**1. Introduction**

Over the past few decades, wearable robotics have been adopted in more and more sectors and, lately, the so called "assistive technology", that is the set of all the products that helps people to live as healthy, productive, independent, and dignified as possible, whatever their condition, has started to be more and more widely used also by the health care system [1–4]. There are more than 1 billion people all over the world who need at least one assistive device, however, and high costs and inadequate funding mechanisms allow only the 10% of the ones in need to have access to these products [5]. Keeping in mind the current state of the art [6–9], the authors have tackled this issue moving a step forward the democratization of the assistive technology by developing a low-cost hand exoskeleton to help and assist people with hand(s) impairments since, as easily verifiable, a key role in carrying out the Activities of Daily Living (ADLs) is played by the hands.

Robotic devices are thought to physically interact with human users suffering from disabilities for long periods of time [10] and, hence, they have to be designed meeting strict requirements in terms of safety, comfort and wearability. This is why one of the most difficult aspects of the human-robot interaction field is nowadays represented by the integration of robotics with assistive products. As if that were not enough, the complex anatomy and the wide variety of possible movements make the hand a grea<sup>t</sup> challenge both for the mechanical design and the control strategy [11].

An accurate state of the art assessment has been conducted, in the very first phase of the research activity, to define the underpinnings which the design process in based on, and, throughout the activity described in the paper, the critical evaluation of the wearable technologies in literature has been kept on to understand the research trends in designing exoskeletons responding to the patients' needs which, consequently, has paved the way to the development of an actually usable device.

An important aspect that must be considered is the clustering of the aimed technology. The design phase can be thus conducted heading to the fulfilling of each request group of the whole project (i.e., the exoskeleton design).

In accordance with the state of the art [6–9], hand exoskletons are classified using various criteria: linking system, Degrees of Freedom (DOFs) and actuation type.

As regards the linking system between the hand and the exoskeleton, there are two main different types: multi-phalanx devices [12,13], which directly control each phalanx separately, and single-phalanx exoskeletons [14], which actuate only that part of the hand they are connected to. The multi-phalanx approach exploits mechanisms made up of several parts and, thus, presents more complex control strategies [15–17]. Usually, these devices are not totally portable and they are supposed to be used for rehabilitative purposes [15,18] or in haptics [19], where the portability requirement is not a strict constraint. Nevertheless, this kind of devices allows to actuate the patients' hands exactly as well as they would do if they could by themselves. Single-phalanx devices use, instead, simpler actuation systems and control algorithms despite of less control capabilities than the multi-phalanx ones.

Another possible classification is based on the number of DOFs of the mechanisms. Rigid multi-DOFs kinematic chains are widely reported [20–22], while the number of rigid single-DOF mechanisms is not so large [23,24]. Since exoskeletons using a rigid multi-DOFs kinematic architecture demand multi-phalanx approaches, they usually present the same pros and cons. Current single-DOF devices present a very simplified kinematics [7,25], which is quite far from the physiological hand kinematics.

In recent years, soft-robotic applications have, then, increasingly been developed. They present a totally different type of mechanism based on elastomeric materials or fluid structures [26–30]. These devices result very lightweight and safe for the user because of their limited stiffness.

Concerning the type of actuator, hand exoskeletons may be driven by electric actuators [31,32] or pneumatic actuators [33]. The former actuation provides smaller forces to the hand than the latter, which, in turn, leads to higher weight and size due to its actuation system.

The proposed assistive and rehabilitative device for the hand focusing on the long fingers [34] has been designed considering the aforementioned research scenario. In particular, throughout the paper, three prototypes will be described to define the step by step design process that has lead to a novel single phalanx, rigid, single-DOF and cable-driven mechanism especially developed within this research activity.

In this manifold context, the use of optimization-based methods for the mechanical design, the exploitation of additive manufacturing technologies for the fabrication and considered choices of materials and electronics have proved to be effective tools for the development of well-performing prototypes of hand exoskeletons even in a low-cost perspective.

In this paper, the development process of a low-cost and fully wearable hand exoskeleton is discussed. The remainder of this section will present the overall framework this research activity has been carried out in. Then, starting from the same structure and kinematic architecture reported in Section 2), three different versions of the prototype have been sequentially developed to ge<sup>t</sup> closer to

the user's needs. Sections 3–5 will describe the main accomplishments of each version in mechanical design, actuation system and control strategy.
