A Smart Material is designed to change its properties based on stimulus in its environment. This can be from a range of different sources such as; heat, light, sound, stress, magnetic or electric fields. Importantly the material will return to its original state when the stimulus is removed and can react many times.
Piezoelectric material produces a small voltage when stress is applied to it and will expand/contract when the reverse happens. They can detect incredible small amounts of pressure making them ideal for microphones as a way of converting sound waves to electrical signals. Equally the are used in reverse to produce tonal sounds in a range of audio outputs such as sirens 
Shape memory alloys  & shape memory polymers are materials which return to their original form after large amounts of deformation by returning them to the conditions they were made in. This is known as 'superelasticity' and prevents the material from reaching its elastic limit providing it is returned to its original state, typically temperature. They therefore have a 'temporary state' and a 'permanent state'. Nitinol (Nickel/Titanium) is an alloy used in dental braces for example to apply constant force to help re-align teeth
Quantum-tunnelling composites​are flexible polymer materials that contain small metal particles. When pressure is applied to the material it goes form an insulator to a conductor allowing the communication of signals to/from electronic devices. They are ideal for water proof membrane switches which require no mechanical movement and often used in textiles to control phones/MP3 players & sports analysis.
Electroluminescent materials have the ability to emit light when a small voltage is applied to them. Much like deep sea creatures the material takes advantage of Electroluminescence(EL)  an  electrical phenomenon whereby a current passing through a material creates an electric field that produces light. Often used as a flexible and uniform alternative to LED's for use in textiles, screen backlights, advertising, emergency lighting, and creatively in the film 'Tron'
Thermochromic Materialschange there pigmentation depending on the temperature applied to them using liquid crystals. This allows for a visual indication of the temperature of an environment without the use of a thermometer or electronic device. Typically used in kitchenware, fish tank thermometers, batteries, and paint finishes.

Photochromic Materials are similar but rely on photons of light to change the pigmentation. Often used for reactive lenses in glasses to adjust depending on the light level to protect the user from UV light
Reactive Glass  uses a similar photochromic effect for use in range of industries from automotive to construction. Translucent under normal conditions, glass can alter its opacity based on; heat, light, electrical current and nano particles (SPD). The latter uses a thin layer of nano particles suspended in liquid between the glass panes. When no current is applied the particles randomly align blocking light. Current can be controlled to alter the opacity of the glass

SPD can result in huge savings commerically by reducing the need for air conditioning in summer and heating during the winter as it allows greater control over the level of sunlight. Opacity can be changed within 3 seconds and block 99% of UV rays. It is equally suited to the automotive industry for car sunroofs and aircraft for their many windows

Many smart materials have been inspired by nature and the incredible adaptations animals have developed to overcome the natural world. Below is a 5 part mini series about this link...

Modern and Smart Materials - Smart Materials


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