Unlocking Versatility: Magnetic-Actuated Deployable Suction Gripper for Complex Surface Handling

Abstract

Suction grippers offer a distinct advantage in their ability to handle a wide range of items. However, attaching these grippers to irregular and rough surfaces presents an ongoing challenge. To address this obstacle, this study explores the integration of magnetic intelligence into a soft suction gripper design, enabling fast external magnetic actuation of the attachment process. Additionally, miniaturization options are enhanced by implementing a compliant deploying mechanism. The resulting design is the first-of-its-kind magnetically-actuated deployable suction gripper featuring a thin magnetic membrane (Ø 50 mm) composed of carbonyl iron particles embedded in a silicone matrix. This membrane is supported by a frame made of superelastic nitinol wires that facilitate deployment. During experiments, the proof-of-principle prototype demonstrates successful attachment on a diverse range of curved surfaces in both dry and wet environments. The gripper achieves attachment on curved surfaces with radii of 50–75 mm, exerting a maximum attachment force of 2.89 ± 0.54 N. The current gripper design achieves a folding percentage of 75%, enabling it to fit into a Ø 12.5 mm tube and access hard-to-reach areas while maintaining sufficient surface area for attachment forces. The proposed prototype serves as a foundational steppingstone for further research in the development of reliable and effective magnetically-actuated suction grippers across various configurations. By addressing the limitations of attachment to irregular surfaces and exploring possibilities for miniaturization and precise control, this study opens new avenues for the practical application of suction grippers in diverse industries and scenarios.

Introduction

Challenges in conventional suction gripping

Attachment to rough, wet, and irregular surfaces and shapes poses a challenge across a wide range of applications spanning the medical field, robotic manipulation, and industrial sectors. Suction grippers have emerged as prevalent tools for grasping and manipulating diverse objects and surfaces, including large sheet materials such as glass and metal, as well as various types of packaging boxes. In comparison to mechanical grippers, suction grippers offer the advantage of accommodating different object types and sizes, unconstrained by gripper dimensions. Moreover, suction grippers enable the secure handling of delicate objects without compromising their structural integrity, a crucial aspect in industries like the food sector.

The primary obstacle faced by conventional suction grippers lies in their inability to adhere to surfaces that are rough, irregularly shaped, and porous. Establishing an airtight seal between the suction disc and these surfaces proves challenging, as even the slightest gap can lead to leakage channels, resulting in the gradual reduction of the pressure difference between the disc and the surrounding environment.¹ Sandoval et al.² showed that the adhesion of a commercially available suction cup drastically drops with increasing surface convexity and concavity. To enable the adaptation of suction grippers in applications that would greatly benefit from this gripping mechanism, ongoing research focuses on developing suction grippers that can effectively adhere to rough, irregularly shaped, and soft surfaces.

Despite progress in attaching to rough and irregular-shaped surfaces by researchers such as Ditsche et al.,³ Sandoval et al.,² and Koivikko et al.,⁴ significant challenges remain. These challenges include the miniaturization of suction grippers, precise control over gripper actuation, reducing energy consumption, and the development of universal grippers for diverse surfaces and shapes.⁵ The miniaturization of suction grippers is hindered by the inherent relationship between attachment forces and the size of the vacuum chamber.⁵ Fine control over gripper actuation is crucial for delicate objects and versatile applications, especially in fields like minimally invasive surgery.⁶ The use of smart soft materials responsive to magnetic fields holds promise for advancing these developments.⁷

Opportunities for advanced magnetic materials

Adding magnetic intelligence to suction grippers enables external control of its attachment using a magnetic field instead of a vacuum pump. This magnetic actuation offers precise control over the attachment’s intensity and direction. Furthermore, multiple suction discs can be managed individually without the need for complex tubing systems or multiple vacuum pumps. Additionally, magnetic fields possess the ability to travel through various mediums like air, human tissue, and water, opening interesting external operation possibilities. Remote control of the gripper becomes achievable, improving system miniaturization by eliminating the need for a direct connection between the actuation mechanism and the gripper. Finally, magnetic interaction is fast and reversible, enabling swift attachment and detachment of the suction gripper.

