Paper-based thermometer wraps around objects: monitors temperature on all sides

March 15, 2017 // By Julien Happich
Researchers from the Renmin University of China have devised a very cheap and scalable temperature sensor made out of paper and traces of gold (for the electrodes) joined by a line of inkjet printer-ready ionic ink as the thermistor.

Publishing their results in the ACS Sensors journal under the title "Ultrafast Paper Thermometers Based on a Green Sensing Ink", they detail a very simple implementation where they leverage the benefits of paper as a flexible substrate and the ionic liquid, 1-ethyl-3-methyl imidazolium bis(trifluoromethylsulfonyl)imide ([EMIm][Tf2N]) for its non-volatile and hydrophobic nature.

Transferred to regular A4 paper by pen writing or inkjet printing, the ionic liquid holds onto the paper through the capillary effect, showing no leakage during bending and folding. The electrical conductivity of the ink trace is measured between two sputtered gold electrodes (5mm wide and 1.5cm long in the experiment) connected to an external power supply. It happens to be ultrasensitive to external temperature changes and provides reliable temperature readings despite numerous folding and bending cycles. "The paper thermometer's response time against thermal heating is about 8 seconds", report the researchers, "regardless of the difference of conductivity change at different testing temperatures."

The researchers then pushed the new thermistor concept to another level, patterning eight parallel gold electrodes on one side of a A4 paper, manually writing 64 dots of ionic liquid on the other side of the paper, aligned with the electrodes (so the ink would reach the gold electrodes through the paper) and depositing another set of eight parallel gold electrodes crossing over the dots, perpendicular to the electrodes from the opposite side.