Chris Phoenix, over on the CRN blog, in reply to a comment of mine, asked an interesting question that I replied at such length to that I feel moved to recycle it here. His question was, given that graphite is a very strong material, and given that graphite sheets of more than 200 carbon atoms have been synthesized with wet chemistry, why is it that life never discovered graphite? From this he questioned the degree to which biology could be claimed to have found optimum or near optimum solutions to the problems of engineering at the nanoscale. I answered his question (or at least commented on it) in three parts.
Firstly, I don’t think that biology has solved all problems it faces optimally – it would be absurd to suggest this. But what I do believe is that the closer to the nanoscale one is, the more optimal the solutions are. This is obvious when one thinks about it; the problems of making nanoscale machines were the first problems biology had to solve, it had the longest to do it, and at this point the it was closest to starting from a clean slate. In evolving more complex structures (like the eye) biology has to coopt solutions that were evolved to solve some other problem. I would argue that many of the local maxima that evolution gets trapped in are actually near optimal solutions of nanotechnology problems that have to be sub-optimally adapted for larger scale operation. As single molecule biophysics progresses and indicates just how efficient many biological nanomachines are this view I think gets more compelling.
Secondly, and perhaps following on from this, the process of optimising materials choice is very rarely, either in biology or human engineering, simply a question of maximising a single property like strength. One has to consider a whole variety of different properties, strength, stiffness, fracture toughness, as well as external factors such as difficulty of processing, cost (either in money for humans or in energy for biology), and achieve the best compromise set of properties to achieve fitness for purpose. So the question you should ask is, in what circumstances would the property of high strength be so valuable for an organism, particularly a nanoscale organism, that all other factors would be overruled. I can’t actually think of many, as organisms, particularly small ones, generally need toughness, resilience and self-healing properties rather than outright strength. And the strong and tough materials they have evolved (e.g. the shells of diatoms, spider silk, tendon) actually have pretty good properties for their purposes.
Finally, don’t forget that strength isn’t really an intrinsic property of materials at all. Stiffness is determined by the strength of the bonds, but strength is determined by what defects are present. So you have to ask, not whether evolution could have developed a way of making graphite, but whether it could have developed a way of developing macroscopic amounts of graphite free of defects. The latter is a tall order, as people hoping to commercialise nanotubes for structural applications are going to find out. In comparison the linear polymers that biology uses when it needs high strength are actually much more forgiving, if you can work out how to get them aligned – it’s much easier to make a long polymer with no defects than it is to make a two or three dimensional structure with a similar degree of perfection.
I do not think that biology / natural evolution has discovered the best solutions for engineering at the nano-scale for two fundamentally different reasons.
First, biology has only explored the worm wet nano-world, as we both know there are many other possible nano-scale environments, ionic liquids, non-polar liquids, super critical system, solids in a vacuum, gas phase, materials in a strong magnetic field, etc.
Secondly engineering goals are human goals, natural selection goals are survival and reproduction .
You wrote a lot more than this on the other blog so I would encourage readers to look there.
The claim that biology solutions become closer to optimal as you move to the nanoscale is weak, and is compatible with complete incompetence on the part of biological evolution.
It seems that what you really want to say is that nanotech won’t do better than biology. But if that is your point, how strongly do you mean it? Can you give specific tasks at specific scales where you are confident that no artificial system will do better than what biological evolution has already created?
Hal, as you say, maybe it wasn’t a good idea to run the same conversation on two blogs, so I have already, I think, conceded Jim’s point about the importance of environment in my replies on the CRN blog.
As an example of a specific task for which evolution has delivered something pretty close to an unsurpassable solution , I would cite the efficiency of ATP-synthase in converting energy stored in hydrogen ion gradients into the chemical energy of ATP. This efficiency seems to be more that 97% of the maximum permitted by thermodynamics. Any improvements that an artificial system can deliver on this aren’t going to amount to much. I suspect your view of evolution as being completely incompetent derives from a rather eukaryocentric viewpoint, and if the only life-forms you knew about were bacteria you would take a different view. They seem fantastically competent organisms. So in terms of how strongly I mean the statement “nanotech won‚Äôt do better than biology” I probably do mean it quite strongly in the context of micron scale devices that live in an aqueous environment at 300K – I do think it’s going to take us a very long time to do better than a bacteria at this job.
