Electrophoresis is the movement of particles or molecules by an electric field.

Once just a neat trick in the Victorian parlour, the effect has been harnessed into displays that are quietly nudging aside a few LCDs, and even creating their own niches.

The displays work by attracting various coloured powders to a front viewing surface by electrostatic force. This means they can only work in reflective mode, and therefore will never work in the dark without additional lighting.

However, they have three big advantages compared with reflective LCDs: wide viewing angle, high contrast and low power consumption.

The first two come inherently from the user looking directly at coloured powder - LCDs instead manipulate the polarisation of incident light.

Low power on the other hand comes from the image remaining once power is removed, a characteristic that electrophoretic displays share only with zenithal and cholesteric LCDs.

There are two ways to make a pixel inside an electrophoretic display: either a single powder in a contrasting fluid, or two oppositely charged contrasting powders. E Ink and SiPix, currently the dominant electrophoretic technology providers, represent both of these approaches.

“We use two particles at E Ink,” says E Ink marketing director, David Jackson, “SiPix uses a white particle and a black background.”

Electrophoretic displays are already found in products. E Ink, for example, sells into Motorola’s F3 phone, eBooks from firms including Sony, and a USB memory stick from Lexar.

Electronics Weekly has handled the phone and an eBook. Both displays are clear and easy to see, and the 170dpi eBook display is a pleasure to read compared with existing LCD PDAs operating in reflective mode.

A further attraction to product designers is that electrophoretic displays are thin. “The first hook is the look. Then power consumption, then the whole thing is only 1.3mm thick,” says Jackson. “And that is in glass.”

The electrophoretic materials - the particles and the fluid - in E Ink displays are trapped in spherical voids - effectively bubbles - in a plastic film. “The microcapsules are between 40 and 100µm,” says Jackson.

SiPix on the other hand embosses ‘microcups’, separated by thin walls, into a plastic film to create voids, then seals them with a capping layer.

In either case, the voids do not have to align with pixel electrodes and half a void can happily function as part of one pixel, while the other half takes a different state in another pixel.

How big are the charged particles? “Very small,” is all E Ink’s Jackson will reveal.

Once positioned, particles stay in place for a long time. “With no power they will stay there for about a year with sub-ten per cent image degradation,” says Jackson.

This stability allows grey scales to be developed. “Instead of pulling the particles all the way to the top, we use a waveform that leaves them part way through the liquid,” says Jackson. “We have three-bit and four-bit pixels giving us eight or 16 levels.”

With electrophoretic displays, the time to change an image is measured in hundreds of milliseconds. This is not a problem in eBooks, but is restrictive if moving pictures are required. “We are a little slow with updates. In future we will change the electronics and use the next generation of ink,” says Jackson. “Also, instead of the whole screen, we will do regional updates.”

Commercial E Ink displays are already twice as fast as earlier versions, while white reflectance has climbed from 34 to 40 per cent - offering a contrast ratio of 8:1, says Jackson, who points out that this lowly contrast ratio should not be confused with the 400:1 achieved viewing emissive displays in the dark. Contrast of real ink on real paper is no better than 50:1, and can sink to around 15:1.

As a sign of things to come, says Jackson, E Ink showed “full motion NTSC colour video” on a 150mm panel at the Society for Information Display in May - consuming under 1W.

Colour was achieved using black and white pixels plus colour filters, rather than coloured particles.

E Ink would like to see electrophoretic displays used for the second screen encouraged by Windows Vista. “The displays are so thin they could be included in the top of a [closed] laptop without re-profiling the moulding,” claims Jackson.

Unlike OLEDS, the up and coming competitor to emissive LCDs, lifetime is not an obvious challenge for electrophoretic displays, but Jackson sees some potential for improvement. “The drive electronics has to be tri-state, and the zero needs to be zero for the long-term health of the display,” he says.

For backplanes, amorphous silicon-on-glass is the most obvious technology to start on, but many think the true future of electrophoretic displays lies in flexible displays.

For example, LG.Philips showed a 14.1in. flexible 4,096 colour electrophoretic display earlier this year.

LG.Philips used a metal substrate, however it is likely that plastic substrates will be needed to fully realise the technology’s potential at a reasonable price.

Philips spin-out, Polymer Vision, and Cambridge-based Plastic Logic are both making all-plastic electrophoretic displays.

Plastic Logic’s displays, being made in Dresden, are thick enough to stand-alone and so can only be flexed rather than bent.

Polymer Vision’s are thinner, rollable, and are being made in Southampton by partner firm Innos. “We aim to produce 100,000 to 200,000 displays in Southampton in the first year, depending on the market,” says Polymer Vision chief operating officer, Guido Aelbers.

Innos’ speciality is plastic transistor backplanes which it combines with bought-in electrophoretic frontplanes. “Our polymer transistors have donor and acceptor mobilities very similar to amorphous silicon,” claims Dr Alec Reader, Innos business development manager. “The mobility is our big differentiator. Some other guys have a couple of orders of magnitude less.”

With polymer transistors on plastic substrates, moisture ingress is the enemy and barrier layers the defence.

So hot is the topic that no one wants to give away their barrier secrets, including Reader. “We use proprietary barrier techniques and proprietary techniques to encapsulate. We have been making devices for more than five years now and not had a major issue,” he states.

Polymer substrates are intended to flex - Innos’ are 50µm thick - which can make alignment during manufacture a headache.

To get around this, Innos processes its substrates bonded to a silicon wafer. “As you go to colour, you decrease pixel size,” says Reader. “With the holding technique we use, pixel size is not an issue. And we use an I-line stepper from the semiconductor industry, which is a bit of overkill - way too good for what we are doing.”

He does not expect pixel size to drop far below 100µm. “The eye cannot resolve much better than that,” says Reader.

With Innos’ carrier mobility and equipment to make small features, Reader sees more than an active matrix on substrates of the future. “We could go to logic and memory circuits. It is an enabling technology,” he says.