Molecular machines are devices capable of manipulating single atoms and molecules. For example, to transfer them from one place to another, to bring them together so that a chemical bond is formed, or pull them apart so that the chemical bond is broken, to assemble molecular structures or, conversely, to disassemble molecular structures, and so on. It is easy to see that the size of a molecular machine cannot be too large in relation to the object with which it manipulates, which means that we are talking about functional molecular structures that have a characteristic size of the order of several nanometers.
What is a molecular machine?
Let’s start from the usual ideas about cars. For example, a car. It has two main modules: a power unit, which converts the energy released during the combustion of fuel into mechanical movement, and the second module is a functional one, which converts the mechanical movement removed from the power unit into a functionally useful movement. In a car, this is everything that turns the linear displacement of the piston into the rotation of the wheel.
Molecular machines are actually the same devices, only very small, 100 times smaller than a micron. Since perpetual motion machines cannot exist either at the macroscopic level or at the nanolevel, a molecular machine must also have a power unit capable of converting external energy into something similar to mechanical movement – something similar, because at the scale, when an atom becomes an individual object, real mechanics no, there is no “solid body” – everything “shakes and twitches”, as Richard Feynman wrote. But on slightly larger scales, “quasi-mechanics” is already possible – a directed displacement of sufficiently large molecular fragments. From all this, by the way, follows a fundamental limitation from below on the size of the machine as such – it cannot be less than a few nanometers, this is the limit. So it turns out that molecular machines are machines of extremely small size. Hence all the problems with their design. It is necessary to somehow manage to make a power unit out of a couple of hundred atoms, and even stick to it a functional module that could accurately operate with atomic-scale objects in conditions where “everything is shaking and twitching.”
Such machines, if you learn how to make them, open up absolutely fantastic prospects. The point here is this. Ordinary chemistry is a kind of constructor, which is engaged in the assembly of molecules, when everything is “shaking and twitching.” He achieves success through a large number of attempts. The scale of the available number of attempts is set by the famous Avogadro number. This is a large number, one with 24 zeros, and if, say, out of a million attempts, only one is successful, then the result will still be visible. But luck-based technology only works for assembling molecules that aren’t too complex. If you need to assemble a complex, large structure in which molecules of several types must be arranged in a strictly defined way, then even Avogadro’s number does not save. There is only one way out – to assemble accurately, excluding errors at each assembly step.
Everything is just like in life. No matter how much you shake the bag with parts from disassembled watches, by chance they will never add up.
Modern assembly technologies are primarily those machines that provide the required assembly accuracy. Having molecular machines, it is possible to assemble molecular structures, relatively speaking, of unlimited complexity and obtain materials with unprecedented characteristics. On the contrary, it is possible to carefully disassemble large molecules, such as polymers, into atoms without polluting the environment with fragments, which inevitably appear when they are disposed of by the “undermining” method. Figuratively speaking, molecular machines can replace all of today’s “stochastic chemistry” with “algorithmic chemistry”. In addition, molecular machines can be embedded in very small objects. A machine a few nanometers in size can crawl, for example, into a living cell without destroying it, and at the atomic-molecular level do what it is prescribed to do there. Here, no less fantastic prospects open up:targeted drug delivery, target correction of biochemical functions and so on. Of course, all this can be used not only for the benefit, but also to the detriment of a person. It’s always been that way. All the inventions that allowed a person to increase a thousandfold their very modest natural abilities to move, communicate and influence – all of them were used against the person himself. It would be naive to believe that this will not be the case with molecular machines. But there are a number of natural, so to speak, limitations. For example, it may be possible to create devices with high energy density using artificial molecular machines, but they will hardly be interesting as destroyers. Yes, there is no particular need for this, they have already thought of it. Weapons of the “bacteriological” type from artificial molecular machines are also unlikely to be obtained, at least in the next 20-30 years. Truth, some opportunities for new terrorist threats are quite visible here. But those special materials, the creation of which molecular machines will make possible, will undoubtedly arouse the interest of the military-industrial complex.
Dreaming, of course, is not harmful, but are such devices possible? Are machines so small possible?