Few and limited implementations of magnetically actuated suction grippers have been reported in literature. Zhang et al.⁸ presented a suction disc using a magneto-rheological elastomer and a permanent magnet for wall-climbing robots. In a subsequent study, a softer actuation system with a silicone edge and electro-permanent magnet was introduced.⁹ Koivikko et al.¹⁰ developed a suction gripper capable of switching between soft and hard modes using magneto-rheological fluid to improve structural integrity. Iwasaki et al.¹¹ designed an elastomeric suction disc controlled by a magnetic field for medical treatments, and Hu et al.¹² proposed a magnetic suction disc with a deformable magnetic polymer membrane. However, attachment of most of these magnetic suction cups was limited to smooth surfaces.

Goal

This study aims to develop a suction gripper utilizing advanced magnetic materials for actuation by an external magnetic field. The objective is to design a versatile gripper that can attach to irregularly-shaped objects by incorporating magnetic responsiveness into a soft matrix to enhance the sealing capability. To allow for use in tight spaces and improve the load carrying capacity, we propose to explore deploying mechanisms, similar to the one proposed in our previous study.¹³

Magnetic Suction Gripper Design

Design requirements

To ensure attachment to a wide range of complex-shaped objects, this study should present a suction gripper design that accommodates surfaces with single and double curvatures, and varying radii (Fig. 1). The gripper design should incorporate the basic components such as the gripper body, rim (outer edge of the gripper), and footprint (bottom part of the gripper in contact with the substrate) while integrating soft and rigid materials. Soft materials facilitate adaptation to non-flat surfaces, while rigid components maintain structural integrity and provide the necessary pressure for a secure seal. Furthermore, sufficient friction at the gripper-object interface is vital for successful attachment and lifting. The gripper surface should, therefore, be designed to provide adequate friction, enhancing gripping performance while delaying failure. Safety measures regarding the use of magnetic fields are crucial, therefore, a maximum flux density of 400 mT is set for actuation, which is in accordance with the International Commission on Non-Ionizing Radiation Protection.

FIG. 1.

Complex-shaped object examples. (a) Different curved surfaces, from a flat surface to a single- and double-curved surface. (b) Objects for attachment in different potential use applications, with their corresponding dimensions.

For some applications, the suction gripper is required to lift an object instead of just attaching to a surface. Taking examples from applications in the fruit industry, different fruits and vegetables have different weights, requiring different lifting forces. Most current developments in this industry focus on the harvesting of strawberries, tomatoes, and apples, as they are harvested in large quantities.^14,15 For instance, strawberries typically weigh between 15 and 20 grams, while large tomatoes and apples range between 125–220 grams and 177–235 grams, respectively.^16,17 This would require a suction force of, approximately, 2.3 N. The attachment force of the suction gripper F_s is determined by the pressure difference (ΔP) and the suction surface area (A_s), as shown in Equation 1. Theoretical limits for the attachment force of a suction gripper are, therefore, determined by the maximum suction surface area and maximum vacuum pressure that can safely be applied. Unfortunately, the maximum surface area is often restricted by the entry point into the human body or the available space inside marine or space vehicles. A foldable structure can mitigate these challenges as this would allow the usage in hard-to-reach environments and provide more miniaturization options, for example for usage in the medical field to perform attachment inside the body, or in marine biology, for deep-sea attachment. The smaller the gripper dimensions can be in a folded state, the smaller entrances could be used while maintaining equal suction forces. $F_{s} = Δ P \cdot A_{s}$ (1)

Design overview

The final design of the magnetic suction gripper consists of a magnetic responsive suction mechanism, a deployable frame, and a handle, see Figure 2a–b. The magnetic gripper is connected to a tube and handle for comfortable two-finger grasping, while the thumb controls the position of the permanent magnet attached to a spring-positioned rod. The small permanent magnet actuates the suction gripper. Figure 2c shows the three phases when operating the magnetic suction gripper.

