1 Introduction

Soft robotics are commonly powered by flexible, elastic materials that can withstand large strains, diverging from the traditional rigid robotics that rely on rigid materials, rigid structures, complex sensing and control systems, and mechanical joints [1,2,3,4,5]. Soft robotics offer a wide range of control over their own external structure, stiffness, and motion patterns. As a new type of intelligent robot that imitates the shape or locomotion patterns of soft organisms in nature (such as worms, caterpillars, jellyfish, mimosa, and octopuses), soft robotics feature high degrees of freedom, continuous deformability, and safe human–machine interaction [6,7,8,9]. In response to external stimuli, including air pressure, electricity, heat, light, and magnetic fields [10,11,12,13], they can produce complex deformations such as stretching, bending, and twisting to achieve flexible and non-destructive grasping, biomimetic crawling, swimming, and jumping, and exhibit body intelligence comparable to that of living organisms.

The Earth’s magnetic field provides information that seems to be everywhere and can help animals navigate. The magnetic field direction (polarity) and/or magnetic inclination angle (the angle between the field lines and the Earth’s surface) can be used to determine favorable directions of locomotion [14,15,16,17]. As accomplished navigators, sea turtles can navigate toward the nesting area to avoid getting lost using magnetic polarity and/or inclination angle as reference directions for the magnetic compass. Similarly, they can also detect magnetic induction parameters to determine their positions [18,19,20]. Some researchers have attempted to design and study the kinematic behavior of soft robotic turtles inspired by the movement of sea turtles [21, 22]. However, the actuation methods are primarily based on fluid variable pressure [21] or electric fields [23], making it challenging to escape the constraints of external gas/liquid conduits or wires.

Magnetic soft robotics have advantages over other actuation methods, including programmability, remote control, high output force, fast response speed, high penetrability, and ease of manipulation [24,25,26,27,28]. Soft magnetic particles, such as carbonyl iron and soft ferrites, and hard magnetic particles, such as neodymium-iron-boron, samarium-cobalt, and chromium dioxide, are combined with elastic materials in magnetically controlled biomimetic micro-soft robotics. The magnetic force or torque generated by the magnetic filler particles under the influence of the applied magnetic field is utilized for remote actuation. There are two mechanisms involved in this operation: magnetic torque actuation and magnetic gradient actuation [29]. Due to their low coercivity and inability to maintain magnetization in the absence of applied magnetic fields, magnetorheological elastomers (MREs) soft robotics embedded with soft magnetic particles exhibit relatively low programming flexibility and the shape deformation of them is limited to simple expansion or contraction [30]. Recently, research on programmable actuation of magnetically controlled biomimetic soft robotics embedded with high coercivity hard magnetic particles (neodymium-iron-boron) [12, 31, 32] has progressed considerably. Currently, polydimethylsiloxane (PDMS), hydrogels, Ecoflex, and other commercial silicone elastomers have been used as the soft matrix for the magnetic filler particles [33,34,35,36]. When the pre-arranged magnetic domain polarization direction within the material does not align with the applied magnetic field, a magnetic torque distribution is produced, and the spatially varying magnetic field can be used to remotely actuate and control the soft robotics to achieve pre-programmed shape deformation or locomotion. The application of magnetic soft robotics in biomedicine has great potential. For instance, magnetically responsive microneedle robotics can be used for intestinal macromolecule delivery [37], and a magnetically controlled guidewire robot system can be used for vascular interventional surgery [38]. This provides valuable information for the design and operation of new magnetically programmed biomimetic sea turtles.

