A new movie, I Robot, opens July 16th with a vision of lifelike robots that do all our work for us. Amazingly, one nanotechnologist has made a start: he's built an incredibly tiny walking robot out of living material, DNA. As this ScienCentral News video reports, this minute robot eventually could help design drugs, molecule by molecule, to fight specific diseases in individual patients.
Building with DNA
In the movies, robots often are clanking metal menaces. But the latest real-life robot is invisible to the naked eye.
At New York University, chemist Nadrian Seeman and his post-doctoral associate William Sherman reported in the journal Nano Letters that they've built a sub-microscopic robot, or "nanowalker," that walks on two legs just ten nanometers long, made of fragments of DNA. And their tiny stroller has taken its first baby steps—two forward and two back.
Seeman says the team uses DNA for a reason: "We all know that DNA is a double helix. The two strands of DNA wrap around each other and form this double helix," he says. The structure makes DNA a particularly strong molecule – and also a very versatile one. "DNA is the easiest molecule to work with," Seeman explains, "because we can control the interactions between DNA molecules in the same way that nature controls the interactions between the two strands of one DNA molecule."
Seeman was inspired to begin building with DNA in a bar in Albany, New York, in the fall of 1980, where he was working as an X-ray crystallographer. Sitting in the bar, he realized that the flying fish in a familiar woodcut by M.C. Escher were similar to a DNA molecule with six branches. To Seeman, Escher's image, which now hangs in his NYU office alongside intricate knots and posters of DNA, was familiar from his crystallography work. Escher's picture shows "a whole bunch of flying fish that look just like a jack, a six-armed junction arranged like the molecules in a crystal."
Over the next two decades, Seeman has been using DNA to make intricate structures that resemble elaborate crocheting, including a cube and a truncated octahedron. Last year, as reported in the Journal of the American Chemical Society, he and his team were able to "knit" nylon with DNA. "What we are trying to do is to control the structure of matter on a very small scale, to actually design molecules on the most fundamental levels, and then get them to assemble into specific three-dimensional molecular structures." To do that, Seeman takes advantage of the fact that DNA's two strands like to pair up with other strands, the a zipper's tracks do, but only if the sequences of bases in each strand are complementary. "If DNA can do so, it will form these complementary helices at all sites that are available to it," says Seeman, "and these double helices are very predictable. If we know what's on one strand, we know what's on the other strand." By controlling the sequences, the chemists can control where each strand attaches.
DNA image: Drew Berry, The Walter and Eliza Hall Institute of Medical Research
In the case of the nanowalker, the robot's DNA legs, each 36 bases long, are linked to each other by DNA. Each leg has a foot-like extension that is single stranded, and can get a foothold on a track that's also made of DNA. Each foot can pair up with a complementary DNA strand on the special sidewalk. To make the foot take hold of the sidewalk, Seeman and Sherman add a "set strand"—complementary to both strands—that zips the foot to the foothold. To make the robot walk, they then add an "unset strand" to unzip the foot. Then the released foot grabs another set strand to move forward and zip up to its next hold on the track. A new set strand upzips the foot to allow it to step backwards.
Once Seeman and Sherman's tiny biped is able to take more than a couple of steps, it could "man" a molecule-sized assembly line. "I believe we can make molecular-level assembly lines to do the same sort of things that are made on a human scale," says Seeman. "Ultimately I think this will lead to nano-manufacturing, or 'nanofacturing', where we can take a molecule and move it along, just like a car on an assembly line in Detroit. And just the way a car reaches a certain point and a spot welding robot comes down and works on it, our nanoscale robot can do some chemistry on a molecule. Then the molecule can move on, and at the next point where we need more chemistry, the same thing can happen."