Artificial muscle

Due to their high flexibility, versatility and power-to-weight ratio compared with traditional rigid actuators, artificial muscles have the potential to be a highly disruptive emerging technology. Though currently in limited use, the technology may have wide future applications in industry, medicine, robotics and many other fields. While there is no general theory that allows for actuators to be compared, there are 'power criteria' for artificial muscle technologies that allow for specification of new actuator technologies in comparison with natural muscular properties. In summary, the criteria include stress, strain, strain rate, cycle life, and elastic modulus. Some authors have considered other criteria (Huber et al., 1997), such as actuator density and strain resolution. As of 2014, the most powerful artificial muscle fibers in existence can offer a hundredfold increase in power over equivalent lengths of natural muscle fibers. Researchers measure the speed, energy density, power, and efficiency of artificial muscles; no one type of artificial muscle is the best in all areas. Artificial muscles can be divided into three major groups based on their actuation mechanism. Electroactive polymers (EAPs) are polymers that can be actuated through the application of electric fields. Currently, the most prominent EAPs include piezoelectric polymers, dielectric actuators (DEAs), electrostrictive graft elastomers, liquid crystal elastomers (LCE) and ferroelectric polymers. While these EAPs can be made to bend, their low capacities for torque motion currently limit their usefulness as artificial muscles. Moreover, without an accepted standard material for creating EAP devices, commercialization has remained impractical. However, significant progress has been made in EAP technology since the 1990s. Ionic EAPs are polymers that can be actuated through the diffusion of ions in an electrolyte solution (in addition to the application of electric fields). Current examples of ionic electroactive polymers include polyelectrode gels, ionomeric polymer metallic composites (IPMC), conductive polymers and electrorheological fluids (ERF). In 2011, it was demonstrated that twisted carbon nanotubes could also be actuated by applying an electric field. Twisted and coiled polymer (TCP) muscles also known as supercoiled polymer (SCP) are coiled polymer that can be actuated by electric power. A TCP muscle looks like a helical spring. TCP muscles are usually made from silver coated Nylon. TCP muscle can also be made from other electrical conductance coat such as gold. TCP muscles should be under a load to keep the muscle extended. The electrical energy transforms to thermal energy due to electrical resistance, which also known as Joule heating, Ohmic heating, and resistive heating. As the temperature of the TCP muscle increases by Joule heating, the polymer contracts and it causes the muscle contraction.. 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 encircle the bladder, translating the volume expansion to a linear contraction along the axis of the actuator. PAMs can be classified by their operation and design; namely, PAMs feature pneumatic or hydraulic operation, overpressure or underpressure operation, braided/netted or embedded membranes and stretching membranes or rearranging membranes. Among the most commonly used PAMs today is a cylindrically braided muscle known as the McKibben Muscle, which was first developed by J. L. McKibben in the 1950s.