## 4 Rigid Robotic Exoskeletons for Hand Rehabilitation
### Functional Features  
* _Actuated movements._ Most of the rigid hand exoskeleton devices focus on replicating human finger flexion/extension movements by neglecting the abduction/adduction motions. Typically, each finger is actuated using at least one motor and presents the same mechanical structure, except for the thumb. The thumb presents a more complex and particular kinematic structure compared to the other fingers [55]. The literature review shows that the thumb exoskeletons vary depending on the specific design requirements and there are no rigid hand exoskeletons capable of performing the three movements of the thumb, i.e., flexion/extension, abduction/adduction, and opposition. In some applications, the thumb is designed to perform flexion/extension and abduction/adduction motions by using two actuators [53]. In other cases, the thumb exoskeleton is able to perform flexion/extension, while abduction/adduction motions are performed passively or not included at all [22; 27; 29; 32; 33; 34; 35; 36; 42; 45; 46; 52]. There are also examples where the thumb is entirely passive or not considered in the rigid hand exoskeleton design [37; 41; 47]. In order to let the user perform natural and coordinated hand movements, the concept of synergies introduced by [PERSON] et al. [56] is exploited in some solutions in the design phase of the exoskeleton [32; 48].
* _Range of motion._ The combination of ROM and stretching exercises is essential for patients during the rehabilitation process. ROM maintenance plays an important role in preventing complications such as finger contractures [57; 58]. Therefore, achieving a suitable ROM is a key feature when designing hand exoskeleton devices. Ideally, such rehabilitative devices would cover the complete hand ROM and all of the DOFs. However, in practice, the solutions presented in the state-of-the-art technology are always a trade-off between size, wearability, weight, strength, and the specific type of movement actuated. As a result, rigid hand exoskeletons able to cover all hand ROMs and DOFs are currently unavailable, as can be observed in Table 3. In the literature, various works lack explicit information about ROM data [23; 25; 33; 38]. In other cases, only partial details are provided regarding the ROM of the device [22; 29; 30; 46]. Meanwhile, some studies provide more comprehensive insights and detailed information [24; 27; 28; 31; 32; 42].
* _Force._ Force is another aspect to consider when designing a hand exoskeleton. Human hands can exert various types of force and adjust their grip strength depending on the object to be grasped. In exoskeleton devices, the force is typically measured through force sensors at the fingertips [27]. Solutions exploiting other points on the exoskeleton are presented in the literature, for example, embedded into the linear actuator supports [32]. Usually, the information provided by the sensor is used as feedback for the therapist, enabling a constant analysis of the rehabilitation process. In rigid hand exoskeletons, such as those with a remote center of rotation or those employing consecutive link mechanisms, the force is applied perpendicular to the contact point between the device and the phalanges. This force may not be uniformly distributed, depending on the number of activated DOFs. According to the literature review, the minimum force needed in grasping and manipulation tasks ranges between 10 N and 13 N. Meanwhile, for daily life activities, a force value of 20 N is usually indicated [8; 29]. The device presented in the work by [PERSON] et al. [31] exhibits a force output of 10 N, whereas the devices in other papers [27; 29; 30] can exert maximum forces of 8, 15, and 30 N, respectively. However, in most of the works analyzed in the literature review, there are no data and tests regarding the maximum force exerted by the devices. Usually, the authors prefer to use actuators able to deliver more than 20 N in order to be able to use the device for daily life activities [23; 32; 52].