Transformable Soft Gripper: Uniting Grasping and Suction for Amphibious Cross-Scale Objects Grasping

Abstract

Robotic grasping plays a pivotal role in real-world interactions for robots. Existing grippers often limit functionality to a single grasping mode—picking or suction. While picking handles smaller objects and suction adapts to larger ones, integrating these modes breaks scale boundaries, expanding the robot’s potential in real applications. This article introduces grasping modes transformable soft gripper capable of achieving amphibious cross-scale objects grasping. Despite its compact and fully scalable design (20 mm in diameter prototype), it morphs into two configurations, gripping objects from 10% (2 mm) to over 1000% (200 mm) of its size, spanning a vast 100-fold range. To enhance its grasping efficacy, we derived theoretical analytical models for the two distinct grasping modes. Subsequently, we present a detailed illustration of the gripper’s fabrication process. Experimental validation demonstrates the gripper’s success in attaching or detaching everyday items and industrial products, achieving high success rates in both air and underwater scenarios. Amphibious grasping and card manipulation demonstrations underscore the practicality of this transformative soft robotics approach.

Introduction

Grasping, an essential skill involving the lifting and secure handling of objects, stands as a critical aspect enabling robots to physically engage with the real world.¹ A robot’s efficiency and maneuverability hinge greatly on a reliable and adaptable robotic end-effector.² Over the past decades, various types of grippers have emerged.³ Grippers and suction cups emerge as the dominant types, serving as pivotal tools for effective interaction in various applications.

Grippers, engineered for secure grasping through force or form closure, span various types—finger-based,^4–6 jamming-based,^7–9 origami-inspired,^10–13 and bio-inspired tentacle grippers^14,15—showing remarkable versatility in handling objects smaller than their own size. This adaptability allows precise manipulation across scales, from industrial tools to nano-scale cells. Conversely, suction cups utilize vacuum pressure to attach and adhere to grasped objects.¹⁶ Their flexible cup-shaped structure establishes an airtight seal on surfaces, excelling in handling larger objects surpassing their dimensions, especially on irregular surfaces. Palletizing and bin-picking commonly rely on suction cups, exemplified by a 12.5 mm diameter cup effortlessly handling objects over 95 mm in size with just 30 kPa negative pressure.¹⁷

In real-world applications, robotic end-effectors face limitations in targeting different scales, often necessitating switching between grippers and suction cups. Integrating both grasping and suction capabilities into a single end-effector emerges as an intuitive and promising approach to enhance versatility. The Amazon Picking Challenge champion¹⁸ and numerous high-performing participants adopted a hybrid grasping and suction solution. One typical example involves a gripper with an insertable suction cup:¹⁹ extending the suction cup enables suction mode, while retracting it transitions the hand to grasping mode.²⁰

However, integrating two independent grippers and suction cups mechanically often results in a large and bulky hand, limiting scalability and application potential. The critical challenge lies in compactly integrating grasping and suction modes within a mechanism and structure, pivotal to achieving a successful hybrid-mode robotic hand.

The evolution of soft robotics technology offers a promising avenue for achieving an optimal fusion of grasping and suction capabilities within robotic grippers.²¹ These soft robots epitomize the integration of actuation and execution, enabling the consolidation of multiple functions within a singular body. Specifically, soft grippers demonstrate morphological computation by leveraging material softness and mechanical compliance, simplifying intricate control processes.^22–24 Some of the grippers combine finger grasping and other mechanisms such as jamming,²⁵ suction,²⁶ and micro-structure²⁷ to enhance the grasping performance or the grasping range. Through the use of elastic polymers, these grippers excel in delicately handling fragile objects and navigating intricate surfaces. Pneumatic-driven methods play a vital role in activating these soft grippers, boasting traits such as lightweight design, impressive efficiency, minimal environmental impact, and adaptability to diverse environments.

While prior integration of grasping and suction in soft robotics has enhanced gripping by augmenting attaching force and range between objects and fingers, the limitations of large workspace requirements for finger grasping still exist. Achieving both jamming grasping and suction on a single robot tip like a traditional suction gripper remains a critical pursuit in this field.²⁸ One example of the mentioned gripper is universal vacuum gripper (UVG).²⁹ It combines jamming and suction, which allows the gripper to achieve cross-scale grasping. However, since the jamming area and suction area of UVG are both distributed on the grasping area, each of the grasping methods cannot 100% utilize the grasping area. As the jamming area is distributed in the skirt of the main plate, it is hard to apply jamming grasping in a narrow space.

