## 2 Materials & Methods  
### Float System Co-Design Approach  
The current version of Float derives from a user-centered design process (Figure 1) that extends the steps presented in [15] in order to provide the developers with different implementation options that directly derive from a co-design approach, involving clinicians, technicians and patients along three phases. The goal established by the clinicians was to provide an ecologically valid device to simulate real-life tasks and work-related activities within clinical centers. Indeed, the device was created to be used in the clinic, under the supervision of a therapist and not at home or at work. The initial discussions with the clinicians focused on requirements that could have been satisfied by a list of macroscopically different systems, not necessarily leading to an exoskeleton or an end-effector. A series of interviews during the \"exploratory phase\" enabled an initial understanding of the user needs, considering as users both the clinicians and the patients according to the rehabilitative goals. The main result of this phase is constituted by 3 user requirements that became design principles of Float:  
Figure 1: The user-centered design process adopted to achieve the current version of Float. 1. Wide freedom of movement of the patient during rehabilitation;
2. Possibility of rehabilitation while sitting, standing and bent over;
3. Weight of the exoskeleton not perceived by the patient. The principles 1 and 2 (which depends on 1 too) were posed by the clinicians for easing the future exploitation of Float in occupational therapy with ecologically valid settings and tasks. On the other hand, principle 3 was generally requested by the patients too (always with the goal of making the technology supportive yet, as much as possible, \"transparent\" in the user experience). Accordingly, the designers and the developers generated--\"generative phase\"--a set of mockups of the device. These solutions were evaluated by technicians and clinicians in order to match the three principles listed above alongside general requirements of effectiveness, comfort and safety in order to select the basic configuration of the device. This phase was especially useful for clarifying the opportunity of creating a hybrid configuration of exoskeleton (the wearable component) and end-effector (the polyarticulated arm) that offer major advantages of both robotic systems in rehabilitation procedures. Finally, the first prototype of Float was implemented and the \"formative phase\"--based on the CEI EN 62366:2015 standards for usability engineering (Medical devices--Part 1: Application of usability engineering to medical devices)--started as an iterative and participatory design and evaluation process conducted to progressively refine the device through user tests. Such a process involved both hardware and software components of Float. A series of usability tests involved 2 physiatrists and 5 physiotherapists (at the INAIL Motor Rehabilitation Center--Crm--of Volterra, Italy). Several tests were conducted, including the usability of the graphic user interface (GUI), the control, mechanical features and interfaces with the patients. Usability methods were exploited for understanding how the users interact with the devices: the participants were invited to describe the steps of their reasoning while creating a user ID and a sequence of exercises (thinking aloud technique). Patients' safety during Float utilization was our top priority. Therefore, we applied a risk management process, appropriate technical standards and regulations during the generative (considering the mockups) and formative phases (assessing the prototypes). In particular, we identified, estimated, evaluated and mitigated all risks connected with the device's use according to CEI EN 14971:2019 (Medical devices--Application of risk management to medical devices).