FIG. 2.

Final Design Magnetic Suction Gripper. (a) Exploded view of the developed magnetically actuated suction gripper. (b) Dimensions of the gripper design. The nitinol wires have a diameter of 0.5 mm. The magnet has a diameter of 6 mm and a height of 10 mm. The tube has an inner lumen of 12.5 mm. (c) Operation modes of the magnetic suction gripper, consisting of: (1) Folded state inside a tube. (2) Deployed state upon attachment to a substrate. (3) Attachment state upon magnetic actuation by the permanent magnet (PM) of the shape change of the membrane resulting in attachment forces to the substrate.

Folded state of the magnetic actuated gripper: In this phase, the magnetic gripper is folded in a smaller tube to enable the insertion of the gripper into hard-to-reach places.

Deployed and deactivated magnetic gripper mode: When the gripper arrives at the target location, it is extracted from the tube. Due to the superelasticity of the nitinol wires, the gripper deploys. When there is no magnetic field near the gripper membrane, the magnetic membrane will remain in its flat resting state.

Activated magnetic gripper mode: When a magnetic field is brought in close contact to the magnetic membrane, the membrane is attracted to the magnet, creating a cavity. Due to the negative pressure difference between this cavity and the surrounding environment, the gripper attaches to the substrate. By removing the magnetic field from the proximity of the magnetic membrane, the membrane will return to its original resting state.

Magnetic responsive membrane

An advanced magnetic material, composed of silicone and magnetic particles, was designed to respond to magnetic actuation. The magnetic composite material contains magnetic particles randomly distributed within the silicone matrix. During the curing process, no external magnetic field was present. As a result, the magnetic particles dispersed uniformly in the material, yielding an isotropic magnetic composite that can be attracted by an external magnetic field. This material was employed to create a thin membrane, designed to be attracted to an external magnetic field.

The magnetic actuation force must be capable of overcoming both the suction force in the gripper cavity and the elastic forces in the magnetic membrane. The elastic force of the membrane opposes the magnetic force interacting with the membrane and should be minimized to enhance the effect of the magnetic force. The design challenge lies in finding the right balance between the magnetic particle content and the material’s elasticity, ensuring sufficient deformation of the volume cavity can occur. In the final design, the magnetic membrane is made of silicone-type Dragon Skin (Smooth-on, USA), with a shore hardness of 20 A. This silicone is mixed with 70 wt% carbonyl iron microparticles of 5–9 µm (Sigma–Aldrich, USA) in an isotropic configuration. Research by Hu et al.¹² and Zhang et al.^8,9 have found that a weight ratio of iron powder exceeding 75% causes a loss of elasticity in the material. Although the stretching ratio drops with higher particle content, the material with 70 wt% particles still performed well.¹² The selected thickness for the membrane is approximately 0.5 mm, which provides strong magnetic attraction, sufficient strength to prevent tearing and flexibility to adapt to curved surfaces.

To enable magnetic actuation in the final design, a permanent magnet was used. The shape of the permanent magnet affects the shape and strength of the magnetic field. Therefore, an optimal size and capacity magnet is crucial for a successful functional design. The magnetic force between two nearby surfaces usually depends on the flux density, the magnetic surface area, and the permeability of space. This underscores a crucial point: the size of the magnet imposes constraints on the maximum magnetic force. This limitation arises because the maximum flux density and permeability of space are predetermined. A larger magnet creates a stronger magnetic field but also attracts a larger portion of the magnetic membrane, leaving less membrane attached to the substrate, potentially affecting the seal. A smaller magnet can lift less of the magnetic membrane, but might be more beneficial as it allows for a larger portion of the membrane to contribute to the seal. Furthermore, a smaller magnet improves the ability to attach to objects smaller than the nitinol wire frame diameter, as the inner magnetic membrane can create a seal around the object. The current proof-of-principle prototype, therefore, uses a 6 mm cylindrical Neodymium grade 45 magnet (Supermagnete, Germany). The magnetic membrane has an outer diameter of 50 mm.