The answer here is completely discouraging. It turns out that in nature there is already a gigantic world of molecular machines, and it arose without our participation. This is all wildlife. Moreover, the very idea of molecular machines is “stolen”, if you like, from living nature. The amazingly ordered inner life of the cell is organized by molecular machines. Some machines copy genetic texts almost without errors, others build complex units for assembling molecular machines and maintain their work, others perform a transport function – they drag subcellular structures along special “rails” laid in the cell. There are several thousand molecular machines in a living cell, each of them doing its own thing, and each of them converts chemical or thermal energy into quasi-mechanical motion and uses nanomechanics to accurately perform a certain operation. This is an amazing picture. It not only gives us impressive examples of nanotechnology, but changes our whole worldview.
And everything would be great if not for one “but”. In living nature, everything is arranged in such a way that biological molecular machines are created with the help of the biological molecular machines themselves. That is, together with molecular machines, wildlife has built a very special technology for assembling them, and biological molecular machines are very finely tuned for this technology. It, this technology, is used in genetic engineering, for example, on this, in fact, it stands. In general, all biotechnologies – in pharmaceuticals, medicine, agriculture – based on biological molecular machines and their assembly technology, created by nature itself. This simplifies the task, but the technologically closed circle also generates fundamental limitations. Using biotechnology and genetic engineering, we cannot go beyond the boundaries outlined by nature itself. We can make certain biological or similar substances in the quantity we need, but we cannot break out of the scope of the set of functions that are predetermined by biology. We cannot make other machines: the very technology of assembling biological molecular machines does not allow this. There is, of course, a lot of space left for biotechnology, but there is a limitation, and it is strong. Everything that is outside of biology turns out to be inaccessible on this path.
It is clear that if it were possible to break out of this “vicious circle”, then the broadest horizons for the application of molecular machines in completely different areas would open up. Is it possible? Until very recently, it seemed that there was no hopeful way, because it seems that it is possible to assemble molecular machines only with the help of other molecular machines, and there are no other molecular machines except biological ones. But in 2013, everything changed dramatically.
It has been shown that the idea of making artificial molecular machines by self-assembly is not so crazy.
It turned out that a small polymer globule (chain) with a fractal structure can be a molecular machine. The fact that fractal globules should exist was predicted by physicists Alexander Grosberg, Sergey Nechaev and Evgeny Shakhnovich back in 1988. Everyone treated this as a subtle joke of theorists. Only 20 years later, quite recently, the international group of Leonid Mirny from the Massachusetts Institute of Technology showed that two-meter DNA is packed in the cell nucleus just into a huge fractal globule. Therefore, it does not get mixed up anywhere, and you can quickly read genetic information from any part of it. But DNA is a huge molecule, and it’s not a machine at all. So in 2013, it turned out that if a relatively short polymer chain is placed in a fractal globule, that is, in the same way as DNA is laid, you get a molecular machine. A small fractal globule is capable of converting stochastic thermal motion into strict nanomechanics, and it does it exactly the same way as a heat engine does. And it seems that the chemical structure of the polymer chain is not so important here, that is, such fractal globules can be made from various synthetic polymers. So physics seems to have found a way to create truly artificial molecular machines, and the first such machine can be created in the next 2–3 years.
The creation of technology for the design and production of artificial molecular machines is quite capable of leading to a technological explosion in almost all areas of human activity. Super-revolutionary things are possible here, perhaps on a larger scale than even the microelectronic and communication revolutions we are experiencing. The most daring ideas: to build ultra-compact factories of molecular machines for the waste-free production and processing of various chemical substances, to create materials with amazing properties using molecular machines, to penetrate into a living cell and perform functions there that are inherent or, on the contrary, unusual for a living organism, and so on. etc. Instruments for precise manipulation at the atomic-molecular level can be used in almost all areas of activity,
But perhaps the most exciting things may be molecular operating systems, that is, systems of molecular machines that, as a whole, act as “machines” themselves.
In the same way as electronic operational elements with an elementary step-switching function, when combined into a functionally connected system, they give rise to a supercomputer with a huge set of functional capabilities. There is a very interesting, albeit very distant prospect of creating something that would look like “intelligence in a flask”, based on, say, non-carbon polymers, like in a living brain, for example, silicon polymers. Such an operating system could also be computer-compatible, opening the way to hybrid intelligence systems in which “strategic thinking” based, like biological intelligence, on “images and intuitions” is combined with detailed calculations obtained with with powerful computing power.