# Recent Improvements in the Development of Soft Grippers
## 4 Discussion  
Soft grasping is a branch of soft robotics that allows the manipulation and handling of various objects. It is the evolution of classical grasping towards creating versatile devices for general objects' manipulation independent of their shape or deformability, with a safe interface for humans and the environment.  
The most common actuation technology used in soft grippers is FEA followed by servo motor actuation, tendon-driven actuation, SMA actuation, and particle jamming. In the review, both anthropomorphic and non-anthropomorphic grippers appear. The sensors are mainly pressure sensors and some few grippers have tactile, flexion sensors, and force/moment sensors.  
Due to the existing limitations in the modeling, control, perception, and manufacture of soft robots, soft grasping presents difficulties in task repeatability, movement precision, and grip force. An approach used to overcome these difficulties is the hybridization of existing technologies. It has been studied to vary soft actuators' stiffness or to improve perception by the introduction of sensors in the actuator matrix. Examples of hybridization strategies have been reviewed in Section 2.4, along with Figure 2 and Table 1. Different solutions have been reviewed in Section 2.4, Figure 2, and Table 1, which compare the performance of various hybrid grippers.  
Soft grippers' dexterity depends on their capacity to manipulate an object in 6 DOFs. With this criterion, among the reviewed grippers, the BCL-26 developed in [29] stands out with its 26-DOF space, and its maximum lifting force of 21.9 N, making it a performant gripper.  
For all the grippers, the reported achievements are normally related to maximum forces. Other achievements, such as the bending angle, the sliding resistance, and the workspace, are equally reported. Regarding dexterous manipulation, real dexterity is not evaluated in all cases, and is normally evaluated with real tasks. On the other hand, potential dexterity is mostly evaluated by performing the feasible primitive movements discussed in Section 3.1 and the evaluation techniques exposed in Section 3.3.1. In future works, it could be interesting to develop a formalism between these two aspects to improve the design of dexterous soft grippers.  
A number of methods are proposed for dexterity evaluation or the ability to manipulate various objects skillfully. These methods can be classified into real dexterity and potential dexterity. Moreover, the evaluation of potential or real dexterity in a gripper is determined by what is makeup with its control system. The authors in the literature have not yet chosen a global reference method for this. Thus, dexterity assessment has been performed differently in each publication. In terms of a specific assessment of grippers, anthropomorphic hands typically measure potential dexterity with the Kapandji test, and in general, the Feix test is used as a measure of potential dexterity. Various manipulation  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Ref.** & **Real Dexterity Evaluation** & **Potential Dexterity Evaluation** & **Other Evaluations** \\ \hline \multirow{4}{*}{[15]} & - & Rotation of an object without & & \\  & releasing it & & & \\  & - & Movement of a ball with & - & Kapandji test & \\  & - & two fingers & - & Grasping various objects & Force measurement under \\  & - & Rotation of an object with the & & (sphere, cylinder, fruit, etc.) & actuation \\  & - & Oscillation of a thin rectangular object & & & \\ \hline \hline \end{tabular}
\end{table}
Table 3: _Cont._tasks like opening a bottle, writing with a pen, breaking an egg, rotating a rubric cube, pouring from a cup, and others, helped assess real dexterity.  
Overall, this review points out the relationship between the degrees of freedom and the corresponding type of dexterity, potential or real. The survey proves that soft grippers which pass potential dexterity tests are capable of real dexterous manipulations.