Different Temperatures Revealing Different Images

A research group in China has come up with a novel approach to product authentication, with a label that can be internally heated to reveal a changing image.

A research team led by Prof Ji Shulin from the Hefei Institutes of Physical Science of the Chinese Academy of Sciences has developed a transparent heating film made of silver nanowires that can be used to fabricate a paper-based silver nanowire film heater.

When a low voltage is placed across the nanowire structure, the resistance in the wire causes rapid temperature changes which in turn change the colour of thermochromic inks. Multiple and overlapping high-resolution thermochromic patterns are printed onto the substrate so that various image elements are revealed, and change colour, at different nanowire temperatures.

Prof Ji has been working on this project for several years and now expects this mature technology to be marketed commercially.

The heating system is based on the heat exchange between the silver nanowires and the thermochromic inks; the film is optically transparent and does not interfere with whatever is printed on the paper or plastic label.

The temperature difference between the nanowires and the inks can be adjusted to control the speed at which the thermochromic patterns appear. The researchers are confident that the effect cannot be duplicated by other discolouration attempts by counterfeiters.

The research team expects that the effect will find applications in the fields of anti-counterfeiting and intelligent displays.

Different temperatures reveal different image elements (© Hefei Institutes of Physical Science).