Deployable frame

When combining the smart magnetic material in a suction configuration, a gripper body is required. To include opportunities for miniaturization, a deploying frame should be implemented. This frame expands the suction surface when it reaches the target location and provides sufficient pressure to create a seal with irregular surfaces. It should also serve as a base to attach to the magnetic membrane.

Different deploying mechanisms were explored for the frame of the suction gripper, such as joint mechanisms, origami mechanisms, umbrella mechanisms, rotational deploying mechanisms, and compliant mechanisms. Although each mechanism has its advantages, with further miniaturization in mind, it was chosen to adopt pre-bent ultrathin superelastic nitinol wires as a deployable frame. The superelastic nitinol wire frame allows for passive folding and deploying of the suction gripper and provides stability for the magnetic membrane. To suffice the double functionality of the nitinol wires, various designs for the nitinol folding frame were created and evaluated for their level of deployability and ability to exert sufficient pressure onto the magnetic membrane, see Figure 3. It was found that the outer dimensions of the suction disc in folded state were determined by three elements: the nitinol wire frame, the silicone magnetic membrane, and the permanent magnet. To minimize the folded state dimensions, the diameter and quantity of nitinol wires are minimized while still ensuring sufficient stability and pressure on the membrane for a secure seal formation.

FIG. 3.

Iterations of Nitinol Frame Configurations. (a) Single wire structure with a two-step bending sequence. (b) Flower-like structure with too little pressure on the rim. (c) Flower structure with sharper angles and steeper incline, where leakage channels still resulted in failure of attachment. (d) Structure with overlapping wires for pressure on the complete rim. (e) Final design of the nitinol frame.

Leakage of the suction disc is prevented by the formation of a stable rim. By overlapping the nitinol wires and creating a flat ring of these wires on the footprint, it is expected that leakage channels can be minimized so that a pressure difference can be created upon magnetic actuation, see Figure 3d. Using two wires in this configuration creates a stiff, flat frame that is suitable for sealing on flat surfaces but not on irregular shapes. Besides, the two-wire structure was difficult to fold into a smaller tube owing to its stiffness. To address this issue, a structure with three nitinol wires was developed, see Figure 3e. The nitinol wire used in the final design is a super-elastic nitinol wire (55% Ni, 45% Ti) (Titaniumshop, the Netherlands) with a diameter of 0.5 mm, and the heat treatment temperature is set to 500°C. The nitinol wire is encapsulated in silicone and cured onto the magnetic membrane. The silicone membrane protrudes the nitinol frame to improve the sealing of the gripper.

The final prototype is illustrated in Figure 4. The handle and nitinol connector were 3D-printed with a Stereolithography (SLA) printer (Formlabs, Form 3B) in clear resin. The magnet holder with a button was printed in clear polylactic acid (PLA) on a Fused Deposition Modeling (FDM) printer (Ultimaker 3). The outer tube (stainless steel) has an outer diameter of 12.5 mm. In the proof-of-principle prototype, the magnetic membrane has a total diameter of 50 mm, and the nitinol frame rim has a diameter of 25 mm.

FIG. 4.

Final Proof-of-Principle Prototype Magnetic Suction Gripper. (a) Illustration of the components included in the final design. (b) Instrument configuration of the magnetic suction gripper. (c) The instrument gripper lifting of a bell pepper.

