Is that paper really blank? A new technique using special reusable paper makes water an undetectable invisible ink. Researchers in China developed a rewriteable paper coating that can encrypt secret information with relatively low-tech invisible ink — water.

A message printed out by a water-jet printer on a manganese-complex-coated paper is invisible to the naked eye, but the message reveals itself under 254 nm UV light. The paper can be ready for another round of printing after erasing the message by heating it with a blow dryer for 15-30 seconds. The method, presented September 25 in the journal Matter, allows reversible secure printing for at least 30 cycles. The recorded information is invisible under both ambient light and typical UV light. The data will only appear under a photoluminescence lifetime imaging (PLIM) technique, protecting it from general decryption methods. (PhysOrg)

The water-jet printing method is cost efficient ($0.002/print) and environmentally safe, although the short-wavelength UV light used to read the page is still potentially harmful to humans.
What will be the first book published using invisible water printing?

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