## 1 Introduction
[PERSON] et al. [5] stated that \"There are mainly two types of hand exoskeletons developed over the years, which are classified as rigid and soft exoskeletons or a combination of the two\", but they fundamentally identified two classes: rigid, in which the forces or torques are transmitted to the required joints through a mechanical structure, and soft, with flexible materials or elastic structures to transmit the forces to the joints of the fingers. Instead, [PERSON] and [PERSON] [15] distinguished three classes: rigid (linkage-based), cable-driven, and soft exoskeletons. Exoskeletons in which the movement produced by the actuator is transferred to the joints with hard and inelastic materials are rigid. If the movement produced by the actuators is transferred to the joints with the help of cables and pulleys, the exoskeletons are hybrid; this also occurs for tendon-driven exoskeletons. They defined soft exoskeletons as devices that are made from materials compatible with the human anatomy. [PERSON] and [PERSON] [9] reviewed soft exoskeletons and defined them as robots made from easily deformable materials such as fluids, gels, and soft polymers that have better biomimetic properties due to their greater compliance and versatility while conforming to the contours of the human body. Specific definitions of hybrid exoskeletons can be found in articles presenting specific solutions for hybrid devices. For [PERSON] et al. [16], hybrid soft-rigid exoskeletons use deformable materials in combination with a rigid frame. For [PERSON] et al. [17], an exoskeleton that integrates soft-rigid hybrid actuators consisting of flexible elements and rigid or semi-rigid elements is also considered hybrid. Our definition of the different typologies does not fully coincide with any of those existing in the literature but integrates the definitions proposed by different authors. We agree with the definition of \"rigid\" in [15] as linkage-based exoskeletons, i.e., with rigid transmission mechanisms. For the \"soft\" class, we agree with the definition of [PERSON] and [PERSON] [9], the exoskeletons formed by easily deformable materials, but with the addition of textile materials to the list of specified materials. The definition of \"hybrid\", in our view, merges the definitions proposed by [15; 17]. We classify hybrid cable or tendon-drive exoskeletons or exoskeletons with soft-rigid hybrid actuators. After the investigation of the main technical features unique to each exoskeleton family, namely rigid, soft, and hybrid architectures (Figure 1), a cross-analysis between device families and features is presented to provide a supporting tool for both the design and selection of exoskeletons, according to an impairment-oriented rationale. The main technical features were divided into two main categories: functional features and interactive experience features. The functional features are related to the main motor functions of the hand, and among these, we identified the following: movements performed by the exoskeleton, the ROM allowing for the different movements, and forces. The interactive experience features concern the performance of the interaction of the user with the device, and among these, we include the following: wearability, control mode/possibility, and portability. The rest of the paper is organized as follows: Section 2 describes the PICO model and the proposed impairment-based approach for the design and selection of a robotic exoskeleton for hand rehabilitation. A detailed description of the impairment-based design process is presented in Section 3. Sections 4-6 provide a detailed overview of the state-of-the-art technology for the three classes of robotic hand exoskeletons: rigid, soft, and hybrid. The analysis focuses primarily on the functional features and interactive experience features discussed in Section 3 to determine how the different device classes relate to these features.