Experimental Validation

Attachment performance

Goal of the experiment

To ensure the gripper’s versatility across various applications, it must successfully attach to an array of complex-shaped objects. By combining single and double curvatures, concave or convex surfaces with different radii are formed. Demonstrating the gripper’s adeptness with complex-shaped objects involves showcasing its attachment capabilities to different single and double-curved surfaces separately. Furthermore, to allow for its use in different application fields, the gripper must exhibit attachment capabilities in diverse environmental settings. These settings include airborne scenarios and liquid mediums like water or blood. Showcasing its adaptability in these environments necessitates the evaluation of its capacity for attachment both in air and when submerged in water. The objective of the attachment experiment was twofold: (1) to examine the magnetic suction gripper’s capacity to adhere to different surfaces and (2) to measure the lifting force achieved by the gripper during attachment.

Two dependent variables were measured:

The attachment force: which was measured as the maximum force required to detach the suction gripper from the substrate.

The pressure difference underneath the suction gripper.

Three independent variables were considered:

The shape of the curved substrate: ten different surfaces were tested, varying from a flat surface to a maximum curvature of 25 mm, both convex and concave (Fig. 5).

The environmental condition: air or submerged in water.

The presence or absence of a magnetic field.

FIG. 5.

Test Surface Illustrations with Reference Objects.

Each condition was repeated five times.

Experiment setup

The magnetic suction gripper was affixed to a holder to ensure consistent magnetic actuation during the validation process (Fig. 6). To control the actuation, a permanent magnet (Ø6 mm) was employed. The gripper’s attachment was accomplished by moving the magnet downward and then lifting it to a designated reference point on the holder, which pulled the membrane upwards and created a suction chamber underneath it. At this position, the magnet was securely fixed using butterfly screws. The holder was connected to a linear stage, which, in turn, was connected to a load cell (Futek, model LSB205) to measure the gripper’s attachment force. The linear stage was controlled using ControlDesk (dSPACE). To integrate the load cell with the linear stage, a custom-made aluminum block was manufactured. The load cell was then connected to a computer via an amplifier module (Futek, model CGS110). For the test surfaces, 3D-printed PLA surfaces were manufactured. These surfaces featured a hole connected to a silicone tube, which, in turn, was linked to a pressure sensor (NXP, model MPX4115AP) for vacuum measurements. To facilitate the experiments conducted in the air environment, the surfaces were mounted on top of a plastic box, providing space for the vacuum tubing. For the experiments conducted in the submerged environment, the surfaces were positioned inside a plastic container, which was filled with water. In these experiments, no pressure tests were conducted.

FIG. 6.

Test Setup for the Attachment Performance Experiment. (a) The magnetic gripper assembly with magnetic actuation mechanism for the experimental validation. (b) Schematic overview of the experimental setup. (c) Two-step procedure to apply a constant actuation force. (1) Loosening the screws and lowering the magnet. (2) Raising the magnet to top position and tightening the screws.

Experiment protocol

Before commencing the experiment, the load cell underwent calibration to account for the weight of the suction gripper. To initiate the experiment, the linear stage was carefully adjusted to lower the gripper until its magnetic membrane made contact with the test surface. The gripper was then affixed perpendicularly to the linear stage, positioned at the center of each test surface. To achieve magnetic attachment, the butterfly screws securing the magnet were loosened, allowing the magnet to descend towards the magnetic membrane. Once the magnetic attraction occurred, the magnet was raised to the desired position and securely fixed. For measurements taken without magnetic actuation, the magnet was positioned at the top of the holder, ensuring no magnetic attraction influenced the membrane. Subsequently, the linear stage was elevated at a steady speed of 5 mm/s over a distance of 100 mm. This sequence of steps was repeated five times for each experimental condition.

Data analysis

The data of the load cell was collected via ControlDesk. The data from the pressure sensor was collected via LabVIEW 2013. The data was analyzed by a one-way Analysis of Variance (ANOVA) test to examine if significant differences in attachment force were found for the different surfaces in both environments separately. Consistent performance between conditions is favorited for a highly versatile magnetic gripper. To analyze the influence of the environment on the attachment performance in air or water, an independent two-sided t-test for each of the test surfaces was performed (α = 0.05).

