r & d BREAKING DOWN TECH —PRESENT AND FUTURE
60 MAXIMUMPC FEBRUARY 2007
B
ut Stephenson doesn’t get all the credit
for this idea: The technology—formally
known as electrophoretic display—was
conceived of in the 1970s. Research efforts
were pretty much abandoned in the face
of what seemed to be insurmountable
challenges, however, until a group of MIT
students under the tutelage of Professor
Joseph Jacobson made a key breakthrough
in the 1990s.
Jacobson later founded E Ink to commer-cialize the technology. Today, E Ink’s product
can be found in everything from Sony’s Reader
(a handheld electronic book) to Motorola’s
new Motofone (a cell phone that’s thinner and
lighter—by half—than the company’s Razr, yet
delivers 500 minutes of talk time).
Electrophoretic displays operate by
moving tiny electrically charged particles of
pigment in a liquid sandwiched between two
conductive plates. When voltage is applied
across the two plates, negatively charged
particles migrate to the plate exhibiting
positive polarity, while positively charged
particles migrate to a negatively charged
plate on the opposite side. If the white-
pigmented particles bear a negative
charge and a negative electrical charge
is applied to the bottom plate, the white
particles will move to the top (view-
ing) plate and become visible. Black
particles bearing the opposite charge,
meanwhile, are drawn to the bottom
plate and become hidden. Reversing
the charge has the opposite effect.FAILURE ANALYSIS
Three phenomena were responsible
for the failure of earlier prototypes:
gravity, adhesion, and fl uid con-
vection. Since the particles are
denser than the liquid they fl oat in,
gravity would eventually overcome
electrical attraction and the par-
ticles would sink to the bottom of
the display. Some particles wouldinevitably adhere to the surfaces of the con-
ductive plates, resisting the forces of both
gravity and electrical attraction. And fl uid
convection—the dynamic of some particles
boiling up to the top when voltage is applied
to the plates, while others migrate to the
bottom—rendered the displays too slow.EUREKA!
The breakthrough came when the MIT stu-
dents encased ceramic pigment particles
in microcapsules. Microencapsulation has
been used for timed-release drug delivery
and other purposes for decades, but most
microcapsules are designed to break down
over time. E Ink’s microcapsules act as
durable containers in which the pigment par-
ticles are suspended in organic oil; the black
particles are made of the pigment used in
newspaper ink, while the white particles are
made of the pigment used to create white
paint. Since each microcapsule is less than
50 microns in diameter, the particles don’t
settle because they can migrate only as far
as the bottom of the microcapsule; they
don’t stick to the surfaces of the microcap-
sule, and the microcapsules are so small
that the particles contained within them are
not subject to fl uid convection.
The next step in the process is to mix
the microcapsules with additional polymers
to create a paste. E Ink rolls out a fl exible
plastic substrate with an adhesive backing,
applies the paste to the other side, allows
it to dry, and then rolls it up on a secondWhite Paper: Electronic Paper
Each 50-micron microcapsule contains organic oil and approximately one million negatively charged black-pigment particles and
positively charged white ones. When a positive charge is applied to the electrodes, the white particles fl oat to the top and become
visible, while the black particles sink to the bottom and are hidden.HOW IT WORKS
Top transparent
electrodeE Ink technology
ILLUSTRATION COURTESY E INK CORPORATIONWe’ve been jonesin’ for
electronic paper ever
since we read Neal
Stephenson’s 1992 sci-fi
novel The Diamond Age.
BY MICHAEL BROWNPositively
charged
white
pigment
chips
Clear
fl uidSubcapsule addressing enables
supercrisp display capabilityNegatively
charged
black
pigment
chipsBottom
electrodeNegatively
charged
black
pigment
chipsBottom
electrodeLIGHT STATE DARK STATE+ + + - - -Electrophoretic displays operate by
moving tiny electrically charged particles of
pigment in a liquid sandwiched between two
conductive plates. When voltage is applied
across the two plates, negatively charged
particles migrate to the plate exhibiting
positive polarity, while positively charged
particles migrate to a negatively charged
plate on the opposite side. If the white-
pigmented particles bear a negative
charge and a negative electrical charge
is applied to the bottom plate, the white
particles will move to the top (view-
ing) plate and become visible. Black
particles bearing the opposite charge,
meanwhile, are drawn to the bottom
plate and become hidden. Reversing
the charge has the opposite effect.MAMAMAXIMXIMXIMXIMUUUUMMPPPCCFEBRUARY 2007Sony’s Reader uses
E Ink’s technology.