EPD is a reusable display material having many of the properties of both conventional wood-pulp paper and electric display monitors. It differs from conventional Cathode Ray or LCD displays in that it stores images over time, has a wide viewing angle, and the display comes by reflected light with the power source used only to change images. It is also inexpensive, lightweight, and portable. It improves the wood-pulp paper in that the materials can be erased and rewritten any number of times.
The earliest electronic papers used the Gyricon technology, developed by Nick Sheridon at Xerox's Palo Alto Research Center (PARC) in the 1970s. This technology involves suspending tiny rotating spheres of plastic in bubbles of oil embedded in transparent silicon sheets. The spheres have a black side with negative charge and a white side with positive charge, charged using transparent electrodes. The polarity of the voltage applied to each pair of electrodes determines which side faces up, giving the pixel a white or black appearance and forming images. Gyricon’s poor contrast and resolution remained a big obstacle to the popularity of electronic paper.
E-Ink’s Electrophortic Technology
The electrophoretic display technique involves two parallel conductive plates with thickness of 10 to 100 millimeters, filling the gap with a mixture of hydrocarbon oil, any dark-colored dye, surfactants, and charging agents, and dispersing millions of one millimeter diameter titanium dioxide microcapsules into the mixture. Each microcapsule contains white particles with a positive charge and black particles with a negative charge. The application of negative electric fields causes the white particles to move to the top of the microcapsule and the black particles to hide at the bottom of the microcapsule, causing the area to appear to the viewer as a white dot. Similarly, application of the positive electric field causes black particles to migrate to the top and the white particles to move to the bottom, generating black text or a picture. The images once struck remain embedded there, until applying new voltage to change a page. This ability to stick, while ensuring low power utilization, also causes a low refresh rate, making this technology unsuitable for displaying animation or video.
Although Oliver Smithies discovered the elecrophoretic technology as early as in 1957, it was only in 1997, when the Michigan Institute of Technology spun off the MIT media lab as E Ink Corporation that the technology began to apply to electronic paper displays. E-ink that uses electrophortic technology is a big improvement on Gyricon as it bears close resemblance to ordinary paper in display and resolution. E-ink lacks the high-contrast available to backlit computer screens, making reading difficult in dim light, but have a reflective surface that allows reading in daylight situations, which is not possible with conventional laptop displays. Most e-books such as the Amazon Kindle, Barnes & Noble Nook, Sony Librie, Sony Reader, and iRex iLiad use e-ink.
Of late, Philips Research has made further advances in this technology by applying the Electronics on Plastic by a Laser Release (EPLaR) process that allows creating flexible plastic displays. This makes e-ink almost as flexible as conventional pulp-based paper. E Ink has joined hands with the Japanese company Toppan Printing to produce color filters, and thereby produce electronic pages with color displays.
Electro-wetting display (EWD) technology, like Electrophotic technology controls the shape of a confined water or oil interface by applying voltage. When no voltage is applied, the colored oil forms a flat film between the water and a hydrophobic water-repellent insulating coating of an electrode, resulting in a colored pixel. Application of voltage between the electrode and the water causes the water to move the oil aside resulting in a partly transparent pixel, or a white pixel if the surface is reflective white.
Electro-wetting technology is a considerable improvement over both Gyricon and electrophotic technologies. The small size of the pixel allows high brightness and high contrast, with the displays four times brighter than reflective LCDs. The switching between white and colored reflection is fast enough to display video content.
The new Electro fluidic display invented at the Novel Devices Laboratory at the University of Cincinnati is a variation of the electro-wetting technology. This technology uses ambient lights and pigments to generate displays as bright and vibrant as displays of a glossy magazine.
An electro fluidic display involves two sheets of plastic with mesa-like polymer structures printed to form pixels on the top sheet. A tiny reservoir containing aqueous pigment dispersion remains hidden from view between the sheets. A tiny hole inside each pixel allows the pigmented fluid to get in and out. Another sheet of plastic containing a transparent electrode covers the pixel sheet. The application of voltage pulls the pigment out of the reservoir and spreads it as a film directly behind the viewing substrate and on to the pixels. Removing the voltage produces liquid surface tension, causing the pigment dispersion to recoil to the reservoir.
Electro fluidic displays, while in its early stages have the potential for dominance. Its ability to deliver durability, brightness of color, and transfer pixels at video speed, and all at very low power consumption, make it a technology to watch out for.
The Cholesteric liquid crystal (ChLCD), under development by IBM, Philips, HP, Fujitsu, and others is based on the technology in use for LCD displays. The technology involves application of voltage to spiral-shaped liquid-crystal molecules embedded within silicon sheets to change it from vertical to horizontal position and thereby create images.
The high level of maturity this technology has already achieved owing to its application in LCD monitors and its many benefits over other technologies make it another candidate to become the most dominant and best electronic paper display technique of the future.The many advantages include unmatched flexibility, lightness, ability to bend, extremely thin size with a thickness of just 0.8 millimeters, a bi-stable nature requiring no power to maintain images, excellent brightness, contrast, and resolution, vivid color, and refresh rates capable of displaying animation and video.
A review of the different electronic paper display technologies suggest that while Glycon is passé and Electrophortic technology is on its way out. The competition for the future could very well be between the electro-fluidic and ChLCD technology, or any one of the other nascent technologies under incubation.
- ieeeSpectrum. “Ohio Engineers "Ink" New E-Paper.”https://spectrum.ieee.org/semiconductors/materials/ohio-engineers-ink-new-electronic-paper-technology. Retrieved June 11, 2011.
- "The Skinny On Electronic Paper." https://sitn.hms.harvard.edu/sitnflash_wp/2009/11/issue60/. Retrieved June 11, 2011.
Image Credit: flickr.com/Yutaka Tsutano