Colour Changing Inks

It's like finger painting, only without the messy paints. Pressing a sensor on paper printed with colour-changing inks could bring extra interactivity to books and wallpaper.

Adding stiff wires and LEDs to a sheet of paper is one way to create colour-changing paper. But Kohei Tsuji and Akira Wakita, both at Keio University in Japan, wanted to create the same effect without affecting the paper's softness and flexibility. 

They printed colour-changing inks on one side of a sheet of paper and painted conducting pastes to make an electric circuit on the other side. Touching the paper activates a copper pressure sensor taped on the back of the paper. This sends electricity through the painted silver wires, which warms electrodes made from carbon paste. Heat radiates through the paper to the colour-changing ink on top.

This ink is a liquid crystal and the molecules within it organise into twisted helices when warmed. The natural structure of the helices interacts with light and reflects only some wavelengths. The material changes colour from red to blue to green as it warms because the liquid crystal structure changes with temperature. 

In one demonstration, Wakita and Tsuji printed a butterfly with colour-changing ink in its wings. Pressing the centre of the butterfly causes waves of colour to spread across its wings.

Wakita and Tsuji also used an opaque ink that becomes transparent when warmed. They created an interactive star atlas that shows the shapes of constellations and reveals their history with just a touch. Diagrams printed in rainbow ink outline the stars in the constellation. Opaque ink covers both the words and a picture describing the constellation. 

The opaque ink appears to dissolve when the paper is touched, revealing the rainbow ink atop a picture of the subject of the constellation and a text description of its histor

The paper artworks will be shown at SIGGRAPH ASIA in December.