Micro-Engines and their Significance

Year 1959: the event was an annual meeting of the American Physical Society, at Caltech. A scientist of superlative talents, who was far ahead of his time, delivered a talk entitled "There is Plenty of Room at the Bottom". The talk turned out to be the cornerstone of a singularly important area of research, that has attracted scientists like no other field probably ever has. If you are still wondering about who the speaker was, let me abate your suspense: he was none other than Richard Feynman, a Nobel Laureate and one of the finest ambassadors of Physics in the second half of the twentieth century. The classic talk predicted how controlling systems at extremely tiny scales can achieve far-reaching consequences, and change the way we are used to living. By tiny, we mean dealing with systems that are of the dimensions of one-millionth of a metre, or even less. There are practical examples of such systems, so much so that we are made up of myriads of them. Each and every biological cell and its components constitute living examples of such systems and preclude any doubt about the possibility of manufacturing them in principle.

The benefits of this ultra-miniaturization can hardly be overestimated. Just think of the gigantic computers of the 1950s, and the modern cellphones that fit into our palms. If we add the fact that the latter can process information thousands of times faster than the former, one can easily imagine the benefits of constructing smaller and smaller systems, and simultaneously coming up with new technologies that improve their efficacy. Remember that what we are dealing with here is miniaturization gone berserk: scaling a system down by a factor of a million!

An excerpt from Feynman's talk summarizes what was about to follow in the next few decades:

"In the year 2000, when they look back at this age, they will wonder why it was not until the year 1960 that anybody began seriously to move in this direction."

Fast-forward to 2020: nanoscale systems are routinely developed in laboratories. The much enhanced surface-to-volume ratio and the quantum nature of these particles have paved the way for developing materials having very diverse properties. Nanoscale crystals called quantum dots have found widespread applications in fluorescent biological labelling, solar cells, TV displays, and cellular imaging. Graphene quantum dots have potential applications in producing nanoscale transistors. Titanium dioxide and zinc oxide are used in cosmetics and paints. In the automobile industry, they find usage in the form of scratch-resistant paints and anti-reflective coatings. In medicine, they are often used in the delivery of drugs to targeted sites. Carbon nanotubes, which are essentially sheets of graphene rolled into cylindrical shapes, yield conducting wires of very high strength. The nanoparticles come in various shapes and sizes, leading to materials having drastic differences in their characteristics and thereby making space for applications across many different fields.

And then there are nanobots: tiny machines that can be programmed to function in a certain manner. Imagine a little robot injected into the bloodstream, in order to repair a damaged tissue! True, they are still in the developmental stage in reality, and highly developed nanobots have till date existed within the confinements of science fiction novels and movies. But challenges like these have time and again been overcome in the history of science, and there is no reason to believe that the one we are discussing presently is any different.

One of the primary hurdles in the manufacture of useful nanobots is the lack of an efficient method of powering them when they are deployed to perform a given task, hence the vertiginous activity that is observed to be directed towards the making of extremely small engines. Certainly, a large engine cannot be used for this purpose, since that will spell disaster for our painstakingly prepared nanobot in no time. Our engines must then be tiny as well, in order to be commensurate with the machines that they power. Is that a big issue as far as theoretical framework is concerned? Wouldn't it suffice to simply apply the thermodynamics of large-scale engines to these little ones as well? Not really! What we forget while dealing with macroscopic objects (a sophisticated term for the stuff of sizes that are typically encountered in our daily lives) is that they are at some finite temperature, and so the molecules in the immediate surroundings must be incessantly striking the object. Large objects (we may consider a wooden block as an example) remain immune to these bombardments, because their effects cancel out on average and no noticeable change is perceived. For nanoscale objects, the situation is very different. Such objects can actually be observed to be wiggling around randomly, owing to the kicks received from the surrounding particles. Consequently, what is often taken for granted for large objects ceases to be true as we demagnify them to smaller and smaller dimensions.

The biological motors that operate within our cells do work under such drastic conditions, and yet have found ways to maintain a high efficiency. Nature has endowed them with highly sophisticated mechanisms to counter the inclement environment, and even to use it to their benefits. So you see, we must incorporate a certain degree of stochasticity (randomness) in our thermodynamics for small systems in order to proceed. Our engines meant to power nanobots must abide by this so-called Stochastic Thermodynamics.

Such engines are a reality now, and that is a stepping stone towards developing realistic functional nanobots. They have been prepared in laboratories, and descriptions of their behaviour are slowly finding firm theoretical platforms. The job is far from being over, and yet scientists are far ahead of where they were, say, a couple of decades back. It seems pertinent to end with the famous lines of Vince Lombardi: Perfection is not attainable, but if we chase perfection we can catch excellence.