# Recent Improvements in the Development of Soft Grippers
## 3 Dexterous Manipulation
### Achieving Dexterity for Soft Grippers
#### 3.2.2 Stable In-Hand Manipulation  
Once a gripper is capable of the stable grasping of various objects, some other techniques can be applied to ensure that it manipulates the objects without breaking contacts and that some standard known performances are achieved during manipulation. According to [24], one reliable way of achieving dexterity with soft grippers is by increasing the DOF space of the end effector. The 3-DOF finger developed in [15] is said to substantially reach a larger area than single-chamber conventional soft actuators, making it capable of various grasping and in-hand manipulations. In addition, the passive compliance of most soft grippers adds a passive reachable area to the active workspace. This idea is better explained in [15]. For tendon-driven foam hands, the possibility of the tendon being routed anywhere along the hand's surface allows complex families of deformations and facilitates dexterous in-hand manipulation [12]. For pneumatically actuated grippers, one way of increasing the DOF space is by increasing the number of pneumatic chambers in the fingers, as explained in [72], where the bio-inspired soft anthropomorphic hand has 22 DOFs. Dexterity could also be enhanced by the introduction of sensors in the actuator matrix. For example, the soft gripper in [33] uses tactile sensing and active shape-changing to achieve precise in-hand manipulation. The tactile sensors avoid object damage and provide the closed-loop control of the fingertips' positions. The grasping quality of the fingers developed in [21] is increased by the installation of the capacitive tactile sensor developed by the same authors in [73]. They use the sensors' data as feedback for the fingers' control. A force-feedback control method is also used in [32]. They adjust the joint angles or link lengths of their elongatable fingers based on the contact pressures measured by some hyperelastic soft sensors. According to [34], most grippers incorporate force sensors that only detect one direction force. As a result, they propose a six-axis force/moment sensor that detects the force and moment in three directions, i.e., Fx, Fy, Fz, Mx, My, and Mz. Detecting the force in the three directions allows the gripper to safely grasp random objects, while detecting the moment in the three directions allows the gripper to precisely detect the object's position with the gripper. One other way to achieve dexterity is by setting the axis normal to the palm, parallel to the object's central axis, as explained in [1]. This allows the object rotation and translations to occur in the reference frame attached to the palm. In [26], it is explained that the grippers are configured such that the instantaneous screw axes always intersect at the same point relative to the palm, which produces an instant motion about this common point independent of the object geometry in a stable grasp. The integration of hybridization strategies that enable attributes to change over time also improves the dexterous capabilities of soft grippers. Figure 4 shows a number of dexterous hybrid soft grippers that are capable of stiffness variation. In [31], the combination of different actuation systems is proven to overcome the weaknesses and shortcomings of soft grippers based on only one actuator system. They combine SMA actuators with DC motors and electromagnets to actuate a soft hand that is capable of grasping objects of different shapes and objects from inside, securing smaller objects, and a higher grip force. In [21], the authors proposed a dexterous finger whose dexterity is achieved by varying the bending point and the stiffness of its soft pneumatic actuator. They argue that the variable effective length of the fingers and their bending capability increases the hand's dexterity. The tunable rigidity elastomeric fingers that constitute the gripper in [55] confer the gripper with twisting manipulations, which enhances in-hand manipulation.