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MEMS Technology and the Microhand
February 2007
The Microhand
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From the minds, and laboratory, of Dr. Yen-Wen Lu and
Dr. Chang-Jin Kim comes the Microhand, a cutting-edge
example of micro-fabrication techniques and quite
probably the future of non-invasive surgery. The
Microhand measures about 1mm across when clenched and
is, according to CJ Kim, the world’s smallest robotic
hand.
The hand itself is comprised of four fingers, each
consisting of 6 silicon wafers like those used in
electronics. The silicon wafers have polymer balloons
between them that can be inflated to force the hand to
curl closed and drained to allow the fingers to release.
The balloons are all joined by narrow channels etched in
the silicon wafers through which air is pumped, allowing
very controlled and equal pressure to be applied to the
digits of the hand. We believe the balloons are made of
a polymer called parylene, which is highly suited to
forming skins of consistent thickness that trap air well
and function well in chemical and biological
environments. |
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In testing, the hand was used to grasp
a single fish egg from a clutch of capelin roe, which is
also called masago by those of us with a taste for the
sushi. Capelin roe is extremely small and particularly
delicate, yet binds itself protectively into its clutch
through high surface tension. The demonstration, which can
be viewed in a livescience video following a short but
annoying commercial, demonstrates the impressively precise control possible
with the hand.
It was able to grasp the roe gently enough not to crush it,
yet maintained a firm enough grip to overcome the egg’s adhesion to the rest of
the clutch. The digits appear to be structurally rigid, not deforming or
flexing while pulling against the surface tension of the Capelin roe, providing
the human-hand equivalent of a “firm but gentle grip”.
The Microhand was the PhD work of Yen-Wen Lu, currently
an assistant professor at Rutgers University, and he
says that one of the biggest challenges in the
development of the Microhand was in devising a simple
but effective method of closing the hand. |
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Parylene
Parylene is a polymer which is
used in the construction of electronic sensors and
components, coils, keypads, wire, fiber optics, and
disk drive components.
It is produced by
“vapour-phase deposition”…this is more or less fancy
term for “condensation”. A cloud of appropriate
chemicals are allowed to settle out and form parylene, which is often used as a coating for its
ability to completely and uniformly coat surfaces
with edges, sharp corners, and protrusions. The
coating is often used as a long-life lubricant or
protectant. Parylene does not run or sag and forms a
pin-hole free layer that resists penetration by
gases and liquids.
It
is biologically inert and resistant to funguses and
bacteria, very resistant to electricity and chemical
attack and solvents, and is mechanically strong. It
has a low coefficient of friction and makes a
wonderful lubricant. Its properties remain
consistent with changes in temperatures.
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Silicon
Silicon is extremely abundant, making up ¼ of the
earth’s crust. It is non-toxic, mechanically strong, and biologically and
chemically inert. Its manipulation and manufacture is well known and used
extensively through the semiconductor industry. Silicon is a natural
semiconductor, whose electrical properties can be easily manipulated by
mixing it with other materials at the molecular level, in a process called
“doping”.
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He needed to devise a method that incorporated inert
materials and friendly processes that could be used in
biological environments, where things are invariably wet
and chemically sensitive. If the materials or methods of
operation were poorly chosen, the hand could end up
reacting with or damaging the environment it was working
in. Conversely, choosing the wrong materials or methods
of operation could create a hand that constantly becomes
fouled and inoperative or which degrades over time in
harsh environments. The method of controlling the hand
also needed to be compatible with the fabrication
techniques used to create such a tiny device and simple
enough to work as part of the rugged, monolithic design.
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Physically, the Microhand chip was
essentially glued together with Plexiglas. The parylene bags
are joined by tiny channels etched into the silicon wafers
that make up the fingers of the hand. The hand contains
larger air ducts that connect the microchannels to a gas
cylinder which uses non-reactive gas, such as air or
nitrogen. The controlled release of the gas which expands
the polymer balloons and forces the hand closed is well
measured and well documented by its developers in the
micro-fabrication labs at UCLA. A reliable mechanism for
releasing the gas and extending the digits has not been
developed.
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The hand is clearly in the mind of the
scientific and news community as a pioneering invention in
the field of microsurgery.
"I must say that the microhand is a wonderful
achievement," says Albert Pisano, a mechanical engineer
at the University of California, Berkeley. "The field of
microsurgery and minimally invasive surgery is currently
dominated by grippers and tools that are mounted at the
end of long, rigid aluminum rods. Certainly these are
adequate for many purposes, but now that functional
microhands have been developed, one can visualize a new
set of minimally invasive surgical tools that allow the
surgeon additional dexterity in complicated procedures."
Currently, the hand is years away from
actual use in surgery. In the long run, the hand could be
used at the end of a catheter and could eventually include
other micro-scale technologies like fiber optic viewers,
which would allow doctors to remotely see and manipulate
deep within a patient. The hand is small enough to grab and
manipulate a single nerve bundle.
