## 1 Introduction  
Robot-based hand therapy, training, or assistance is increasingly being used to treat or support various impairments related to pathologies, accidents, or advancing age. Meanwhile, through pilot studies, numerous authors have shown the effectiveness of this approach for improving neuromotor recovery [1, 2, 3, 4]. From a technical design perspective, the literature presents several works on robotic exoskeletons for the human hand for medical purposes, namely rehabilitation and assistive applications, and not for performance augmentation or haptic and sensing [5]. Literature reviews especially tend to focus on specific kinds of hand exoskeleton or applications, e.g., active robotic exoskeletons for hand rehabilitation, since the functional domain heavily influences the design constraints of these devices [6]. In previous years, particular attention has been paid to soft exoskeletons [7, 8, 9, 10]. Most of these review papers investigate, through the literature, the state-of-the-art hand exoskeletons and classify these devices according to a set of parameters, mainly related to technical characteristics, such as performance specifications or design architecture. For instance, typical evaluated aspects are the presence and number of actuators or actuated degrees of freedom (DOFs), the number of actuated fingers, the kinds of allowed movements, the actuation technology, or the presence of sensors. In a review by [PERSON] et al. [5] of 2021, these characteristics are further classified into general requirements, common to all devices of the analyzed category, and specific requirements, i.e., those characteristics related to a specific application. This work depicts, in particular, an interesting overview of active hand exoskeletons developed for the specific purpose of rehabilitation and assistance in the last decade. The analysis is performed by describing the requirements captured by the investigation of hand exoskeletons currently mentioned in the literature. The limited time frame of the analysis allows the devices and their technologies to be compared, providing a general overview of these specifications with an observational approach. The same distinction between general and application-specific requirements is presented in a paper published in 2019 by [PERSON] et al. [11]. The focus of this work is slightly different, since the authors aim to identify guidelines and best practices for the design of generic exoskeletons, i.e., purpose-independent devices. Coherent with this final goal, the main design requirements are extracted by the analysis of exoskeletons in the literature, and then a selection of design solutions to fulfill those requirements is proposed. The same deductive approach was adopted by [PERSON] et al. [6] in a paper published in 2016. In their work, the authors present an original classification strategy based on three alternative parameters, i.e., the number of actuated DOFs, the number of mechanical connections between the subject and the device, and the mechanism architecture (namely external, internal, or lateral). These parameters are recognized by the authors as being particularly relevant for the mechanism synthesis, and therefore, they are drivers for the definition of the most proper technical solutions. As for the work by [PERSON] et al., the paper by [PERSON] et al. aims to provide the designer with indications for the design phase but focuses, in this case, on the synthesis of the mechanism itself. In 2020, [PERSON] et al. [12] instead published a paper focused on the definition of the design requirements of hand exoskeletons for assistive purposes based on end-user needs and expectations. The study investigated both qualitative and quantitative data through surveys to clinicians and patients on one side and through experimental measurements on the other. Among the paper's findings, the authors conclude that some preliminary criteria do exist, but the differences and peculiarities of patients demand tailored requirements, translating into custom solutions.