A3.1.8 Smart Materials

The material: Lead zirconate titanate (PZT), quartz, polyvinylidene fluoride (PVDF).

The stimulus: Mechanical Stress or Electric Field

What it does: Apply a mechanical force — the material generates a voltage. Apply a voltage — the material physically deforms. The response is bidirectional and immediate (Callister & Rethwisch, 2018).

Why it is selected:

The piezoelectric effect eliminates the need for separate sensing and actuation components. In an inkjet printer head, a PZT element deforms by nanometres when a voltage pulse is applied — precisely enough to eject a single ink droplet. In structural health monitoring of aircraft wings, PVDF film bonded to the surface generates voltage spikes when stress waves from micro-cracks pass through — acting simultaneously as sensor and signal generator with no moving parts and no power input required for sensing (Janocha, 2007).

Design selection logic:

• Context requires: Precise mechanical actuation at small scale, or vibration sensing without external power

• Stimulus present: Mechanical stress or applied voltage

• Material response: Voltage output or dimensional change

• Outcome: Sensor and actuator functions embedded in the material itself

The material: Nitinol (NiTi), copper-zinc-aluminium alloys, shape memory polymers.

The stimulus: Temperature

What it does: The material is deformed — bent, compressed, or stretched — at low temperature. On heating above its transformation temperature, it recovers its original programmed shape with significant recovery force (Otsuka & Wayman, 1998).

Why it is selected:

The body itself provides the stimulus. A coronary stent manufactured from Nitinol is crimped to a small diameter at room temperature and threaded into a blocked artery via catheter. Once in position, the patient's body heat — 37°C — exceeds the alloy's transformation temperature. The stent expands to its programmed diameter, pressing against the arterial wall and holding it open. No inflation mechanism, no external tool, no surgeon manually expanding it. The material does the work (Otsuka & Wayman, 1998).

In aerospace, Nitinol actuators replace hydraulic systems in morphing wing structures — changing aerofoil geometry in response to aerodynamic heating at different flight speeds.

Design selection logic:

• Context requires: Actuation triggered by a temperature threshold with no external mechanism

• Stimulus present: Temperature change crossing transformation temperature

• Material response: Controlled shape recovery

• Outcome: Self-actuating component with no moving mechanical parts

The material: Silver halide or organic dye compounds embedded in glass or polymer lens substrate.

The stimulus: Ultraviolet or Visible Light

What it does: UV radiation triggers a reversible photochemical reaction — the material darkens. Remove the UV stimulus (move indoors or into shade) and the reaction reverses — the material clears (Schwartz, 2002).

Why it is selected:

Transition® photochromic lenses contain organic photochromic molecules that undergo a structural rearrangement when struck by UV photons — the new molecular configuration absorbs visible light, producing a tinted appearance. The lens darkens proportionally to UV intensity and clears in its absence. The designer selects this material specifically because the stimulus — UV light — is precisely the condition under which glare reduction is required. No electronics, no manual switching, no user intervention needed. The material self-regulates in direct response to the relevant environmental condition (Schwartz, 2002).

Design selection logic:

• Context requires: Variable light transmission proportional to UV exposure

• Stimulus present: UV radiation intensity

• Material response: Reversible darkening of optical transmission

• Outcome: Autonomous light-adaptive optic with no mechanical or electronic components

The material: Carrier fluid (typically silicone oil) containing suspended carbonyl iron particles, 3–5 microns in diameter.

The stimulus: Magnetic Field

What it does: In the absence of a magnetic field, the iron particles are randomly dispersed — the fluid flows freely with low viscosity. Apply a magnetic field and the particles instantly align into chains parallel to the field — the fluid transforms into a near-solid state, resisting flow with forces up to 100 kPa (Janocha, 2007).

Why it is selected:

The Delphi MagneRide® suspension system fitted to performance vehicles uses MR fluid in the shock absorbers. Wheel-mounted sensors detect road surface conditions 1,000 times per second. The control system adjusts the magnetic field across the MR damper — stiffening it for cornering loads or softening it for rough road absorption — within 1 millisecond. No mechanical valve, no hydraulic switching. The material changes state at electronic speed, providing ride and handling performance mechanically impossible to achieve with conventional damper designs (Janocha, 2007).

Design selection logic:

• Context requires: Continuously variable, electronically controllable mechanical stiffness

• Stimulus present: Applied magnetic field of variable strength

• Material response: Viscosity change from near-liquid to near-solid

• Outcome: Variable-stiffness structural element with millisecond response time

The material: Non-conducting carrier fluid containing suspended dielectric particles — typically cornstarch, silica, or polymer particles in silicone oil.

The stimulus: Electric Field

What it does: In the absence of a voltage, the fluid flows freely. Apply a high electric field (1–5 kV/mm) and the dielectric particles polarise and align into chains — the fluid stiffens dramatically and resists flow (Janocha, 2007).

Why it is selected:

ER fluids function similarly to MR fluids but are actuated by electric fields rather than magnetic fields — making them more suitable for applications where generating a magnetic field is impractical or adds excessive mass. ER fluid clutches and torque-transfer devices can be engaged and disengaged purely by switching a voltage — with no friction plates, no mechanical actuation linkage, and no wear-generating contact between surfaces. In haptic feedback gloves for surgical simulation, ER fluid pockets stiffen when the virtual instrument contacts a virtual surface — feeding physical resistance back to the surgeon's hand in real time (Schwartz, 2002).

Design selection logic:

• Context requires: Electrically switchable mechanical resistance with no moving mechanical parts

• Stimulus present: Applied electric field

• Material response: Reversible stiffening of fluid

• Outcome: Electronically controlled force-transmission or damping component

The material: Bismuth telluride (Bi₂Te₃), lead telluride (PbTe), silicon germanium alloys.

The stimulus: Temperature Differential

What it does — Seebeck Effect: A temperature difference across a junction of two dissimilar conductors generates a voltage. The greater the temperature differential, the greater the voltage produced.

What it does — Peltier Effect: Passing an electric current through a thermoelectric junction actively pumps heat from one side to the other — one face becomes cold, the other becomes hot (Callister & Rethwisch, 2018).

Why it is selected:

In deep-space spacecraft — including the Voyager probes and the Mars Curiosity rover — solar panels are useless. NASA selects radioisotope thermoelectric generators (RTGs): plutonium-238 decays, generating continuous heat; thermoelectric modules convert that temperature differential directly into electrical power with no moving parts whatsoever. The Curiosity rover's RTG has operated continuously since 2011 with zero mechanical failures (NASA, 2012).

In consumer electronics, Peltier coolers are selected where silent, vibration-free cooling is a priority — wine refrigerators, CPU coolers, and portable medical sample storage units — because the Peltier effect requires only a current, produces no moving parts, no refrigerant gas, and no compressor noise (Ashby & Jones, 2012).

Design selection logic:

• Context requires: Power generation from waste heat, or precise silent cooling without moving parts

• Stimulus present: Temperature differential (Seebeck) or electrical current (Peltier)

• Material response: Voltage generation or heat pumping

• Outcome: Solid-state energy conversion with no moving mechanical components