Mechanical actuators are defined as mechanical devices that convert an input energy into motion. Since the 1990s, advancements in the fields of robotics and automation have produced a critical need for the development of lightweight and efficient actuators capable of human-like motion. In the past few decades, extensive research activities in the fields of materials science and smart materials have led to the development of a novel type of actuator known as artificial muscles. This review paper describes the evolution of mechanical actuators from conventional technologies such as electric, hydraulic, and pneumatic actuators, to bioinspired artificial muscles. The working mechanism, manufacturing process, performance, and applications of different artificial muscles are described and compared with those of conventional actuators. Details on the cost, input sources, activation modes, advantages, and drawbacks of each artificial muscle technology are also provided to guide the reader through the intricate selection process of the best-suited actuator for a specific application.

There is a strong need for compact artificial muscles capable of applying large contractile strokes and lift heavy weights. Coiled fibers recently emerged as attractive candidates for these purposes, owing to their simple construction and the possibility of their thermal, electrical and chemical actuation. An intuitive theoretical understanding of the mechanics of actuation of these muscles is essential for the enhancement of their performance and can pave the way for the development of new applications and technologies. In this paper, a complete theoretical model for the tensile actuation of fiber reinforced artificial muscles is presented and experimentally validated. The model demonstrates that all muscles made from the same material have a universal behavior, which can be described by a single master curve. It enables the systematic design and understanding of coiled muscles for specific performance owing to a comprehensive mathematical correlation among the geometry, materials properties, and actuation. Carbon fibers (CF)/polydimethylsiloxane coiled muscles are demonstrated as simple to fabricate yet powerful muscles owing to the availability of high strength CF. In addition to showing excellent agreement with the theoretical models, they can be actuated by joule heating or chemical swelling, lift up to 12 600 times their own weight, support up to 60 MPa of mechanical stress, provide tensile strokes higher than 25%, and a specific work up to 758 J kg ⁻¹ , the latter is more than 18 times higher than that of natural muscles.

Twisted and coiled artificial muscles (TCAMs) represent an emerging class of actuators. These lightweight muscles provide high power/weight ratio, close to that of skeletal muscles, and can substitute heavy electromagnetic and pneumatic motors in applications requiring low weight, fine motion, and high contractile work. In a recent work we proposed a theoretical model to describe the static behavior of electro-thermally actuated TCAMs. In this study we use the previous static formulation to develop a physics-based dynamic model able to estimate the time varying tensile actuation of TCAMs, according to a specific thermal and electro-thermal input. The proposed model is experimentally validated on a new type of low-cost TCAMs that we recently developed from Carbon Fibers and Silicone Rubber (CF/SR). Satisfactory agreement is shown between the theoretical predictions and the experimental results (maximum errors of 3.4% and 13% are measured on the TCAMs displacement during heating and cooling, respectively). The proposed dynamic model allows to describe the time-varying actuation of TCAMs, fundamental to analyze their response to time-varying phenomena such as high frequency inputs, disturbances, or noises injected into the system. This model represents a useful tool for the design and control of TCAMs-based devices, for applications ranging from the robotic to the biomedical field.