What Are Artificial Muscles?

Artificial muscles are actuators, sometimes called muscle-like actuators, that convert an applied stimulus directly into contraction, extension, or bending in a soft or compliant material, imitating the way skeletal muscle produces motion. They differ from conventional electric motors and hydraulic cylinders in generating linear or bending motion without gears, bearings, or rigid linkages, and in being distributed through the structure they move rather than concentrated at a joint. The stimulus may be electrical, thermal, chemical, pneumatic, or optical, and the working material is typically a polymer, elastomer, gel, alloy, or fiber assembly.

Interest in these devices comes from two directions. Biomedical engineering wants actuators that can be implanted or worn against tissue without the stiffness mismatch that rigid mechanisms impose, and robotics wants compliant motion that is inherently safe around people and adaptable to unstructured surroundings.

Actuation Mechanisms

Several distinct physical mechanisms are in use. Dielectric elastomer actuators sandwich a soft elastomer film between compliant electrodes so that electrostatic pressure at high voltage compresses the film and expands it in area. Ionic polymer-metal composites and conducting polymer actuators work at low voltage by driving ion migration that swells one side of the material, which makes them attractive for aqueous and implantable use. Shape memory alloys such as nickel-titanium contract on heating through a martensitic phase transformation and deliver high force in small cross sections. Twisted and coiled polymer actuators are made by inserting twist into ordinary polymer fiber until it coils, after which thermal expansion of the fiber untwists the coil and produces contraction; the mechanics and physics of these actuators explain how the anisotropic thermal expansion of the drawn fiber is geometrically converted into large axial strain. Pneumatic artificial muscles, including the braided McKibben design, contract when an inner bladder is inflated inside an inextensible braided sleeve.

Performance and Characterization

Comparison across these mechanisms requires a common set of figures of merit: actuation strain, blocked stress, work and power density per unit mass, energy conversion efficiency, bandwidth, hysteresis, and cycle life. Biological skeletal muscle sets the reference point, producing roughly 20 percent strain at about 0.3 megapascals of stress with efficiencies near 20 percent, and several artificial mechanisms exceed it on individual metrics while falling short on others. Reported device performance is highly configuration dependent, and a twisted and coiled polymer actuation module built for soft robotic use delivered about 0.7 newtons of contraction force with roughly 6.8 millimeters of displacement when driven at 24 volts. Because loading conditions change these numbers substantially, characterization studies examine behavior under compression and off-axis load as well as in simple tension, as in work on reversible actuation of fibrous artificial muscle under external compression.

Fabrication and Control

Manufacture ranges from fiber twisting and coiling on modified spinning equipment to solution casting, electrospinning, and additive processes that print elastomer and electrode layers together. Control is complicated by the same properties that make these actuators useful: the materials are nonlinear, hysteretic, and often slow to reset, and thermally driven types depend on heat removal for their bandwidth. Practical systems therefore pair the actuator with embedded strain, resistance, or optical sensing so that position can be estimated without a rigid encoder, and with model-based or learned controllers that compensate for creep and temperature drift.

Applications

Artificial muscles have applications in a range of fields, including:

• Soft robotics, grippers, and compliant manipulators
• Prosthetic limbs, orthoses, and wearable exoskeletons
• Implantable and assistive medical devices
• Active textiles and haptic interfaces
• Micro air vehicles and biomimetic swimming and flying robots
• Adaptive optics, valves, and microfluidic pumping