Artificial muscle is a generic term used for materials or devices that can reversibly contract, expand, or rotate within one component due to an external stimulus (such as voltage, current, pressure or temperature). The three basic actuation responses – contraction, expansion, and rotation – can be combined together within a single component to produce other types of motions (e.g. bending, by contracting one side of the material while expanding the other side). Conventional motors and pneumatic linear or rotary actuators do not qualify as artificial muscles, because there is more than one component involved in the actuation.
Artificial muscles can be divided into four major groups based on their actuation mechanism:
Electric field actuation
Pneumatic artificial muscles (PAMs) operate by filling a pneumatic bladder with pressurized air. Upon applying gas pressure to the bladder, isotropic volume expansion occurs, but is confined by braided wires that encicle the bladder, translating the volume expansion to a linear contraction along the axis of the actuator.
In this category, in addition to the application of electric fields, ions are required for the actuation; therefore, the actuation must occur in a wet environment. Ionic electroactive polymers and ionic polymer metal composites (IPMCs) belong to this group. In 2011, it was demonstrated that twisted carbon nanotubes can be actuated by applying an electric field.
Shape-memory alloys, metallic alloys that can be deformed and then returned to their original shape when exposed to heat, can function as artificial muscles. In 2012, a new class of electric field-activated, electrolyte-free artificial muscles called "twisted yarn actuators" were demonstrated, based on the thermal expansion of a secondary material within the muscle's conductive twisted structure. Materials with negative thermal expansion coefficients can also be engineered to actuate upon Joule heating.
Artificial muscle technologies have wide potential applications in biomimetic machines, including robots and industrial actuators. EAP-based artificial muscles offer a combination of light weight, low power requirements, resilience and agility for locomotion and manipulation. Pneumatic artificial muscles also offer greater flexibility, controllability and lightness compared to conventional pneumatic cylinders.
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