Therefore, this study is motivated by the characteristics and movement patterns of sea turtles that navigate using the Earth’s magnetic field. Herein, we developed a novel centimeter-scale magnetically programmed soft robotic turtle (12.50 mm in length and 0.24 g in weight) to achieve biomimetic locomotion patterns such as straight-line swimming and turning swimming, using an acrylic elastomer VHB (very high bond) as the soft matrix for the magnetic filler particles (samarium-iron–nitrogen). The effect of magnetic field strength on swimming speed, swimming amplitude, bending deformation angle of the front and back limbs, and other movement parameters is studied. Notably, when the bending deformation magnitudes of its left and right limbs differ, the soft robotic turtle switches from straight-line (0.476 times body length per second) to turning swimming (6.334 rad/s). The soft robotic turtle was able to achieve tracking swimming behaviors along specific planned paths such as square-shaped, S-shaped, and double U-shaped maze-like trajectories, by adjusting the magnetic field parameters. The soft robotic turtle is anticipated to be utilized for special detection and targeted drug delivery, among other applications.

1.1 Magnetic Programming Design and Fabrication of Limbs

When sea turtles swim in nature, their four limbs play important roles. The forelimbs are mainly used to support and propel the torso, while the hindlimbs are used to adjust direction and speed. It is worth noting that the forelimbs are longer than the hindlimbs. In the case of calm water surface, sea turtles can adopt a kayaking-style swimming method, where the forelimbs paddle in the water while the hindlimbs control the direction and speed, similar to the way sailboats use paddles. When facing large waves or in need to move quickly, sea turtles adopt a continuous swimming method, where all four limbs move simultaneously to propel the torso forward. As shown in Fig. 1(c), a layer-by-layer assembly strategy was used to mix and stack samarium-iron–nitrogen magnetic particles (Hubei Haopeng Magnetoelectricity Co., Ltd.) and VHB (3 M Company, USA), in 1.9:1 weight ratio. Its magnetic properties (magnetization curve and hysteresis curve) are shown in Fig. S1. The stacked magnetic VHB was then cut into suitable shapes and sizes, to imitate limbs of the magnetic soft robotic turtle, applying a length of 7 mm for the forelimbs and a shorter length of 6 mm for the hindlimbs, both 3 mm wide. Subsequently, a clamp-type magnetizer QS-10–7 (Shanghai Tianduan Technology Co., Ltd.) was used to set the current at 2.5 A, voltage at 38 V, and overvoltage/overcurrent/overload protection at 110 V, 6 A, and 100 W, respectively. The magnetizer was energized for 10 s to magnetize the material in two directions, 45° and 90°, so as to adjust the ordered distribution of magnetic domains in the limbs. The experimental results show that the locomotion effect of the magnetic soft robotic turtle with 90° programming magnetization is not as stable as that of the 45° programming magnetization, and the analysis is shown in Fig. S2.

Fig. 1
figure 1

a The schematic of magnetic soft robotic turtle. b Torso structure making process. c Mix and stack samarium-iron–nitrogen magnetic particles and VHB by a layer-by-layer assembly strategy. d The programmable magnetization schematic of magnetic soft robotic turtle. (e) The splicing of torso parts and limbs of magnetic soft robotic turtle

Full size image

1.2 Design and Fabrication of Torso

Using 3D printing technology, a mold for the magnetic soft robotic turtle torso was designed and produced (size: length 12.5 mm, width 8 mm). Ecoflex-30 A and B were mixed in a 1:1 ratio and injected into the mold. After fully curing at room temperature for 4 h, the silicone torso of the soft robotic turtle was removed from the mold. Finally, the silicone torso of the soft robotic turtle was assembled with the respective limbs, using silicone adhesive. The derived magnetic soft robotic turtle has a mass of 0.24 g and a torso length of 12.5 mm. It is noteworthy that, to achieve high swimming speed, the assembly is performed according to points A and B shown in Fig. 1(c) and (d), resulting in the magnetic domain distribution, on the left and right sides of the biomimetic soft turtle limbs, to be in opposite direction. Figure 1 shows the manufacturing and magnetization schematic of the soft robotic turtle.

