Study into ARTIFICIAL MUSCLES
PNEUMATIC ARTIFICIAL MUSCLES (PAM)
They are known under many names, such as McKibben muscles – after their inventor J. L. McKibben, air muscles, or Pneumatic Artificial Muscles (PAM). In principle, they consist of an inflatable rubber bladder inside a Chinese finger trap-cylindrical, hectically wound braid. When air pressure is applied to the bladder, it inflates inside the constraining braid, which redirects the expansion force into a contractive force through changing it’s braiding angle. However, the more the muscle contracts, the steeper the braiding angle gets, and less force is delivered along the muscle’s length axis. This also stresses the braid material, which must withstand several times the contractive force exercised by the muscle. ELECTROACTIVE POLYMERS (EAP)
Polymer-based artificial muscles save themselves from a lot of the drawbacks of pneumatic artificial muscles. Those materials typically experience a shape change in response to a high voltage electrical field, delivered through electrodes that are attached to the material.
Most EAPs are blends of polymers, copolymers, and oils, assembled in complex 3D geometries. The mixing ratio determines the molecular structure and properties of the material. Ferroelectric or piezoelectric materials are used to obtain their electroactive properties. In terms of strain ratio, strength, and efficiency, EAPs are currently one of the more promising artificial muscle technologies.Yet, after decades of research, the fundamental obstacle for EAPs is still in place: power output. There are piezoelectric polymers, dielectric actuators (DEAs), electrostrictive graft elastomers, liquid crystal elastomers (LCE), ferroelectric polymers, and many more. Their operation principles are throughout based on intramolecular, intermolecular or electrostatic forces. These forces act on very short distances and decrease disproportionally with increasing deformation. Also, EAPs operate in a single cycle, in which the entire mechanical work output of a full contraction must be delivered: Once an EAP is fully contracted or expanded, it can not perform the same molecular interaction again to contract or expand further. THERMALLY ACTUATED ARTIFICIAL MUSCLES
Analogous to EAPs, thermally actuated materials change their shape depending on temperature.
The fishing line experiment is a popular example for such a device. It shows that a plain polyethylene string, basically fishing line, is already some sort of muscle: simply by heating it up, it will contract by a few percent.
By twisting and coiling up the string, it is possible to create a thermally actuated artificial muscle that can contract by about 50%, and produce up to 4 kN / cm2 of pressure. Unfortunately, a fishing line muscle will contract significantly less under load. Still, their light weight and fast response time – given a fast active heating and cooling system – leads to specific power outputs that may even exceed the performance of natural muscles. Shape Memory Alloy (SMA). SMAs are metallic alloys that can change their shape depending on their temperature. This is possible, because Shape Memory Alloys, unlike other alloys, can co-exist in two phases: Martensite and austenite.
In its cool state, an SMA remains in the martensite phase and can be plastically deformed. If the SMA is then heated above its transition temperature, it will change to its austenite phase which “remembers” and restores its original form. Even after it cools down again, it will remain in its original, undeformed shape. This operation mode, where the SMA remembers only its original state, is called one-way shape memory. Two-way shape memory can, among other training methods, be achieved by severely deforming the SMA in its cool state. It will then still return to its original state when heated above the transition temperature, but fall back to the deformed state once it cools down again.
