Actuation Technologies for Soft Robot Grippers and Manipulators: A Review

The new paradigm of soft robotics has been widely developed in the international robotics community. These robots being soft can be used in applications where delicate yet effective interaction is necessary. Soft grippers and manipulators are important, and their actuation is a fundamental area of study. The main purpose of this work is to provide readers with fast references to actuation technologies for soft robotic grippers in relation to their intended application. The authors have surveyed recent findings on actuation technologies for soft grippers. They presented six major kinds of technologies which are either used independently for actuation or in combination, e.g., pneumatic actuation combined with electro-adhesion, for certain applications. A review on the latest actuation technologies for soft grippers and manipulators is presented. Readers will get a guide on the various methods of technology utilization based on the application.


Introduction
Soft robotics has introduced the world to a new class of robots that has helped the researchers in finding solutions to the problems that rigid systems are not able to cope with. Owing to their soft nature, they are safe to interact with human beings as eradicating the danger of harm during operation. Their ease of adjusting with the environment, picking and placing of delicate objects without damaging them, and flexibility to operate in complicated environments, as compared to their rigid counterparts, are some other benefits they offer. This novel field is demonstrating these advantages in a number of applications such as biomedical and industrial ones, especially through the development of soft manipulators and grippers. In this work, soft manipulator refers to continuum arm that can morph its shape to perform different operations, while soft gripper refers to end-effector used to handle, hold, or grasp objects through the use of fingers or suction cups. It can also be attached to the manipulator for high degree of maneuverability.
In [1], the authors provided an overview for the readers about existing soft robotic grippers, the materials used in their fabrication, and their different designs. A classification of soft grippers based on configuration, actuation, application, size, and stiffness was presented by Samadikhoshkho et al. in their work [2]. Hughes et al. reviewed the state of the art for soft manipulators in their paper, focusing on the material and fabrication processes, actuation technologies, sensing methods, and structures. They mentioned that further research is needed to develop systems with increased speeds and precision that can expand the applications of manipulators in the fields of agriculture and industry [3].
Different actuation technologies have been developed and are widely in use. Soft grippers employ actuators that make them adapt to objects of various shapes, material, and stiffness [4]. In this paper, focus has been laid on the different actuation technologies and how the main physical principle has been exploited by each research group for targeted applications.
Starting from pneumatic actuation, the paper progresses by discussing vacuum, cable-driven, and shape memory alloy actuation. Also included are other actuation types based on electroactive polymers and electro-adhesion followed by discussion and conclusion. The main operating mechanism associated to the performances of the developed devices and the target applications have been also collected and presented in different tables related to each actuation technology, to provide the reader a clear overview of the design choices in the development of soft grippers and manipulators. The performance hexagon has also been made part of the paper (Fig. 1) so that the reader can understand and compare the performances of the discussed actuation technologies.