Drawing inspiration from nature’s adaptive capabilities observed in amphibians and insects, soft robotics delves into the concept of transformation.³⁰ The idea of a universal gripper, positioned between traditional grippers and suction cups, holds tremendous promise in achieving transformative grasping and suction capabilities.³¹ The transformative potential is realized through the gripper’s infill material, which, when in place, acts as a jamming gripper for handling smaller objects and transforms into a flat surface suitable for suction cup functionality when extracted. Both grasping modes can fully use the grasping area independently. Therefore, this innovative approach effectively integrates both grasping and suction capabilities into a single robotic gripper, introducing a transformative potential that we refer to as the transformable soft gripper (TSG).

Our proposed TSG embodies a versatile capacity to seamlessly transition between suction and jamming grasping functionalities, ensuring reliable performance even in amphibious scenarios. This transformative capability enables the TSG to adeptly handle a broader spectrum of object sizes, ranging from 2 mm to over 200 mm, surpassing the capabilities of conventional suction and caging grippers (Fig. 1). A pneumatic system governs the attachment and detachment processes even in narrow space, granting precise control. Moreover, the TSG’s streamlined design enables operation underwater, significantly broadening its potential applications.

FIG. 1.

Comparison of gripper size and corresponding graspable object range. Gripper sizes indicated by dots labeled “D,” while the range between arrows denotes the spectrum of graspable object sizes. Numbers in brackets refer to their respective reference numbers.

Experimental measurements of the maximum grasping force across different morphing forms validate its efficiency. The main contributions of this innovation can be summarized as follows: 1.

Proposed a novel soft gripper (TSG) enabling transformation between caging grasping and suction for amphibious cross-scale objects grasping and be able to achieve grasping in narrow space.

Established analytical models to predict the performance of the gripper in caging grasping and suction modes, facilitating task-based design guidance.

Outlined a comprehensive fabrication methodology for fully customizable and scalable TSG realization.

Materials and Methods

Gripper design and working principle

To achieve amphibious operation with both grasping and suction forms, the TSG comprises a central chamber filled with coffee grounds and two pear-shaped side chambers designed to facilitate the jamming mechanism. In addition, it incorporates a suction cup sealed with a silicone contact membrane to create a vacuum on the object’s surface, preventing medium flows inward (Fig. 2A).

FIG. 2.

Design and working principle of TSG. (A) Components of TSG. (B) Caging grasping process with TSG caging form can transform to suction form when the pressure of the middle chamber changes to P < 0. (C) Adhesive grasping process with TSG suction form can transform to caging form when the pressure of the middle chamber changes to P > 0. (D) Highlighted features of TSG.

The transformation between the caging grasping and suction forms relies on the deformation of the contact membrane, a transformation regulated by the input pressure directed into the middle chamber. By pressurizing the middle chamber with positive pressure, the contact membrane will protrude outward, causing the coffee grounds in the middle chamber to fall into the bulging membrane cavity, thus, transforming into the caging gripper form (Fig. 2B). After the protruded membrane approaches and deforms around the object in a soft state, the pressure of side chambers is increased to prevent the coffee grounds from returning to the middle chamber by their deformations. Then, the pressure of middle chamber will switch to negative, making the membrane cavity much stiffer. In this way, the object will be caging grasped in the membrane cavity. The advantage of this caging grasping form lies in its ability to securely grasp objects of nearly any surface shape, including those smaller in size than the contact membrane. For small objects, this grasping form can envelop the object akin to phagocytosis, ensuring robust grasping capability.

Benefiting from the transformation capability, when confronted with grasping large objects that exceed the size of the contact membrane, the TSG can seamlessly transition into a suction form (Fig. 2C) by implementing negative pressure in both the middle and side chambers. Consequently, the contact membrane deforms toward the suction cup while unblocking the side chambers, enabling the grounds to retract into the middle chamber. Afterward, the side chambers are pressurized and block the grounds again. After emptying the suction cup cavity, the contact membrane protrudes outward with 2 kPa to expel any medium between contact surfaces. With attachment to the objective surface, the input pressure to middle chamber will be adjusted to negative, and the attachment force due to vacuum between the contact membrane and the object surface is created.

Taking advantage of the transformation between caging form and suction form, TSG can grasp small objects such as electronic components and large objects such as logistics boxes. In addition, with the contact membrane sealing the opening of suction cup, TSG can be used amphibiously. The attachment and detachment can be also controlled by adjusting the pressure of the middle chamber (Fig. 2D).

