How To Make Electronic Displays With Mood Ring Ink

Soychemist writes "Print some thermochromic ink onto a sheet of paper, put metal heating elements on the other side, and you have a rudimentary color-changing display. Chemists in the Whitesides Group at Harvard think that the devices could be used to provide a simple readout from cheap medical tests and kits that check water for pollutants. In the past year, the same scientists have made a three-cent medical test and improvised a centrifuge with an egg beater. Their aim is to invent useful gizmos with everyday materials."

Resistive ink

[Rorschach1]

I was thinking about making a thermochromic display for a custom watch, actually. Didn't know if anyone else had tried it before, but I guess I'm on the right track. It's going to take a lot of batteries to power, but I only really want it to run for Burning Man and Maker Faire.

I'm going to see if I can screen print resistive ink onto a PCB to make the heating elements. Failing that, I'll just go with thin film SMT resistors. Anyone know if that has been done before?

Two questions

[raktul]

What temperatures are needed to change the ink? Obviously if it needs to be really hot that is bad since it might burn the person holding it, while cooler temperatures can be affected by the atmosphere around the page causing problems. Would it make more sense to use a different material then paper to print the ink on to avoid accidental colour changes?