The big idea: A smart material has one or more properties that change significantly in response to a change in its environment.
Learn them as pairs: Every one is a pair: a stimulus going in, a response coming out. Learn them as pairs and the six stop blurring together.
Six stimulus-and-response pairs, with the reversible and both-ways ones marked at the end.
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| Smart material | Stimulus | Response | In products |
|---|---|---|---|
| Piezoelectricity | A squeeze or vibration | Produces a voltage | Gas-lighter spark, ultrasound |
| Shape memory | Heat above a set point | Returns to a remembered shape | Spectacle frames, stents |
| Photochromicity | Ultraviolet light | Darkens, then clears again | Reaction lenses, tinting glass |
| Magneto-rheostatic | A magnetic field | A fluid stiffens instantly | Adaptive suspension |
| Electro-rheostatic | An electric field | A fluid stiffens instantly | Fast clutches, haptics |
| Thermoelectricity | A temperature difference | Produces a voltage | Camping-stove charger |
Two run both ways: Piezoelectric and thermoelectric materials work in both directions.
Squeeze a piezo crystal and you get a voltage; apply a voltage and it moves.
Put a temperature difference across a thermoelectric and you get power — and supply power and you get a temperature difference, which is how a cooling plate works.
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A smart material replaces a mechanism. That is the reason to use one, and it is the answer an exam is looking for.
Conventional solution
- Sunglasses: carry a second pair
- Suspension: valves, sensors and a controller
- Spark: a flint and a striker that wears out
- More parts, more assembly, more to fail
Smart-material solution
- Photochromic lens: one pair that responds by itself
- Magneto-rheostatic fluid: one coil, no valves
- Piezo igniter: one crystal, no moving parts
- Fewer parts, no mechanism to wear out
The trade: Smart materials cost more per unit, respond only within a limited range, and can be slow or fatigue over many cycles. They win where removing a mechanism is worth the price.
Real uses the papers name
Shape memory alloy: stents
A nickel-titanium stent is squeezed small to go into an artery, then body heat makes it open back to its remembered shape.
Piezoelectric: energy floors
Pressure from footsteps or passing trains squeezes piezo crystals and makes a small voltage.
Photochromic: windows and lenses
Light darkens the glass, so windows and glasses cut glare without blinds.
Magneto-rheostatic: dampers
A magnetic field thickens the fluid in a damper or brake in milliseconds, used in vehicle suspension and heavy-vehicle braking.
How this is tested — explaining how materials can be selected to react to external stimuli. It comes up two ways:
Paper 1 — multiple choice
- Match a smart material to the stimulus it responds to.
- Identify the smart material in a described product.
Paper 2 — analysing a product
- Explain how a smart material improves a named product.
- Explain why a smart material replaces a mechanism.
The trap: Naming the material without naming the stimulus. "It uses shape memory alloy" is half; "it uses shape memory alloy, which returns to its remembered shape when heated above 40°C" is the answer.
A greenhouse vent must open automatically when the temperature rises, with no power supply. Explain how a smart material could achieve this.
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