Fabrication process of the TSG

The fabrication of TSG is divided into three steps as shown in Figure 3. In step 1, we 3D print a set of Mold A, where Mold A_Btm forms the bottom cavity of the mold, which constitutes the bottom part of the TSG core. It also allows for the insertion of a premade suction cup to be combined with other parts of the TSG core. Mold A_Left and Mold A_Right form the side chamber and outer wall of the TSG core. Mold A_Top is used to align the position of Mold A_Left and Mold A_Right and provide space for degassing. After the Mold A assembly, silicone gel (Ecoflex™ 0030, Smooth-On) is poured from the top opening of Mold A_Top. Poured Mold A is then placed in a vacuum box for degassing. After 10 mins of degassing, the mold is placed in atmospheric for curing. In step 2, we flip over the cured TSG core and place it on Mold B with silicone gel and cast the cap for TSG Core. In step 3, we use computer numerical control machining (CNC) to manufacture the Mold C with a pattern and cast it with silicone gel. After curing, it forms the contact membrane. Finally, the contact membrane is adhered to the opening of the suction cup, and the TSG core is adhered to a Polylactic Acid (PLA)-printed shell with silicone adhesive (Sil-Poxy™, Smooth-On), and we can get the completed TSG after curing.

FIG. 3.

Fabrication process of the TSG.

Pneumatic control system

In order to achieve attachment and detachment for caging and suction form, and transformation between those two forms for TSG, an electric-pneumatic pressure control system is developed. The system is shown in Figure 4A, it consists of an MCU (Arduino, UNO board), a 4-channel relay module (TONGLING, JQC-3FF-S-Z), two pressure sensors (CFsensor, XGZP6847A), two air pumps (Fspump, 370B), four 2/3-way electric valves (Fspump, 0520F), and the TSG.

FIG. 4.

(A) The electric-pneumatic system for TSG. (B) Schematic of the TSG pneumatic system, the numbers on the 2/3-way electric valves symbol representing the ports on the valves. (C) An inflated side chamber can block the way between CC and MC. (D) A deflated side chamber can open the way between CC and MC. CC, suction cup cavity; MC, middle chamber cavity.

For the electric part, we use Arduino UNO to control the “on/off” statuses of the four valves via the relay module, the pressure data of all chambers of TSG are measured by the pressure sensor and recorded by Arduino UNO for further control with our model.

For the pneumatic part (Fig. 4B), two sets of pneumatic components control the pressure of the side chambers and middle chamber individually. In the side chamber control set, the outlet of $Pum p_{A}$ is connected to the port 2 of $Valv e_{1}$ .When $Valv e_{1}$ is in “on” status and $Valv e_{2}$ is in “off” status, the port 2 and port 1 of $Valv e_{1}$ and the port 2 and port 3 of $Valv e_{2}$ are connected. Hence, $Pum p_{A}$ can pump air from the surrounding and transmit positive pressure to the side chamber, therefore, causing the side chamber to inflate toward the middle chamber (Fig. 4C) and preventing grounds from passing through the middle chamber.

When $Valv e_{1}$ is in “off” status and $Valv e_{2}$ is in “on” status, the port 2 and port 3 of $Valv e_{1}$ and the port 2 and port 1 of $Valv e_{2}$ are connected. Hence, $Pum p_{A}$ can pump air to the surrounding and transmit negative pressure to the side chamber, therefore, causing the side chamber to deflate (Fig. 4D) and allowing grounds to pass through the middle chamber. The middle chamber control has the same setup, therefore, when $Valv e_{3}$ is in “on” status and $Valv e_{2}$ is in “off” status, resulting in the inflation of the contact membrane, which can create a large suction cup cavity (CC), allowing coffee grounds to drop from the middle chamber cavity (MC). When $Valv e_{3}$ is in “off” status and $Valv e_{2}$ is in “on” status, resulting in the deflation of the contact membrane, which can push the coffee grounds back to MC. Thus, by controlling the valves’ statuses, we can control the inflation and deflation of the side chamber and middle chamber and the position of coffee grounds. Following the working principle, we can achieve attachment and detachment for caging and suction form and transformation between those two forms for TSG as shown in Table 1.

Table 1.

Valves Control Table for Realizing Different TSG Status

Valve status				Chambers status			TSG status
$Valv e_{1}$	$Valv e_{2}$	$Valv e_{3}$	$Valv e_{4}$	Side chamber	Middle chamber	Grounds position	TSG status
On	Off	Off	On	Inflated	Deflated	CC	Caging form: attachment
On	Off	On	Off	Inflated	Inflated	CC	Caging form: detachment
Off	On	Off	On	Deflated	Deflated	CC to MC	Caging to suction transform
Off	On	On	Off	Deflated	Inflated	MC to CC	Suction to caging transform
On	Off	Off	On	Inflated	Deflated	MC	Suction form: attachment
On	Off	On	Off	Inflated	Inflated	MC	Suction form: detachment

CC, suction cup cavity; MC, middle chamber cavity.

