## 4 Rigid Robotic Exoskeletons for Hand Rehabilitation
### Interactive Experience Features  
* _Wearability_. The wearability of rigid exoskeletons is consistently regarded as a fundamental requirement by researchers in the field: keeping the exoskeleton worn and used for prolonged periods of time reflects a lot on the rehabilitation process and the user's autonomy [25; 59]. Addressing the wearability requirement necessitates careful consideration of various factors. One important aspect is the choice of materials for exoskeleton manufacturing, as these should guarantee a long-lasting performance over time. For instance, in the work by [PERSON] et al. of 2010 [22], the authors used a combination of materials, i.e., Acrylonitrile Butadiene Styrene (ABS) and aluminum. In alternative cases [32; 38], the device is made in ABS, while in others [31; 32], a combination of plastic and flexible materials is used. Furthermore, to guarantee the wearability, the weight of the exoskeleton should be as limited as possible by choosing the appropriate actuators and defining the best position for the actuation unit. In rigid exoskeletons, the actuation unit tends to be positioned on the back of the hand to minimize interference as much as possible during the activities of daily living by connecting the actuators directly to the mechanical structures. The mechanical structure responsible for finger movement is designed to have a limited height and length and should not exceed the size of the user's fingers. The works analyzed in the literature review show that certain types of finger movement, such as finger abduction/adduction, are often restricted with respect to others, i.e., finger flexion/extension. This choice allows the number of required actuators to be limited. Furthermore, often, the thumb is partially implemented or excluded completely from the design due to its complex kinematic structure. There are also solutions aimed at limiting the number of actuators per finger, e.g., the one presented in the work by [PERSON] et al. [40] where a differential mechanism is employed to couple the motion of two adjacent fingers. \begin{table}
\begin{tabular}{l l l l l l l l l} \hline \hline
**Ref.** & **Actuator Type** & \begin{tabular}{c} **FF** \\ **[deg]** \\ \end{tabular} & \begin{tabular}{c} **TF** \\ **[deg]** \\ \end{tabular} & \begin{tabular}{c} **IDP\({}_{f}\)** \\ **[deg]** \\ \end{tabular} & \begin{tabular}{c} **PIP\({}_{f}\)** \\ **[deg]** \\ \end{tabular} & \begin{tabular}{c} **MCP\({}_{f}\)** \\ **[deg]** \\ \end{tabular} & \begin{tabular}{c} **CMC\({}_{t}\)** \\ **[deg]** \\ \end{tabular} & \
\begin{tabular}{c} **MCP\({}_{t}\)** \\ **[deg]** \\ \end{tabular} &
\begin{tabular}{c} **IP\({}_{t}\)** \\ **[deg]** \\ \end{tabular} \\ \hline \hline
[53] & Brushless DC & & 85 & 81 & 90 & 30 & 20 & 50 \\ \hline
[22] & Linear actuator & & & 65 & 55 & & & \\ \hline
[24] & DC & & all range & all range & all range & & \\ \hline
[25] & Linear actuator & & & & & & & \\ \hline
[23] & Linear actuator & & & & & & & \\ \hline
[35] & DC & & 85 & 89 & 93 & & \\ \hline
[36] & DC & & & all range & all range & & \\ \hline
[27] & Linear actuator & 57 & 86 & 73 & & & \\ \hline
[28] & DC & & & 64.5 & 67.8 & & \\ \hline
[30] & Linear actuator & & & 100 & 90 & & \\ \hline
[31] & Linear actuator & & & 76 & 63 & & \\ \hline
[32] & Linear actuator & & & 87 & 65 & & \\ \hline
[37] & Servomotor & 10 & 80 & 52 & & \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Actuator type and ROM achieved by rigid exoskeletons presented in the literature. FF: finger flexion; TF: thumb flexion; DIP\({}_{f}\): DIP fingers; PIP\({}_{f}\): PIP fingers; MCP\({}_{f}\): MCP fingers; CMC\({}_{t}\): CMC thumb; MCP\({}_{t}\): MCP thumb; IP\({}_{f}\): IP thumb. In human/exoskeleton interaction contexts, a device is defined as transparent when it does not modify the nominal behavior of the user in terms of the end-effector, joint trajectories, and patterns of muscle activation [60]. Transparency can be obtained in rigid exoskeletons by employing suitable control strategies. For instance, [PERSON] et al.