As an aside to Jim’s remarks, I’m fascinated by the idea of artifical evolution, as exemplifed by Sol Spiegelman’s experiments on RNA. In these, by experimental design and control of the environment in which the evolution can take place, one can actually align a human design goal with the natural selection goals. I think this is a tremendously powerful principle that we need to think hard about how to exploit elsewhere.
To clarify, I did not mean to say that I thought evolution was incompetent, but rather that the statement “the closer to the nanoscale one is, the more optimal the solutions are” is so weak that it would still be true even if biological evolution were exceedingly incompetent, as long as it was slightly less incompetent at small scales. I was criticizing the statement on rhetorical grounds, not complaining about biological efficiency.
That’s a good point about ATP synthesis, but wouldn’t it make more sense to look at what makes the proton gradient as well? From what I understand these electrical gradients don’t exist in nature as a fuel source, but have to be maintained artificially by the life forms. In eukaryotic cells it goes back to oxidation of glucose. If you look at the whole chain as glucose + oxygen => ATP, how efficient is that, and would you still claim that no artificial system could produce ATP more efficiently?
Hal, the best example of a more complete system to analyse would be photosynthesis at the chloroplast level, where you have light making a hydrogen ion gradient that in turn is turned into ATP. I’m sure such an analysis has been done but I don’t have a figure to hand.
Biological energy systems are pretty complicated and I don’t know that much about them. From a few student-oriented web pages I gather that photosynthesis can be thought of as producing glucose, which is then oxidized in the respiration cycle to produce ATP. Here is an analysis that says that the maximum efficiency of converting light energy into glucose is 28.5%. It is emphasized that this is a maximum value. Among other things it ignores the fact that plants absorb the full spectrum, but only use the amount of energy of red light, the rest being lost as heat. This should decrease the efficiency in terms of sunlight absorption by perhaps 20%. Then the glucose has to be converted to ATP. According to this other student page, the efficiency in cells of this step is approximately 50%. Putting these together, the overall efficiency of sunlight to ATP is about 12%.
Of course ATP is not likely to be used in a nanotech system unless it were specifically designed to assist a cell’s energy metabolism. Nanotech devices would probably use some other energy storage. It seems difficult to come up with a complete but simple task where a nanotech device could be compared with biological systems.
Section 6.3.4.4 of Nanomedicine describes a glucose engine, basically an internal combustion engine that burns glucose instead of gasoline. It’s problematic because one of the few things that doesn’t scale properly as you shrink mechanical devices is heat dissipation. Nanoscale devices have much higher heat conduction, even in a constant-velocity scaling analysis. I guess it’s because they have fewer atoms. So Freitas has to go to some lengths to make his heat engine work. He even resorts to electromagnetic levitation(!) to suspend the engine without physical contact, coating it with shiny material to try to reduce radiative heat loss. It doesn’t look like a very promising design direction to me.
Hal, I don’t think you’ve found the most reliable sources there. My understanding is that the direct products of photosynthesis are the energetic molecule NADPH and the pumping of hydrogen ions across a membrane, with the energy stored in this hydrogen ion gradient subsequently being used by ATP-synthase to make ATP. Glucose synthesis occurs later.
As you say, heat engines work very badly on the nanoscale because of the scaling of thermal diffusion. That’s why biological motors all work at constant temperature, directly extracting mechanical work from a chemical potential gradient. We should try and do something similar for a Nanotech device. I discuss this point at some length in my book.
Freitas is nothing if not encyclopedic. He was not recommending that heat engine, just analyzing it. Many other ways have been proposed to convert chemical to mechanical energy, including mechanosynthetic mills running in reverse. For some reactions (perhaps many), this might be nearly 100% efficient. The requirements are 1) very low friction 2) no abrupt state transitions. Another way to convert chemical to electrical energy is the fuel cell; this works quite well with protons. I think a nanoscale fuel cell could be very efficient because it would not have to waste energy driving chemicals to diffuse through many microns of fluid; in other words, it could run near the theoretical maximum voltage at high current.
Mechanosynthetic mills differ from biological mills in that parts of the system can be strained without loss of efficiency. Biochemicals are too floppy to use that approach, so the system has to be carefully tuned to maintain equilibrium between every piece in every state. (ATP synthase is very highly conserved–implying there’s not much room to experiment.) Note also that one parameter of the system is the concentration gradients of the reactants–so the enzyme would not be as efficient if not for the buffering effects of a surrounding volume of water. A mechanochemical mill could produce a constant force at each displacement, easy to use efficiently.
So please, let’s ignore microscale heat engines and talk about more useful designs.
Chris