Future of Printing: This Rewritable Paper Prints Using “Light” not Ink

Richard P. Feynman's Quote on a light printed reusable paper. Image Credits: Nano Letters
REWRITABLE PAPER IS HERE

Scientists in China and USA have developed rewritable paper that uses light induced inkless printing. This new technology promises to be cheaper and less taxing on the environment than traditional ink based paper printing.

At the core of this technology is a coating of Nanoparticles (NP), which changes colour on exposure to Ultra-Violet (UV) radiation. The best part is that the colour change is reversible.

On heating the coating to 120°C, the colour change is reversed and this can go on for 80 cycles. This repeatability is increased 4-fold since the team’s previous attempt.

Chemistry Professor at the University of California, Yadong Yin says, “The greatest significance of our work is the development of a new class of solid-state photoreversible colour-switching system to produce an ink-free light-printable rewritable paper that has the same feel and appearance as conventional paper, but can be printed and erased repeatedly without the need for additional ink.

PRINTING WITH UV LIGHT AND REMOVING WITH HEAT

The coating comprises of NPs of coloured dyes (Prussian Blue and its analogues) integrated with photoactive TiO2 NPs. Upon UV light exposure, reduction takes place, and the dye looses its colour turning white. TiO2 acts as a photoactive catalyst mediating effective electron transfer.

This colour changes lasts up to a few days before the dye oxidizes naturally gaining back its colour. Heating can speed up the erasing process. Just by heating the paper and film to about 120 degrees Celsius (250 degrees Fahrenheit) the oxidation reaction speeds up, erasing the printed content completely within about 10 minutes.

Since the dye is blue, coating turns a plain piece of paper into blue colour. Upon UV exposure, photo-active TiO2 looses electrons which are picked up by the dye (Prussian blue NP), eventually loosing colour.

Reading a blue text on a white background is easier compared to the other way round. Hence, a reverse mask is used which exposes the ‘inverse’ of the writing such that the writing appears blue on a white paper. The team experimented with different analogues of Prussian blue in the coating to obtain different colours also.

Rewritable paper samples
Different samples of the light printable paper. Image Credits: Wang et al., American Chemical Society, 2017

The NP coating can easily be applied by soaking the paper in the dye or by spraying.The researchers confidently state that the high-resolution printing (5-µm) can be used on most kinds of paper.

Since the process is reversible, it saves the cost of ink and paper significantly.

GREAT FOR THE ENVIRONMENT

The simplest and the most hard hitting issue of the paper manufacture industry is deforestation. Close to a third of harvested trees in the US are sacrificed for the paper and pulp industry.

As of now, production and disposal of paper hugely impacts the environment in a negative way. Discarded paper constitutes to almost 40% of landfills.

Even if the paper is recycled, ink removal adds to growing chemical pollution. It is needless to say that rewritable paper would really benefit our environment greatly.

Prof. Yin adds, “Our work is believed to have enormous economic and environmental merits to modern society.

The team believes that the costs involved with the technology will go down when they scale up. In the end it is important that this technology evolves and finds its way to common households.

 

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Source American Chemical Society University of California, Riverside Nature

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