Scientists have created a new form of invisible ink, but it has a big problem: it may be toxic. Aut Exit Sliding Guide,Wear-Resistant Sliding Exit Guide,Wire Rod Mill Exit Sliding Guide,Sliding Guides For Steel Profiles JINGJIANG EASTSUN METALLURGICAL MECHANICS CO.,LTD , https://www.jesguide.com
Invisible ink or "smart ink" can be used to prevent simple storage of information. These inks should not be visible unless they are activated by light or heat, but in practice you can usually still see something written in ink. In one study, scientists created a truly invisible ink until you add salt. 
The new material is not perfect: the basic compound is lead and toxic. But scientists are trying to design a safer alternative.
This lead-based compound is invisible. However, once a special type of salt has been added, lead will turn into nanocrystals when you shine a UV lamp on paper. Once you remove salt again, the ink will become invisible again, so this information can be kept confidential and transmitted again. This particular technology may not be widely used, but it is closer to better safety and important information, or just passing notes in class.