Simply, they are materials with properties that can be activated to display a special effect.
Often, the change is made by specific environmental stimuli such as temperature, sunlight or water. Most effects are reversible but some materials cannot change back. The most common smart materials include Hydrochromic, Thermochromic, Photochromic or Glow in the Dark.
Hydrochromic:
Initially white, Hydrochromic materials lose their opacity when water or moisture is present. Hydrochromic ink and Hydrochromic sheets can cover surfaces to hide and reveal messages, images or colours.
Hydrochromic ink can be used to make umbrellas that changes colour in the rain!
These materials are sensitive to changes in temperature, and change colour or become transparent at a certain temperature. For example, smart straws change colour in cold drinks. Liquid Crystal ink can be used an alternative to mercury thermometers and Thermochromic pigments can be made into Thermochromic coatings when combined with a binder. As mentioned, many smart materials can be used over and over, but some 'smart' health and safety products are required to be irreversible.
Photochromics change colour in the sun or UV light. Starting out pale or off-white, Photochromic pigments and ink burst into colour when outside. Applications include the Photochromic or transitional prescription or sunglasses which darken in sunlight.
As the name suggests, these products glow a certain colour in the dark. No batteries required! By absorbing UV light, Glow in the Dark pigments and inks can charge, meaning they can glow for many hours after. The effect is called photoluminescence. Industrially, switches and power boxes can be applied with Glow in the Dark products in the case of power failures.
Which smart materials have you tried? What is your favourite effect?