In this blog post, we’ll explore the ink technologies behind e-paper, specifically the principles and applications of electrophoretic and electro-wetting methods.
You may have imagined yourself flying on a broomstick and casting spells as a child. J.K. Rowling’s “Harry Potter” series sparked that desire, and the scene in the novels where photos in the newspaper moved remains etched in many people’s memories. Now, an experience similar to those moving photos in the newspaper is becoming a reality through electronic newspapers and e-paper.
E-paper is created by integrating ink technology with display technology; it is flexible enough to be rolled or folded like real paper, yet can be written on and erased like a standard LCD screen. It offers a wider viewing angle and greater durability than conventional LCDs, and because it uses reflected light, it consumes less power. As a result, it is expected to be utilized in various fields, such as e-books, magazines, and posters. Ink technology is broadly categorized into two methods: electrophoresis and electro-wetting.
An electrophoretic display (EPD) is a display that applies technology using colloidal particles that move toward the anode and cathode under the influence of an electric current to form text or images. Here, colloidal particles refer to particles that are larger than ordinary molecules or ions and are small enough to pass through filter paper but not semi-permeable membranes.
The basic operation of an electrophoretic display involves placing white and dark-colored pigments, each about 1 micrometer in diameter, inside small capsules and applying an electric current. To make the particles carry a charge, a charge-imparting agent is added to an oil medium, dispersing the particles within it. When a voltage is applied, each particle moves toward the electrode carrying the opposite charge to its own. For example, if white particles are designed to carry a negative charge and black particles a positive charge, when a voltage is applied, the white particles move in one direction and the black particles in the opposite direction. To display text or images, a specific electrode is applied only to the text area, causing black particles to gather there, revealing black on the surface and creating the desired pattern. Additionally, to display colors, color filters are incorporated into the glass substrate; when black particles are positioned in front of a color filter, that color is suppressed, and when white particles gather, the filter’s color is revealed.
Electrowetting displays (EWD) utilize the property that water and oil do not mix. By placing water and oil together inside the panel and applying a voltage to the water, the spreading area of the oil droplets can be controlled to represent pixels. Here, a cell refers to a single unit of the display panel, while a pixel refers to the smallest point that makes up an image.
In the electro-wetting method, oil droplets colored black or red, blue, and green are used. When no voltage is applied, the oil spreads widely across the panel, causing the entire cell to display the color of the oil. Conversely, when voltage is applied, the oil clusters on one side, displaying color only in certain areas; in the areas where no color is displayed, the light reflected from the underlying reflector or the white light from a reflective backlight becomes visible.
The most common application of e-paper and ink technology is the e-book reader. Most e-book readers, such as the Amazon Kindle, use e-ink for their displays. In 2009, Fujitsu unveiled color e-ink, but full-color display has not yet been fully realized. Additionally, current products are often designed to display text stored from other sources, so they have limitations in terms of their ability to generate text on their own.
In the future, once e-paper and ink technology achieves full color capability, low battery consumption, and the ability to generate text independently, we will likely see e-paper used in various fields, such as textbooks and printed newspapers.
