139 © Springer Nature Switzerland AG 2021 P. J. Bártolo, B. Bidanda (eds.), Bio-Materials and Prototyping Applications in Medicine, https://doi.org/10.1007/978-3-030-35876-1_8 Chapter 8 Exoskeletons for Lower Limb Applications: A Review Mohammed S. Alqahtani, Glen Cooper, Carl Diver, and Paulo Jorge Bártolo 8.1 Introduction Exoskeletons can be defined as wearable or robotic devices that assist people in performing their daily life movements, thus boosting the user’s performance. They are used for regaining mobility, thereby allowing people to walk, stand and sit [13]. Exoskeletons, consisting of sensors, actuators and control elements, are utilised in various applications requiring carrying heavy loads or for rehabilitation and assis- tance of paralysed patients [32, 34]. Different classifications of exoskeletons have been proposed as shown in Fig. 8.1 [1, 12, 30, 34]. One classification is based on the part of the body the exoskeleton supports. This classifies exoskeletons as an upper limb, lower limb or full body. Exoskeletons can also be classified as active, passive or quasi-passive. Active exo- skeletons require an energy source to actuate sensors and actuators, whereas passive devices do not require any energy source as they are only formed by mechanical elements such as linkage, springs and dampers. The quasi-passive devices lie between these two types [53]. Depending on the application, exoskeletons can be classified as devices for gait rehabilitation, human locomotion assistance and human strength augmentation. Finally, depending on the type of actuators, exoskeletons can be classified as electric, pneumatic or hydraulic actuators. M. S. Alqahtani · G. Cooper · P. J. Bártolo (*) School of Mechanical, Aerospace and Civil Engineering, The University of Manchester, Manchester, UK e-mail: mohammed.alqahtani-4@postgrad.manchester.ac.uk; glen.cooper@manchester.ac.uk; paulojorge.dasilvabartolo@manchester.ac.uk C. Diver Department of Engineering, Manchester Metropolitan University, Manchester, UK e-mail: c.diver@mmu.ac.uk