1.3 Actuation Method for Magnetic Soft Robotic Turtle

The magnetic field actuation experimental platform is constructed by 3D communication coil 3HLY7.5–100 (Shanghai Tianduan Technology Co., Ltd.), power amplifiers HEAS-50, HEA-200C, and HEA-500G, waveform generator (Nanjing Fone Technology Industry Co., Ltd.) and Gaussmeter (Guangzhou Weilai Electronic Technology Co., Ltd.). The Multi-Axis Magnetic Field Control System, which was jointly developed by the authors’ team and Tianduan Technology Co., Ltd., is used to accurately control the intensity and direction of the external uniform rotating magnetic field, by adjusting the waveform, frequency, phase, peak value, and other parameters of the experimental magnetic field actuation platform, in real-time (Fig. S3, Supporting Information). This system enables the realization of biomimetic motion behaviors, such as straight-line swimming, turning swimming, and load-carrying swimming of the magnetic soft robotic turtle. The motions are captured and observed using a camera.

2 Results and Discussion

2.1 Magnetic Deformation Mechanism

Under the remote control of the external alternating magnetic field, the magnetic soft robotic turtle can achieve pre-programmed body deformations, according to the deformation mechanism as shown in Fig. 2. The magnetic domain arrangement in the right forelimb is opposite to the magnetic domain arrangement in the left hindlimb, and the magnetic domain arrangement in the left forelimb is opposite to the magnetic domain arrangement in the right hindlimb. Notably, the magnetic domain arrangement in the left forelimb is perpendicular to the magnetic domain arrangement in the left hindlimb, and the magnetic domain arrangement in the left forelimb is perpendicular to the magnetic domain arrangement in the right forelimb. By applying an external alternating magnetic field, the limbs on both sides of the torso are deformed, causing the soft robotic turtle to swim forward or turn. When the magnetization direction of the pre-arranged internal magnetic domains (45° programming magnetization) in the limbs does not align with the applied magnetic field, the limbs will deviate in the direction of the external magnetic field, generating the magnetic torque that causes the free end of the limbs to be subject to bending deformation, positively correlated with the magnetic field B. Since the magnetization direction of the left limbs is opposite to that of the right limbs, when a uniaxial uniform alternating magnetic field is applied (Fig. 2(a)), the magnetization direction is symmetric about the magnetic field direction. In this case, the axis of the soft robotic turtle remains unchanged, so it moves straight toward the head. When a combined alternating magnetic field of x and y axes is applied, as shown in Fig. 2(b), during forward swing, the angle between the magnetization direction of the right limb and the magnetic field direction is smaller than left side, so the right limb swings forward more vigorously than the left limb. During backward swing, the angle between the magnetization direction of the left limb and the magnetic field direction is smaller than right side, so the left limb swings backward more vigorously than the right limb, resulting in counterclockwise turning locomotion of the soft robotic turtle. By applying spatially varying magnetic fields, remote control of the soft robotic turtle can achieve pre-defined biomimetic motion behaviors, such as straight-line swimming or turning swimming.

Fig. 2
figure 2

a The deformation mechanism of magnetic soft robotic turtle with 45° programmable magnetization when applying a uniaxial uniform alternating magnetic field. b The deformation mechanism of magnetic soft robotic turtle with 45° programmable magnetization when a combined alternating magnetic field of x and y axes is applied

Full size image

2.2 Straight-Line Swimming Performance

When the magnetic field strength (B) is 13/3 mT, the magnetic torque drives the limbs of the magnetic soft robotic turtle to align with the external magnetic field direction and induce a sinusoidal bending deformation (Fig. S4(a), Supporting Information). The average swing amplitude of the forelimbs is 25.794°, while the maximum swing amplitude of the forelimbs is 26.66°. The average swing amplitude of the hindlimbs is 14.472°, whereas the maximum swing amplitude of the hindlimbs is 16.495°. At the same time, the swing amplitude of the forelimbs is significantly higher than that of the hindlimbs. Similar to the straight-line swimming mode of turtles, as achieved by the front and rear limbs paddling forward and backward, both the front and hind limbs of the soft robotic turtle exhibit sinusoidal bending deformations, while in a uniform alternating magnetic field, generating a forward thrust at a certain frequency and enabling continuous swimming motion with an average speed of 0.907 mm/s. The maximum speed reached 1.161 mm/s.

