DNA Robots Sort and Carry Molecular Cargo

Just when you might have thought robots couldn't get any smaller, scientists at the California Institute of Technology announced today they have developed a new kind of biological robot made from a single strand of DNA. The nanoscale robot — or nanobot — is so small that it can pick up and sort individual molecules.

While the DNA strand isn't a robot in the electromechanical sense, it's designed and built much in the same manner as a traditional robot, with moving parts that stand in for legs, arms, and hands. It's also programed like a robot, although in this case the programing is all done in the language of chemicals.

It works like this: Strands of DNA have unique chemical and physical properties determined by their nucleotides. Designated by the letters A, C, G and T, these compounds are arranged in a sequence with specific pairings — A bonds with T, or G with C. When a single strand encounters a compatible other strand — technically a “reverse complimentary strand” — the two zip together, creating the familiar double-helix shape of DNA.

This zipping action is what makes synthetic DNA robots possible, said senior author Lulu Qian, assistant professor of bioengineering at Caltech.

“Many molecular components in biological organisms can be viewed as machines that perform mechanical tasks at the nanometer scale,” Qian said in press materials issued with the new research. “Inspired by these natural molecular machines, we wanted to design and build artificial molecular machines that have similar capabilities, but allow their functions to be programed by humans.”

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Each Caltech DNA robot is made of components designed to act as robotic limbs. The robot's “foot” component zips and unzips as it moves across a molecular surface, bonding with complimentary chemicals.

So long as the designers know the chemical makeup of the robot, and the surrounding molecular environment, they can predict when the robot will attach and when it will release. This same principle allows the robot's “hands” to grab and carry individual molecules.

In order to test the robots, the Caltech team built a kind of molecular playground, a 58-by-58-nanometer pegboard structure. (A sheet of paper, by comparison, is about 100,000 nanometers thick.) As the robots roam the pegboard, they grab molecules tethered to the pegs, then drop them off when they receive a chemical signature at a different peg.

In a series of experiments backed by Caltech, the National Science Foundation, and the Burroughs Wellcome Fund, the DNA bots sorted six different molecules and placed them in a designated drop-off zone. Qian said a single robot might take as long as 24 hours to explore the entire pegboard surface, but multiple bots were able to get the job done more quickly. The results of the experiments were published today in the journal Science.

“We used two fluorescent dyes, one is yellow and another is pink, as two types of molecular cargo,” Qian said. “However, in principle, a cargo could be any molecule that is chemically linked to a DNA strand. For example, it could be a protein, a small chemical, or a metal nanoparticle.”

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The ability to sort and carry molecular cargo could make DNA bots valuable for future applications in manufacturing or medicine. For example, a DNA robot might build compounds in an artificial molecular factory, working on scales otherwise impossible. Or a DNA bot could synthesize a medicine in the bloodstream, releasing it only when it recognizes a chemical signal generated by a diseased cell.

While researchers have previously built molecule-carrying DNA strands, the Caltech project is intended to make general-purpose DNA robots a reality, Qian said.

“Many of the potential applications are still science fiction, and there are many practical difficulties that need to be overcome,” Qian said. “I hope the work will inspire more researchers to develop modular, collective, and adaptive DNA robots for a diverse range of tasks — to truly understand the engineering principles for building artificial molecular machines, and make them as easily programmable as macroscopic robots.”

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