The printing of body parts (see Sunday’s SR) 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 […]