Furthermore, adjusting the magnetic field strength can control the swing amplitude of the limbs of the magnetic soft robotic turtle. When the magnetic field strength is 26/3 mT, the average swing amplitude of the forelimbs of the soft robotic turtle increases to 38.192°, while the average swing amplitude of the hindlimbs reaches 20.92° (Fig. S4(b), Supporting Information). As shown in Fig. 3(a), when the magnetic field strength is 13 mT, the average swing amplitude of the forelimbs is 68.633°, whereas the maximum swing amplitude of the forelimbs is 70.979°. As the magnetic field strength increases, the swing amplitude of the limbs also increases and the straight-line speed shows a significant rising trend. The average swing amplitude of the robot’s hindlimbs is 47.984°, while the maximum swing amplitude of the hindlimbs is 51.943°. As shown in Fig. 4(a), the soft robotic turtle swims straight along a distance of 46.964 mm within 12 s, at an average speed of 3.914 mm/s (0.313 times body length per second), which is 4.315 times the speed in the case of B = 13/3 mT. Between t = 10 s and t = 11 s, the soft robotic turtle reaches a maximum speed of 5.946 mm/s (0.476 times body length per second).

Fig. 3
figure 3

a The limb swing amplitude of magnetic soft robotic turtle when swimming straightly during one cycle at a magnetic field strength of 13 mT. b The swing amplitude of the magnetic soft robotic turtle when turning during one cycle at a magnetic field strength of 13 mT

Full size image
Fig. 4
figure 4

a Straight-line swimming performance of magnetic soft robotic turtle under different magnetic field strengths. b Turning swimming performance of magnetic soft robotic turtle under different magnetic field strengths. c Straight-line swimming performance of magnetic soft robotic turtle under different loads. d Straight-line swimming performance of magnetic soft robotic turtle under different loads

Full size image

It is worth noting that, during the process of increasing the magnetic field strength, there is a slight increase in speed (Movie S1), during the middle time period, compared to the respective values achieved at both ends of this timeframe. This phenomenon occurs because, when the magnetic soft robotic turtle moves into the central area of the applied magnetic field, the magnetic field strength increases, compared to the edge of the applied magnetic field. At the same time, if the magnetic field strength is too low, it may not provide sufficient actuation force and limit the straight-line speed of the soft robotic turtle.

2.3 Turning Swimming Performance

When the alternating magnetic field with a combined magnetic field strength of 13/3 mT, along the x and y directions, is applied, the left and right limbs of the magnetic soft robotic turtle swing differently, due to the different angles between the magnetization direction of each limb and the applied alternating magnetic field (Fig. S5(a), Supporting Information). The average swing amplitude of the left forelimb is 19.956°, with a maximum swing amplitude of 23.24°, while the average swing amplitude of the right forelimb is 22.065°, reaching a maximum swing amplitude of 23.488°. When the magnetic field strength increases to 26/3 mT (Fig. S5(b), Supporting Information), the swing amplitude of the forelimbs increases compared with that of B = 13/3 mT. As shown in Fig. 3(b), when the magnetic field strength increases to 13 mT, the average swing amplitude of the left forelimb reaches 58.104°, with a maximum swing amplitude of 60.594°, while the average swing amplitude of the right forelimb increases to 71.29°, with a maximum swing amplitude of 75.621°. Therefore, it can be concluded that, during turning movements, the soft robotic turtle’s right limb swings more intensely than its left limb, resulting in a counterclockwise turning motion of the torso.

As the magnetic field strength increases, the swing amplitude of the limbs also increases (Movie S2), while the turning speed shows a significant rising trend. As shown in Fig. 4(b), when the magnetic field strength is 13/3 mT, the magnetic soft robotic turtle reaches 2.791 radians per second, while the maximum turning speed reaches 2.949 radians per second. When the magnetic field strength is 13 mT, the soft robotic turtle reaches 6.334 radians per second, which is 2.269 times faster than the one in the case of B = 13/3 mT, whereas the maximum turning speed reaches 6.559 radians per second. This means that, under stronger magnetic fields, the soft robotic turtle can change directions more easily and achieve larger turning angles.

