IT MAY look like a line-dancing bar stool鈥攂ut scientists at MIT reckon
their nanowalker will make even Michael Flatley look flat-footed. Their agile
robot can take 4000 steps per second. It can tiptoe along in 4-nanometre steps
or stride around at up to 20 centimetres per second.
The three-legged, 3-centimetre-tall walker is the brainchild of Sylvain
Martel at MIT鈥檚 Bio-Instrumentation Laboratory. Martel wants swarms of
nanowalkers to act as precision tools to manipulate individual atoms or
molecules to build nanomachines. 鈥淵ou can use this for manipulating atoms,
changing molecules, developing new materials or studying DNA,鈥� says Martel.
The first tool he has built into the nanowalker is the probe and imaging
circuitry of a scanning tunnelling electron microscope (STM). He chose the STM
because the tip of the microscope is used to nudge atoms around, as well as
perform microscopy on the atomic scale.
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Martel plans to use the nanowalker鈥檚 accurate footfalls to take pictures of a
single atom, recording precisely where it is, so that other robots can come back
to the same atom and manipulate it. While Martel does not claim to have invented
a new instrument, he has managed to slash the size of the business end of an
STM.
The nanowalker also contains a water cooling system, flexible circuits that
drive the walking mechanism and an infrared communications system for sending
and receiving control and microscope image signals. 鈥淭his is the most
complicated thing I have ever had to do in my life,鈥� he says.
Each of the nanowalker鈥檚 three legs is divided up into four piezoelectric
ceramic quadrants which change their length depending on the voltage you apply
to it (see Graphic).
If you apply a similar voltage to all four quadrants, the
leg will extend. Reverse this voltage and they will shrink. But if you apply
opposite polarities to opposite quadrants the leg will bend, as one side extends
and the other shrinks. The nanowalker also has a number of different gaits,
ranging from bouncing to stepping. It all depends upon the structure of the
surface鈥攁nd the speed and length of the strides it needs to take.
The robot will take forever to get from A to B with tiny steps, says Martel.
But the walker can vary the length of its stride from a few nanometres for
accurate work to 50 micrometres for moving round at speed.
You can exploit the fact that the piezoelectric actuators react much more
slowly than the robot鈥檚 electronics to curtail the length of the robot鈥檚 stride,
say Martel. When you tell the legs to move, they barely have enough time to
start moving before the next step command arrives. This limits the size of step
which can be taken.
Because each nanowalker can carry out 200,000 measurements a second, Martel
says hordes of them could work in concert, performing millions of operations
every second. To control them, Martel鈥檚 broadband infrared communications system
acts like a kind of air traffic control radar, recording the position of the
nanowalkers.
The walker gets its power through its legs. It operates at 15 watts, so heat
is a big problem, says Martel. It needs to be cooled continuously to prevent it
burning out, so to dissipate the heat each device comes with its own
water-filled jacket.
Martel hopes that future nanowalkers will carry tools that will deposit and
etch the materials used to build up a microchip. This will avoid the need to use
photolithography鈥攁 technique that is running out of steam as demand
increases for denser and denser circuits.