Attachment performance results

The experiments illustrated that the gripper shows higher attachment forces for curved surfaces with larger radii (see Table 1). The measurements are visualized in Figures 7 and 8. One-way ANOVA tests revealed that there was a statistically significant difference in the attachment force of at least one concave surface in air [F(2,12) = 41.73, p = 3.9e-06] and in water [F(2,12) = 60.56, p = 5.4e-07], at least one convex surface in air [F(2,12) = 7.86, p = 0.007] and in water [F(2,12) = 32.62, p = 1.4e-05] and at least one ball-shaped surface in air [F(2,12) = 44.21, p = 2.9e-06] and in water [F(2,12) = 22.56, p = 8.6e-05]. Independent t-tests showed no significant difference between concave 50 mm and 75 mm radius surfaces in air [t(8) = 1.803, p = 0.11] and in water [t(8) = 2.241, p = 0.055], no significant difference between convex 50 mm and 75 mm radius surfaces in air [t(8) = 1.086, p = 0.309] and in water [t(8) = 1.167, p = 0.277] and a significant difference between 50 mm and 75 mm radius ball in air [t(8) = 3.354, p = 0.010] and water [t(8) = 3.200, p = 0.0126]. Independent t-tests showed a significant difference between air and water tests for the concave 75 mm radius surface [t(8) = 2.8207, p = 0.0225] and the convex 25 mm radius surface [t(8) = 4.4622, p = 0.0021]. For the other types of curved surfaces where the attachment was achieved, no significant difference between the air or water environment was found. For the experiments where no magnetic actuation was used, the attachment was possible on all surfaces except the 25 mm curved surfaces, although this force was always smaller compared with the attachment with magnetic actuation (see Table 2).

FIG. 7.

Attachment Forces of the Magnetic Suction Gripper on the Single Curved Surfaces and Environments. The stars in red and blue represent the individual maximum attachment force for the experiments, for respectively the measurements in air and in water. The black boxes represent the mean values of the attachment force. The green crosses show the average maximum attachment force without magnetic actuation for the surfaces where the attachment was achieved. A = Air, C = Concave, V = Convex, W = Water. The numbers indicate the radii of the surfaces.

FIG. 8.

Attachment Force of the Magnetic Suction Gripper on the Double-Curved “Ball” Surfaces. The red stars represent the results of the measurements in air, and the blue stars from the measurements in water. A = Air, B = Ball, W = Water. The numbers indicate the radii of the surfaces.

Table 1:

Experiment Results of the Attachment Experiment with Magnetic Actuation (n = 5)

Type of surface	Radius	Attachment force air	Pressure difference in air	Attachment force water
Flat	Infinite	1.93 ± 0.75 N	2.57 ± 0.30 kPa	3.51 ± 1.0 N
Concave	25 mm	0.13 ± 0.05 N	0.99 ± 0.34 kPa	0.27 ± 0.15 N
	50 mm	1.34 ± 0.32 N	3.38 ± 0.77 kPa	1.82 ± 0.53 N
	75 mm	1.74 ± 0.33 N	1.49 ± 0.20 kPa	2.33 ± 0.25 N
Convex	25 mm	0.87 ± 0.09 N	0.86 ± 0.63 kPa	0.21 ± 0.04 N
	50 mm	1.85 ± 0.91 N	3.30 ± 1.13 kPa	1.95 ± 0.53 N
	75 mm	2.36 ± 0.74 N	2.48 ± 0.93 kPa	1.66 ± 0.24 N
Ball	25 mm	0.26 ± 0.07 N	0.63 ± 0.46 kPa	0.20 ± 0.07 N
	50 mm	1.75 ± 0.53 N	2.62 ± 0.76 kPa	1.28 ± 0.85 N
	75 mm	2.89 ± 0.54 N	1.98 ± 0.74 kPa	2.82 ± 0.85 N