2.4 The Load-Carrying Performance

In nature, the backs of turtles are often parasitized by barnacles. An excessive number of barnacles can lead to a lower swimming speed and reduced swimming ability in turtles. As shown in Fig. 4(c, d), a magnetic soft robotic turtle was used to simulate the load-bearing movement patterns of turtles, while the effect of different loads on the swimming speed of the soft robotic turtle was studied. Under the same magnetic field strength conditions, the average speed of the load-free soft robotic turtle (total weight 0.24 g) was 3.914 mm/s, while the maximum speed reached 5.946 mm/s. In the case of carrying a load of 0.5 times its own weight (total weight 0.36 g), the average speed decreased to 2.781 mm/s, whereas the highest speed reached only 4.946 mm/s. In the case where the soft robotic turtle carried its own weight (total weight 0.48 g), the average speed was 0.770 mm/s, that is 0.197 times the respective value under load-free conditions, while the highest speed was only 1.175 mm/s (Movie S3).

Under the same magnetic field strength conditions, the load-free magnetic soft robotic turtle (total weight 0.24 g) reached an average turning speed of 6.334 rad/s, while the maximum turning speed was 6.559 rad/s. When it carried a load of 0.5 times its own weight (total weight 0.36 g), the turning speed of the soft robotic turtle decreased to 2.238 rad/s (0.353 times the one under load-free conditions). When the soft robotic turtle carried its own weight (total weight 0.48 g), the turning speed decreased to 1.476 rad/s (0.233 times the one under load-free conditions). It is evident that, adding a load will affect the turning performance of the soft robotic turtle and reduce its turning speed (Movie S4). In practical applications, the effect of loads on the turning performance of the soft robotic turtle needs to be considered and the magnetic field parameters adjusted as needed, to achieve the desired turning effect.

2.5 The Tracking Performance of the Magnetic Soft Robotic Turtle

Applying precise control of the magnetic field parameters, the magnetic soft robotic turtle can not only swim straight but also turn flexibly. To better verify the adaptability of the soft robotic turtle to environmental changes, its directional control ability (magnetic navigation ability) in water and its path planning ability were studied. The soft robotic turtle was able to achieve swimming behavior along specific planned paths such as square-shaped, S-shaped, and in a double U-shaped maze. When a single-axis (y-axis) magnetic field is applied, the soft robotic turtle realizes a straight-line movement in the planned paths, and when two magnetic fields of x-axis and y-axis are applied perpendicular to each other and equally large, the soft robotic turtle realizes counterclockwise turning motion in the planned paths. As shown in Fig. 5(a), after planning the route of the double U-shaped maze, the soft robotic turtle can swim in a straight line in A–B, C–D, E–F, with a total distance of 150 mm within 74 s; therefore, the average speed of the soft robotic turtle, moving straight in the double U-shaped maze environment, was 2.027 mm/s.

Fig. 5
figure 5

The tracking performance of magnetic soft robotic turtle in specific planned paths such as double U-shaped maze (a, b), S shaped (c, d), and square shaped (e, f)

Full size image

Compared to the double U-shaped maze, the S-shaped path requires the magnetic soft robotic turtle to be able to precisely control its straight-line and turning movements, so as to complete continuous curved swimming in a confined space. Its motion principle is the same as the robot’s motion principle in the maze. As shown in Fig. 5(b), after planning the route of the S-shaped path, the soft robotic turtle can swim in a straight line in A–B, C–D, E–F, with a total distance of 106 mm within 23 s, which means at an average speed of 4.609 mm/s.

The tracking motion experiments of the magnetic soft robotic turtle in the aforementioned maze environments and special scenarios, such as “S”-shaped paths and square-shaped paths, demonstrate the potential applications of the soft robotic turtle in complex environments.

To further evaluate the straight-line swimming ability and turning angle accuracy of the magnetic soft robotic turtle, on a specific planned path, in addition to the above scenarios, the magnetic field parameters were adjusted so that the soft robotic turtle could swim along a predetermined square-shaped trajectory. Its motion principle is the same as the robot’s motion principle in the maze. As shown in Fig. 5(c), first, the trajectory of the square-shaped route is planned, and the straight-line swimming is realized in A–B–C–D with a total distance of 140 mm, while the total duration of straight-line motion was 24.5 s, resulting in an average speed of 5.833 mm/s.

