# Recent Improvements in the Development of Soft Grippers
## 1 Introduction  
Since robots are constructed to perform human or animal-like tasks, it is essential for them to be capable of grasping and manipulating objects of various shapes in order to interact with their environment. To achieve this, the development of robotic hands has been long-term in progress. The first to be produced were made of rigid materials with rigid links and kinematic joints, integrating a large number of sensors and actuators, e.g., refs. [1, 2, 3]. According to [4], this technology has resulted in a limited number of everyday applications. To respond to some of the challenges faced by this kind of technology, such as safety for humans, adaptability to the environment, the manipulation of fragile objects, cheaper cost of fabrication, etc., soft compliant materials have lately been used in the development of grippers or robotic hands in various reported works.  
For example, the work in [5] presents a review of soft grippers, with the main illustration being the picking up of objects. Their article focuses on the different grasping technologies and the various materials used to develop soft grippers. They present a comparison of gripping performance and discuss the sensing technologies in soft grippers. Reference [6] is another paper that reviews soft grippers to assess actuation methods, sensors, and materials and methods used in their development. They also discuss the fabrication technologies and the morphology of soft robotic grippers.  
The present article focuses on dexterity and dexterous manipulations. Our main contribution is the proposal of a uniform framework for the characterization of dexterous soft grippers and their performance, based on the existing benchmarks. We discuss how to achieve dexterity for soft grippers, the primitive movements that dexterous soft grippers can perform, and their evaluation. We also discuss hybridization strategies and their impact on gripper performance. A review of various dexterous soft grippers found in the literature is made, including their performance, morphology, and assessments. Before discussing the aforementioned points, we present soft grippers and different grasping techniques, the materials used, the main sensors, and the manufacturing methods.  
Focusing on recent work, the different articles reviewed in this article were selected from the works reported in the past decade (2014-2023). However, our review includes older research works that provide foundational methodologies and historical perspectives that are still relevant today. The articles were selected from leading journals and conferences in the field of robotics and automation, through keyword-based searches in the usual search engines and in the search engines of various publishers. The keywords for the article selection included dexterous grasping, dexterous manipulation, soft fingers, soft grippers, stiffness variation, and dexterity evaluation. Some of these keywords were combined in the research process. Since our article does not aim to report every single work published in this domain within the last decade, we selected the articles which, according to us, show the potentiality of dexterous capabilities thanks to the arrangements of their DOFs, towards the proposition of a uniform framework for dexterity evaluation.  
This article's structure is as follows: in Section 2, we discuss soft grippers and those capable of in-hand manipulation, different actuation, sensing, manufacturing techniques, and hybridization strategies. Section 3 presents dexterous manipulations and the primitive movements that can be achieved by dexterous soft grippers and the methods applied in the literature to achieve dexterity. In this section, we also discuss performance evaluation for various soft grippers found in the literature, and we propose a uniform re-usable approach for the characterization of a dexterous soft gripper and its performance. Finally, Section 4 summarizes the article, and in Section 5, we suggest prospects for improvements in the development of soft dexterous grippers.