Micro power lights the way

Low-cost display and fluid manipulation technology has the right ingredients for devices powered by very small batteries. Tom Shelley and Julie Bieles report



New display technologies, based on fluidics, are now appearing that are daylight visible and consume exceptionally little power.
Bartels Mikrotechnik in Dortmund, Germany, has invented and developed one such idea, which is a variant of what are called ‘Electrowetting’ displays. The principle of operation here is to have a droplet of liquid on a hydrophobic surface, which can then be made to spread out and wet by applying a voltage to it. This occurs because the voltage interacts with the electric charge double layer that exists at any liquid-solid interface.
Developed initially in response to a request for a very low power display by a major fire alarm and security company, they also have potential to be used in quite large displays. And the underlying technology can be used to control fluid flow on ‘Lab on a chip’ devices.
Meanwhile Liquavista, spun out of Philips Research Labs, has for some time been developing displays in which applying a voltage causes coloured oil droplets to transform from wetting to balled-up droplets to expose a white reflecting surface underneath.
Its newly launched commercially available electrowetting reflective display technology – ColorBright – is targeted at applications that include watches and mobile phones. The optical stack, or image forming part, is made up of three ingredients – fluoropolymer, oil, and water – and an electrode. In a display, these ingredients are sandwiched between glass or polymeric substrates. When a voltage is applied across the fluoropolymer, it changes from hydrophobic to hydrophilic and the oil is pushed aside by the water, which exposes a reflective surface.
The display technology has a 180-degree viewing angle and is visible in natural light, low light and high light conditions, without needing a backlight. Simon Jones, VP of marketing and sales, told Eureka this was because the displays were based on a more efficient light switch than LCD.
“This means that a greater proportion of ambient light is reflected back from reflective displays,” he explains. “The eye’s sensitivity scales to the ambient light – which is why reflective displays look better in sunlight. In low light conditions, the greater efficiency, which gives greater contrast and reflectivity, allows the display to be read in dimmer lighting conditions.”
The company’s electrowetting displays are manufactured on modified LCD lines. Current production is at an existing factory where Liquavista uses front end and back end LCD processing. “We replace one part – the most expensive part of the LCD process – with our own process,” Jones says. “Ultimately, the manufacturing process has the potential to be lower cost, because it uses much simpler materials – oil, water, dye – and less materials than LCD. Two of the most expensive components are the liquid crystal materials and the polarisers, which we don’t need.”
The company is developing a full-colour active matrix platform, which can be used to make power efficient main displays for mobile phones and other portable applications, according to Jones. Liquavista has also launched a research programme to develop next-generation flexible electronic displays, which support full colour and video, with plastic electronics firm Plastic Logic. The £12m three-year programme is partially funded by the UK Technology Strategy Board and will use Liquavista's electrowetting technology to create thin, flexible and light screens that display bright, colourful images, and show video content with low power consumption, according to the company. Future applications could include e-magazines.
So how does this differ from the Bartels Mikrotechnik technology? Well, the latter actually works the other way round, it would appear, so that a voltage transforms a balled-out droplet to spread out liquid. But what is really clever is that, when an electrode beside a contracted droplet is energised, the droplet moves over to where the electrode is and then stays there when the voltage is switched off again. Power is only consumed in order to move droplets from one reservoir to an adjacent one. Hence the display is bi-stable and requires no power at all to maintain a display configuration – only needing a brief injection of voltage, with very little associated current, to change it.
Eureka saw it demonstrated on a remote locking car key fob to show whether the car was in an unlocked or locked state. Such devices are expected to be able to run on a single battery for a year or more. So, if they are going to incorporate a display, this has to consume much less power than most displays that are commercially available, while being daylight visible.
Other applications are seen as on/off displays or memory state indicators in mobile phones, PDAs, USB sticks or MP3 players. However, according to those in the know, the technology is in no way restricted to small dimensions, since displays can be addressed that have any number of droplets. Temperature is not a limiting factor, since the technology works over a wide range. It thus has the potential to be used in truly large displays, such as advertising billboards.
We’ve also been told that it has the potential to be used in non-display applications, such as arrays of micro lenses whose focal length could be varied, as is already the case for some macroscopic lenses in mobile phone cameras. It could equally be used in lab-on-a-chip devices, where the technology could be used to move either streams of liquid or individual micro droplets. These could be moved around on surfaces in any direction, without the need to be restricted to etched channels.

Pointers
* New display technologies, based on fluidics, are daylight visible and consume very little power
* Bartels Mikrotechnik technology has the potential to move droplets of liquid around on a surface in lab-on-a -chip applications
* Moving droplets could move in any direction and need not be restricted to particular etched channels
* Liquavista display technology has a 180-degree viewing angle and is visible in natural light, low light and high light conditions, without needing a backlight
* Liquavista’s commercially available electrowetting reflective display technology, ColorBright, is targeted at applications that include watches and mobile phones

Author
Tom Shelley

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