Table 2:

Experiment Results of the Attachment Experiment without Magnetic Actuation (n = 2)

Type of surface	Radius	Attachment force air	Attachment force water
Flat	Infinite	1.54 ± 2.09 N	0.91 ± 1.04 N
Concave	25 mm	—	—
	50 mm	0.68 ± 0.70 N	1.01 ± 1.15 N
	75 mm	0.48 ± 0.47 N	0.87 ± 0.92 N
Convex	25 mm	—	—
	50 mm	0.61 ± 0.77 N	1.06 ± 1.41 N
	75 mm	0.67 ± 0.62 N	1.01 ± 1.16 N
Ball	25 mm	—	—
	50 mm	0.18 ± 0.00 N	0.22 ± 0.05 N
	75 mm	0.11 ± 0.01 N	1.15 ± 1.36 N

Deployment performance

The prototype underwent evaluation regarding its deployability. Tests were conducted using a nitinol wire frame connected to the magnetic membrane, mirroring the final design’s dimensions. 3D-printed tubes with inner diameters of 20 mm, 17.5 mm, 15 mm, 12.5 mm, and 10 mm were utilized for testing purposes. The results, depicted in Figure 9, indicate that the nitinol wire frame could fit and stretch inside the smallest tube with a diameter of 10 mm. However, the silicone membrane connection between the nitinol wire and the magnetic membrane did not fit within the smallest-diameter tube. The magnetic gripper design could be fully folded within the 12.5 mm tube, achieving a deployment percentage of 75%. The nitinol deployment mechanism increases the theoretically achievable suction force of the magnetic gripper, with the current rim and suction surface dimension ratio, by 75%.

FIG. 9.

Proof-of-Principle Prototype Folded Configuration. For the tube diameter of 10 mm, the silicone membrane connector limits the withdrawal of the gripper.

Discussion

Main findings

The exploratory design study has resulted in a concept for a magnetically actuated and deployable suction gripper. The prototype demonstrated successful magnetic actuation and attachment to curved surfaces in various environments. Figure 10 shows initial trials that demonstrate attachment to a diverse collection of objects. The design incorporates an advanced isotropic magnetic material composed of carbonyl iron particles and silicone. This combination results in significant shape changes of the membrane when subjected to magnetic actuation. The high friction coefficient of the silicone matrix material enhances the gripper’s frictional support against shear stresses. By combining the thin magnetic membrane with a nitinol frame, the gripper creates vacuum attachment forces. The nitinol frame provides stability, allowing the membrane to be placed on a substrate, forming a vacuum-pressure cavity underneath the membrane. The superelastic properties of nitinol enable the gripper to be folded into a smaller tube, facilitating downsizing and increasing the suction area when deployed.

FIG. 10.

Attachment of the Magnetic Suction Gripper to Various Objects. (a) Lifting a grape. (b) Lifting a strawberry. (c) Lifting an apple. (d) Attach to fine sandpaper with a grid of 600 K. (e) Attach to coarse sandpaper with a grid of 40 K. (f) Attach to human skin. (g) Attach to soft gelatin sample. (h) Lifting a delicate lightbulb. (i) Lifting a delicate soft plastic cup, and performing attachment without substrate deformation.