Pneumatic Actuation
Pneumatic actuation (Table 1) is the most popular type of actuation used in soft robots for gripping various objects by using positive pressures. It is based on pressurizing purposively designed soft chambers so as to have pre-set deformations, like bending (Fig. 2). Researchers have utilized this type of actuation for different targeted applications. Lee et al. used pneumatic networks (PneuNets) type of soft bending actuators comprising of air chambers and constraint layer for actuating soft grippers composed of finger-like parts. The conformable grasping with low contact pressure and high lifting force was attained by the use of stiffness patterning by arranging nodes of different stiffness in a specific manner in the constraint layer of the actuator [5•], which guides an asymmetric deformation. Nguyen et al. made use of pneumatic actuation for controlling a tri-stable robotic finger by employing bistable buckling springs which can maintain two stable states depending on the energy input. They took into account three stable states of grasping: open, pinch, and wrap by employing two bi-stable springs of soft and stiff nature in the finger [6]. Kim et al. replaced the compressor by an origami pump driven by tendons to control the bending angle of a soft robotic finger. The operating mechanism comprised of bending and releasing of fingers through pulling and releasing of tendons [7].
Ariyanto et al. managed the inflation and deflation of their three-fingered gripper by the use of external mini air compressor connected to a solenoid valve which regulated the entry and exit of air into the gripper [8]. The problem of low actuation speed and fingertip force of soft pneumatic actuators (SPAs) was addressed by Park et al. They developed a hybrid PneuNet actuator by integrating rigid structures into the soft part of the gripper. The round edge shape between rigid and soft material was responsible for high fingertip force, while the fast PneuNet with a high number of chambers and channels enhanced the actuation speed of the gripper [9]. Another work by Meng et al. made use of a hybrid approach which combined the characteristics of tendon-driven grippers and the SPAs. The quick release mechanism of soft fingers was controlled by tendons which resulted in grasping objects at high actuation speeds. The pneumatic actuation was used for inflating the three-fingered soft gripper and the soft pneumatic telescopic palm [10]. The telescopic palm structure has multiple segments; the third placed inside the second and second placed inside the first just like the structure of the telescope.  (2020) [2] Pinch and wrap grasps of different objects; twisting of bulb, holding of water balloon, lifting of flat objects, e.g., paper 1. Pressure variation from low to high changed the posture patterns of the gripper (pinch, wrap, and opening posture) • Pull and torque tests were conducted for the tri-stable and control grippers • With the 20-mm diameter test objects, the tri-stable fingers outperformed the control by 312% on average across all three geometries in both tests • For 20-, 40-, 60-mm-sized objects, tri-stable gripper improved performance by: 186% in z axis pull, 375% in x axis pull, 129% in roll torque, and 342% in yaw torque Kim et al.
(2020) [3] Grasping of different objects like ring-shaped snack, cup, • Experiments showed that the gripper based on nano-fiber film array outperformed the one without film in grasping fruits underwater in terms of weight: • Weight lifted by gripper without film underwater: Peach = ∼115 g, banana = ∼103 g, melon= ∼95 g • Weight lifted by gripper with film underwater: Peach = ∼135 g, banana= ∼123 g, melon = ∼103 g Venter et al.
(2017) [9] Handling and manipulation of delicate produce in horticultural applications  [11]. For grasping objects in wet and slippery conditions, Luo et al. modified the surface of pneumatic actuators by making use of a biomimetic nano-fiber array film (polydimethylsiloxane). For the actuation purpose, they used the pneumatic drive. The degree of fingers' bending depended on their internal air pressure [12•]. Venter et al. developed a soft gripper for picking and handling delicate fruits, for attaining lower contact pressure and equal distribution of force around the picked fruit [13]. Soft manipulators and grippers driven by pneumatic actuators are also popular in minimally invasive surgery. De Falco et al. introduced an octopus-inspired minimanipulator equipped with a gripper at its tip. The manipulator was able to bend in all directions along with the elongation. It comprised of three modules connected with each other. Each module was a soft cylinder made of silicone rubber. The opening and closing of gripper depended on the inflation and deflation of air inside the pneumatic cylinder [14]. Tawk et al. enhanced the capabilities of a pneumatically actuated soft gripper by adding structures inspired by Fin-Ray effect (fish fins bend in S-shape when they are subjected to external loadthis way a wrap is formed around the load/object to be grasped resulting in adaptive grasping) such that objects of different shapes, stiffness, and weight could be efficiently grasped [15].

