Molecular machines are nano-scale assemblers that construct themselves and their surroundings into ever more complex structures. Sometimes dubbed "nanotech" in the media, these devices are promising but also widely misunderstood. Here's what separates the science fact from science fiction.

The concepts that underpin this form of nanotechnology have certainly had long enough to percolate through modern science. Richard Feynman first speculated about the idea of "synthesis via direct manipulation of atoms" during a talk called There's Plenty of Room at the Bottom. Looking back, that sparked much of the subsequent thinking about treating atoms and molecules more and more like simple building blocks.

Perhaps most famously, K. Eric Drexler considered the idea of taking the bottom-up manufacturing approach to its atomic extreme in his 1986 book Engines of Creation: The Coming Era of Nanotechnology. There, he posited the idea of a nan-oscale "assembler" that could scuttle around, building copies of itself or other molecular sized objects with atomic control; one which might in turn be able to create larger and more complex structures. A kind of microscopic production line, building products from the most basic ingredients of all. Coming when it did, in the mid-eighties, it felt very much like science fiction.

So much so, in fact, that even Drexler acknowledged that it was prudent to tread carefully in a nano-scale building site. "Imagine such a replicator floating in a bottle of chemicals, making copies of itself," he explains in Engines of Creation. "The first replicator assembles a copy in one thousand seconds, the two replicators then build two more in the next thousand seconds, the four build another four, and the eight build another eight. At the end of ten hours, there are not thirty-six new replicators, but over 68 billion. In less than a day, they would weigh a ton; in less than two days, they would outweigh the Earth; in another four hours, they would exceed the mass of the Sun and all the planets combinedif the bottle of chemicals hadn't run dry long before."

That ruthless efficiency could, Drexler argued, make some nano-robots "superior" to naturally occurring organic beings, at least in an evolutionary sensethough, crucially, not necessarily as valuable. Indeed, he suggested that omnivorous bacteria could out-compete real bacteria, reducing the biosphere to dustor 'grey goo'in a matter of days. That hypothetical end-of-the-world scenario, where nanobots turn our world and us into an amorphous sludge, was as tempting to skeptics as the promise of nanotechnology was to scientists. Still, almost thirty years on we're still here and, while some of us may be a little more ashen of face, we're yet to be submerged in the biological by-product of engineered molecular machines.

Truth is that scientists have been very busy indeed over those past thirty years, creating a host of molecular-sized structures that can manipulate and assemble themselves, move, and even work together. It's not always easy, of coursebuilding at the molecular levels requires atomic accuracybut mercifully chemistry and physics has advanced to a point where it's increasingly possible. And there's a rich pool of molecular machines, some inspired by nature, others by mechanical engineering principles, to show for it.

The majority of successes have been built from DNA molecules. Here, DNA isn't being used to carry genetic information; rather, it's a structural material in its own right. Its four basesadenine, cytosine, guanine and thyminebind more or less strongly to one another depending on how they're paired up along the length of a DNA double helix, allowing scientists to tweak the way in which they join together. "We can direct the associations of molecules through Watson-Crick base pairing. Intermolecular interactions using sticky ends have a well-defined geometry," explains Professor Ned Seeman, a nanotechnologist in the Department of Chemistry at New York University, who's widely regarded as inventing the field of DNA nanotechnology. "DNA is like Lego."

The fundamental building blocks of life already have the features required to fold, join, build and growso they're perfectly suited to building things at the nano-scale. By creating strands of DNA with carefully controlled base sequences, the binding can be specifically tailored so that customized strands can be combined to bind with each other and construct exotic structures. Geometries are first modelled on computers to work out what molecules are required, then the appropriate can be DNA synthesised in order that they can be put togetherjust like a Lego kit.

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What Will the Future of Molecular Manufacturing Really Be Like?