In the case of a dexterous-type gripper, it offers the advantage of delicately manipulating objects using its high-degree-of freedom (DoF) structural characteristics. The dexterous-type gripper can also be further classified into the following types: a high-DoF gripper consisting of multiple dexterous fingers, and a robotic hand that mimics the configuration and motion of the human hand. In the case of the robotic hand, its advantages arise from operating tools by mimicking the motion of the human hand [17,27–30]. On the other hand, the high-Dof gripper, unlike the configuration of human fingers, is designed to enhance work efficiency. It can manipulate objects by gripping them in ways that differ from the movements of the human hand. However, these dexterous-type grippers require delicate motion control to grip or manipulate an object, and these motions need to be optimized depending on the characteristics of each object. Therefore, these types of grippers require gripping strategies to effectively utilize the ability of dexterous motion of fingers, and a sensor system to detect the 3-dimensional precise position, orientation, and gripping status of objects using vision, force sensors, tactile sensors, etc. However, if the work process environment is not fully controlled, errors may occur in estimating the 3-dimensional position and orientation of objects. Furthermore, work time can increase, and the probability of failure may also rise as the complexity of the gripping strategy increases. Due to these limitations, it is difficult to use in manufacturing applications that require robust performance in variable production environments and a short takt time with a high success rate.

Because of the complexities of high DoF gripper, relatively simple types of grippers are predominantly used in the industrial field. Grippers with a pair of opposing jaws or fingers can be classified as Clamp-type gripper, which generate gripping force by simply moving the jaws (tips) toward the target object. Depending on the jaw shape and gripping method, these grippers can be further divided into two categories: Friction (Force-fit) and Encompassing (Form-fit) grip as in Fig. 2 [31]. The friction grip generates gripping force based on the friction between the surface of the target object and the gripping jaw. Therefore, the compression force exerted by each jaw, commonly referred to as ‘gripping force’, along with the friction coefficient between the surface of the object and the jaw, determines the holding force of the gripper. Since the holding force is generated by friction, the jaws typically have a simple, flat shape and do not require specialized design. Consequently, the friction-grip based grippers offer advantages in their versatility and ease of application across various object shape, without the need for complex gripping strategies. On the other hand, a disadvantage exists in that determining the appropriate compression force can be challenging without prior knowledge of the optimal holding force. Therefore, a sufficiently high compression force needs to be applied to the target object to prevent gripping failure. However, because high compression force cause breakage of the object, the friction-grip based grippers are not appropriate for handling fragile or delicate objects, such as glass, ceramic, food, and electric components. Additionally, an object with a complex surface shape can lead to failure in the friction-grip method, as uneven surfaces may result in unbalanced net forces being applied to the object, causing movement of its gripped position.

In contrast, with the encompassing-grip, the shape of the jaw is designed to match the shape of the target object. This jaw design allows for a much higher holding force, even if the direction of the compression force applied by the gripper jaw does not align with the direction of the external force applied to the object, as shown in Fig. 3. Meanwhile, the shape of the gripper jaw needs to be changed whenever the shape of the target object changes. Additionally, another limitation is that the direction or position in which the object is gripped is predetermined by the shape of the jaw, making it difficult to adapt to changes in the gripping environment.

Impactive grippers can be further categorized by the method of jaw transfer, such as parallel or angular, and by the type of power source, including electric or pneumatic options.

A finger-type soft gripper has also been proposed for gripping delicate, irregular shaped objects, especially in the food industry. Pneumatic finger-type grippers, including those developed by Soft Robotics Inc., are typically actuated using air pressure through a chamber within the polymeric fingers [32]. The fingertip can consist of a flexible structure, such as the fin-ray gripper developed by Festo Ltd. [18]. This design offers high adaptability and minimizes the risk of damaging delicate objects due to its flexibility

