Forget lemon juice and hot irons, there's a new way to write and read invisible messages – and they can be used over and over again.
The approach developed by researchers in China involves the use of water to print messages on paper coated with manganese-containing chemicals. The message, invisible to the naked eye, can be read by glittering UV light on the paper.
Using hot air from the hair dryer, the message can be erased and the paper can be reused.
Although digital communications and data security are becoming more complex, the study's authors said finding ways to securely send messages on paper remains important.
"Data leaks have become a global problem with dire consequences, including outbreaks of war and severe economic and social problems," they say in the study.
"Although electronic media have become indispensable in our daily lives, it is still the most common storage medium and many important documents are still paper-based. "
Writing in Matter magazine, the team says other" secure printing "approaches include the use of fluorescent inks that can see each other only in UV light. However, they say they have flaws, including that they cannot be erased and paper reused, and are predictable, meaning that messages can be easily cracked.
The new approach involves coating filter paper with a polymer that contains manganese-containing chemicals that can absorb UV light and emit green light and then print messages on that paper with clean water using an inkjet printer. The team said the materials have very low toxicity.
When pure water interacts with manganese-containing chemicals, it disrupts their structure and therefore their photoluminescent properties. The result is that, although the message is not visible to the naked eye, it appears as a dark pattern in short-wavelength UV light.
If the paper heats under the hair dryer for about 30 seconds, the message disappears. So far, researchers have been able to reuse paper up to 30 times before it breaks down – with each print costing a penny.
"If we can find a good substrate to replace the filter paper, we believe that the cycle numbers will be further improved," says Dr. Yun Ma, co-author of the study at Nanjing University of Post and Telecommunications.  The authors said that the message does not simply disappear with the evaporation of water in the absence of a hairdryer, and may continue for more than three months under ambient conditions. to provide the necessary movement for the jet
The team also took a different approach, using two manganese-containing inks to print a message and background on a different type of coated filtered paper.
The message cannot be seen with the naked eye or, importantly, under UV light, since the similarity in the ink means both it and the background emit light of very similar wavelength and intensity under UV light. The message, however, emits light for a longer period than the background, which means it can be read using time-based image techniques.
Developing this further, the team came up with an even more sophisticated approach, which included printing a series of figures on paper using several manganese containing inks, each of which emits light for a different period of time after UV illumination.
By tracking the intensity of light emission over time, a new order of numbers allowing a crack code is revealed. The paper can then be washed in a special solution and the same string of numbers, printed with different ink order, to encode a different message.
"This method of decrypting information has an extremely high level of security and can be promising for military and economic fields," the team says.