Modeling

An accurate and comprehensive model plays a crucial role in studying the characteristics of TSG. As shown in Table 1, the TSG has three kinds of statuses: transformation status, caging form status, and suction form status; we proposed a set of models for each of them.

Characteristics of the transformation status

The model for transformation status focuses on analyzing the deformation of the contact membrane against different input pressures. As illustrated in Figure 5A, the undeformed contact membrane is a cylindrical silicone membrane that has an initial diameter of $2 L_{o}$ and an initial thickness of H. By using a polar coordinate system, any arbitrary point $Q$ on the undeformed membrane can be expressed as $(R_{0}, ϕ, Z)$ . This point will move to the position $(r, ϕ, z)$ after the deformation of the membrane, where $ϕ = ϕ$ . Considering the geometric symmetry, the deformation can be assumed to be axisymmetric about $r$ -axis. The deformed membrane can be approximated as a partial sphere with a circular cross-section as shown in Figure 5B The principal stretches $λ_{i}$ are defined in axial, circumferential, and radial directions, which are indexed by Equations (1), (2), and (3), respectively: $λ_{1} = λ_{2} = λ = \frac{s}{2 L_{o}}, λ_{3} = \frac{h}{H},$ (1)

FIG. 5.

Schematic of the parameters’ presentation. (A) Coordinate setting of the TSG contact membrane. (B) Geometry parameters of the membrane bulge on the longitudinal section of the TSG. (C) Principal stretches λ, surface tensions T, internal pressure P, and Cauchy stresses acting on the unit volume at point Q₀. (D) Cross-section of the TSG in caging form with grasped object. (E) Cross-section of the TSG in suction form. The r is for the height of the membrane convex and z stands for position along the deformed membrane profile.

where $r_{0}$ is the inflated height and $s$ is the arc length of the deformed membrane; $h$ is the average thickness of the deformed membrane. As shown in Figure 5B, the distance between the arc center and the undeformed membrane is $u$ , the cross-section of the deformed contact membrane is a circular arc with radius $v$ , the circumferential angle of the deformed membrane is 2φ. Hence, we have: $φ = π - 2 t a n^{- 1} (\frac{L_{o}}{r_{0}}),$ (2) $u = \frac{L_{o}}{- \tan φ},$ (3) $v = \frac{L_{o}}{\sin φ},$ (4) $s = \frac{2 L_{o}}{\sin φ} φ .$ (5)

According to the theory of nonlinear elastic membranes presented by Green and Adkin,³² as the deformed configuration is axially symmetrical, the equilibrium of deformation at unit volume element can be formulated as: $\frac{d}{d s} (T r) = T \frac{d r}{d s},$ (6) $2 k T = P,$ (7)where $T$ is the stress per unit length in axial direction and circumferential directions, $P$ is the pressure difference between the outer and inner surfaces of the membrane. In the case of free inflation, $P = P_{a} - P_{atm},$ where $P_{a}$ is the pressure inside the chamber and $P_{atm}$ is the atmosphere pressure. In Equation (7) $k$ is the axial and circumferential curvatures at point $Q_{0} (r_{0}, 0, 0)$ on the deformed membrane. They can be defined as: $k = v^{- 1}$ (8)

It is supposed that $σ_{1}$ , $σ_{2}$ , and $σ_{3}$ are the principal Cauchy stresses of the unit volume element along axial, circumferential, and radial direction, respectively, as shown in Figure 5B. According to the membrane theory,³³ the assumptions $σ_{3} ≪ σ_{1}$ and $σ_{3} ≪ σ_{2}$ are generally accepted for a thin membrane. In addition, the weight of the inflating gas is ignored. Therefore, we have $T = h (σ_{1} - σ_{3}) = h (σ_{2} - σ_{3})$ (9)

In order to establish the stress–strain relationship for the deformable membrane, a constitutive equation for the silicone material is firstly derived by using the strain energy density function, $W (λ_{1}, λ_{2,} λ_{3})$ and by adopting the 3rd-order Yeoh model³⁰ with the material propriety of Ecoflex-0030: $W (λ_{1}, λ_{2,} λ_{3}) = \sum_{i}^{3} C_{i} {({λ_{1}}^{2} + {λ_{2}}^{2} + {λ_{3}}^{2} - 3)}^{i}$ (10)

Assuming the material is incompressible, we have $λ_{1} λ_{2} λ_{3} = 1$ . The strain energy density $W$ can be formulated as a function of $λ$ . Hence, the strain energy density function $W (λ)$ can be rewritten as: $w (λ) = \sum_{i}^{3} C_{i} {(2 λ^{2} + λ^{- 2} - 3)}^{i}$ (11)where $C_{i}$ are the material constants. Furthermore, based on Cauchy’s first law of motion, the Cauchy stresses can be used to define the deformation of the membrane by taking the partial derivatives of $w (λ)$ with respect to $λ$ . It yields: $σ_{1} - σ_{3} = λ \frac{\partial w}{\partial λ}$ (12)