Astrictive grippers can be categorized according to the method of generating adhesion force: vacuum suction, magneto adhesion, electro adhesion, and gecko-like adhesion [12]. Electro-adhesion gripper utilizes the electrostatic attraction force generated by strong electric fields at electro adhesive pads. This technology is effective on rough surfaces, including fibrous materials like cloth and carbon fibers [23], and is can be suitable for use in vacuum environments. such as in space. The gripping force of an electro-adhesion gripper is mainly influenced by the electrode pattern design and the distance between the adhesion pad and surface of target object. Consequently, research into various electrode pattern and flexible substrate structures to maximize adaptation to the shape of the object has been conducted. However, the high voltage required by electro-adhesion grippers poses safety issues and produces relatively low gripping force [33]. Additionally, their performance is highly sensitive to environmental factors such as dust and varies with surface roughness. These factors are barriers to their widespread use in the industrial field.

The magneto-adhesion gripper generates adhesion force from the magnetic field created by electromagnet or permanent magnet [24]. It offers the advantage of a simple mechanism and relatively high gripping force, and fast gripping speed [33,34]. However, it can only be applied to object consisted by ferromagnetic materials, which are capable of generating a strong attraction force. Additionally, limitations such as residual magnetism in the gripped object due to the strong magnetic fields, and the high energy consumption required to generate an effective magnetic field in electromagnet-based grippers, restrict its widespread usage in the industrial field.

The Gecko-like adhesion uses the van der Waals effects mimicking the multiscale architectures on the gecko feet [25,35]. The advantage of gecko-like adhesion gripper is that it does not generate an electric or magnetic field to produce adhesion force, thus preventing potential damage to electric components from strong electric or magnetic fields. In addition, this type of gripper does not require a power source such as air supply or electricity, as the adhesion force is generated from the structural characteristics of the pad itself. However, it can only generate adhesion force on flat and clean surfaces.

The vacuum suction-based gripper is widely used in the industrial field due to its simple system, high gripping force, and versatility with various material characteristics of the target object. Compared to the clamp-type gripper, it offers advantages when gripping objects located in small spaces or stacked closely together, where executing a pinching motion can be challenging. This is because a clamping motion requires space on both sides of the object, allowing the jaws to approach and apply a balanced net force to maintain the gripped position. In contrast, a suction gripper only needs access to a small area on the surface, such as the upper part of the object, and is therefore less affected by spatial constraints.

A vacuum suction gripper generates adhesion force from the negative pressure between the surface of object and the gripper. Therefore, the effectiveness of the seal between the gripper and the object surface, which prevents air leakage, primarily influences the holding force. Consequently, the suction gripper typically includes flexible or soft components to maximize adaptation to the surface shape of the target object. To achieve this, elastomeric suction cup [21], sponges with multiple holes [22] or specially designed structure for gripping complex-shaped objects [36–38] are used in the industrial field. However, if the suction cup or sponge structure cannot effectively seal around the contact surface of the object due to its complex shape, air leakage through the gaps can occur, causing a drastic decrease in holding force. Especially, objects with holes, grooves, small or large bumps, or step shapes are difficult to grip due to air leakage. To solve this problem, arrays of multiple suction cups are specially designed to avoid areas where the suction cup has difficulty adhering to the target object [39,40]. This approach is commonly applied in the industrial field. However, in this case, different configurations of the suction cup array need to be prepared in advances for each target object, and these arrays need to be changed using a tool change system whenever the target object changed. Therefore, frequent changes in the objects to be gripped make existing suction cup array structures unusable, complicating the tool changing system significantly when multiple types of objects are involved. To mitigate air leakage due to the complexity of the contact surface to which the suction gripper is attached, a ball check valve can be implemented in each channel of the suction gripper [41]. This valve can block any specific air channel where significant air flow occurs due to the improper adhesion in the contact area, thereby minimizing pressure loss from external air inflow. However, this method can only be used in limited applications because the air channel may become blocked when the gripper is tilted, and it requires a minimum flat contact surface to generate sufficient holding force. The characteristics of these most widely used commercial grippers in the industrial field are shown in Fig. 4.

