Scratch and sniff technology can be applied to all sorts of things other than just stickers, including clothing, compact discs, postcards and advertisements. Scratch and sniff technology involves treating an object with a microfragrance coating.
When the coating is scratched, it releases fragrance molecules into the air that you can smell. The coating usually includes an image related to the fragrance it carries. For example, a scratch and sniff sticker might feature a picture of a banana. When you scratch it, you can smell a banana as if you had just peeled one!
Scratch and sniff stickers first became really popular in the late 1970s. They were fun to put on school notebooks and clothing. Advertisers started to use the technology, too. For example, many perfume makers began to use scratch and sniff technology to coat cards to insert into magazines to promote their fragrances.
Scratch and sniff technology actually has some practical uses beyond just fun, though. For example, some utility companies use them to help people understand what a gas leak would smell like. Researchers use the technology today to help test patients who may be at risk for Alzheimer's disease, since one of the earliest symptoms is a loss of the sense of smell.
If you're wondering how they get smells to release upon scratching a coated surface, the secret is a process called microencapsulation. Smells — whether natural or created with chemicals — are captured in the form of molecules that must be distilled into tiny bubbles of liquid.
These bubbles are then microencapsulated in a secret process that turns them into ink-like goo that can then be printed onto stickers and other objects. When the object is then gently rubbed, the microencapsulated bubbles break easily and release their smells. Microencapsulation technology allows the trapped smells to last for years and years.