e–Paper or Electronic paper is a kind of display technology used to mimic the appearance of an ordinary ink on the paper. Electronic paper displays the content using the reflected light so that reading is comfortable like the reading of a book. The e- Paper devices have large surface and wider viewing angle than other types of displays like LDC and LED displays. E-Paper can give clarity and without fading even in sunlight. Applications of e-Paper technology include e-Reader, Mobile phone display, Electronic bill boards, digital displays in commercial establishments etc. The Electronic paper technology can hold the static text indefinitely without using electricity.
The first electronic paper called Gyrion was developed in 1970 by Nick Sheridon and used polyethylene spheres of 75-106 micrometers .Each sphere has a negatively charged black coloured plastic on one side and a positive white plastic on the other side. Its interior is filled with a Janus particle. Thus each sphere act as a dipole. The spheres are then embedded in a transparent silicon film and the spheres are suspended in a bubble of oil so that the sphere can rotate freely. The polarity of voltage applied to the spheres determines the direction of their movement. Depending on the polarity, either black or white side faces up. So each sphere act as a pixel with white or black colour.
Many types of e- Paper technologies are used to now to make various kinds of displays.
1. Electrophoretic display
In this technology, Titanium dioxide particles having one micrometer are dispersed in hydrocarbon oil. A dark coloured dye is added to the oil, along with surfactants and charging agents that cause to take on an electric charge. The mixture is then placed between two parallel plates. Early electrophoretic displays used small transparent capsules containing oil with a black dye called electronic ink and numerous white Titanium dioxide particles suspended in the ink. The titanium particles are slightly negatively charged and appear pure white. The electrodes sends current to the oil suspension and the ink turns on/ off according to the voltage levels. This creates movement of the titanium particles.
When a voltage is applied in the plates, particles of opposite charges move to the respective plates. When the particles accumulate in the front plate, it appears as white since light from the plate scattered back to the viewer. At the same time, the particles accumulate in the back plate gives black appearance since the incident light is absorbed by the dye. The back plate is divided into many small segments, like Pixels, image cam be produced by applying varied voltage across the plates. This is due to the reflection and absorption of light at varying levels. Current consumption of the electrophoretic display is very low and the display looks like a paper.
Another similar technology is the Electrowetting in which a water – oil interface changes its shape based on the applied voltage. When a voltage is applied, the oil becomes a flat film between the water and electrode, resulting in a coloured pixel. Electrowetting technology is more attractive since the display pattern changes very fast between white and colour so that it is ideal for the video display.
Electrofludic technology uses aqueous pigment dispersion inside a small reservoir. When voltage is applied, the pigment pulls out of the reservoir electromechanically and spreads as a film behind the viewing surface. So the display shows white and coloured pixels similar to the colours letters or drawings on a white paper.