How shape memory alloy artificial muscles work

Shape memory alloy is a metal that returns to its original shape when heated above a certain temperature, even after being deformed by force. Nitinol (NiTi), an alloy of nickel and titanium, is the most widely used.

It is already familiar from medical stents and eyeglass frames. Using it as an artificial muscle applies that same restoring force as a source of motion.

The restoring behaviour comes from a phase transformation — the crystal structure changes with temperature. At lower temperatures it takes a structure that deforms easily; at higher temperatures, one that remembers its original shape.

Current is the means of raising the temperature. Passing current through the metal generates heat through resistance, and that heat drives the transformation, contracting the material. Cut the current and heat escapes to the surroundings, returning it to its original length.

The repeated contraction and relaxation resembles how muscle behaves. That is not to say it produces force the same way muscle does, but being able to control contraction with an electrical signal is why it is used as an artificial muscle.

Three challenges are characteristic of shape memory alloy actuators: heat during operation, response speed limited by cooling time, and lifetime under repeated use.

Response speed improves as the wire gets thinner, because a higher surface-area-to-volume ratio lets heat rise and fall faster. Fabric Muscle uses wire on the order of 25–40 μm in diameter to improve this.

Designing cooling structures alongside has also been studied. Heat is not something that can be removed entirely, so products are designed with the garment layer insulating between skin and actuator.

Pneumatic artificial muscles (PAM) inflate with air pressure and contract. They are strong and fast, but the pump, valves and air lines make the overall system large.

Dielectric elastomers (DEA) are polymers that thin and stretch under applied voltage. They respond quickly and weigh little, but need voltages in the kilovolt range. Ionic polymers (IPMC) run on low voltage but produce small forces.

Shape memory alloy is structurally simple and produces large forces at low voltage, but because heat is the medium, cooling time governs response speed. These approaches suit different conditions rather than ranking against each other.

Wearable robots are the most active application: they need light, soft actuation, and noise and bulk are problems in anything worn against the body.

Research also continues in soft robotics, medical devices and haptic devices that convey touch. What they share is an environment where rigid mechanisms are hard to use.