Like similar molecular-scale efforts, the tweezers' arms merely open and close: they can't move around. But for nanoscale manufacturing to become a realistic prospect, mobile microscopic robots will be needed to assemble other nanomachines and move useful molecules and atoms around. The New York team's biped can "walk" because its DNA-based legs are able to detach themselves from a DNA-based track, move along a bit, then reattach themselves. Why DNA? Two reasons. First, unlike other polymers, DNA chains like to pair up. However, two DNA strands will only "zip" together if the sequences of bases in each strand complement each other in the right way- so by tweaking the sequences chemists get a high degree of control over where each strand attaches. Second, researchers hope that cells can one day be engineered to manufacture these DNA-based machines. Each of the legs in the walker is 36 bases long and is made from two strands of DNA that pair up to form a double helix (Nano Letters, DOI: 10.1021/n1049527q).
At the top, a springy portion of each DNA strand runs across from the left leg to the right, linking them together. At the bottom, one of the two strands pokes out of the helix to serve as a sticky foot.
The track, or "footpath", the walker travels on is also made of DNA, and is designed so that unpaired sections of DNA strands stick up like spikes along its length. These act as footholds for the walker. The feet attach to the footholds via "anchor" strands of DNA that match up with the foot sequence at one end and with the foothold at the other. Because the left and right foot/foothold sequences are unique, each requires a different anchor. So to make the walker take a step, a free piece of DNA called an unset strand is introduced to peel away one of the anchors (see Graphic), releasing the foot. The anchor has a handle at the top - a short length of the DNA strand which does not bind to the foot or foothold. The unset strand sticks to this handle and then binds with the anchor all the way down. The anchor comes away easily because it prefers to have partners for all its base pairs - including the sequence in the handle. The free foot grabs a new anchor sequence, which latches onto the next foothold, stepping the foot forward. Repeating the procedure to move the back foot forward completes the walker's shuffle. The walker takes its nanostroll in a bath of a liquid called a "nondenaturing buffer", which stops the DNA falling apart. To start with, millions of walkers and tracks are floating around freely in this liquid. Only when the researchers add the DNA anchors do the bipeds' feet fix onto the footpaths. Then the unset strands can be added to begin the walking process. The researchers were able to confirm that the nanowalkers had taken their first steps by taking small samples of the solution after each DNA addition. By feeding the material through a gel which separates DNA molecules by size and shape, they confirmed where the feet were attached- it is the same technique that gives "DNA fingerprints" in forensics. Persuading the walker to ferry a load, such as a metal atom, is the team's next challenge.
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JENNY HOGAN, BOSTON