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 bestsuited actuator for a specific application.
Stroke, cerebral palsy, and spinal cord injuries represent the most common leading causes of upper limb impairment. In recent years, rehabilitation robotics has progressed toward developing wearable technologies to promote the portability of assistive devices and to enable home rehabilitation of the upper extremities. However, current wearable technologies mainly rely on electric motors and rigid links or soft pneumatic actuators and are usually bulky and cumbersome. To overcome the limitations of existing technologies, in this paper, a first prototype of a lightweight, ungrounded, soft exoskeleton for wrist rehabilitation powered by soft and flexible carbon fibers-based twisted and coiled artificial muscles (TCAMs) is proposed. The device, which weighs only 0.135 kg, emulates the arrangement and working mechanism of skeletal muscles in the upper extremities and is able to perform wrist flexion/extension and ulnar/radial deviation. The range of motion and the force provided by the exoskeleton is designed through simple kinematic and dynamic theoretical models, while a thermal model is used to design a thermal insulation system for TCAMs during actuation. The device’s ability to perform passive and active-resisted wrist rehabilitation exercises and EMG-based actuation is also demonstrated.
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