# Recent Improvements in the Development of Soft Grippers
## 2 Soft Grippers
### Hybridization Strategies  
In order to improve soft grippers' performance and overcome specific difficulties such as task repeatability, movement precision, and grasping force [6], different technologies have been used. Soft grippers could be developed using active materials and particle jamming technologies to overcome some of these difficulties. The combination of different existing technologies to exploit the best capabilities is considered a more promising approach to overcoming the aforementioned difficulties [5; 27]. This approach, called hybridization, confers the grippers with properties such as stiffness variation, auto-perception, and others shown in Table 1. Hybrid soft grippers can either have a combination of rigid and soft structures like in [18; 21], or a multi-material structure integrating several active materials. In the following works, [10; 11; 30; 44; 45; 46; 47; 48; 49; 50; 51; 52; 53], the stiffness variation of the soft grippers is achieved by integrating active materials such as LMPA, LMPP, SMP, SMA, and jamming approaches in actuators with FEAs or EAPs. Figure 2 shows hybrid soft grippers with variable stiffness. Some soft-rigid hybridization strategies have been found in the literature. The fingers of the gripper developed in [54] are McKibben muscles, whose stiffness is proportional to the air pressure inside them. Pneumatic muscle actuators are used in [18], with rigid articulable bones in each finger constituting the gripper. This confers the fingers with a length-constraining mechanism that allows them to conform to different object shapes and sizes, hence facilitating handling. The soft gripper in [55] uses tunable rigidity elastomers as ligaments. When electrically activated, the ligament's rigidity decreases by approximately 35\(\times\), causing more extension. The work in [21] is based on the use of a translating rod inside the finger to produce a movable bending point and, at the same time, modify the stiffness of the fingers as their effective length varies.  
Although hybridization considerably enhances the dexterous capabilities for soft grippers, the complexity of multi-material integration remains a major trade-off. Active materials, for example, can provide non-desired property shifting, producing challenges in fabrication. The modeling of active materials or combined rigid and soft structures remains underdeveloped. As a result, the control of hybrid soft grippers remains complex.  
Figure 2: Different technologies for soft grippers integrating hybridization strategies. (**a**) Two-finger non-anthropomorphic soft gripper with FEA and layer jamming technologies [44]. (**b**) Three-finger non-anthropomorphic soft gripper with FEA and particle jamming technologies [45]. (**c**) Two-finger non-anthropomorphic soft gripper with DEA and LMPA technologies [46]. (**d**) Soft pneumatically actuated manipulator combined with tendon-driven actuation [47]. (**e**) Two-finger non-anthropomorphic soft gripper with FEA and electrostatic layer jamming technologies [48]. (**f**) Non-anthropomorphic soft gripper with 3 SMA fingers and 2 SMP per finger [49]. (**g**) Three-finger non-anthropomorphic soft gripper with FEA and particle jamming technologies [50].  
Table 1 shows some soft grippers that integrate hybridization strategies. Some of which are capable of dexterous manipulations. The table shows the impact of hybridization on the grippers' performance, from the maximum bending stiffness for some, through the maximum bending angle, and to the maximum pulling force for others.