Vacuum Actuation
Vacuum actuation (Table 2) has also been employed in different grippers. Conceptually, it is the same as pneumatic actuation, but here, the negative pressure instead of positive pressure is utilized for actuation (Fig. 3). Jain et al. worked on an actively controlled soft reconfigurable palm with three fingers and retractable nails. The fingertip as well as the finger nail orientations was changed by controlling the vacuum input of the soft palm. Moreover, the nail grasping forces were also enhanced by changing the vacuum input to the active palm [16]. This system had an added feature of pinch grasping of mini and flat objects. 3D-printed linear soft vacuum actuators (LSOVA) were manufactured by Tawk et al. LSOVA possessed multiple advantages including ease of manufacturing, scalability, and increased actuations speeds. The authors successfully tested these actuators in a number of soft systems like crawling robot inside a transparent plastic channel, soft manipulator, soft artificial muscle, as well as soft gripper and prosthetic fingers [17••].
Vacuum bending actuators (VBA) for continuum manipulators were designed and fabricated by Katugampala et al. They tested a single VBA as well as a bimorph actuator comprising of two VBAs to get multi-plane motions. The authors successfully developed a continuum manipulator consisting of three VBAs [18]. The characteristics of both positive and negative pressurization were utilized by Fatahillah et al. who proposed a positive and negative pressure (PNP) actuator. The

Cable-Driven Actuation
Another immensely used actuation mechanism in soft robotic grippers and manipulators is the cable-driven actuation ( Table 3). It works by controlling the motion of the soft body by retracting the cables that are embedded in the structure and anchored at some specific points (Fig. 4). Although it is flexible and responsive in action, the design of its setup is a challenging task due to placement of motors, pulleys, force sensors, and encoders. Xiang et al. used a two sectioned continuum arm driven by eight cables, each section containing four cables. The soft gripper attached at the end of the arm was also cable-driven in order to perform different picking operations [21]. Yan et al. also used cable-driven actuation for actuating the modules of their manipulator. In their design of soft gripper, they used three modules for gripping of objects. By combing modules in series through connectors, manipulators of different length could be designed. The cables passed through the modules for introducing bending for grasping [22]. For grasping two objects at a time just by pulling a single cable, Honji et al. designed a soft gripper consisting of thin and thick parts; thin parts acted as bending joints, while the thick parts served as the links. The pulling pattern of the cable decided the shape of the gripper for grasping objects [23]. The actuators of a gripper were made smarter by Chen et al. who integrated the soft triboelectric nanogenerators (TENGs) into the cabledriven actuator. TENGs use triboelectric effect to convert small mechanical deformations to electrical signals. Two different types of TENGs were used: one was responsible for measuring contact pressure upon grasping and the other one for detecting bending. A three-fingered gripper design was proposed in this study [24].
Another three-fingered semisoft gripper inspired by origami design was developed by Lee et al. They used an under- Gripping of different objects 1. Authors utilized a central vacuum pump to control suction of multiple holes at the bottom of gripper surface 2. This provided an active grasp control in lifting objects with a large surface area with ease and enhanced grasp stability • Gripper could lift 100 times its own weight with great ease • Could withstand a maximum force of 40 N actuated system for driving the actuators where one single motor was connected to three cables, each cable passing through a single finger [25]. In contrast to this underactuated gripper, Chen et al. developed a gripper with three fingers, each actuated by a separate motor. The gripper was designed through the topology optimization method, a performance enhancement technique that removes excessive material from the subject under special conditions using mathematical methods [26]. Cable-driven actuation was employed in a different manner by Jiang et al. They developed a cylindrical manipulator comprising of three sections, each section made up of a different material. They aimed to explore different motion capabilities of the manipulator through the use of multi-materials in a single structure. By pulling the cables that passed through each section, the bending of manipulator was controlled [27].
To make surgical tasks easier and to improve the accuracy of surgical procedures, Wang et al. developed a cable-driven soft robotic manipulator for cardiac ablation surgery on the beating heart. The manipulator was made up of silicone rubber with no electric wires and rigid structures inside, making it completely safe for the human body. The control of the soft manipulator was made through the propulsion plant (responsible for forward, backward, and rotatory motion) and the cables (responsible for controlling end and middle section direction). The manipulator could be pulled out of the body in any particular situation by simply losing the cables [28]. Zhang et al. gave more controllability and flexibility to the surgeon during surgery by dividing the manipulator into two parts, each part driven by four cables. Thanks to two divided parts of the manipulator, the surgeon can decide the cables of which part to be strained, depending on the internal cavity. Different shapes of the manipulator were achieved by pulling different cables. Experiments showed that this soft manipulator was capable of reaching almost all regions of heart [29].
Roels et al. developed a multi-material tendon-driven soft gripper which could recover any structural damages/cuts during its operation. The two materials that were used had different mechanical properties with one thing in common; both were self-healing Diels-Alder polymers. These kinds of polymers involve reversible covalent bonds that form and break through heat-cool cycle which in turn assist in the healing process. The gripper comprising of four fingers opened and closed by pull and release mechanism of tendons and was capable of handling and gripping objects of various shape and size [30].

