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THE printing of body parts (see article) will probably remain a bespoke industry for ever. Printed lighting, though, might be mass produced. That, at least, is the promise of a technology being developed in Sweden by Ludvig Edman of Umea University and Nathaniel Robinson of Linkoping. Dr Edman and Dr Robinson have taken a promising technique called the organic light-emitting diode, or OLED, and tweaked it in an ingenious way. The result is a sheet similar to wallpaper that can illuminate itself at the flick of a switch.

An OLED is a layer of semiconducting polymer sandwiched between two conductive layers that act as electrodes. When a current is passed between these electrodes, the polymer gives off light. The light is created by electrons released from one electrode layer falling into positively charged “holes” that have been generated by the polymer's interaction with the other layer. These holes are gaps in the polymer's electronic structure where an electron ought to be, but isn't.

Semiconductors are strange materials. Both holes and electrons can move around within them. (The holes move in a manner analogous to the gap in a sliding-tile puzzle.) They are also finicky. Only some sorts of conductors will work as sources of electrons. Only some sorts will work as sources of holes. And no known material works well for both.

The electron source needs to be a metal, and the usual choice is aluminium. Of course, metals are opaque, so the other electrode must be transparent. Fortunately, there are two materials that are both transparent and good hole-generators. One is indium tin oxide. The other is known as poly(3,4-ethylenedioxythiophene).

OLEDs are, however, awkward to make. First, a precisely crafted layer of aluminium has to be created. Then the other two layers are sprayed onto it using an inkjet printer. If the metal electrode could be replaced then it might be possible to make the whole thing using just a printer. That would simplify matters enormously. And that is what Dr Edman and Dr Robinson believe they have achieved.

To do so, they have gone back to their high-school physics lessons. As every schoolboy knows, carbon in the form of graphite is the exception to the rule that metallic elements conduct electricity and non-metallic ones do not. Graphite, which is black in bulk, consists of layers of carbon atoms arranged in a hexagonal grid. When the substance is only a few of these layers thick, though, it is known as graphene and is transparent.

Graphene sheets are not easy to handle. But Dr Edman and Dr Robinson found another team of researchers, led by Manish Chhowalla of Rutgers University, that was working on making graphene electrodes. Unfortunately, they found that graphene by itself will not do the job. But they overcame its reluctance by blending the semiconducting polymer with potassium trifluoromethylsulfonate. This compound consists of positively charged potassium ions and negatively charged trifluoromethylsulfonates. When the current is switched on, the two sorts of ion move in opposite directions to the junctions between the polymer and the electrodes. The trifluoromethylsulfonate ions assist the process of hole formation in poly(3,4-ethylenedioxythiophene) at one junction. That is useful. What is crucial, though, is that the potassium ions liberate electrons from graphene at the other junction.

The result, as the team describe in ACS Nano, is a sheet that emits light in both directions and which it should be possible to make using industrial inkjet printers to spray layer upon layer. It is a truly enlightening idea.