During the A–B movement, the magnetic soft robotic turtle appears able to swim basically along the straight square-shaped trajectory. Reaching point B, it deviates from the expected vertex by 2.2 mm. However, when the robot moves from B to C, its motion trajectory deviates from the original square-shaped trajectory, whereas at point C, it deviates from the expected vertex by 2.6 mm. Then as the soft robotic turtle moves from C to D, it quickly adjusts its motion trajectory so that it returns along the straight square-shaped trajectory in the D–A segment. At point D, it deviates from the expected vertex by 2.5 mm, whereas finally the soft robotic turtle returns to the starting point A, coinciding with the expected endpoint.

At the same time, the turning motion in the three environments of the magnetic soft robotic turtle is compared. When the soft robotic turtle is in the double U-shaped maze, it can achieve turning swimming in B–C and D–E, at the first turning point (B–C), the soft robotic turtle turned π rad in 7 s, at an average turning speed of 0.449 rad/s; at the second turning point (D–E), the soft robotic turtle turned 3π/2 rad in 11 s, at an average turning speed of 0.428 rad/s (Movie S5). However, it is noteworthy that, the motion speed of the soft robotic turtle in complex environments decreased, compared to the speed of simple straight-line swimming or circular movements.

When the magnetic soft robotic turtle is in the S-shaped path, it can achieve turning swimming in B–C and D–E, at the first turning point (B–C), it took 5 s to turn π/2 rad, at an average turning speed of 0.314 rad/s; at the second turning point (D–E), it took 2 s to turn π/2 rad, at an average turning speed of 0.785 rad/s (Movie S6).

When the magnetic soft robotic turtle is in the square-shaped path, it can achieve turning swimming in point B, C, D and E, at the first turning point (B), the soft robotic turtle turned π/2 radians in 2 s, at an average turning speed of 0.785 rad/s; at the second turning point (C), the turn of π/2 radians took 2 s, at an average turning speed of 0.785 rad/s; at the third turning point (D), the soft robotic turtle turned π/2 radians in 2 s, at an average turning speed of 0.785 rad/s (Movie S7).

In summary, when the magnetic soft robotic turtle performs turning motion in the double U-shaped maze, its turning arc is different from that in the S-shaped path and square-shaped path, with a turning arc of π rad in B–C and 3π/2 rad in D, E. In S-shaped and square-shaped path, the soft robotic turtle both has a π/2 rad radian each turn. Moreover, when the soft robotic turtle turns in the square-shaped path, its turning method is different from that in the maze and S-shape, and it turns in place.

The aforementioned experiments on the motion of the magnetic soft robotic turtle in mazes, as well as special scenarios such as “S” shapes and squares, demonstrate the potential application of magnetic soft robotic turtles in complex environments.

3 Conclusion

Inspired by the way sea turtles rely on the Earth’s magnetic field for navigation and locomotion, we have developed an untethered magnetic soft robotic turtle (12.50 mm in length and 0.24 g in weight) with programmable magnetization, integrated with an Ecoflex-based torso and four magnetically programmed VHB-based limbs. Similar to how sea turtles swim forward by paddling their forelimbs back and forth, the magnetic torque causes their limbs to exhibit sinusoidal bending deformations. As the magnetic field strength increases, the amplitude of the limb swings of the soft robotic turtle also increases, and its straight-line swimming speed shows a significant increasing trend, with a maximum swimming speed of 0.476 times body length per second. Notably, when the bending deformation magnitudes of its left and right limbs differ, the soft robotic turtle switches from straight-line (5.946 mm/s) to turning swimming (6.334 rad/s). Furthermore, the tracking swimming activities of the soft robotic turtle in the abovementioned mazes and special scenarios such as “S”-shaped paths and square-shaped paths demonstrate the potential applications of the soft robotic turtle in complex environments.