rules of physics, not least because there isn’t actually a proposal that’s spelled out in enough detail to criticise. So at the moment we have two types of argument against hard, dry MNT. There’s this high level argument that mechanical engineering provides an inappropriate design philosophy for the nanoscale world. At the level of details, one can only point out the many areas in which physical constraints that have not been adequately taken into account will give new design constraints that will shrink the parameter space in which hard, dry MNT will have to work. Areas of vulnerability that I know about include friction; the calculations of friction in Nanosystems prove to be substantial underestimates compared to computer simulation because they omit what turns out to be the dominant mechanism for wearless friction. There are the practical difficulties to do with mechanochemistry that Philip Moriarty has highlighted. There is the problem of the stability or otherwise of intricately shaped nanoscale components with respect to phenomena like surface reconstruction. There’s the issue of the effect of Brownian motion on the effective tolerance of mechanical systems. None of these issues at the moment constitutes a proof that hard, dry MNT won’t work, but they indicate areas in which more detailed design work, better computer simulations and experiments are needed to convince sceptics that workable proposals can be found. At the moment it’s the lack of this experiment/theory feedback loop that means that the hard dry MNT proposals aren’t moving forward.

  7. Hi Richard — Thanks for answering. After a couple of preliminary comments, I have a specific proposal to make. Regarding whether mechanical engineering provides an appropriate or inappropriate design philosophy for the nanoscale world, this would seem to depend on what problem one is trying to solve. There may be challenges for which only such a design philosophy will do the job, regardless of how difficult it is to use at the nanoscale. Second, one can’t assume that an increase in friction beyond that calculated in Nanosystems will automatically lead to MNT designs not working — many of those designs include substantial design margins (i.e., they are “conservative” designs) which may well be able to absorb even major adverse changes in an area such as friction. One would need to study the designs to find this out. However, a more basic suggestion is this: while this discussion is useful, blog software is not very good at representing the current state of technical discussion of a complex topic such as MNT. I suggest we move your list of issues to some different software where we can have a clear, point by point comparison of the best case on each side. Even a stable webpage with two columns in a table format would be very helpful, so that readers could compare the best statement available on each side, for each technical point. You are putting substantial time into this discussion; let’s put it in a format where readers can grapple with it.–CP

  8. Christine. my apologies for being so long replying.
    I completely agree about the need to choose your design philosophy to match the task you need to solve. I guess we already discussed this to some extent here.
    As to whether MNT designs are conservative enough or not, I think the question is still open, because I don’t think we yet have enough detailed designs of reasonable complexity to test that.

    You’re quite right about the shortcomings of blog software – I’ve been meaning for some time to extract some of the most useful stuff and put it in a better format but I just haven’t been able to get round to doing it yet.

  9. A thought I had today, which presumably is nowhere near original, is that even if a Drexler-style nano-factory assembler does work, in the sense of replicating itself from atoms or simple molecules, it’s likely to be at least as complex as a bacterium doing the same thing, especially in a natural environment. The bacterium works with the environment and exploits sloppiness; the factory tries to have precise motions despite the environment, and in a general assembler is supposed to be capable of more manipulations than even a fully loaded E. coli, plus what ever communication channels it has so as to be useful en masse.

    What this may say about a pure-carbon assember working in vacuum is another matter.

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