Machines lace almost all social, political cultural and economic issues currently being discussed. Why, you ask? Clearly, because we live in a world that has all its modern economies and demographic trends pivoting around machines and factories at all scales.
We have reached the stage in the evolution of our civilization where we cannot fathom a day without the presence of machines or automated processes. Machines are not only used in sectors of manufacturing or agriculture but also in basic applications like healthcare, electronics and other areas of research. Although, machines of varying types had entered the industrial landscape long ago, technologies like nanotechnology, the Internet of Things, Big Data have altered the scenario in an unprecedented manner.
The fusion of nanotechnology with conventional mechanical concepts gives rise to the perception of ‘molecular machines’. Foreseen to be a stepping stone into nano-sized industrial revolution, these microscopic machines are molecules designed with movable parts that behave in a way that our regular machines operate in. A nano-scale motor that spins in a given direction in presence of directed heat and light would be an example of a molecular machine.
What are molecular machines?
Molecular machines have been defined as “an assembly of a distinct number of molecular components that are designed to perform machine-like movements (output) as a result of an appropriate external stimulation (input)”. Put simply, molecular machines, also known as molecular motors are either synthetic or natural molecules that convert chemical energy (fuel) into mechanical motion and forces. As any other machine, these molecular-level ones will require a supply of energy for their operation and can be driven by suitable energy sources.
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Consider a minuscule world comprising of nanoscale factories and self-repairing molecular machines that are capable of performing complicated and responsive processes with perfect control and efficiency. Efficacy in terms of precise translational and rotational movement and the ability to replicate any possible macroscopic function in a nano-world fueled by chemical and light energy is what Molecular machines have to offer. The Nobel Prize for Chemistry 2016 went to three chemists who envisioned ‘the design and the synthesis of molecular machines’ viz. Jean-Pierre Sauvage, J. Fraser Stoddart, and Ben L. Feringa.
Microscopic Machinery – Updates
After repeated failures and setbacks, a major breakthrough was seen in the year 1983. Sauvage linked two ring-shaped molecules to form a chain with one part of it that could move around the other stationary part. He used an ordinary copper ion to develop molecular complexes. Sauvage and his team of French researchers used photochemistry to create active complexes that capture energy contained in solar rays and use it to drive chemical reactions. The structure was similar to a molecular chain as it comprised of two molecules intertwined around a central copper ion. The insights from this experiment were further used to construct a ring-shaped and a crescent-shaped molecule so that they can be welded to the copper ion using the cohesive force that held the molecules intact.
The researchers then removed the copper ion which had solved its purpose of providing a base to construct these chains. In the late 1980s, the idea of creating molecular chains was no longer an abstract curiosity. Research teams now had started building assemblies of molecules with interlocked components. The most common one of these is known as rotaxane- a dumbbell-shaped molecule with a ring wrapped around the center that could slide freely along the axle, but was too small to come off at the either end.
While Sauvage discovered rotaxane, the credit for familiarizing chemists to molecular machines goes to Fraser Stoddart. Stoddart used the basic architecture of these nano-scale complexes to sketch the possibilities of large amplitude-controlled motions. He observed that on application of heat, the ring in rotaxane jumped like a tiny shuttle between the two electron-rich lobes of the dumbbell. He controlled this movement by 1994 and thus eventually led to construction of numerous molecular machines.
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A nano-scale lift that can raise itself 0.7 nanometers above a surface and an artificial muscle were designed in the years 2004 and 2005 respectively. The developments increased by leaps and bounds and Fraser, with the help of others developed a rotaxane-based computer chip with a 20kB memory capacity. Sauvage also went on to explore opportunities using rotaxane as the core molecular complex and in 2000, his research group managed to string two looped molecules together forming an elastic structure similar to the filaments of human muscle.
In simultaneous efforts to build molecular motors, Feringa used the idea of added energy to create a molecular rotor blade that would spin in only one direction. After overcoming the problem of basic random movements of molecules, he worked on the speed and by 2014, he had a motor that could spin at 12000 revolutions in a second. He used these motors to spin a glass cylinder 10,000 times bigger than the motors. His team constructed a ‘nano-car’ by linking several motors and axles.
With the radical developments made in this field, it is important to understand the dynamics of these nano-sized motors or machines. Since molecular machines find a huge scope of application in medical and biological scenarios, it is important to note that a certain functions can be linked to certain characteristic of some complexes in particular. Linear motor proteins find a pivotal role in many biological processes including muscle contraction, intracellular transport and signal transduction.
However, proteins replicating unidirectional rotary motors find use in synthesis and hydrolysis of ATP. These observations have been extended to the artificially created molecular machines to construct industry-grade sensors, actuators and transporters. To construct multicomponent mechanical machines, it is required to combine mechanical motions of different units, for instance the rotary motion of one part when coupled with the linear motion of another creates the molecular elevator.
Are we entering a new era of technological revolution?
When it comes to biological motors, a key disadvantage of their application ex vivo lies in their inherent instability and downsides in the environmental conditions they operate in. It is a known fact that nature is capable of maintaining and repairing damaged molecular systems on its own. Hence, to integrate such complex repair mechanisms would be a challenge for the current researchers in nanotechnology. On the positive side, synthetic systems have a higher tolerance towards more diverse range of conditions than their biological counterparts, thus offering considerable advantages in the development of complex nanomachinery.
While designing molecular machines, paramount importance should be given to the design such that the motion at a microscopic level can be translated to ‘visible’ motion at a macroscopic level. To translate nanoscopic movement to macroscopic levels, several units of these tiny molecular motors must be able to work cooperatively. A major difficulty in operating molecular machines lies in controlling their directionality. It is important to find a balance between the random Brownian movement of microscopic molecules and the controllable Newtonian motion at a macroscopic level.
The discovery of ‘molecular machines’ has been compared to significant game changers like the invention of the wheel, invention of the flying machine by the Wright brothers and invention of the first crude electric motor. Molecular machines might just be ushering us into an era of unprecedented technological revolution. The design of molecular machines heralds endless opportunities in industrial, scientific and theoretical innovations.
Although researchers working in this field have already built knots, shuttles, rotors, switches and chains at a microscopic level, a car powered by a molecular engine is still a distant dream, of course. Many working in the field speculate that molecular machines could find significant application in computing, novel materials, and energy storage. To quote Stoddart, ‘We’re on a very early part of a very steep learning curve. Chemistry is a fundamental science and it needs some space in which to develop the fundamentals. It’s going to be a slow process and it may take decades to develop the field to a stage where it’s applied to whatever the technology of the day is, but then suddenly it will take off, and people will see what all that fundamental development can lead to.’