In the short run, the innovations that
come from developing such a device will inevitably lead to
many more new breakthroughs in the manufacture, design, and
application of a variety of technologies. The creators of
the hand are currently working with industry to develop a
slightly larger, fully functional version of the hand. CJ
Kim mentions that the technology could also be upsized to
help develop stronger, more precise manipulators for use in
bomb defusal robots. In fact, in many ways, the Microhand is
just the beginning of the story.
Our Fearless Heroes
The microhand was developed by Yen-Wen Lu as his PhD
work under Professor Chang-Jin Kim at UCLA Henry Samueli
School of Engineering and Applied Science.
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Chang-Jin "CJ" Kim is a Lead Researcher at UCLA and
holds a PhD in Mechanical Engineering. He has also earned a
BS, MS, and a Graduate Research Excellence Award. His
Micro-Electro-Mechanical Systems (MEMS) program is one of
the fastest growing research projects at the school. It
emphasizes the principals and techniques used in
designing and manufacturing useful devices that are less
than 1mm in size.
He established the MEMS PhD field major for the
Mechanical and Aerospace Engineering Department at UCLA and
directs the Micro and Nano Manufacturing Lab. He currently
oversees work on digital microfluidics, nano-engineered
surfaces, micro-droplet dispensing systems, RF liquid
switches, micro fuel cells, 3-D microbatteries, and the
on-chip encapsulation of micro devices. What did you do at
work this week?? |
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Yen-Wen Lu studied under CJ Kim at
UCLA, where he earned a PhD in Mechanical and Aerospace
Engineering. He also holds a MS and BS. He is currently an
assistant professor at Rutgers University and is helping to
develop a PhD program in Mechanical and Aerospace
Engineering there, with a focus on design, fabrication, and
system integration in MEMS and nano-technology. |
MEMS
So really, MEMS is the true hero of this story. “MEMS”
stands for Micro-Electro-Mechanical Systems. It is the
science/art of combining mechanical devices, sensors,
actuators, and electronic circuits on the same chunk of
silicon, a strategy called “monolithic design” which tends
to create very reliable and durable goods. It deals with
creating tiny devices that are less than 1mm and often far,
far smaller. MEMS manufacturing techniques are based on the
well known and proven processes used in manufacturing
electronics and semiconductors but are being used to
integrate a wider variety of components, which vastly
multiplies the number of innovative devices possible. The
fabrication techniques are geared towards mass producing
exceptionally functional, reliable, and sophisticated
devices. MEMS is not only incredibly interesting, it will
soon change your life.
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The electronics portion of the
monolithic chip is fabricated using established integrated
circuit techniques like CMOS and Bipolar. Mechanical
components, such as electric motors thinner than a human
hair, are fabricated using “micro-machining” processes that
selectively etch away parts of the silicon wafer or add new
structural layers to form microscopic, functioning
mechanical devices. On a single chip, MEMS combines
electronics that can monitor and make decisions about the
environment and then activate on-chip mechanical devices to
manipulate that environment. Sensors measure temperature,
pressure, magnetism, chemical and biological levels.
Electronics process this information, make intelligent
decisions, and trigger the mechanical devices that move,
pump, filter, and regulate conditions in the real world.
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MEMS Manufacturing Techniques
It relies on depositing thin films of materials.
Using chemical or physical processes like
evapouration or electroplating, it can build up new
material on silicon wafers.
It
uses lithography, which is a fancy word for
printing. MEMS transfers patterns onto silicon
wafers through exposure to light or other radiation.
The patterns can change the physical properties of
underlying films or the pattern can be used as a
mask in an etching process that removes material
from the device being constructed.
Etching is the third fundamental process and
involves using ions or chemical vapours to remove
materials from the device being constructed. This
process is also referred to as “micro-machining”,
allowing nano-technologists to shape silicon in the
same way a machinist shapes metals. |
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In modern automobiles, accelerometers
are used to detect collisions and activate air bags. The
assemblies consist of different modules for the sensors and
trigger electronics. They place an accelerometer assembly
near the bumpers, consisting of the accelerometer sensor and
supporting electronics, and electronic modules near the
airbags themselves that consist of more electronics and the
mechanical mechanisms that actually trigger the airbags. The
whole complicated assembly generally costs around $50. Using
MEMS technology, the accelerometer and electronics are all
configured onto a single chip that costs around $5 or $10.
The MEMS single chip technology is smaller, lighter, more
reliable, easier to maintain, and cheaper.
One of the coolest aspects of MEMS is
that it’s not really using new technology… most of the
processes, materials, and techniques have been used in the
manufacture of electronic components for years. In fact, the
use of the ever-present silicon is often not critical to the
function of these physically tiny devices except that its
properties and techniques for manufacture are well known
from the semiconductor industry, allowing devices to be
mass-produced and easily integrated with electronic
controls. Many of the fundamental machines being integrated
into MEMS devices are simple ones like motors, switches, and
pumps. All this “old news technology” is being miniaturized
and used in innovative and ground-breaking new ways and will
be changing your life shortly. In the same way that the
transistor allowed the proliferation of televisions and
computers, MEMS will create revolutionary technology we can
only guess at right now. MEMS takes old technology and turn
it on its head, making it intelligent, tiny, cheap, mass
producible, and exceptionally versatile. Everything old is
new again.
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