A critical review of transitioning from conventional actuators to artificial muscles in upper-limb rehabilitation devices

Brain injuries resulting from spinal cord injuries, strokes, or cerebral palsy are among the traumas most capable of compromising the motor activities of human limbs, hence the necessity for the development of exoskeletons dedicated to the rehabilitation of these organs. This review examines the landscape of actuators essential for the design of cutting-edge upper-limb rehabilitation exoskeletal structures. Beyond merely surveying the current types of actuators available, the paper aims to provide guidelines for selecting actuators that fit optimally with the objectives of upper-limb rehabilitation. The description starts with a brief discussion on the biomechanics of the upper limbs, focusing on the kinematics of pivotal joints (wrist, elbow, shoulder). Subsequently, the existing actuators are systematically reviewed, offering detailed insights into their primary features, operational principles, strengths, weaknesses, and noteworthy applications within the realm of rehabilitation robotics. After the discussion about the actuators, the paper advances by furnishing valuable guidelines for actuators' selection tailored for upper limb rehabilitation. These guidelines discuss crucial factors, such as the forces required and the natural Range Of Motions (ROMs) of upper limb joints. Finally, the manuscript serves as a valuable resource for researchers, engineers, and practitioners involved in the development of innovative upper-limb rehabilitation devices.