Therefore, based on the circular arc assumption, Equations (9) and (12), the relationship between the inflated pressure and the principal stretches of the membrane in free space can be derived as follows: $P = 2 k (\frac{H}{λ}) \frac{d w}{d λ}$ (13)where $\frac{d w}{d λ} = \sum_{i}^{3} i C_{i} (2 λ - 2 λ^{- 5}) {(2 λ^{2} + λ^{- 2} - 3)}^{i - 1} .$

Characteristics of the caging status

The model for caging status focuses on analyzing the trend of the caging grasping force against objects of different sizes. For small targets (with a radius smaller than 20 mm), when the TSG inflates to the size of the target, the robotic arm will move the TSG above the target and cover half of its height. At this point, the pneumatic system will adjust the side chamber to positive pressure and the center chamber to negative pressure. In this situation, the coffee grounds will be held by the side chamber, and only gas can pass through the center chamber. The TSG will use the shape-adaptive feature of the soft membrane to form a high-stiffness hard shell around the target, thus achieving the purpose of grasping the object. With the deformation response of the inflated membrane to the pressure described in the previous section, we can estimate the maximum grasping force. This method utilizes hardened coffee bean clumps to provide support for the surrounded object. However, at the same time, because of the deformable membrane’s own resilience, it provides a certain amount of pushing force to the surrounded object. Therefore, we can use the formula for transformation status to estimate approximately at which air pressure the membrane’s tension changes the least, to find the most suitable pressure for expansion.

When the gripper covers the object and degas, the interaction between the deformed contact membrane and the object can be explained by two contacting forces: friction around the object and tension against the object by the deformed membrane, which can be expressed mathematically as follows: $F_{caging} = f_{s} - f_{t},$ (14)where $F_{caging}$ is the caging force of the TSG. Moreover, $f_{s}$ and $f_{t}$ denote the shear force and the surface tension of the contact membrane, respectively.

The shear force $f_{s}$ is caused by the disengaging from the contact membrane. The shear force is related to the contact area $A_{c}$ of the contact membrane and the object, and the shear strength of the membrane material: $f_{s} = τ A_{c}$ (15)

The tension $f_{t}$ is caused by the deformation from the contact membrane. According to Equation (7) the tension $T$ of contact membrane in free inflation status is related to the internal pressure and the curvature of the contact membrane. In this caging case, it is complex to analyze the exact tension of the contact membrane, but it is proportional to the free inflation tension $T$ of transformation status in Equation (7) and the tension caused by the deformation due to the caged volume of the grasping object $T_{v}$ . $f_{t} \propto T + T_{v}$ (16)

From Equations (7) and (16), we can know that higher initial pressure can cause higher $f_{t}$ , which means lower caging force. From Equation (13), we know the volume of CC is proportional to the initial pressure $P$ . Hence, there should be an optimal initial pressure for the biggest CC size and greatest caging force.

Characteristics of the suction form status

The model for suction status focuses on analyzing the trend of the suction grasping force against different input pressure. When negative pressure is applied to the middle chamber, a conical vacuum space is created between the contact membrane and the contact surface, which generates suction. Since the deformation of the contact membrane is relatively small, the deflection of the silicone can be treated as a linear deformation. Hence, we use any other set of parameters to simplify the deformation model. The deflection³¹ caused by the pressure difference between the pressure in the conical vacuum space $P_{s}$ and the pressure of the middle chamber $P_{i}$ of the contact membrane can be expressed by the formula: $r (z) = \frac{3 (1 - {v_{p}}^{2})}{16 E t^{2}} (P_{s} - P_{i}) {(z^{2} - {L_{0}}^{2})}^{2},$ (17)where E, $v_{p}$ , t, $L_{0}$ , and z represent Young’s modulus, Poisson’s ratio, thickness and radius of the contact membrane, and the distance from the center of membrane (Fig. 5E), respectively. Therefore, the volume of closed space $V_{s}$ is expressed by: $V_{s} = V_{s 0} + \int_{0}^{2 π} \int_{0}^{L_{0}} r (z) dzd ϕ .$ (18)

Here, $V_{s 0}$ is the initial volume of at resting status, which is thin cylinder with a height of $h_{0}$ . $V_{s 0} = π {L_{0}}^{2} h_{0} .$ (19)

By Boyle–Charle’s Law: $P_{0} V_{s 0} = P_{s} V_{s} .$ (20)

Substituting Equations (18) and (20) we can express the vacuum space pressure $P_{s}$ by: $P_{s} = \frac{P_{0} V_{s 0}}{V_{s}} .$ (21)

The suction force can then be indicated by: $F_{s} = P_{s} A_{s} .$ (22)

As $A_{s}$ is the area of the suction cup, which is a constant, combining Equations (17 )–(22), we can say the suction force $F_{s}$ is positively correlated to the pressure in the middle chamber $P_{i}$ .

