The evolution of textual communication, from cave walls and stone tablets to ink on parchment, from the first Gutenberg press to the latest high-speed printing presses, has always embodied a common thread of concern: readability. Once taken for granted, the issue jumped back to the fore in the past few decades when text leaped off the page and onto the computer screen.

Recently, new technologies have emerged that seek to straddle both realms and provide the best of both: the readability of printed matter and the refreshability of an electronic display. Such technologies are generally referred to as electronic paper, or e-paper.

Eye-catching, curved Seiko watch design is made possible by using E Ink electronic paper technology.

E-paper has been on many drawing boards for years, and was originally envisioned as a means of enabling e-books and e-magazines that would replace their printed counterparts. (See the Sony e-reader above.) But the attributes of the e-paper technologies make them suitable for a wide range of other applications as well. Their low power consumption makes them ideal for any portable, battery-powered device. Their high legibility makes them attractive for any device that must display complex information, from upscale cooking appliances to critical pieces of medical equipment. And their ability to operate in something other than a flat plane opens the door to entirely new applications, such as a curved wristwatch. (See photo.)

The research and development of electronic paper has been pursued for a long time. One of the more prominent e-paper technologies is electrophoresis, which has been under development for about 40 years. Xerox developed electrophoretic technology in 1969, about the same time that LCDs were being developed.

Enlarge this picture White and black particles move within a fluid when subjected to a specific charge. Negative moves the white particles to the top, rendering the pixel white. A positive charge reverses the position, rendering the pixel black. Photo: E Ink

While LCDs have become commonplace, e-paper has experienced a slower march to market. One reason that electrophoretic technology took so long to enter the market is because at that time the voltages that were required to move the particles in the fluids were in the hundreds of volts. Only recently, with new material advances and improved manufacturing techniques, have voltage requirements dropped to the point where commercially available integrated circuits can be used to drive this technology, says Bryan Chan, director of marketing for SiPix Imaging Inc. of Fremont, Calif. SiPix is an e-paper manufacturer that uses electrophoretic technology.

In spite of e-paper’s many other advantages, the ability to display video remains a weak spot for the display technology. Boosting switching speeds and other advancements must be made in order for it to catch up to LCD technology in terms of video. Currently, e-paper cannot switch from image to image fast enough to be used for video-type broadcasts.

Telecom Italia and Polymer Vision developed the “cellular-book,” a mobile device with rollable display. The display is made with E Ink’s e-paper.

The ability to display color was another traditional weak spot for e-paper, but that now is changing as the differences between LCD and e-paper are shrinking. Some e-paper technologies are now demonstrating full color and grayscale capabilities.

The most widely touted advantage for e-paper is readability, which is considered better than traditional display technologies. E-paper can be read at any angle and reflects light like ordinary paper. Unlike conventional reflective displays that need a backlight to illuminate the material, e-paper can be read in ambient light.

Another key advantage of e-paper technologies is that they are bistable, which means they only consume power when the image is changed or updated. An unchanging image can be held indefinitely without drawing power. For applications where the display is infrequently refreshed, bistability can significantly cut power consumption.

Enlarge this picture Nemoptic BiNem technology works on the principle of “surface anchoring breaking.” The BiNem cell has a conventional polyimide “strong anchor” layer at the top, but a proprietary “weak anchor” alignment material at the bottom. Changing states is accomplished by means of applying a specially shaped electrical pulse. The front end of the pulse will be steep and that will break the weak anchor, allowing the liquid crystals to be realigned. When the pulse has a steep falling edge, the liquid crystals will realign in a twisted state that allows the reflecting of light off the reflective polarizer at the cell bottom, rendering the cell into its light state. When the trailing edge of the electrical pulse is smooth, the liquid crystals will realign into a uniform state that blocks reflected light and places the cell into a dark state. The liquid crystal materials re-anchor after the pulse and, therefore, maintain their new configuration until another pulse is applied.

The advantage of bistability can be observed clearly in a retail shelf pricing display application where the battery-operated display would only use power when the price is changed. Shelf pricing and marketing displays often feature e-paper because it combines the ability to display text for long periods of time while being easy to change the text without having to manually change the price.

Wristwatches represent another growing market for e-paper, but work best for devices without a second hand. This is opposed to a watch that ticks off every second that passes and would require constant updating.

The electronic paper can be rigid, such as with the e-books that use glass backplanes. Or, they can be flexible; flexible to the point where they can be rolled up or folded, adding durability and design flexibility. For designers, e-paper isn’t just relegated to 2-D displays, they can be curved, contorted or conformed to another 3D surface such as on a curved wrist watch that fits better to a wrist.

Enlarge this picture Demonstration of flexible display by Plastic Logic, Cambridge, U.K., using E Ink Imaging Film.

“The flexible displays not only offer conformability and ruggedness,” says Sriram Peruvemba, vice president – marketing, E Ink Corp. of Cambridge, Mass., “They offer appliance designers the freedom to think outside the rectangular format they were restricted to with conventional displays.”

