Scent stickers are one of those novel technologies that sounds like a really good idea... But other than appearing in children's books and occasionally as scent strips in magazines, they never really caught on. Why can't olfactory information be as important as auditory and visual information?
That may be because no one has figured out how to encode language in taste. Still, if you have children, there's a good chance you'll have a scent sticker book at home. Even though this book is 20 years old, it still smells good! The scent of the stickers lasts because they are produced using microencapsulation technology. The basic principle behind the scent stickers is to pack the chemicals responsible for the smell into very small gel or plastic spherical capsules, which are on the order of a few microns in diameter. When you scratch off the sticker, you break up some of these balls and release the flavor. The flavor is actually encapsulated in millions of tiny capsules, and every time you scratch off the sticker, you break some of those little capsules. These tiny capsules can preserve the scent for years.
The microencapsulation technology used in fragrance stickers was originally used to produce carbon-free carbon paper. Carbon paper is coated with microcapsules containing colorless ink. When you write on the paper, it punctures the microcapsule and releases the ink. The ink mixes with developing chemicals on the next sheet of paper to produce dark colors!