Result

Experiment 1: validation of transformation status model

To validate the deformation model of the transformation status, a contact membrane with a radius of 20 mm and a thickness of 1.15 mm of the TSG is being cast using EcoFlex-0030 and it is glued to a suction cup. A syringe pump (370-B, fspump Ltd) is used to inflate or deflate the contact membrane and a pressure sensor (XGZP6847, CFsensor Ltd) is used to measure the pressure inside the middle chamber. The inflated height of the contact membrane is measured by an optical tracking system (Supplementary Data S1). Pressure data are recorded when the inflated height of the anchoring section increases by 1 mm. In the simulation, coefficients in the elastomer model³⁴ of EcoFlex-0030 are selected to be $C_{1} = 17 k P a$ , $C_{2} = - 0.2 k P a$ , and $C_{3} = 0.023 k P a$ .

The simulated model and real data are shown in Figure 6B. We can see that the model can predict the real inflation well, and that the inflation into a half-spherical shape of soft silicone can be divided into three stages (Fig. 6A). The first stage (green highlight in Fig. 6B) occurs when the pressure difference between the inside and outside of the membrane is from 0 to 7 kPa. At this stage, the volume of the caging cavity grows with the pressure difference almost linearly. The second stage starts from 7 to 9 kPa (blue highlight in Fig. 6B). The pressure nearly not change with the caging cavity growth, the most pressure-stable area is around 8 kPa. The third stage occurs after 9 kPa (red highlight in Fig. 6B), the pressure significantly increases with the growth of caging cavity.

FIG. 6.

Experiment of the inflation of the contact membrane. (A) Image of the inflated membrane with different pressure. (B) Simulation and experimental validation of the contact membrane inflation model.

As the inflation model can well predict the deformation of contact membrane, from Equation (7) we know the surface tension of the contact membrane is proportional to the pressure difference. During caging form, contact membrane should be able to wrap the target as much as possible with minimal surface tension. Therefore, this model may help us find the initial gas pressure in the second stage, thereby providing the highest grasping force under the caging form.

Experiment 2: validation of caging form status model

According to the results of experiment 1, at an initial pressure of 8 kPa, the contact membrane reaches its maximum capacity to accommodate the largest volume, resulting in the maximum contact area with the object while having the smallest surface tension. Therefore, based on Equations (15) and (16), the optimal grasping pressure for using caging form can be predicted as 8 kPa. To verify this idea, we 3D printed a series of test balls with different diameters (2, 4, 6, 8, 10, and 12 mm). Each ball is connected to a fixture fixed on a force sensor. In this experiment (Fig. 7A and B), the TSG is pre-pressurized to different pressure levels and then moved down from above the target object until the contact membrane completely covers the object, at which point the vacuum is activated, completing the grasping process. Once the pressure stabilizes, the linear motor pulls up the TSG. Since the target object is fixed on the force sensor, the maximum grasping force provided by the TSG at this initial pressure can be measured.

FIG. 7.

Experiment for investing the relation between grasping force of the caging form and object size. (A) Experimental setup. (B) Illustration of the experiment. (C) Experimental result of the caging force with different object sizes and initial pressure.

Figure 7C demonstrates the results of the experiment. Based on the results, the optimal grasping pressure of the TSG aligns well with our predicted results. From the perspective of grasping the same object size with different initial pressures, initial pressure with 8 kPa has the best performance, since it can provide the largest shear force $f_{s}$ with the smallest surface tension $T$ . From the perspective of grasping different object sizes with same initial pressure, as the contact area of objects growing with the object size, resulting in a higher shearing force $f_{s}$ for grasping, the grasping force also grows with the object size. However, as the size of an object goes over a certain diameter, 12 mm in our case, the size effect of casing surface tension $T_{v}$ is higher than the effect of shearing, overall resulting in a performance drop for the caging grasping.

