North AmEricAN

the displ aYs and microfluidic systems
I’ve described are far from the only applications
being explored. My group and others, for exam-
ple, are actively building light-emitting devices on
paper to make luminous displays [see images, “On
Display”]. Others are investigating novel ways to
construct flexible RF antennas attached to curvi-
linear surfaces that boost their performance. Of
course, regardless of what we choose to build,
paper electronics will always be limited unless
we can find a way to deliver power to the devices
in a way that’s just as mobile, thin, lightweight, and
flexible as paper itself. What we’d really like to do is
build batteries, capacitors, or photo voltaic cells di-
rectly onto the same paper that bears the devices.
One potential way to store energy on paper
is to take advantage of its long, thin cellulose fi-
bers, which offer a lot of surface area that could
potentially be used to store charge. Paper can
be soaked with electrolytes to make a variation
on the traditional battery. Alternatively, it can be
coated with inorganic metal or carbon to store
charge. The work is far enough along that it is
being pursued commercially by firms such as
the Paper Battery Co., in Troy, N.Y., Power Paper,
in Israel, and Enfucell, in Finland. The stor-
age specifications already seem promising:
a1-millimeter thick, 10- by 10-centimeter
square patch can store a few hundred milliam-
pere hours at 1.5 volts, about 10 to 20 percent
the capacity of a typical AA battery.
Once all these components—power, back-
end electronics, and front-end devices—are in
place, I believe it will be possible to develop fully
integrated, complete systems on paper that can
power themselves and communicate with the
outside world.
But finding ways to perform this integration
will be a significant challenge. The ideal paper
substrate for back-end circuitry might be very
Patrick gillooly/mit
different from what’s required to build, say,
a front-end display or marry a microfluidic
device with logic and communications
circuits. Certain features, in particular the
wires used to connect components, are
especially fragile and will have to be care-
fully constructed, probably using materials
and geometry different from those in con-
ventional rigid integrated circuits.
But think of the possibilities if we suc-
ceed. We could fill an important economic
gap in the technological spectrum of elec-
trical devices, between the low-tech realm
of incandescent lightbulbs and electric mo-
tors and the high-tech world of computer
chips and flat panel displays. Although the
cost of making an individual transistor has
been declining for decades, the overall
fixed costs of materials, fabs, and equip-
ment are substantial and growing. We need
a fundamentally new approach if we want
to shake up the industry.
Paper is likely to emerge slowly in elec-
tronics: You’ll see it first in markets where
low cost—not high performance or small
area—is the main consideration. Along the
way, paper will face competition: Plastic is
more rugged and electronics-friendly, and
glass can now be made so thin and bend-
able that it’s not impossible to imagine it
could one day be fed into roll-to-roll ma-
chines. Despite this, paper has the poten-
tial to extend the reach of electronics into
areas we might never have considered
before, offering consumers a much wider
range of choices when it comes to per-
formance, reliability, and price. On paper
(ifyou’ll pardon the pun), there’s little rea-
son to think that this technology will stay
in the lab for long.
■
Integration
paper power: Flexible,
foldable arrays of solar cells can
be built on a paper substrate
using vapor deposition. The solar
array pictured here incorporates
five layers and uses organic
photovoltaic materials to convert
light into electricity with roughly
1percent efficiency.
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02.PaperElectronics.NA.indd 61 1/17/13 1:16 PM