It might be impossible to quantify the number of machines involved in daily life. We use machines to control the climate in our homes, move between places, and heat up water for our morning cup of coffee or tea, among other things. Somewhat less obvious, though, is the fact that our very ability to get up in the morning and make a caffeinated drink relies on a more hidden kind of machinery: molecular machines that convert the energy and carry the signals that power our bodies. Those tiny biological machines serve as inspiration for work that extends human engineering capacity down to the molecular level.
“The molecular scale, obviously, has very different rules than the macroscopic scale. Everything flies around in this Brownian hurricane all the time—everything moves and vibrates and wiggles,” says Michael Kathan of Humboldt University of Berlin. In an environment awash with the noise of thermal motion, directing energy into specific tasks requires different strategies than the ones used in the macroscopic world (see the article by Dean Astumian and Peter Hänggi, Physics Today, November 2002, page 33). And just as the development of complex machinery began with simple tools like wheels and levers, making machines that work at the molecular scale required the establishment of basic components.
An early step was the synthesis of mechanically interlocked molecules. In contrast to covalently bonded atoms, which share valence electrons in a covalent bond, mechanically interlocked molecules are connected by their physical shapes, as shown in figure 1, in what’s known as a mechanical bond. Mechanically interlocked molecules come in a few shapes, including knots, rings on an axle, and intertwined rings. Just like macroscopic metal chains, ring-shaped molecules linked together, known as catenanes, can combine the benefits of strength and flexibility and exhibit other emergent properties. Catenanes’ shape flexibility, for example, could make them promising catalysts. Mechanically interlocked molecules’ ability to move in relation to each other also makes them useful building blocks for nanoscale machines.
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