I spent a couple of days earlier this week at a conference in Manchester called “Printing of Functional Materials”. The premise of this meeting was the growing realisation that printing technologies, both the traditional, like silk-screen and gravure, and modern, like ink-jet, offer scalable, cheap and flexible ways of precisely depositing small quantities of materials on surfaces. Traditional inks are just vehicles for pigments to create static images, but there’s no reason why you can’t use printing to deposit materials that are conductors or semiconductors of electricity, which are electro-luminescent, or which have some biological functionality. Indeed, as one of the organisers of the conference has shown, one can even use ink-jet printing to deposit living human cells, with potential applications in tissue engineering.
The degree of commercial interest in these technologies was indicated by the fact that, unusually for an academic conference, more than a third of the attendees were from the commercial sector. Many of these were from the cluster of small and medium companies developing ink-jet technologies from around Cambridge, but European and American concerns were well represented too. My impression that the sector that is closest to maturity in this area is in electrically functional devices, where there’s a great deal of momentum to drive down the cost of RFID and to develop cheap, flexible displays. But there are still many materials issues to solve. It’s not easy to get a complex fluid to flow in the right way to form the tiny, well-defined droplets that make it ink-jet well, and formulating the ink in a way that makes it dry to give the best properties is awkward too. Silver inks illustrate the problems – commercial inks to write conducting lines sometimes use silver nanoparticles. Making the silver particles very small is helpful in making them coalesce well to make a continuous silver layer; the melting point of materials is lowered when they are in nanoparticulate form, making them sinter at lower temperatures. But then you have to work hard to stop the particles aggregating in the ink (it’s particularly undesirable, or course, if they aggregate in the ink-jet nozzle and block it up). To stabilise them, you need to coat them with surfactants or polymer molecules. But then this organic coating needs to be driven off by a heating step to get good conduction, and this compromises your ability to print on paper and plastics, which can’t take much heating. It seems to me that this technology has a huge amount of promise, but there’s a lot of materials science and colloid science to be done before it can fulfill its potential.