Pneumatic artificial muscles (PAMs) are contractile or extensional devices operated by pressurized air filling a pneumatic bladder. In an approximation of human muscles, pneumatic artificial muscles are usually grouped in pairs: one
agonist
An agonist is a chemical that activates a Receptor (biochemistry), receptor to produce a biological response. Receptors are Cell (biology), cellular proteins whose activation causes the cell to modify what it is currently doing. In contrast, an R ...
and one
antagonist
An antagonist is a character in a story who is presented as the main enemy or rival of the protagonist and is often depicted as a villain.McKibben Artificial Muscles'') in the 1950s for use in artificial limbs. The
Bridgestone
is a Japanese multinational manufacturing company founded in 1931 by Shojiro Ishibashi (18891976) in the city of Kurume, Fukuoka Prefecture, Fukuoka, Japan. The name Bridgestone comes from a calque translation and transposition of (), meaning ...
rubber
Rubber, also called India rubber, latex, Amazonian rubber, ''caucho'', or ''caoutchouc'', as initially produced, consists of polymers of the organic compound isoprene, with minor impurities of other organic compounds.
Types of polyisoprene ...
company (
Japan
Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
) commercialized the idea in the 1980s under the name of Rubbertuators.
The retraction strength of the PAM is limited by the sum total strength of individual fibers in the woven shell. The exertion distance is limited by the tightness of the weave; a very loose weave allows greater bulging, which further twists individual fibers in the weave.
One example of a complex configuration of air muscles is the Shadow Dexterous Hand developed by the Shadow Robot Company, which also sells a range of muscles for integration into other projects/systems.
Advantages
PAMs are very lightweight because their main element is a thin
membrane
A membrane is a selective barrier; it allows some things to pass through but stops others. Such things may be molecules, ions, or other small particles. Membranes can be generally classified into synthetic membranes and biological membranes. Bi ...
. This allows them to be directly connected to the structure they power, which is an advantage when considering the replacement of a defective
muscle
Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...
. If a defective muscle has to be substituted, its location will always be known and its substitution becomes easier. This is an important characteristic, since the membrane is connected to rigid endpoints, which introduces tension concentrations and therefore possible membrane ruptures.
Another advantage of PAMs is their inherent compliant behavior: when a force is exerted on the PAM, it "gives in", without increasing the force in the actuation. This is an important feature when the PAM is used as an actuator in a
robot
A robot is a machine—especially one Computer program, programmable by a computer—capable of carrying out a complex series of actions Automation, automatically. A robot can be guided by an external control device, or the robot control, co ...
that interacts with a human, or when delicate operations have to be carried out.
In PAMs the force is not only dependent on pressure but also on their state of inflation. This is one of the major advantages; the mathematical model that supports the PAMs functionality is a non-linear system, which makes them much easier than conventional pneumatic cylinder actuators to control precisely. The relationship between force and extension in PAMs mirrors what is seen in the length-tension relationship in biological muscle systems.
The compressibility of the gas is also an advantage since it adds compliance. As with other pneumatic systems PAM actuators usually need electric valves and a compressed air generator.
The loose-weave nature of the outer fiber shell also enables PAMs to be flexible and to mimic biological systems. If the surface fibers are very badly damaged and become unevenly distributed leaving a gap, the internal bladder may inflate through the gap and rupture. As with all pneumatic systems it is important that they are not operated when damaged.
Hydraulic operation
Although the technology is primarily pneumatically (gas) operated, there is nothing that prevents the technology from also being hydraulically (liquid) operated. Using an incompressible fluid increases system rigidity and reduces compliant behavior.
In 2017, such a device was presented by
Bridgestone
is a Japanese multinational manufacturing company founded in 1931 by Shojiro Ishibashi (18891976) in the city of Kurume, Fukuoka Prefecture, Fukuoka, Japan. The name Bridgestone comes from a calque translation and transposition of (), meaning ...
and the
Tokyo Institute of Technology
The Tokyo Institute of Technology () was a public university in Meguro, Tokyo, Japan. It merged with Tokyo Medical and Dental University to form the Institute of Science Tokyo on 1 October 2024.
The Tokyo Institute of Technology was a De ...
Exoskeleton
An exoskeleton () . is a skeleton that is on the exterior of an animal in the form of hardened integument, which both supports the body's shape and protects the internal organs, in contrast to an internal endoskeleton (e.g. human skeleton, that ...