The machines we're familiar with operate on some pretty obvious principles. It's easy to supply them with energy, whether it's chemical or mechanical. Given parts we've carefully arranged, that energy gets converted into some form of useful work, like pumping water or moving a car. While the environment can have an effect on the machine's performance, the machine can usually operate over a wide range of conditions.

Absolutely none of this is true at the level of individual molecules, yet people have figured out how to make machines out of little more than a few dozen atoms. This year's chemistry Nobel honors three pioneers in this field.

Nearly everything about building a molecule-sized machine is hard. You can't put the parts in their proper places by hand or even with a small machine; they have to be built by chemical synthesis. The environment dominates at the molecular level, with Brownian motion tending to force molecules to move about at random rather than in the directed manner needed to perform work. And providing energy to a molecule is much more challenging than giving an internal combustion engine something to burn—the energy for a molecular machine needs to be precisely tailored to act at specific chemical bonds.

The process of solving these challenges was a gradual one, and it required a lot of work by many people. But there were some key landmarks along the way, and the three Nobel honorees were behind a number of them. Jean-Pierre Sauvage of the University of Strasbourg gets credit for helping set the field in motion by being the first to synthesize molecules linked by a mechanical bond rather than a chemical one. The class of molecules that were developed along these lines have been termed catenanes.

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Sauvage was working on carbon ring structures that were attracted to copper atoms. If you linked two of these rings at the right distance, the resulting molecule would wrap partway around the copper. Sauvage's group extended one of these molecules so that it curved around and linked to itself, creating a ring with copper off on one side. He then supplied a similar structure in two halves, both of which also wrapped themselves around the copper atom. This put the ends of the two halves in close proximity, allowing them to be linked. The result was two ring-shaped molecules that were interlocked, each threaded through the center of the other.

Sauvage's team went on to demonstrate that if you give the rings the right chemical structure, you can get one to rotate around the other. And rather than purely being driven by Brownian motion, the rotation could be controlled and driven by energy supplied through either chemicals or light. Sauvage's group also demonstrated that it was possible to get molecules to expand and contract, acting a bit like muscles.

But while his work helped create the field, he was followed by other labs, among them Fraser Stoddart's (currently at Northwestern University). He created what are called rotaxanes through a similar synthesis approach. In his case, the starting material was a dumbbell-shaped molecule with a long central bar. The bar was engineered to contain an internal ring structure that would attract a series of similar molecules. Stoddart arranged these molecules so that they were part of a single molecule with ends that could be connected to form a loop. When this loop was closed, it closed around the bar of the dumbbell.

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Using this basic design, Stoddart's group made a series of molecular motors. In one, the ring could be shifted back and forth between two locations on the bar of the dumbbell, and then it was possible to control the ring's location. Once the group figured out how to read the ring's location, it was possible to create a form of digital memory using this molecule.

Ben Feringa at the University of Groningen figured how to translate a basic mechanical device—the ratchet—down to molecular scales. With the right chemical groups, he was able to ensure that a molecular motor could only rotate in one direction, ensuring that the environment couldn't randomize the motion of these molecules. With further modifications, it became possible to power the rotation through UV light, which created a series of internal chemical rearrangements that drove rotation.

With some refinements, he was ultimately able to get his motors to turn at 12 million revolutions a second. By combining four of these rotors with a central support structure, he was able to create an all-wheel drive molecular car. Other refinements include figuring out a way to power the car through chemistry.

Overall, the field has generally followed a similar pattern. Figure out how to build a basic structure, figure out how to get it to move, then figure out how to direct and power that motion. What's missing so far are the applications, as we don't currently have an obvious need to go four-wheel driving at the molecular level. But the people behind the prizes note that the steam engine didn't have much obvious utility when it was first developed, and it was originally put to use to power a pump for draining mines—the industrial revolution took a while to follow.