Soluble semiconductor polymers are bringing a new versatility to the printed circuit, which has to date been little more than a template of interconnections onto which components can be soldered. Now components themselves can be inkjet-printed to produce a wide range of devices, from intelligent labels to 'cyberskin' and other smart surfaces.

Inkjet-printed transistor. See here for full caption

It is becoming possible to draw up a logic circuit or even a custom chip in a computer-aided design package and print it out fully formed on an inkjet, according to Tracey Stephens, marketing manager of Plastic Logic , a spin-off from the same Cavendish Laboratory that spawned Cambridge Display Technology (CDT).

The technique is never going to replace silicon but it looks good for mass-produced throwaways, such as smart tags, or for niche products that would be uneconomical in silicon. A near-term target application is to produce active-matrix drivers for Lep screens of CDT, which owns a slice of Plastic Logic. This would enable both the light-emitting and driver layers to be sprayed on, avoiding a series of costly lithographic processes.

The idea of printing logic is even older than Leps; but researchers quickly found that inkjets, though precise enough to print pixel matrices, lack the resolution needed to trace the microstructure of a useful transistor. Finding a way round this has been one the major breakthroughs exploited by Plastic Logic (follow link for full caption above).

Another breakthrough was to find a way of allowing an inkjet head effectively to 'see' where it is printing, avoiding the tricky and expensive registration issues of lithographic printing.

A third has been to find a way of producing 'via holes' to hold non-printable components or connect different circuit layers. Stephens stresses that the technology is still under development and will not go commercial until late 2003 at the earliest. First applications are likely to include electronic tags; next will come Smart cards, low-capacity memory and sensors - one suggestion, post 11 September, was for an Anthrax detector. Oled displays, custom chips and perhaps processors are possible within four or five years.

Other suggestions include smart delivery systems for drugs, giving patients the correct dose on schedule, packages that announce when they are getting empty or games printed on the back of food packages.

Printed circuits are not limited to logic. ID tags could include an RF module that allows wearers to be tracked round a site. (Perhaps something similar could be used to allow lost socks to trace their partners.) A combination of Leps and printed logic could turn almost any object, including items of clothing or jewellery, into a dynamically changing display.

Printed circuits do not share the encapsulation problems of Leps and can use plastic substrates, which leads to other intriguing possibilities. Set sensors into a flexible mesh of surface logic and you get a kind of cyberskin that could increase the resolution of devices that transmit or receive body movements and tactile pressure. The possibilities are endless.

More prosaic are smart surfaces of a kind also being investigated at Cambridge. They consist of a matrix of contacts that automatically configure themselves to deliver services to devices placed in any way on the surface. This would enable your mobile phone, for instance, to synchronise and start charging itself as soon as it is put down. One thing Plastic Logic certainly does not lack is ideas on how to use the technology. 'It seems everyone who comes to visit us comes up with a new suggestion,' said Stephens.

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