Experiment 3: validation of suction form status model

Unlike ordinary suction cups, TSG suction cups do not achieve adhesion by directly sealing the periphery of the suction cup and then using negative pressure. Instead, our suction cups first ensure that the medium (such as air or water) within the suction cup area is mostly evacuated through the inflated contact membrane. Then, negative pressure is applied to the other end of the contact membrane, creating a maximum vacuum between the suction cup and the contact surface. This ensures that the suction cup can be used with any gas or liquid. To verify the relationship between negative pressure and adhesion force within the TSG, we designed a simple testing platform. Similar to experiment 2 (Fig. 8A and B), we installed the TSG on a linear motor, and a pressure sensor monitored real-time pressure changes in the TSG middle chamber. Below the TSG, there is an acrylic plate connected to a force sensor. During the experiment, the TSG is first inflated to 2 kPa, and then it descends to firmly contact the acrylic plate. Subsequently, negative pressure is applied to initiate adhesion in the middle chamber. Once the internal pressure reaches the preset value (ranging from 0 to −20 kPa with an increment of −2.5 kPa), the motor starts to ascend, and the force sensor records the maximum adhesion force before the suction cup detaches from the acrylic plate.

FIG. 8.

Experiment for investing the relation between the grasping force of the suction form and suction pressure. (A) Experimental setup. (B) Illustration of the experiment. (C) Experimental result of the suction force with different suction pressures.

Figure 8C demonstrates the results of this experiment. The result shows the suction form of TSG can generate adhesive force up to 4.85 N by applying −20 kPa to the middle chamber. Compared with the analytical data, TSG can generate adhesive force (around 0.5 N) even if there is no negative pressure applied to the middle chamber. This is because the soft suction cup will be deflected even if the contact membrane is not deformed by negative pressure once the suction cup is sealed. It also leads to less force output with the same suction pressure in theory since the deflection of the suction cup will cause less vacuum area.

Demo 1: Grasping cross-scale objects amphibiously

During tests, the TSG shown in Figure 4A is installed on a robot arm as an end effector. For grasping objects whose shortest length is smaller than 20 mm, we use the caging form of the TSG and suction form for grasping objects whose shortest length is larger than 20 mm. Figure 9 shows some representative results of grasping tests. The grasping performance of TSG is demonstrated by grasping selected daily objects, we can observe TSG successfully grasped various daily objects with not only spherical and flat shapes but also complex and deformable shapes, ranging from millimeter-sized green beans, medication pills, electronic components, to centimeter-sized apples, medicine boxes, and plastic sheets with 100% successful rate in the provided demonstration videos (Supplementary Videos S1 and S2).

FIG. 9.

Some representative results of the grasping tests: The caging form of TSG can grasp items with both primitive shapes and complex shapes of different sizes while the suction form can grasp large items with flat surfaces or small curvature surfaces (completed amphibious grasping test can be found in Supplementary Videos S1, S2, S3, and S4).

Furthermore, in underwater conditions, the grasping effectiveness of the TSG is still reliable. We present the underwater grasping with two grasping forms (Supplementary Videos S3 and S4), the TSG still functions properly as the contact membrane seals the TSG as a close-form system. Thus, compared with traditional suction cups, the TSG is more reliable and endurable in a complex working environment for cross-scale objects.

A limitation of TSG was found during the test. As the TSG caging form can handle different shapes of objects with a shortest length that is smaller than 20 mm, the suction form of TSG is limited to only handling large objects with flat or small curvature surfaces. This is due to the nature of suction grasping, surfaces with significant curvature, the cup cannot form a complete seal, leading to air leakage and a loss of suction.

Demo 2: Unbox and grasp cross-scale objects amphibiously

In industrial production or underwater object retrieval, actuators often face numerous challenges, such as the ability to operate both on the water surface and underwater, or the need to remove larger debris before picking up smaller objects. In this demonstration (Fig. 10A–F), we utilized a robotic arm installed with a TSG to open an underwater acrylic box using TSG’s suction form. With the same grasping form, the gripper picks up a pear with diameter of 86 mm in a box with side length of 95 mm. After detaching the pear, the TSG is transformed into caging form and picks up a 2.4 mm diameter green bean in the box. This demonstration showcases the robustness and amphibious capabilities of our TSG, enabling it to overcome buoyancy differences between underwater and in air, while also demonstrating excellent tolerance for grasping various object sizes. (complete demonstration can be found in Supplementary Video S5)

FIG. 10.

Three demonstrations showing the capabilities of TSG. (A-1) Experimental setup of cross-scale objects amphibiously unboxing and grasping; (A-2) TSG used suction form to open a box in the water; (A-3) TSG detach the box cover. (A-4) TSG used suction form to pick a big pear; (A-5) TSG used caging form to pick a 2.4mm green bean; (A-6) TSG detached items with different scales. (B-1) Experimental setup of the card manipulation; (B-2) Suction form cannot insert the care; (B-3) TSG re-positioning the card. (B-4) TSG transforming to caging form; (B-5) The card is picked up by caging form; (B-6) The card successfully inserted by caging form; (C-1) Experiment of grasping in narrow space; (C-2) TSG can barely enter the beaker and reach the bean with caging form; (C-3) The small green bean is picked out; (C-4) TSG can also reach the plastic sheet at the bottom of the beaker and attach the sheet with suction form; (C-5) The plastic sheet is picked out.