The impactive universal gripper developed at KIMM consists of a shape-adaptive, variable-stiffness tip (jaw) based on the clamp-type gripper system as shown in Fig. 5(a). The initial modulus of this tip is extremely low (~46 kPa), even lower than the tofu (~57 kPa). Additionally, the honeycomb-based structure facilitates deformation according to the shape of target object. The initial shape of the tip can be freely configured into the desired shape; therefore, the initial shape of the tip in this gripper could be designed as the general flat shape. Therefore, the gripper can function similarly to a friction grip, which does not require precise consideration of the exact gripping position and orientation, but simply moves each tip toward the object. Consequently, the advantages of the friction grip, including its versatile application and simple operation, are also feature of this gripper. During the gripping motion, the surface shape of the tip changes to match the shape of the object, facilitated by the low-surface-tension structure embedded in the gripper. After the gripping procedure is completed and the tip reaches its target position, the modulus of the tip increases. Consequently, the deformed shape of the tip, which now conforms to the target object, is retained. This allows the tip to function as if it were specially designed for the target object, demonstrating the characteristics of an encompassing grip. In other words, the gripper begins to benefit from the advantages of encompassing grip, which involves achieving higher holding force with minimal compression force. Therefore, this gripper can incorporate the advantages of both friction and encompassing grip methods while minimizing the disadvantages of each. For example, unlike the friction grip method, where the minimum required compression force to generate frictional holding force needs to be known in advance, a very small compression force - sufficient to deform the tofu-like modulus tip - is enough to securely hold the object. Therefore, the risk of damaging delicate objects can be significantly reduced with this simpler gripper system.

Although this gripper has the advantages described above, there were several problems in applying it to actual industrial sites. The first limitation was the size of its tip. One of the major requirements demanded by manufacturing application is sufficient working space. When gripping objects inside a machine or transferring a gripped object to a certain manufacturing system, working space becomes a critical factor.

In addition, bin-picking of stacked objects in a confined, small box also present challenges due to the gripper size. However, the relatively large gripping tip becomes a barrier for these applications.

Another limitation was the difficulty in picking up thin object located on the ground. Due to the relatively blunt external shape of the gripper tip, it was challenging to meet the needs of companies that required gripping thin objects placed on the floor. Specifically, many of these thin objects were included among the molded products that companies targeted as their application areas.

To overcome these limitations identified during the application of the gripper in actual industrial process, the configuration of the gripper has been modified as shown in Fig. 5(b). In case of the previous gripper tip design, the rigid jig structure that held the gripper tip consisted of both upper and lower sides, which posed a problem: the rigid jig would collide with the floor when the gripper approached too close to grip thin and flat objects. Therefore, the rigid jig structure on the bottom was eliminated, and the bottom of the gripper tip was redesigned to be sharp and flexible for effective adaptation. In addition, the modulus of tip’s sharp edge side can now be varied in real time, allowing the shape of the tip to change according to the shape of the object and then retain its deformed shape. The thickness and width of the gripper tip were also reduced by up to half, as required by the company. Due to this modified tip design, thinner objects such as disks or rods can now be gripped, which was not possible with the previous design (Fig. 6).

We used vacuum suction as the source of astrictive force in the gripper due to its high gripping force and versatile usage, as described in the previous section. However, one of the major drawbacks that hindered the expansion of the application area of the suction gripper was the decreased holding force on objects with irregularly shaped surfaces or through-holes due to air leakage. To solve these problems, the gripper was designed by mimicking the two-level (macro-and meso-scale) shape adaptation of an octopus’s leg as shown in Fig. 7 [12]. These distinctive characteristics of KIMM astrictive gripper could be achieved by using an orthotropic surface tension mechanism and the flexible hexagonal holes composed of an orthotropic modulus structure. Therefore, the developed gripper grips objects using the following procedure: First, the macro adaptation allows for shape deformation according to the overall shape of the object, similar to how an octopus wrap its leg around an object. Then, the meso-scale adaptation causes deformation according to the detailed shape of the object, similar to how an octopus’s sucker conforms to the detailed contours of the object as shown in Fig. 7(a). Therefore, the developed KIMM astrictive gripper can grip a wider variety of objects that previous suction grippers could not handle. Additionally, the variable stiffness structure incorporated into the gripper helps maintain the initial grip position, even when external forces are applied to the object. Consequently, the developed gripper can perform precise assembly processes and complex sequential tasks (Fig. 7(b)), which were difficult to achieve using traditional suction grippers.

