## 5 Soft Robotic Exoskeletons for Hand Rehabilitation
### Soft Robotic Exoskeletons Actuation  
Pneumatics is the predominant actuation technology used in robotic gloves [70], which are therefore mainly based on pneumatic actuators (PAs). The choice of actuator type has a decisive impact on the final structure of the device. Different categories of soft actuators have been developed and presented in the literature for the actuation of soft gloves. Pneu-Nets Soft Actuators (PNSAs) made of elastomers, whose first concept goes back to Polygerinos [71], have been studied by several authors and used in a considerable number of soft glove solutions [66; 72; 73; 74; 75; 76]. Pneu-Nets soft actuators move the finger joints directly by positioning them over the fingers. Different constructive versions have been developed to improve the movement and thrust and to achieve a movement of the fingers that is as close as possible to the physiological one. In [72], [PERSON] et al. present an optimized design of Pneu-Nets soft actuators based on the finite element analysis. Using material tensile tests, they first determined the hyperelastic constitutive model, which was then used in the FEM analysis. From the optimization, they concluded that a geometry with a semicircular chamber has a better performance than one with a conventional rectangular cross-section. The authors also presented an innovative design for a soft rehabilitation glove consisting of adaptable parts and newly developed soft actuators. The new actuator can move each finger joint, because it is small enough to be attached to the knuckle of the glove. PNSA structures with variable levels of stiffness were designed to achieve bending at the finger joints and straight behavior at the segments, according to the skeletal characteristics of the human hand [73]. Two different technological processes are usually used in the production of Pneu-Nets: casting in a mold or direct 3D printing. Curing of the silicone in a plastic mold made by additive manufacturing is the most common  
Figure 4: Examples of soft hand exoskeletons. (**a**) Soft exoskeleton with Pneu-Nets soft actuators. Source: [PERSON] et al., 2023 [66]. (**b**) Example of a fabric-based soft exoskeleton. Source: [PERSON] et al., 2018 [67]. (**c**) Wearable exoskeleton with Machine-Knitted Seamless Pneumatic Actuators. Source: [PERSON] et al., 2021 [68]. (**d**) Soft robotic glove with actuators consisting of molded elastomeric chambers with fiber reinforcements. Source: [PERSON] et al., 2015, open manuscript [69]. process, but recently, with the continuous innovations in 3D printing, direct printing of the silicone-based actuator has also been applied. This has greater simplicity and speed of production (since the actuator is made in a single body), but it has the disadvantages of a limited choice of materials and expensive equipment. The bending motion of this family of soft actuators is due to the presence of a strain-limiting layer. The classical solution for casting in a foundry process involves fabricating the strain-limiting layer separately from the actuator body and then bonding it. Some authors propose solutions for monolithic cast-in-a-mold fabrication processes, in which a core of wax is inserted and then removed by heating [66] or a soluble TPU. PNSAs bend well and easily, but can only exert relatively low forces, around the 1-2 N range. Fiber-reinforced actuators belong to another category of soft actuators. They have a compact structure and can provide high forces, reaching values of 8 N, compared to Pneu-Nets. Fabrication based on a lost-wax process allows monolithic molding, and the use of an inverse-flow injection (IFI) method reduces unwanted bubbles in the final actuator [77]. A soft glove designed with fiber-reinforced actuators can be more compact than one using Pneu-Nets or other categories of soft actuators [78; 79; 80; 81].