Shape Memory Alloy Actuation
Shape memory alloys (SMA) ( Table 4) have also been used in soft grippers for actuation purposes owing to a number of merits including low noise, high force to weight ratio, small size [31], and the ease of their usage (Fig. 5)   (2020) [20] For picking different kinds of objects 1. Upon receiving pressure or bending values, an electric signal was produced by the TENGs which was used for monitoring the states of the soft actuator. This electric energy could also be harvested for powering other electronics 2. Cable that drove the actuator had its one end connected to a mini DC motor, while the other end was fixed to the tip of the finger 3. Actuator bending was achieved by pulling cables Application not yet explored (in paper till 2016) 1. Three independent cables placed symmetrically passed through the three sections of manipulator 2. By pulling each cable with a different displacement, the manipulator bent to a 3D curve For cardiothoracic endoscopic surgery 1. Cables passed in the longitudinal direction throughout the manipulator 2. Propulsion plant was added to give a better motion performance 3. Through the cables and the propulsion plant, the soft manipulator was completely controlled • Ablation performance was found to be satisfactory after using the manipulator • No numerical performance parameters were discussed Zhang et al.
(2015) [25] Application: soft surgical robot 1. Manipulator was divided into two parts; each part driven by four cables 2. By straining the cables, different shapes/patterns of manipulators were achieved • Two-part manipulator was controlled better than one-part • Had the ability to reach almost every regions of the heart • No numerical performance parameters were discussed Roels et al.
(2019) [26] Grasp and pick up soft objects of various shapes and sizes 1. The bending of actuators was obtained by pulling single tendon cable going in and out of each finger and into the next one Hellebrekers et al. also developed a SMA actuated soft gripper. They integrated sensors into the soft elastomeric body of the gripper which were responsible for sensing temperature, pressure, orientation, and proximity. The gripper was designed to hold objects passively through minimizing activation time such that the energy required for operation is conserved [33]. Another work of Liu et al. combined the variable stiffness property of paraffin with the shape memory effects of SMA wires. They developed a soft gripper consisting of three fingers. Each finger had two joints whose stiffness could be changed. The SMA wire passed through the fingers [31]. Besides having only two conditions of on and off through SMAs, Hadi et al. developed a module which could achieve any desired setpoint by using a proper control strategy. In other words, any configuration of the module was possible. By heating the SMA springs individually or together, a differential actuating system was obtained [34]. SMA springs also find their application in biomimetic systems. Golgouneh et al. developed a 2-DoF SMA actuated robotic arm which mimicked the real-time arm movements of the user. The current supply in the SMA springs was controlled by a controller which in turn controlled the arm bending [35]. Yin et al. used three different kinds of SMA wires in their two-fingered soft robotic gripper for multiple purposes. Upon heating the wires, SMA-1 wire changed its modulus, and SMA-2 wire changed its length, while SMA-3 wire showed good elasticity [36]. Obaji et al. developed a three-fingered gripper. In each finger, three SMA springs were embedded inside the silicone elastomer. Two out of them were placed in a way that upon receiving electric current, they bent in a U-shape resulting in bending of finger with a greater force. The direction of third spring was fixed in a way that it did not assist the bending but helped in attaining a stable grasp upon activation [37].