As the technology has improved, the number of manufacturers are starting to creep up. Still, electronic paper has only a handful of key players, the best known is E Ink. The company offers the E Ink Vizplex Imaging Film and the film can be used in a range of full display sizes or in segmented display applications. It is produced using the electrophoretic process and is sold for use in a whole host of applications.

E Ink was the first to offer the e-paper in mass quantities and to well-known customers for use on the Kindle e-book as well as the Sony Reader. Other applications include wristwatches, weather alert devices, eNewspapers, Web browsers like the Polymer Vision Readius, cell phones like the Motofone, memory indicators like the Lexar USB drive, an automotive key fob like the Delphi bidirectional key fob, point-of-purchase devices, outdoor signage, and more.

Enlarge this picture A color display using Nemoptic liquid crystal display technology.

E Ink has developed agreements with a number of companies such as Prime View International, Samsung and Epson. Recently, LG Display introduced a 14.3-in. flexible color display at the January 2008 CES show. In addition, Polymer Vision, a spin-off of Philips Electronics, announced plans to mass-produce a “rollable” display device, Readius, which is aimed at cell phone applications.

SiPix also uses electrophoretic technology to make its e-paper. The company, which started with a focus on two main areas – smart cards and shelf displays – now find its technologies used in a range of products from high-resolution displays to consumer devices such as clocks, watches, and board games.

Another player that will be able to produce the e-paper material in high volume is France-based Nemoptic, which uses a different approach to developing e-paper. Nemoptic’s product is called BiNem and is a variant of LCD technology. The bi means that it is bistable, and nematic means that it uses nematic liquid crystal materials. Nemoptic is one of the first company’s to use LCDs and achieve bistability, says Jacques Noels, CEO of Nemoptic. The company has subcontracted the manufacturing of its BiNem e-paper technology to Japan-based Seiko Instruments Inc., which has the technology to produce the material in high volumes, which will help to keep costs down as is critical to e-paper’s continued growth, says Noels.

Motorola Motophone with ClearVision display based on E Ink technology.

The technology is suitable for electronic shelf labels, point–of-purchase systems, home automation, and handheld devices such as e-books smart cards, and mobile phones.

As can be seen, different types of technologies are used to make e-paper. Electrophoretic and LCD are two of the most often used.

An electrophoretic display forms images by rearranging charged pigment particles using an applied electric field. The solid particles are introduced into a liquid “carrier medium” and when the electrical field is applied, the particles move. The liquid allows the particles to be printed using existing screen-printing processes onto virtually any surface, including glass, plastic, fabric or even paper.

Nemoptic’s new A4 e-paper display is 210 mm x 297mm and has a resolution of 200 dpi.

E Ink and SiPix use electrophoretic technology, although they both have their own methods. E Ink’s two-particle system features a microcapsule that contains white and black particles that are suspended in clear fluids. When a negative electric field is applied, the white particles move to the top of the microcapsule where they become visible to the user. This makes the surface appear white at that spot. At the same time, an opposite electric field pulls the black particles to the bottom of the microcapsules where they are hidden. By reversing this process, the black particles appear at the top of the capsule, which now makes the surface appear dark at that spot, says Peruvemba.

SiPix’s electrophoretic technology is based off of a single particle method as opposed to E Ink’s two-particle method. SiPix creates the e-paper by inserting electrically charged white particles and dielectric fluids within a matrix of what the company calls Microcups. The Microcups can be filled with a variety of colored dyes. The electric field charges the particles and then they migrate through the dielectric fluid. If the white particles are at the surface, the pixel will appear white. Otherwise, the pixel will reflect the color of the fluid. Currently, SiPix displays can show a variety of monochromatic colors, which means each display can be black/white, red/white, blue/white, and so on. The company is working on a product  where each Microcup can change from white, black, and also a color (i.e. three colors per pixel), and this approach will eventually serve as the basis for its full color displays.

While SiPix and E Ink use electrophoretic technology, other e-paper developers, such as Nemoptic, have achieved similar effects using variations of conventional LCD technology. Nemoptic, for example, utilizes standard nematic liquid crystal materials, but in unique fashion. Nemoptic BiNem technology works on the principle of “surface anchoring breaking.” The BiNem cell has a conventional polyimide “strong anchor” layer at the top, but a proprietary “weak anchor” alignment material at the bottom.

A point-of-purchase display with area color developed by SiPix Imaging.

Changing states is accomplished by means of applying a specially shaped electrical pulse. (See illustration above.) The front end of the pulse will be steep and that will break the weak anchor, allowing the liquid crystals to be realigned. When the pulse has a steep falling edge, the liquid crystals will realign in a twisted state that allows the reflecting of light off the reflective polarizer at the cell bottom, rendering the cell into its light state. When the trailing edge of the electrical pulse is smooth, the liquid crystals will realign into a uniform state that blocks reflected light and places the cell into a dark state. The liquid crystal materials re-anchor after the pulse and, therefore, maintain their new configuration until another pulse is applied.

