Electrophoretic moves - Brief Article

The electrophoretic display has the potential to make digital paper a reality. But, at the same time, it may be another blind alley for the displays industry.

An Arizona newspaper has invested in one company, ostensibly to make interactive advertising hoardings, but the technology being developed by US start-up E-ink may go further into personalised newspapers. E-ink is combining plastic transistor technology from Bell Labs with an electrophoretic ink developed at the Massachusetts Institute of Technology (MIT).

The ink works by spreading coloured microcapsules on a plastic surface, which carries an array of electrodes. The microcapsules contain a dye and a collection of microscopic chips of white pigment, each carrying a positive charge. If the microcapsule layer is covered with a transparent array of electrodes, it becomes possible to attract or repel the pigment chips to either surface of the microcapsule.

By attracting the chips to the bottom of the microcapsule, it takes on the colour of the dye. If they are attracted to the top, the microcapsule appears to turn white. E-ink will use the plastic transistors developed by Bell Labs to spread the microcapsules on a range of flexible surfaces and control their colour dynamically.

Zikon, another start-up, has developed a technology that it calls reverse microemulsion, largely aimed at conventional display panels.

The basic components that make up the reverse emulsion electrophoretic display are two glass plates, each with etched ITO electrodes and a coloured reverse emulsion. The glass plates are sandwiched together, leaving a space of a few tens of micrometres between. The reverse emulsion is then injected in the space, and the sandwich is sealed.

Appropriate voltages are then applied across the emulsion through the ITO electrodes to change the colour of the display. Effectively, the microemulsion is a lot of coloured droplets floating in a clear liquid that can be controlled electrically. If they are spread out, the pixel takes on the colour of the droplets. If compacted, the liquid seems to clear.

Zikon has developed two ways of controlling the colour of the suspension: frequency addressing and electrophoretic addressing. When the field is removed or a high-frequency field is applied, the emulsion turns to a clear liquid. Intermediate frequencies cause the droplets to spread and the liquid to take on the colour of the dye.

Electrophoretic addressing uses two types of electrode. For this to work, the suspended droplets must be charged. The electrodes on the two plates are etched differently. One plate has wide, pixel-size electrodes.The other has much smaller electrodes.

When an electric field is applied in one direction, the droplets all concentrate and flatten out on the wide electrode and the pixel seems coloured. When the field is applied in the opposite direction, the droplets move to the other electrode and arrange themselves in narrow lines. The pixel is transparent.

Although electrophoretic inks are at an early stage, they may make it into very cheap or even disposable displays. But they are unlikely to challenge other flat-panel technologies directly for some years.