Electroactive Polymer Actuation
Electroactive polymers (EAPs) ( Table 5) make use of elastomeric materials that can be actuated upon electrical stimulation on two side electrodes that tend to attract and produce large deformations (Fig. 6). Xu et al. got inspiration from the Venus flytrap to develop their soft gripper based on this actuation technology. They designed a two-leaf structure that could create opposite axial elongation to open and close the gripper. They employed dielectric elastomer (DE) as reconfigurable matter so that it can change its shape and properties. Upon applying voltage, DE exhibits high actuation pressure, short response times, and high expansion efficiencies [38]. Wang et al. also developed their Venus flytrapinspired gripper to handle objects up to 15 g ranging from strawberries to plastic cups. Their gripper was multilayered made of dielectric minimum energy structures (DEMES) which is a subclass of DE actuators [39]. Hwang et al. made use of the benefits of both electro-adhesion and electroactive polymers for the design and development of their soft gripper. Specifically, the dielectric polymer was used to enhance the grasping force of the gripper by expanding in areal directions [40]. Variable stiffness dielectric elastomer actuator (VSDEA) was used to develop a soft gripper able to hold objects up to 11 g by Shintake et al. The DE actuator in this case was introduced in the structure to achieve bending actuation during the grasping, while a low melting point alloy (LMPA) was employed to introduce variable stiffness by switching between hard and soft states through Joule heating. Through the combination of fine bending actuation and variable stiffness, the gripper performed efficiently with a good response time [41]. Zhou et al. also used DEA for fabricating their soft gripper. For constructing the frame of the gripper, they directly used FDM, 3D printing technique, over the DEA membrane. This did not require any kind of adhesives between the DEA and the elastic frame. The authors developed three different actuators. The most optimized design produced a maximum change in tip angle of about 128°with the maximum blocked force of 25 mN [42]. Application planned for medical purposes (due to bio-adaptability of SMA) 1. Three SMA springs were mounted in 120°a round a base spring, all springs passing through the module 2. Heating SMA springs individually or together resulted in a differential actuating system • SMAs usually provides only two points (on and off) in the work space of the system • Through the control strategy of this work, any desired set point is achievable Used for actuation of robotic arm that mimicked a wearable sleeve (for rehabilitation purpose) 1. Two SMA springs controlled the robotic arm movement 2. Two IMUs were also attached to the arm for obtaining different measurements. The arm bent due to SMA actuation • Weight of robotic arm = 59 g with a wide controllable range of motion of 119°for the first joint, 123°for the second joint • Steady-state errors of 2.7°and 2.1°for the first and the second joints lead to average RMSE of 11.2 mm for end-effector displacement Yin et al. (2017) [32] Gripping of different objects 1. Four SMA-1 wires were used to vary the stiffness of the fingers for enhancing the grasping ability 2. SMA-2 wire was used for actuation, while SMA-3 wire was used for restoring the fingers' posture after every unloading • Grasping ability of the soft robot gripper increased with the growth of its stiffness by at least 40% Obaji et al. [33] Gripping objects of complex shapes 1. Three SMA springs were embedded inside the silicone elastomer 2. Two SMA springs upon heating formed a U-shape for grasping while the third enabled stable grasp • Experiments proved that SMAs are a good replacement of actuators • No numerical performance parameters have been presented

Electro-adhesive Actuation
Electro-adhesion (EA) ( Table 6) is a further actuation technology that finds application in soft grippers. The principle behind this actuation is that upon applying voltage to the electrodes embedding the dielectric material, an electric field is built which in turn energizes the substrate due to electric induction (Fig. 7). Due to the presence of opposite charge on the substrate and the electro-adhesive pads, the attractive forces help in developing a tight grip. Guo et al. made use of EA actuation as well as DEA   [34] Test was made to grip ping-pong ball