Through meetings with multiple companies across various fields, it is expected that the developed gripper could be applied to various manufacturing processes where robotic automation could not previously be implemented due to the limited performance of existing grippers. Particularly, the developed gripper shows significant advantages in the bin-picking process when multiple types of objects are randomly stacked in a bin (Fig. 8). In contrast to the typical bin-picking process, which relies heavily on the performance of the vision system, the developed gripper can effectively grip objects without a specific picking strategy, even with inaccurate position data due to uncontrolled environmental conditions.

Traditionally, when gripping sheet metal products, jigs were used to avoid areas with features such as holes, high steps, and rough surfaces where suction grippers struggle, and to selectively position the grippers on easier-to-grip areas like flat, smooth surfaces. However, this approach requires a different type of jig each time the object shape changes. In contrast, this KIMM gripper can effectively grip large steps and through-holes that existing grippers cannot, allowing it to grip various types of sheet metal products with a single gripper system. The developed gripper can also effectively grip complex-shaped rocks, which was not possible with the previous suction grippers. In the case of paper cups or parcels wrapped in plastic bags, which were difficult to grip with previous suction grippers, the developed gripper can effectively grip and maintain a gripped state even when the center of gravity is not centered (Fig. 11). Furthermore, fragile objects such as battery cells or glass plates also can be effectively gripped by having the gripper encompass the contact area using only the thin side parts, without applying compression force.

However, several issues were discovered while implementing the gripper in the actual manufacturing process. Especially, in the pick and place process, which is one of the most frequently requested manufacturing process, short takt time is required, thereby increasing the usage frequency of the gripper compared to general cases. Notably, the measured lifespan of the developed gripper, approximately 30,000 to 40,000 cycles, is significantly shorter than that of the typical suction cup. Additionally, the maximum holding force of the developed astrictive gripper was about 2 kg, but this value decreased to under 1 kg when handling objects with either long sizes or complex surface patterns. In particular, tilting the gripper while holding a long object caused a gripping failure, then limitations in object manipulation were discovered.

The primary issue that needed to be resolved for the actual implementation of the gripper in manufacturing process was its lifespan. Three major durability issues were identified during its application in actual manufacturing settings: first, the tearing of the gripper surface due to continuous friction as it contacted the object being grasped; second, tearing caused by the stiffness difference between the internal rigid structure and the flexible body’s outermost wall structure; and finally, as the number of deformations increased, the same outermost position of the gripper consistently deformed, leading to increased fatigue at those locations and eventually resulting in fractures. For the first issue, we controlled the friction coefficient of the material comprising the gripper. By reducing the friction between the gripper and the surface of the object, we were able to minimize surface wear caused by friction. For the second and third issues, to minimize performance degradation while enhancing durability, the thickness of the lower part of the outermost structure, where fractures frequently occur, was selectively increased. Additionally, the stiffness of the polymer material used in the gripper was increased from Shore 00–30 to 00–50. At the same time, to maintain the overall modulus of the gripper, the size of the individual holes was increased by 1.2 times. As a result, the lifespan of the modified gripper design has been extended to up to 150,000 cycles, while minimizing performance degradation.

However, even the modified version of the gripper still lacks the lifespan compared to the general suction cups commonly used in the industry. Additionally, there are still limitations in maintaining the gripping state when the gripped upper part of long objects, such as water bottles, is tilted-a requirement from the logistics automation industry.