Torsional artificial muscles
Large stroke torsional actuators are the newest class of artificial muscle technology that produces rotary motion or generate torque in response to various stimuli. A number of materials comprising twisted fibres or filaments have been shown to display high degrees of reversible untwist and retwist under various experimental conditions such as heating, electrochemical charging, chemical absorption or photonic excitation. Torsional actuators are of potential application in areas that include microfluidic mixing, microsensors, photonic displays, and energy-harvesting devices. Furthermore, the torsional actuation in fibres can be translated into a linear, or tensile, actuation when the fibres are formed into coils. These coil tensile artificial muscles are of potential use in soft and wearable robotics, medical devices and prosthetics. This review will provide a comprehensive overview of torsional actuators constructed from different functional materials, their actuating mechanism, potential applications, and their current limitations. The review will conclude with recent developments and future trends of torsional actuators as well as critical issues that need to be addressed and resolved.