# Systematic Review of Exoskeletons towards a General Categorization Model Proposal
## 3 Proposed Framework
### General Categorization  
The general categorization suggests several feasible exoskeleton categories. Such categories have general classes, due to the wide quantity of exoskeletons in existence, and the structure, the body part focused on, the action, the power technology, the purpose, and the application area varying from one to another. Thus, we propose general classes in each classification category, allowing researchers to consider all the exoskeletons in existence, regardless of how specific an exoskeleton could be to a pathology, an application area, or a body part. For instance, the exoskeleton's purpose could be recovery, performance, or even both. The purpose of each one is not necessarily tied to the specific pathology. For example, a patient who has suffered a stroke could need a recovery exoskeleton; and after the rehabilitation, the same patient could use a performance exoskeleton for the same pathology [23]. Moreover, explaining each exoskeleton using this classification is an optimal way to understand the capabilities of the same exoskeleton, without further explanations of the device's technicalities. Subsequently, the diagram of Figure 5 will be further explained in detail, for the readers to understand what may or may not be included in each class of each category in order from left to right and top to bottom of the diagram. Firstly, exoskeletons are not only designed for specific body parts; the exoskeletons may be designed more generally for only one hand, a leg, or even the complete body. Thus, the separation of the classes demonstrates the most common body parts exoskeletons can be built for. The full-body class refers to the exoskeletons made to assist all the limbs, or most of the body. The upper body refers to the exoskeletons made for the upper limbs, and involving the chest, head, back, and/or shoulders. The lower body category refers to the exoskeletons made for the lower limbs: thighs, lower legs, and/or hips. Moreover, there are classes for specific limbs and specific joints. These classes include exoskeletons designed for the knee, ankle, hand, arm, foot, etc. Additionally, there is a special class for any other exoskeleton that is not included in the previous classes. The second easiest way to differentiate an exoskeleton is by its structure, resulting in the establishment of only two classes, rigid and soft. Rigid exoskeletons are those whose structural components attached to the user's body are made with hard materials. Such materials include metals, plastics, fibers, etc. On the other hand, soft exoskeletons, also called exo-suits, are made with materials that allow free movement of the structural components. Exo-suits are often made with, yet not restricted to, textiles. Furthermore, the action category describes the type of help the exoskeleton gives the user, dividing exoskeletons into active and passive action. The active class comprises exoskeletons that give \"active\" aid to the user; in other words, these exoskeletons perform the movements without the need for the user to apply energy. The energy needed to perform the movement is supplied by an external source. On the other hand, the passive class comprises exoskeletons that need the user to perform the movement to work; these exoskeletons do not have power sources. Thus, the user has to perform the movement, and while doing it, the exoskeleton facilitates the movement. The next category goes hand in hand with the previous one. The powered technologies are separated into four main classes, with one specific class for hybrid and one for any other non-common power technology. The four main classes comprise the electric, hydraulic, and pneumatic actuators as the active action, and the mechanical systems as the passive action. The electric actuators comprise any kind of electric motor that activates the exoskeleton's movements. Similarly, the hydraulic and pneumatic actuators comprise pistons and soft actuators.