The prototype gripper demonstrated secure attachment to both single-curved and double-curved surfaces with a radius of 50 mm or higher in air and water. Attachment performance was similar between curved surfaces and flat surfaces, indicating effectiveness across different shapes. The maximum lifting forces achieved in air was 2.89 ± 0.54 N, allowing for lifting objects to approximately 350 grams with a single gripper, such as strawberries, large tomatoes, and large apples. A mean maximum pressure difference of 3.38 ± 0.77 kPa was reached, which complies with the relatively small cavity formed underneath the membrane upon magnetic actuation. Attachment to a curved surface with a radius of 25 mm was unsuccessful, as the gripper was not able to create a seal. However, attachment to curved surfaces with a radius less than 25 mm was achieved by sealing the inner part of the magnetic membrane onto the substrate while the nitinol wire frame surrounded the object without making contact, see Figure 10a–b. These observations indicate that attachment is challenging for surfaces where the curvature aligns with the gripper rim. The ability of the gripper to attach to substrates without actuation could indicate that the adhesive forces of the membrane itself also play a role in providing attachment.

Limitations

The experiments performed in this study rest on the constancy of attachment conditions across various measurements. The assumption was made that any deviations in attachment performance were solely attributed to the distinct shapes of the test surfaces and their environmental conditions. Also, a uniform distribution of magnetic particles in the membrane was assumed. A larger sample size is required to draw definite conclusions on the attachment performance. As stated within the goal of this research, the proof-of-principle prototype focuses on attaching to differently shaped surfaces. However, it should be noted that for a wide range of objects, the surface roughness should also be incorporated. The gripper is shown to attach to rougher surfaces, as shown in Figure 10e, where it is attached to sandpaper with a coarse grid, as well as to the surface texture of a strawberry. Even so, attachment to those objects was more prone to failure when compared with the smoother surface of a grape or a glass lightbulb. Future experiments should discover the effect surface roughness on attachment performance further.

Recommendations

The suitability of the gripper in specific applications, such as the medical field, should be further evaluated. The gripper’s foldability and attachment to small configurations, like heart tissue, need to be assessed. Initial trials demonstrated attachment to a gelatin phantom representing the stiffness of heart tissue, suggesting potential feasibility (Fig. 10g). For future research, the magnetic interaction could be improved by programming the magnetic particles in the soft matrix material in a specific orientation. This will enhance the magnetic force and provide the possibility of a more controlled detachment process. To comprehensively understand the gripper’s performance, further evaluations are recommended including a wider range of surface characteristics, such as roughness, and different strategies for magnetic actuation. The dimensions of the membrane and magnet should be optimized for effective sealing and miniaturization by modeling the magnetoactive membrane and its deformation and actuation force, comparable to the analytical model developed by Pan et al.¹⁸ Possibilities for the integration of multi-stimuli behavior could be discovered by embedding the shape-memory effect into the nitinol frame.

Conclusion

In this study, we introduced a magnetically-actuated suction gripper and examined its ability to grasp a wide range of objects. The proposed gripper relies on a small external magnetic field to induce a shape change in a soft magnetic membrane, generating a vacuum on diverse surfaces and shapes. The membrane was manufactured from a mix of silicone and carbonyl iron microparticles. To enhance stability and flexibility, we incorporated three superelastic nitinol wires in the suction gripper, enabling the gripper to fold up to 25% of its original outer diameter. The magnetic gripper showcased remarkable success in grasping various objects of different shapes and sizes, showcasing its versatility and adaptability. Experiments were performed on single-curved surfaces, double-curved surfaces, and flat surfaces, both in air and in water environments. Maximal mean attachment forces of 2.89 ± 0.54 N and 3.51 ± 1.0 N were found in air and water, respectively. This force proved adequate to lift objects weighing around 350 grams, considering the current gripper dimensions and magnetic actuation strength. These results indicate the promising potential of the magnetically actuated gripper for implementation in diverse applications from medical to marine exploration. Further research and development in this domain could lead to enhanced gripping capabilities, particularly on challenging substrates and difficult-to-reach areas.

Footnotes

Authors’ Contributions

Conceptualization: E.V., V.G.K., J.J., A.S. Validation: E.V. Writing—Original Draft: V.G.K. Writing—Review and Editing: J.J., A.S. Supervision: J.J., A.S.

Author Disclosure Statement

No competing financial interests exist.

Funding Information

No funding was received for this article.

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