Other Types of Actuation
Besides the above discussed technologies, several other actuation methods have been used by researchers (Table 7). Electric actuators based on handed shearing auxetics (HSA) were employed by Chin et al. in their soft gripper. HSA-based actuators can both twist and extend upon applying angular input owing to their auxetic design. Their gripper integrated three modes of grasping: parallel jaw, suction, and soft fingers. For the mode of suction, suction cups were incorporated

Discussion and Conclusion
Actuation is a major challenge in general in soft robotics, as without a proper actuation technology, the desired functionality cannot be achieved. At the same time its mechanical design should not affect the soft nature of the device. Soft grippers and manipulators make no exception, and a trade-off is necessary. Pneumatic actuation is the widely used technology as it provides high grasping forces and gives control to the user to attain the desired shape of the actuator. These actuators have no problem of friction and are quick in response. Moreover, the control is easy that is why it is more commonly used over the other methods. However, it is difficult to miniaturize them, and they are easy to fail during trials due to leakages. Also their manufacturing is not simple as they are made in various stages. Cable-driven actuation is also listed as a well-established technology as it provides good force and moment control. In biomedical applications, this technology is preferred owing to the fact that the cables can be pulled at any time out of the body in case of any problem. Cable-driven actuation offers good response speed, motion accuracy, flexibility, and adaptability; however, energy loss due to friction between cables is a limitation. The shape-changing property due to temperature stimulus has made SMAs popular in driving different soft systems. High power-to-weight ratio SMAs can be easily driven by electric current through ohmic heating. They produce low noise, have low driving voltages, are small in size, provide high distortions and smooth movements, and possess simple structures. However, owing to their slow response, they are not preferred in applications that demand quick response. They also have poor fatigue characteristics. Vacuum actuation is another used technology mainly for suction or in applications where variable stiffness is desired. Negative pressure in vacuum actuators provides a fail-safe feature in comparison to pneumatic actuators. They shrink upon actuation that makes them suitable for small space applications and improves the actuators' lifetime and durability. However, miniaturization is a problem for them, and in case of jamming, they demand a dexterous design. EAPs possess the strengths of having high actuation pressure, short response time, high expansion efficiencies, low energy consumption, conformable grasping, and being lightweight. However, these actuators normally generate low forces, cannot pick heavy objects, and require high voltage for their actuation. Electroadhesion has the benefit of giving a firm grip to the objects. This type of actuation is silent. The EA actuators can easily handle lightweight objects, have simple design, and are electrically controllable. However, they are not typically effective on rough surfaces. Also planar EA grippers have difficulty in picking curvy objects and in adhering to non-flat surfaces. This is the reason why many authors have used EA-based actuation but always in combination with other actuation technologies.
Each actuation technology has its own merits and drawbacks. The selection is mainly based on the application. The most optimized solution has to be chosen such that the required purpose of the design gets fulfilled. From the literature used in this paper, the main observation was that most soft grippers and manipulators are mainly tested in the laboratory particularly for pick and place operations and handling of delicate as well as complex-shaped objects. Very few of them made it to the field. Therefore, a lot has to be done yet for developing soft grippers and manipulators for the industrial applications. Another point worth mentioning is that since pneumatic, vacuum, and cabledriven actuations have widely been used by the researchers owing to the fact that they possess more advantages over the other available technologies, this has improved their reliability. The technologies including SMAs, EAPs, and EA are still in a growing stage, and it will take time for them to be mature. However, they can be integrated with the pneumatic, vacuum, or cable-driven technologies such that a more efficient and reliable system could be developed.
Funding Open access funding provided by Scuola Superiore Sant'Anna within the CRUI-CARE Agreement. This work was partially supported by the European Union's Horizon 2020 research and innovation program through the SMART project (contract 860108) and the SoftGrip project (contract 101017054).

Declarations
Conflict of Interest The authors declare no competing interests.
Human and Animal Rights and Informed Consent This article does not contain any studies with human or animal subjects performed by any of the authors.
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