This article was contributed by Lauren L. C. Marotta
As the holiday season approaches, you may be starting to consider gifts for family or friends. A good choice this year might be a product that incorporates the clever technology called “electronic paper,” known best for its use in Amazon’s Kindle, one of several electronic book readers that have recently become popular. Introduced in the U.S. in 2007 and worldwide this October, the Kindle is lighter than a typical paperback novel, but it can store 1,500 books, newspapers, or magazines at once. Its screen displays pages one at a time, and pages can be “turned” with the click of a button. Importantly, devices with electronic paper screens have much longer battery lives than those with conventional screens, and electronic paper can be viewed in bright sunlight without glare and from almost any angle, just like normal paper and unlike other types of screens. These amazing properties are fueling the development and adoptance of the technology, and they are a result of the science underlying it.
How does electronic paper work?
The basis of most electronic paper is a substance called “electronic ink” developed at M.I.T. about 12 years ago by scientists Barrett Comiskey, J. D. Albert, Hidekazu Yoshizawa, and Joseph Jacobson, now made by a company called E-Ink based in Cambridge, Massachusetts. The ink is electrophoretic, meaning that it contains charged particles that can be moved around by electricity. These are microparticles made of polyethylene and either titanium dioxide, which is white, or an inorganic black pigment. Each microparticle is about 5 microns (millionths of a meter) in diameter, the size of the nucleus of a human cell. The white ones are negatively charged, while the black ones are positively charged. They are suspended in a liquid and “printed” onto a transparent, conductive film to make paper-thin sheets called “Vizplex.” A sheet of Vizplex with the particles facing down forms the top of an electronic paper display. A layer of conductive material is then added to form the base of the display. When a pattern of electric current is applied between the top and bottom display layers, these become negatively- and positively-charged in particular regions. Then, the black and white microparticles move towards the negative and positive regions, respectively, and an image is formed by the particles at the top of the display.
Notably, the microparticles in electronic ink are grouped by the hundreds into small capsules about 40 microns across (approximately the width of a human hair). The capsules are made of a strong, clear material containing urea and formaldehyde. Within each capsule, black and white particles are prevented from binding to each other by individual coatings of a polymer that provides strong inter-particle repulsive forces. The use of discrete capsules makes electronic ink printable, allowing for its mass production and thereby helping to gradually reduce the costs associated with this technology. Also, the properties of the capsules are such that the microparticles remain in their last configuration at the top or bottom once an electric field is no longer present.
What makes electronic paper better for reading than other types of displays?
The most common type of screen found in televisions, cell phones, and other electronic devices today is liquid-crystal display (LCD). An LCD screen is composed of molecules called liquid crystals that must usually be illuminated by a hidden light source, either fluorescent tubes or light-emitting diodes (LEDs), to be seen. Because of this backlighting, LCD screens cannot be viewed in bright light, such as that encountered outdoors on a sunny day, electricity must be constantly consumed to power them, and they cannot be viewed well from many angles. Other types of displays, such as cathode-ray tube (CRT) monitors and plasma screens, which emit light themselves, have the same shortcomings. In electronic paper, the pigmented microparticles in electronic ink reflect ambient light just like pen on paper, so the images they form can be observed in dim or bright light, without use of electricity (which is only needed to change the displayed images), and from almost any angle.
The future of electronic paper
More than 5 million electronic book readers are expected to be sold this year, and these may be used for more than just recreational reading. A few schools have begun providing textbooks to children in electronic form so that they do not have to lug so much to school, and one school has replaced its library with 18 Kindles. Electronic ink may become common in cell phones: it is already used in the screen of the Motorola F3 and in the keypad of the Samsung Alias 2. Clocks, watches, and USB memory sticks with electronic paper displays are beginning to appear on the market, and electronic ink (coupled with wireless internet capabilities) may soon be used in household appliances to automatically show the latest headlines, weather, or other data. Similarly, electronic paper may soon be used to make schedules for posting at bus stops, community bulletin boards, and price labels for the shelves of supermarkets that may be changed remotely.
Sheets of electronic ink can be made flexible by enclosing them in thin, durable plastic rather than in glass, as is customary in today’s electronic paper products. Electronic book readers with screens that roll up when not in use are expected to be available soon. Flexible electronic ink is likely to also appear on credit cards (where it could display travel information using electricity obtained when cards are swiped), in ads on the curved surfaces of subway trains, and eventually even on clothing. Electronic paper featuring color is also being developed. This may employ tinted filters incorporated into electronic ink displays or a novel form of LCD screen with properties similar to those of electronic ink. Digital photo frames may one day utilize such technology. Finally, image refresh rates of electronic paper are currently too low to support video, but refinements with that goal in mind are in the works. For now, the major benefits of electronic paper, its low power consumption and readability in all lighting, will continue to attract and intrigue consumers, encourage more scientific research, and maybe even save a tree or two.
For more information, please see:
An article entitled “Electric dreams” from The Boston Globe (November 1, 2009) by Michael Fitzgerald: http://www.boston.com/bostonglobe/magazine/articles/2009/11/01/electric_dreams
The Wikipedia entry on electronic paper: http://en.wikipedia.org/wiki/Electronic_paper
Primary literature referenced in this article:
“An electrophoretic ink for all-printed reflective electronic displays,” Barrett Comiskey, J. D. Albert, Hidekazu Yoshizawa, and Joseph Jacobson, Nature, Volume 394, pages 253-255, 16 July 1998.
“Electronic paper rewrites the rulebook for displays,” Duncan Graham-Rowe, Nature Photonics, Volume 1, pages 248-251, May 2007.