In 1986, Eric Drexler published Motors of Creation, a vision of what the emerging science of nanotechnology could mean for the future. Drexler claimed that nanoscale robots would one day swim through our bloodstream, attacking invaders such as viruses, and removing sclerotic debris from the walls of blood vessels.
Drexler laid out the technical aspects of his vision in a 1992 book. Nanosystems, who argued that a “molecular assembler” would position atoms exactly where they were supposed to build nanoscale devices like motors and gears made of diamond-like carbon. If such fitters could build themselves, this manufacturing process could be scaled down inexpensively to make us masters of matter with atomic precision.
The books had a huge impact on futurologists. In Ray Kurzweil’s 2005 book The singularity is nearDrexlerian nanotechnology played a key role in the imaginary fusion of man and machine, which Kurzweil believed would usher in a form of immortality.
Scientists were less in love. Some accused him of disregarding basic chemical principles by assuming that atoms could be arranged in arbitrary structures. One of the most famous skeptics was Nobel Prize winner Richard Smalley, co-discoverer of fullerenes and a leading expert on carbon nanosciences. He argued in 2001 that Drexlerian nanoassemblers would be undermined if the fingers were too “fat” – bulkier than the structures they were trying to make – and too “sticky” because they couldn’t simply let go of the atoms that position them .
While such arguments went inconclusive, Drexlerian nanotechnology caught the public imagination. His apocalyptic vision of ‘gray goose bumps’ mentioned in Motors of Creation As a threat to the replication of nanoassemblers getting excited and breaking all of Earth’s biomass into carbonaceous fragments, it was irresistible to science fiction writers – villainous Drexlerian replicators featured in Michael Crichton’s 2002 thriller prey. And in Neal Stephenson’s steampunk future in The diamond ageDrexler is a hero in a world powered by carbon-based nanotechnology.
Three decades later Nanosystems, Nanotechnology has gone mainstream, but there is still no sign of Drexler’s vision of it emerging. And yet … the fantastic, precision-engineered rotors depicted in his book, spinning in their lubricated bearings, come to mind when faced with the unearthly elegance of a new structure of the bacterial flagella engine recently developed by a team at Zhejiang University was unveiled in Hangzhou, China.1 Yongqun Zhu and colleagues used cryo-electron microscopy to create an atomic-resolution image of this device, which is perhaps the most famous example of a biological nanomachine.
The motor sits in the bacterial cell membrane, its axis is attached to a long protein thread known as the flagellum. It rotates at several hundred revolutions per second, driven by an influx of hydrogen ions along an electrochemical gradient. As the rotor spins within two protein-based sleeve assemblies (each comprising multiple rings and sub-rings), the flagellum whips around, propelling the bacteria through fluids. When all the motors spin in the same direction, their whip-like flagella spin into a single bundle of screws that acts like a propeller. This movement drives the bacterium towards increasing concentrations of nutrients such as sugars or amino acids. When concentration drops, some motors are briefly shifted into reverse, which will break the bundle of flagella and randomly stagger the bacteria before venturing in a new direction to see if this increases concentration. This is how bacteria achieve chemotaxis: the ability to swim up a concentration gradient.
The engine pictured by Zhu and colleagues – that of Salmonella Bacteria – is about 26 nm in diameter and consists of 175 subunits of about 12 different proteins. (The whole in vivo The assembly is closer to 40 in size.) Excellent balanced electrostatic intermolecular forces between the rod-shaped axis and the top ring embedded in the outer membrane help hold them in place and maintain separation. The lower rings act as a rotor and engage stator units in the inner diaphragm to generate torque.
It is amazing that evolution can produce a device that looks so tiny and yet so finely constructed. Does this mean that we can actually only shrink mechanics down to the molecular scale, as Drexler suggested? Not really. The flagella motor stands out precisely because it is extraordinary. Most of the nanoscale machines in the cell, such as the motor proteins dynein and kinesin, that move objects in cells, are far less like our own miniaturized versions. The mechanics of the cell are stochastic, non-deterministic and fault-tolerant, not precisely structured. It’s a mechanic designed for the molecular scale.
Most importantly, no nanoassembler has ever built this thing: It assembles itself and not in Drexler’s thermally dormant vacuum, but in the chaotic chaos of the cell. What I said when I checked Nanosystems to the nature seems to be confirmed: he suggested “doing the hard way things that could be done easier and faster with a little chemical ingenuity”.