Demo 3: Card manipulation

Since the outbreak of the pandemic, an increasing number of robots have been deployed in the service industry. When robots are performing services, they often need to access restricted areas through an IC card. While wireless sensing access control systems only require the robotic arm to bring the card close, how can we deal with plug-in access controls? In this demonstration (Fig.10.B-1–6), we employed a single robotic arm operating the TSG with a suction form to first attach the card and attempt to insert it into the card slot. However, the suction form itself occupies a sizable portion of the card’s surface, hindering its entry and preventing the robotic arm from inserting the card using this approach. Therefore, the robotic arm first positioned the card vertically to the ground and then switched the TSG to the caging form. This allowed the robotic arm to grasp the card with a minimal contact area, resembling the combination of a human’s index finger and thumb, and push the card into the card slot. This demo showcases TSG as a transformable soft robot with strong adaptability to meet specific requirements. It provides significant possibilities for simplifying end-effector designs (complete demonstration can be found in Supplementary Video S6).

Demo 4: Grasping in narrow space

The common advantage of caging and suction grasping is their minimal operation space. Compared with finger grasping, caging, and suction only need vertical space above the contact surface to perform grasping, not horizontal space is needed. To demonstrate this advantage, we set up a beaker with 30 mm diameter, and put a 25 × 25 mm plastic sheet on the bottom and a 2.4 × 2 mm green bean on the top (Fig. 10.C-1–5). TSG can reach into the inner side of this narrow space easily and perform both caging and suction to pick out the bean and plastic sheet in order.

Conclusion and Discussion

In this article, we introduced a pioneering TSG that integrates both grasping and suction functionalities within a compact structure. The TSG exhibits remarkable effectiveness in grasping amphibious cross-scale objects by seamlessly transitioning between distinct grasping forms. Utilizing the analytical model developed, we meticulously regulate the desired contact pressure for these different grasping modes. Our comprehensive experimental investigations underscore the practical viability of transformable soft robotics. By harnessing the adaptability and extensive deformability inherent in flexible materials, we have engineered transformable structures that respond intuitively to real-world requirements.

The TSG serves as an exemplary illustration of addressing real-world needs, particularly in cross-scale object manipulation. Conventional rigid or flexible structures would typically necessitate employing different end effectors—such as employing a jamming gripper for small objects and a suction cup for larger ones—resulting in additional adaptors and a complex, multistep process of end effector switching. The TSG, through its transformational design, capitalizes on material deformations to adeptly fulfill diverse grasping demands, merging advantages from both transformational forms. This innovative approach significantly streamlines device volume and assembly intricacies.

Further enhancements for the TSG are envisaged, including integrating a microfiber structure onto the contact membrane surface to notably augment the maximum gripping force in both forms. Moreover, given the bio-compatible silicone construction of the contact membrane, the TSG exhibits food-grade capabilities, resistant to extreme temperatures, acidity, and alkalinity. Overall, this article endeavors to introduce and illuminate the transformative potential of soft robotics design paradigms through the TSG concept.

Footnotes

Acknowledgments

The authors thank Mr. Xuchen Wang and Mr. Chen Pu for their help in the experiments.

Authors’ Contributions

T.P.: Conceptualization, investigation (equal), methodology (lead), project administration (lead), software (lead), visualization (equal), writing—original draft (lead), and writing—review and editing (supporting). J.Z.: Writing—original draft (supporting), and writing—review and editing (lead). Z.Z.: Investigation (equal), software (supporting), validation (equal), and writing—review and editing (supporting). H.Z.: Validation (equal). J.H.: Validation (equal). J.A.: Validation (equal). Y.L.: Funding acquisition (supporting), supervision (supporting), and writing—review and editing (supporting). X.M.: Funding acquisition (lead), project administration (supporting), resources (lead), supervision (lead), visualization (supporting), and writing—review and editing (supporting).

Author Disclosure Statement

No competing financial interests exist.

Funding Information

This work was supported in part by the National Natural Science Foundation of China under grant no. 52205032, in part by Guangdong Basic and Applied Basic Research Foundation under grant no. 2023A1515010062. Research reported in this work was supported in part by Research Grants Council (RGC) of Hong Kong (grant no. CUHK 14204423).

Supplementary Material

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Supplementary Material

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