Each company’s product’s has the ability to work in grayscale and monocolor. Traditionally, e-paper has not used color because color can affect brightness levels, which affects readability. To achieve color, there are two primary methods. Nemoptic uses a combination of color filters with gray levels. For SiPix, the colors are created from the dies that are in the Microcup. For its part, E Ink is working with color products in the labs and is doing testing on the product to ensure optimum readability. To date, it has demonstrated full color displays using a RGBW color filter array over high contrast black and white ink.

Because readability is a vital consideration, factors such as contrast and reflectance are critical. On first blush, when looking at the contrast ratios, the numbers would look very small compared to a system that displays video or compared to an LCD monitor, but reflective displays are measured differently. According to Chan,  the contrast ratio is defined as the ratio of how “white” is white compared to how “black” is black. Electrophoretic systems have a contrast ratio that ranges between 6:1 up to 20:1 depending on the system. By comparison, newspaper has a contrast ratio of about 4:1 or 5:1, while a typical, digital wristwatch has a contrast ratio of about 3:1. Chan adds that the differences are not as important as they may appear, as there are limits to the contrast ratio that humans can detect. He says with a reflective display it is difficult to distinguish any ratio above 10:1 unless they are side by side. (A higher contrast ratio is better than a lower contrast ratio.)

A clock that uses SiPix’s e-paper.

Reflectance is another important issue. While contrast ratio compares black and white, reflectance determines how much light is reflected back from the display. Shining a light onto a surface and capturing the amount of light that bounces back determines reflectance. A very white sheet of paper can be anywhere from 80-95 percent. E-paper comes nowhere near that figure, but it doesn’t have to. Typically, reflectance above 30 percent is not noticeable unless they are compared side by side.

Reflectance is also pivotal when it comes to the viewing angle. E-paper’s reflectiveness is different than a mirror where the light bounces back at a predictable angle. In this case, the surface luminance is isotropic and operates under the Lambertian reflectance principle in which light falling on the surface is scattered and the apparent brightness of the surface is consistent to an observer no matter what is the observer’s angle of view. SiPix tends to be about 30 percent reflectance as does Nemoptic’s LCD technology. E Ink is around 40 percent.

Backplanes too have been improved. For the most part, an e-paper display – the front, visible plane of the display -- is “agnostic,” says Chan, and can be used in a variety of backplanes. Often, flexible and segmented displays use a printed circuit board (PCB) backplane while active matrix applications such as the E-books use a backplane made from traditional glass-based thin-film transistor (TFT) technology.

Enlarge this picture A SiPix Microcup contains white particles in a dyed liquid, and various colors can be used. The white particles move to top or bottom in response to a positive or negative charge. With the particles at the top, the pixel is white. With the particles at the bottom, the pixel is the color of the dyed fluid.

According to Chan, backplanes can come on a variety of substrates including glass, plastic, or metal. The technology that drives each pixel also varies. For active matrix displays, where an image is made from individual pixels, each dot (or pixel) needs a transistor, he says. The flexible active matrix displays are nearing commercialization and will likely be made of a plastic substrate. The transistor is also flexible because they are very thin. Another form of flexible transistors are polymer transistors. Unlike TFTs, these can also be printed.

For segmented displays, such as a clock or watch, the image is created by a conductive electrode (either copper or silver) that is on a PCB.  Since flexible PCBs have been around for some time, this technology is already available and on the market.  Flexible direct drive backplanes don’t have any transistors on them. Instead, the electrodes are given a charge from a display driver IC that controls the voltage on each segment, Chan says.

The primary advantage with flexible displays is that there is no glass to shatter, says Peruvemba. Therefore it is easier to design them into applications like wearable computers, handheld devices, military equipment, clothing, restaurant menus or even ladies hand bags that was not possible with a rectangular glass based display, he says.

A flexible display affords an advantage even in applications where it won’t actually be flexed in that it provides a greater degree of impact resistance. A flat, flexible polymer display can withstand an impact that might break a display with glass.

Resolution is another measuring stick for e-paper products. This determines how crisp is the line between the dark and the light. As for resolution, the limitation is in the backplane. The higher the resolution of the backplane, the higher the resolution image can be created. Nemoptic’s resolution is about 150 dpi, and E Ink has demonstrated a resolution of 397 dpi. SiPix has not measured the resolution of its electrophoretic material.

The suppliers say that new and more flexible active matrix backplanes are in the pipeline and might be available as early as this year or next, which might help in developing further applications. With these past and future developments, e-paper may be an option for designers of a range of appliances, large and small, that require a tough, low-paper product that is easy to read and easy to use.

For more information, email:
E Ink: sales@eink.com
Nemoptic: contact@nemoptic.com
SiPix Imaging: bryan.chan@sipix.com