IEEE Spectrum October, 2012 - 51

Fixing a magnetic
"north" pole at the
tumor entices the bacteria
to swim toward it.

the bacteria
converge inside the
tumor and die, releasing
their drug payload.

Flagella

Magnetosomes
Drug payload

The MC-1 bacterium has a chain of magnetosomes (magnetic nanocrystals) that aligns it with a surrounding field.
Anticancer drugs are cocooned in liposomes (fat-like sacs)
and attached to the bacterium using antibodies.

could swallow the surgeon." As Feynman's friend imagined
it, a little mechanical surgeon marches through the bloodstream to the heart and surveys its surroundings. Upon spotting a problematic obstruction or a faulty valve, it simply "takes
a little knife and slices it out."
The problem, of course, is how to make the surgeon. Feynman
envisioned an iterative manufacturing process in which evertinier hands make ever-tinier machines. He also considered the
far-fetched dream of building things atom by atom, the way kids
build toys out of Lego bricks. But he failed to offer any practical
approaches. "I leave that to you," he told his audience.
Four decades later, the closest engineers had come to realizing Feynman's challenge was a diagnostic device called the
PillCam. Introduced by the Israeli company Given Imaging in
2001, the disposable plastic capsule is about the size of a single
piece of Good & Plenty candy and can be swallowed with water.
It contains a light, a camera, batteries, an antenna, and a radio
transmitter. As it passes through the gastrointestinal tract, it
beams pictures wirelessly to a recording belt worn around a
patient's waist. The images are then analyzed for signs of cancers, Crohn's disease, and other disorders. The procedure is
less invasive than a traditional endoscopy and more sensitive
than X-ray imaging.
But while the PillCam is great at inspecting the large, easily accessible cavities of the digestive system, it's much too big
to travel elsewhere in the body. The device is also passive. A
true medical microbot must propel and steer itself through an
intricate network of fluid-filled tubes to tissues deep inside the
body. And because it is so small, it must do so without the convenience of a battery-powered motor.
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You might think that an obvious way to accomplish this
task is to equip the robot with metallic particles and guide it
with a magnet from outside the body. You wouldn't be entirely
wrong. Indeed, some magnet-tipped surgical catheters work
in this way. But a permanent magnet poses several problems
for a smaller, tetherless robot. For instance, the magnet must
be positioned quite close to the tiny particles to exert enough
force to attract them. The robot could operate only near the
skin and in one direction-toward the magnet. It would also
be difficult to control. As it moved toward the magnet, the pull
would increase, accelerating the robot and making it impossible to redirect if it went astray. And how would you know
where it was going, anyway?
The AnSwer iS The Mri MAchine. To understand how an MRI

machine can be used for robotic navigation, you must know
something about how it works. The bulk of the machine is a
powerful, doughnut-shaped superconducting magnet that generates a magnetic field up to about 60 000 times as strong as
Earth's. In medical imaging, the field's purpose is to align the
spin of protons-the nuclei of hydrogen atoms-in the body.
(A spinning proton acts as a sort of bar magnet; like a compass needle, it points in the direction of a surrounding field.)
Tucked inside the big magnet is the RF coil, which transmits
radio-frequency waves. When the frequency of a wave pulse
matches the spin rate of the protons, the spin "flips" direction
by 90 degrees. When the pulse ends, the protons relax back to
their original alignment, a lower energy state. The lost energy
departs as a radio signal, which is picked up by a receiver.
The density of signals gives information about the molecular
october 2012 * Ieee Spectrum * NA

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Table of Contents for the Digital Edition of IEEE Spectrum October, 2012