## 7 Discussion  
The main goal of the current work was to provide a roadmap for both the design and selection of exoskeletons for hand rehabilitation that can capture the peculiarities of soft, rigid, and hybrid devices with an impairment-oriented rationale for the final purpose of matching technical specifications and clinical needs. Accordingly, the first part of the paper focused on the description of the strategies for the identification of the subject's impairments (which define the clinical specifications) and the detection of the main characteristics of state-of-the-art hand exoskeletons (outlining the set of feasible technical features). The PICO model allows for the careful recollection of the set of impairments, both functional and structural, that most likely may affect the end-user of a hand exoskeleton. Those impairments, reported in Table 1, can arise as consequences of very different pathologies, and often, the disease itself is used by the clinician as the first filter for the device selection. However, this approach is less efficient than tracing back the clinical picture to the impairments, since filtering by pathologies can cause the clinician to miss all of the devices compatible with the patient's needs that were developed for a different purpose in the designer's mind. In this context, the knowledgeability of the physician becomes fundamental in recognizing the impairments; actually, a first mapping between pathologies and impairments is outlined in Table 2, but a thorough analysis is still missing. Some investigations in this sense are currently under evaluation. On the other hand, data are available in the literature regarding the technical features characterizing hand exoskeletons. The issues in this context refer to the wide variety of possible features and the peculiarities derived from the different architectures, i.e., rigid, soft, and hybrid devices. The proposed design approach for robotic hand rehabilitative exoskeletons highlights the complexity of the design process, which evolves from an ideally linear procedure towards a set of interconnected iterative optimization phases. At each step of the process, new or updated technical requirements can emerge. The approach proposes a new classification strategy for the characteristics of the exoskeletons, i.e., functional and interactive experience features. The analysis of the devices, performed by the architectural class, allows those concepts to be elaborated in more detail: in fact, a set of features and feature details synthesized in Table 7 and Figure 6 has been identified. In particular, Table 7 enables a direct comparison of the technical requirements among rigid, soft, and hybrid hand exoskeletons for each feature. This kind of schematic is expected to be especially suitable for the designer of devices, who can make use of this matrix-based synthesis to promptly extract detailed technical information. Figure 6 instead presents a qualitative evaluation of the performance assured by the three device families for each feature with a graphical approach. This synthesis is designed to be an immediate supportive tool for device selection, simplifying the interpretation of the technical characteristics, and helping the operator to identify the appropriateness of an architectural solution for specific purposes. For instance, the figure clarifies the tendency for the better performance of the rigid exoskeleton class in assuring the flexion/extension of fingers compared with soft-based devices, whereas better performances in terms of compactness tend to be provided by soft exoskeletons with respect to rigid-based systems. Exoskeletons of the hybrid class generally rank in between these extreme conditions. However, the schematic synthesized in Figure 6 has some limitations, primarily due to the quality of the available data: indeed, the literature presents little data, for some features especially, and there is a high variability for the results within the same class.