Worlds of David Darling > Children's
Encyclopedia of Science > Nanotechnology > 3. Gearing Down
MICROMACHINES AND NANOTECHNOLOGY:
The Amazing New World of the Ultrasmall
a book in the Beyond 2000 series by David Darling
3. Gearing Down
Researchers have found out how to make electronic components incredibly
small. Now they are also learning how to create microscopic versions of
machines like gears, pumps, and motors. These miniature moving gadgets are
known as micromachines or micromechanical devices.
|A tiny mite, barely visible to the unaided
eye, moves toward a micromechanical device
Many people have heard about microchips and how important they are in all
sorts of ways. By comparison, not so much has been said in the media about
micromachines. One of the reasons for this lack of attention is that micromachines
are a lot harder to make and so scientists are still working to perfect
them. Over the next few years, however, micromachines will become more and
more common in everyday life. Eventually, they may just as important a part
in the running of our modern world as microchips do today.
Micromachines took a giant step forward in 1988 when a team for the University
of California at Berkeley demon started a working electric motor that could
be seen only with the help of a microscope. This motor had a main spinning
part, or rotor, that measuring just 60 micrometers, or millionths of a meter,
across. If this rotor had been made ten times bigger, it would still have
been only as wide as a pin.
Since then, even smaller micromotors have been built. Teams of scientists
all over the world are studying how to make these motors work more efficiently
and produce more turning force, r torque, so that they will be able to do
One of the biggest problems with micromotors is friction, the force that
acts when surfaces rub against one another. Friction is what stops your
bicycle when you put on the brakes. We depend on friction to prevent us
from skidding every time we walk. But inside a machine that is supposed
to move freely and easily, friction is a nuisance.
With an ordinary-size electric motor, such as the one in a washing machine,
the frictional forces usually add up to less than a hundredth of the torque
that the motor produces. In micromotors, however, the friction can be as
big as the torque. When this happens, the motor is stopped from moving.
The first micromotors tended to lock up after just a few thousand spins.
Scientists working on these very small devices realized that they would
have to pay special attention to designing bearings that cut down friction
as much as possible. Their efforts have been very successful. The latest
micromotors are still hampered by friction, but the amount of friction can
be reduced to less than 10 percent of the torque. As a result, micromotors
can now run for days on end at low speeds or in short bursts at up to 15,000
revolutions per minute.
Making a Micromachine
Using methods borrowed from siicon-chip technology, researchers started
making micromachines in the 1970s. But they quickly realized that these
methods would have to be adapted in various new ways, because microelectronic
components are more or less flat, whereas micromechanical ones are three-dimensional.
Micromachines have length, width, and height.
|How to make a part for a micromachine
To construct a micromachine, scientists make a multilayer silicon sandwich.
On the bottom is a single crystal of silicon. Above this is a layer of silicon
dioxide. Next comes another layer of silicon, called polycrystalline silicon,
or POLYSILICON, because it is made of lots of little jumbled-up crystals.
On top of this is a second layer of silicon dioxide, followed by a second
layer of polysilicon.
The moving parts for a micromachine are made by masking and dissolving away
specific regions of the second (top) polysilicon layer. Acid is used to
remove the unwanted portions of the silicon dioxide layer below, leaving
the moving parts free to spin or pivot. The underlying polysilicon layer
provides the secure base upon which the micromachine motors and levers can
Alternatives to Silicon
In some ways silicon is a good building material for micromachines. It is
almost twice as hard as iron and more difficult to stretch than steel. Silicon's
biggest drawback is that it is brittle – it can crack or shatter quite
easily. Even little bits of silicon that break off inside a micromachine
can act as grit that stops the device from working.
In the early 1980s a group of scientists at the Karlsruhe Nuclear Research
Center in Germany began developing a new method for making micromachines.
This method is known by the initials – LIGA. Using LIGA, researchers
can build micromachines from nickel and other metals that are much less
brittle than silicon.
Instead of the ultraviolet rays used in microchip technology, LIGA uses
X-rays. The X-rays are shone through a mask – a stencil of the machine
being made – onto a layer of polymer. Then a develop is used to dissolve
the polymer where the X-rays have penetrated it. Up to this point, LIGA
follows steps similar to those involved in making silicon microchips.
Next, however, an electrical process fills the gaps in the polymer with
nickel or a metal like nickel. When this has been done, the rest of the
polymer coating is dissolved away.
The nickel shape that is left behind can be used either directly or as part
of a micromachine or as a master mold from which to make copies. To produce
copies, technicians place a layer called a casting plate over the nickel.
Through tiny holes in this casting plate, hot liquid plastic is injected.
When the plastic has cooled and hardened, it is lifted off and used as a
mold for making duplicates of the original nickel structure.
To make a micromotor with spinning parts that a finer than a human hair
is an amazing achievement. But can these tiny devices do any useful jobs?
One of the problems with micromotors is that they produce very little torque.
Once the friction between their moving parts has been overcome, they can
often barely keep their own rotors spinning around, much less help turn
any larger devices to which they might be attached.
One way to increase the power of a micromotor is to build it less like a
pancake, because the size of a motor's torque increases with the height
of its spinning parts. The methods use for manufacturing silicon micromachines
are not suited to producing structures higher than about 10 micrometers.
However, if nickel and the LIGA process are used, this height limit can
Researchers in Germany made a nickel devices that was only 5 micrometers
wide, but 300 micrometers high. At the University of Wisconsin, another
team of scientists made toothed nickel gears, 50 to 200 micrometers across
and 200 to 300 micrometers high. These toothed wheels were linked together
to form gear trains. Such gear trans could eventually be used to transmit
the motion of a micromotor to other pieces of equipment nearby.
Researchers at Carnegie-Mellon University in Pittsburgh have shown
that microchip manufacturing methods can be used to create what they
call micromechanical Velcro. First they grow a silicon dioxide layer
on a silicon base. Then they repeatedly mask chosen areas and remove
the unwanted parts with acid. The process produces silicon dioxide
arrowheads, or hooks, on silicon pedestals. Each structure is no more
than 18 micrometers wide, and 200,000 of them can be packed onto a
The tiny arrowheads will pierce human flesh and then hold fast because
of their barbed ends. The Pittsburgh team believes the arrowheads
could be used to rejoin severed blood vessels and to close wounds
in place of stitches. But unlike everyday Velcro, the microscopic
version does not pull apart easily. Instead, the hooks tend to break
off as they are separated.
Swimming Bacteria and Medicine
In Japan a team of scientists investigated another kind of micromechanical
device that could eventually be used in medicine. These scientists looked
at ways to propel machines inside a patient's body to deliver lifesaving
drugs. Ordinary micromotors might not be able to move in blood, which is
quite a thick liquid. So, to solve this problem, the Japanese scientists
looked at the possibility of copying the tiny whiplike structures that some
kinds of bacteria use to move around. These structures, which are only 30-billionths
of a meter in diameter, are called flagella.
It has been known for some time that parts of a flagellum work like an ordinary
electric motor. Close to where it joins the main body of the bacterium,
the flagellum has a rotor that spins around inside a fixed ring.
The Japanese scientists want to discover the exact arrangement of chemicals
that help such a tiny structure to work so effectively. With this knowledge,
they hope to make a computer model of the flagellum and then try to figure
out how to produce an artificial version. Such an advanced micromachine
probably lies many yeas in the future, however.