Messages can only be seen under UV light and can be erased using a hairdryer

Forget lemon juice and hot irons, there is a new way to write and read invisible messages – and it can be used again and again.

The approach, developed by researchers in China, involves using water to print messages on paper coated with manganese-containing chemicals. The message, invisible to the naked eye, can be read by shining UV light on the paper.

With the use of hot air from a hairdryer, the message can be erased and the paper reused.

Despite digital communications and data security becoming more sophisticated, the study’s authors said finding ways to securely send hard copy messages remained important.

“Data leakage has become a global problem with terrible consequences including outbreaks of war and severe economic and social issues,” they say in the study.

“Although electronic media have become indispensable in our daily lives, paper is still the most widespread medium for information storage, and many important documents are still paper based.”

Writing in the journal Matter, the team say other approaches to “secure printing” include the use of fluorescent inks that can only be seen under UV light. However, they say these have drawbacks, including that they cannot be erased and the paper reused, and are predictable – meaning the messages, once found, can be easily cracked.

The team report that they have developed a number of solutions to tackle each of these problems. The new approach involves coating filter paper with a polymer that has manganese-containing chemicals, which can absorb UV light and emit green light, and then printing messages on this paper with pure water using an inkjet printer. The team said the materials have very low toxicity.

When the pure water interacts with the manganese-containing chemicals, it disrupts their structure, hence their photoluminescent properties. The upshot is that while the message cannot be seen with the naked eye, it shows up as a dark drawing under short wavelength UV light.

If the paper is heated under a hairdryer for about 30 seconds, the message disappears. So far, the researchers have managed to reuse the paper up to 30 times before it breaks down – with each print costing fractions of a penny.

“If we can find a good substrate to replace filter paper, we believe the cycle numbers will be further improved,” said Dr Yun Ma, the co-author of the study, from Nanjing University of Posts and Telecommunications.

The authors said the message did not simply disappear with evaporation of water in the absence of a hairdryer, and it can last for more than three months under ambient conditions.

One possible explanation, they say, is that it might be necessary to melt the polymer coating to allow the necessary movement for the structure (and hence photoluminescence) of the manganese-containing chemicals to be restored after the water has evaporated.

The team also took a different approach, using two manganese-containing inks to print a message and background on a different type of coated filter paper.

The message cannot be seen by the naked eye or, importantly, under UV light since the similarity in the inks mean both it and the background emit light of a very similar wavelength and intensity under UV illumination. However, the message emits light over a longer period of time than the background, meaning it can be read using time-based imaging techniques.

Developing this further, the team came up with an even more sophisticated approach that involves printing a series of numbers on paper using several manganese-containing inks, each of which emits light over a different period of time after UV illumination.

By tracking the intensity of light emission over time, a new order of the numbers is revealed – allowing the code to be cracked. The paper can then be washed in a special solution and the same string of numbers reprinted with a different order of the inks to encode a different message.

“This information decryption method possess an extremely high security level and might be promising in military and economic domains,” the team said.