A microscopic biped with legs just 10 nanometres long and
fashioned from fragments of DNA has taken its first steps.
The nanowalker is being hailed as a major breakthrough by
nanotechnologists. The biped's inventors, chemists Nadrian Seeman
and William Sherman of New York University, say that while many
scientists have been trying to build nanoscale devices capable of
bipedal motion, theirs is the first to succeed.
"It's an advance on everything that has gone before," says
Bernard Yurke of Bell Labs in New Jersey, part of the team that made
one of the best-known molecular machines to date: a pair of
"tweezers" also constructed from DNA strands (New Scientist
print edition, 12 August 2000). Like similar molecular-scale
efforts, the tweezers' arms merely open and close: they can not move
||One small step for DNA:
How the nano-sized biped walks|
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. 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
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.
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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.
Journal reference: Nano Letters (DOI: