3D Printed Magnetic Bionic Robot Inspired by Octopus for Drug Transportation

Abstract

The octopus has attracted widespread attention owing to its unique underwater movement and its ability to escape with inkjets, which also promoted the development of underwater bionic robots. This study introduces a magnetic octopus robot (MOR) 3D printed with PA6/NdFeB composite material, which has good magnetic responsiveness and rigidity to cope with complex environments. The MOR can roll and rotate through complex terrain and passages because of its eight-claw structure. It also has amphibious locomotion and can pass through narrow gaps of 37.5% of its height by deformation. In addition, the MOR can not only clamp, transport, and release solids but also liquids by adding silicone hollow spheres, which indicates the potential of the MOR to be used in medical applications for transporting solid or liquid drugs. This research will help broaden the application prospects of magnetron robots in the field of medical drug transportation.

Introduction

Biologically inspired methods are increasingly applied in robotics research. The models and structures of bionic robots, such as snake-like robots,¹ jellyfish-like robots,² looper-like robots,³ and octopus-like robots,^4–7 are mostly derived from natural organisms. In the long-term natural evolution, underwater creatures have developed their own unique propulsion methods, the most common of which is the swing propulsion of fish.⁸ However, the arms of octopuses have attracted the attention of many scholars due to their special structure, which can achieve high flexible movement and grasp objects in a complex environment.⁴ The octopus can not only deform its soft body, squeeze into the crevices of the rocks to hunt for hidden shrimp, but also escape through the inkjet.⁹ Currently, bionic octopus research is focused on flexible grasping.^10,11 It can climb columnar objects⁵ and walk underwater.⁶ Despite these encouraging developments, further research into materials and fabrication techniques is needed to achieve the level of motor control observed in biological systems and to achieve the functions of grasping, releasing, tumbling, transporting, and wireless driving.¹² Octopus robots have many characteristics, such as good pass-through, morphing ability, and grab-and-release capability. However, traditional octopus robots usually need to carry energy or work with power cords, which is not only larger in size but also reduces his flexibility and freedom, making it difficult to be applied in clinical medicine.

In recent years, additive manufacturing, also known as 3D printing, has been widely used to manufacture bonded magnets.¹³ With the development of 3D printing technology, a variety of magnetic robots with novel designs, complex geometries, or programmable structures have been progressively developed through 3D printing strategies, including digital light processing (DLP),¹⁴ direct ink writing (DIW),¹⁵ and fused deposition modeling (FDM).^16,17 Among them, although DLP has high printing accuracy, particle settlement is prone to occur during the printing process, and the cost is high.¹⁸ Besides, in the DIW printing process, because of the low modulus of the ink, the samples would collapse under the force of gravity.¹⁹ Compared to other methods, FDM is preferred by researchers due to its low cost, lack of particle settling, good functionality, and effectiveness.²⁰

There are many driving methods for bionic robots, including pneumatic,²¹ optical,²² and magnetic drive.^{15,16,23–26} In particular, the magnetic field can propagate without a medium, and the magnetic force can be controlled in three-dimensional (3D) space and can provide sufficient force.²⁷ The posture, speed, and direction of the magnetic robot can be changed by changing the size and direction of the magnetic field. Small-scale magnetic soft-bodied robots can be designed to operate based on different locomotion modes to navigate and function inside unstructured, confined, and varying environments.²⁸ Owing to these properties, magnetic robots have high potential in the medical field, where they can be widely used in in vivo microsurgery, biomonitoring, and drug delivery.^29,30 The biggest hurdle thus far has been the miniaturization of robotic devices so that they are thin and flexible enough to navigate through narrow and complex neurovasculature.³¹ The use of a magnetic drive allows for good flexibility and controllability of the octopus robot, in the case of smaller sizes. Combined with 3D printing technology, it is easier to create bionic octopus robots with complex shapes.

In this article, we demonstrate a magnetic octopus robot (MOR) 3D printed by PA6/NdFeB composite material, and introduce the preparation method and optimal printing parameters. Through a series of experiments, we have demonstrated that MOR has good magnetic response and rigidity to cope with complex environments. The MOR can roll and rotate through complex terrain and passages, thanks to its eight-claw structure. It also has amphibious locomotion and can squeeze through narrow gaps of 37.5% of its height by deformation. In addition, we have demonstrated the potential of the MOR to transport solid or liquid drugs by showing that the MOR clamps, transports, and releases solids, and transports liquids with the addition of silicone hollow spheres. This research will help broaden the application prospects of magnetron robots in the field of medical drug transportation.

Materials and Methods

Base material and filler

The base material used in this study is PA6 (UBE-1013B) from UBE, Japan, which has a density of 1.14 g/m³ and an average particle size of 38 μm. Its Brinell hardness is 85 in a 23°C environment. Due to its good flow and mechanical properties, PA6 is suitable for use as a base material for 3D printing bonded magnets.

In this study, the isotropic NdFeB(LW-N-12-9) magnetic powder produced by Sinnord, China, was used. According to the information provided by the manufacturer, the density ρ of NdFeB powder is 7.57 g/m³, the remanent magnetization Br is 780 − 800 mT, the endogenous coercivity Hcj is 740 KA/m–800 KA/m, and the maximum magnetic energy product (BH)_max is 97–110 KJ/m³. The NdFeB powder was sieved with a vibrating sieve shaker manufactured by FRITSCH, Germany, to prevent clogging of the nozzles caused by larger particle sizes during the printing process. The sieved NdFeB powder was analyzed with the China King KW510 automatic laser particle size analyzer, and the volume weighted average diameter was 33.25 μm.

Preparation of MOR

MOR is printed using an FDM printer, and the flexible filaments containing magnetic particles is produced by a screw extruder. We have discussed the preparation process and printing parameters in a separate published article.¹³ First, PA6 and NdFeB powders were placed in a drying oven at 80°C for 4 h. Then, the two powders were weighed in the ratio of 25 wt.% for PA6 and 75 wt.% for NdFeB using an electronic balance. The powder was poured into the mixer under air atmosphere and mixed for 10 min. As shown in Figure 1a, the mixed powder was poured into the hopper of a single-screw extruder (SJ-15) manufactured by Orotrim (China) and extruded into a yarn. The composite print filament is extruded through a 1.75 mm diameter circular mold and guided into a filament traction winder manufactured by Orotrim (China) for winding. The parameters of the extruder are as follows: heating temperature 235°C and screw speed 75r/min.

FIG. 1.

Schematic diagram of MOR preparation: (a) Preparation process of flexible filament, including powder mixing, screw extrusion, and filament winding, (b) MOR 3D printing process, and (c) schematic diagram of the magnetization process of the MOR. MOR, magnetic octopus robot.

As shown in Figure 1b, an FDM printer (A5-S) produced by JG-Maker(USA) was used to print MOR with the filament produced from PA6/NdFeB composite material. Printing parameters are as follows: Layer height 0.2 mm, printing temperature 245°C, platform temperature 80°C, printing speed 20 mm/s, and nozzle diameter 0.6 mm. To ensure the bending performance of the connection part, the print thickness of the connection part of MOR is 0.2 mm. To ensure that the claw section has a good magnetic response, the print thickness is 3 mm.

Magnetization of MOR

The schematic diagram of the magnetization process of MOR is shown in Figure 1c. After the printing of MOR is completed, we print a hollow cylindrical fixing fixture with PLA material. Then a force is applied to bend the MOR joint part and make sure the gripper part is parallel to the cylindrical axis and put it into the mold. The fixed MOR is magnetized in a capacitive discharge magnetizer (i Mag™ Master PLUS) produced by Laboratorio Elettrofisico, and the S pole near the connecting part and the N pole at the end of the claw. The average magnetic field strength of the N pole of the eight jaws was measured to be 22.31 mT using a Tesla meter (TD8620) produced by TUNKIA, China. Due to the plastic deformation of the connecting part and the repulsive force between the 8 jaws, the jaws are half-open at an angle of about 48° with the MOR axis.

Motion control of MOR

The MOR is driven by a magnetic field and its linear motion can be controlled in two ways. When the N pole of the cylindrical magnet is close to the S pole of the MOR and the inclination angle is 30° relative to the horizontal plane, the MOR will rotate toward the direction of the magnet movement. Another way of control is to bring the N pole of the cylindrical magnet close to the S pole of the MOR, which keeps the MOR opening up and stable. When the cylindrical magnet rolls forward counterclockwise, the MOR will roll forward clockwise under the effect of the magnetic field.

Characterization

Print dumbbell-type tensile specimens (PA6/NdFeB 75 wt.%, 50wt.%, 25wt.%, Only PA6) according to the printing method described in 2.2, and test the tensile properties of the printed samples using an electronic universal testing machine (UTM4204X) manufactured by SUNS in China. The hysteresis lines of four different mass fractions of PA6/NdFeB filaments were measured at room temperature using a LakeShore 7404-S series vibrating sample magnetometer manufactured in the United States. The magnitude of the magnetic field at the top center of the MOR is measured by a Tesla meter (TD8620) equipped with a Hall sensor probe.

The cross-sectional microstructure of the filaments was photographed with a scanning electron microscope (Regulus 8100) manufactured by Hitachi, Japan. The smartphone (LIO-AL00, Huawei) was used to record optical pictures and motion pictures of the MOR.

Numerical simulation

The MOR was modeled in COMSOL Multiphysics 5.5 finite element analysis software, and the angular response of the MOR under the action of a magnetic field was simulated. In the computational model, it was assumed that the permanent magnetic NdFeB powder is uniformly filled in the base material and the binary PA6/NdFeB mixture was considered a monolithic permanent magnet.^23,32 The simulation results were compared with the experimental results to explore the effect of the driving magnetic field strength on the response angle of MOR.

Results and Discussion

Mechanical and magnetic property characterization

Tensile tests were performed on the printed dumbbell-type specimens to study their mechanical properties, and the result is shown in Figure 2a. Their ultimate tensile strengths of 20.31, 18.50, 16.67, and 18.61 MPa were obtained from the stress–strain curves of the samples with different filling mass fractions of 75%, 50%, 25%, and pure PA6, respectively. Material defects brought by adding fillers, such as pores created during the preparation process, made the ultimate tensile strength of the samples with mass fractions of 25% and 50% lower compared with the pure PA6 samples. However, the modulus of elasticity of the samples increased with increasing mass fraction due to the reinforcing effect of the filler. The sample with 75% mass fraction exhibits good mechanical properties. Robots printed with PA6/NdFeB composites have good stiffness and structural stability compared to the common silicone-based magnetron robots. The increased tensile strength of the material prevents the MOR from losing its ability to work in complex environments due to stretching or crushing.

FIG. 2.

(a) Tensile stress–strain curve and (b) magnetization curve of samples with different mass fractions, inset shows the images of the tensile samples after testing.

The strength of its own magnetic energy affects the magnetic response of the magnetically controlled robot. Figure 2b shows the magnetization curves of NdFeB mass fractions of 75%, 50%, 25%, and pure PA6 samples, which have residual magnetization intensity (Br) of 49.59, 31.78, 10.32, and 0 emu/g, respectively. Due to the use of the same filler, they have similar coercivity (Hc) about 7720Gs, which indicates that they have good magnetic properties. In summary, the NdFeB mass fraction of the material used in the subsequent study was 75% to ensure that the MOR has good mechanical properties and magnetic responsiveness.

Magnetic response analysis

One of the most important features of 3D-printed bionic robots is the ability to deform, caused by different magnetic field sizes. To study the deformation capability of MOR, this section discusses the deformation under different geometrical parameters and magnetic fields. The typical deformation process of MOR under the action of magnetic field in different directions is shown in Figure 3a and Figure 3c. By adjusting the distance d between the permanent magnet (Φ40 mm) and the fixed end of the MOR, magnetic fields of different magnitudes are applied to the MOR, and the degree of deformation is judged according to the angle θ formed between the two jaws of the MOR. In the absence of magnetic influence, the angle θ is 97° due to the combined effect of gravity and the inherent elasticity of the material. The magnitude of the magnetic field at the center of the applied magnetic field is measured using a Gauss meter, and the results are shown in Figure 3g. As the permanent magnet approaches, it shows a nonlinear growth.

FIG. 3.

When the N pole of the permanent magnet is up, (a) schematic diagram and physical diagram of the magnetic response test system, (b) effect of the distance between the permanent magnet and the MOR on its angle at different distances. When the S pole of the permanent magnet is up, (c) schematic diagram and physical diagram of the magnetic response test system, (d) effect of the distance between the permanent magnet and the MOR on its angle at different distances. (e) Effect of the print thickness on the deformation rate. (f) Effect of the connection length to width ratio on the deformation rate. (g) Magnetic field strength of the permanent magnet at different distances. MOR, magnetic octopus robot.

Figure 3b shows the relationship linking the distance between the permanent magnet and the fixed end of the MOR and the angle of the MOR when the S pole of the permanent magnet is upward. As their distance decreases, the angle decreases nonlinearly from 97° to −7°. Contact between the jaws of the MOR occurs and reaches the limit position, after which the angle no longer varies with distance and magnetic field size. Figure 3d shows the relationship linking the distance between the permanent magnet and the fixed end of the MOR and the MOR angle when the N pole of the permanent magnet is up. As the distance decreases, the angle increases from 97° to 241°. When the distance is less than 3 cm, the angle no longer changes due to the limitation of the inherent elasticity of the material. The above results show that MOR has fast, accurate, and stable magnetic response capability.

The deformation rate can be calculated by measuring the change in angle between the MOR jaws, which is defined as follows: $φ = Δ θ / 360 °$ (1) $Δ θ = θ_{\max} - θ_{\min}$ (2) $θ_{\max}$ is the maximum angle that the MOR can reach under the magnetic field and $θ_{\min}$ is the minimum angle that the MOR can reach under the magnetic field.

The deformation capacity of the MOR depends on the geometric parameters of its attached parts, including print thickness and aspect ratio. As shown in Figure 3e, it is clear that keeping the same aspect ratio (8/2) and increasing the print thickness will result in a lower deformation ratio of the MOR. Print thickness increased from 0.1 mm to 0.5 mm and deformation rate decreased from 0.77 to 0.02. As shown in Figure 3f, keeping the same print thickness and increasing the aspect ratio results in an increase in the deformation rate of the MOR. The aspect ratio increased from 4/2 to 12/2 and the deformation rate increased from 0.03 to 0.94. While a thinner print thickness and larger aspect ratio allow the MOR to have the greatest deformation rate, it also reduces the support and stability of the connection part. In summary, to ensure both the deformability and structural stability of the MOR, the printed thickness of the connection part used in our subsequent experiments is 0.2 mm, and the aspect ratio is 8/2.

Motion control in complex environments

Figure 4 shows the schematics diagram and physical images of the motion of MOR driven by permanent magnets, including forward tumbling and rotation around the axis. As shown in Figure 4a and Figure 4b, the MOR can achieve clockwise tumbling, which is driven by a cylindrical permanent magnet that rotates around the x-axis and maintains a positive movement toward the y-axis. When the S pole of the cylindrical permanent magnet is close to the MOR, the MOR opens upward under the action of the magnetic field. By turning counterclockwise and moving the permanent magnet to the right at the same time, the MOR will make a clockwise tumbling motion under the magnetic field force. Continuing this process, when the permanent magnet rotation angle reaches 360°, the MOR resumes its initial state and moves 5.42 cm in the positive direction of the y-axis. As shown in Figure 4c and Figure 4d, in addition to tumbling, the MOR can also achieve rotation around the z-axis in the x–y plane. When the S pole of the cylindrical permanent magnet is tilted upward by 45° near the MOR, the opening of the MOR is tilted forward by 45° around the y-axis. By rotating the permanent magnet counterclockwise, while making a circular motion around the z-axis counterclockwise, the MOR will rotate around the z-axis under the action of the magnetic field along the trajectory of the permanent magnet’s motion. These two types of motion, tumbling and turning, enable MOR to cross obstacles and steer to avoid obstacles, respectively.

FIG. 4.

Schematic diagram of the MOR motion method. (a, b) are schematic and physical images of driving the MOR to roll forward by controlling a magnetic cylinder rotating around the x-axis, respectively. (c, d) are schematic diagrams and physical images of driving the MOR to rotate by controlling a magnetic cylinder rotating around the z-axis in the x–y plane, respectively. MOR, magnetic octopus robot.

As shown in Figure 5, trapezoidal continuous barriers and S-shaped bends were manufactured by 3D printing technology using PLA as the raw material. As shown in Figure 5a, the bionic robot can be driven over obstacles with complex morphology by controlling the MOR tumbling, as mentioned above. First, the N pole of the cylindrical permanent magnet is brought close to the MOR so that its opening is upward. Then, the cylindrical permanent magnet is flipped counterclockwise and moved to the right at the same time, and the MOR is driven by the magnetic field to roll to the right and over complex obstacles. After two cycles of tumbling of the cylindrical permanent magnet, the MOR successfully reaches the end point and resumes its initial state with the opening upward.

FIG. 5.

(a) MOR crossing an obstacle by tumbling motion, (b) MOR passing an S-shaped curve by turning. MOR, magnetic octopus robot.

As shown in Figure 5b, the bionic robot can be driven through a complex-shaped channel by controlling the rotation of the MOR, as mentioned above. The S pole of the cylindrical permanent magnet is tilted upward by 45° near the MOR and moves along the S-shaped curve at the same time. During the movement, the MOR is driven to turn by adjusting the direction of the cylindrical permanent magnet tilt, and finally, the MOR successfully passes the S-shaped curve (Supplementary Movie S1).

Underwater movement ability

In addition to being able to roll and move on land, MOR also has the ability to move amphibiously. As shown in Figure 6a, first, the S pole of the cylindrical permanent magnet approaches the MOR so that its opening is downward, rotates the magnet 180° counterclockwise and moves to the right, and the MOR dives into the water with the opening upward driven by the magnetic field in 3.7 s. Similar to Figure 5a, tumbling across the barrier, the cylindrical permanent magnet rotates 540° counterclockwise, while moving upward along the inclined plane, and the MOR tumbles to reach land under the drive of the magnetic field. Due to the fact that this process requires overcoming gravity and underwater resistance, the time taken was 9.3 s. This process demonstrates the excellent underwater movement of the MOR and its climbing ability.

FIG. 6.

(a) Optical image of MOR entering water from land and then returning to land by tumbling. (b) Optical image of MOR passing through the slit underwater. MOR, magnetic octopus robot.

In addition to the appealing motion method, the MOR can achieve end closure and maintain axis level when the side of the driven permanent magnet is close to the MOR (i.e., magnetic susceptibility level). In this way, the MOR can successfully pass through narrow gaps (6 mm) with a height of 37.5% of its initial height (16 mm). As shown in Figure 6b, the initial state of the MOR is with the opening upward, closing and keeping the axis horizontal under the action of the driving magnetic field. Due to the repulsive force between the 8 jaws, the overall height of the MOR is 8 mm at this point. Then, we shake the drive permanent magnet slightly and move it toward the slit, the MOR creeps toward the slit under the action of the magnetic field. The connection part of the OR touches the upper end of the slit first, and under the influence of the magnetic force, the MOR overcomes the repulsive force between the jaws through the slit with the help of the squeezing force of the slit. Finally, the MOR resumes its initial state on the other side of the slit. The MOR took about 8.0 s to successfully close, squeeze through the gap, and return to its original state. This process shows that MOR has good passability (Supplementary Movie S2).

Transportation of solid drugs

Other than having good motility, the MOR can also entrap, transport, and release drugs in a closed tube. As shown in Figure 7, a horizontally placed S-shaped pipe closed at both ends and filled with light blue liquid was used to simulate the complex environment that may be encountered in the application of MOR. The MOR was located at the beginning of the S-shaped pipe and the blue square drug is located at the end. First, the MOR moves quickly to the end of the S-shaped pipe under the driving of the magnetic field in 4.8 s. Then, the MOR opening was controlled to grasp the drug by adjusting the direction and distance of the driven magnetic field, according to Figure 4. Second, the magnetic induction line was controlled to be parallel to the MOR axis and return along the pipe at the same time, and the permanent magnet was driven close to the pipe to increase the magnetic field strength to maintain the clamping force of the MOR and prevent the drug from falling. Finally, the MOR carries the drug back to the origin of the tube, increasing the distance between the driving magnetic field and the MOR, which opens its eight jaws to release the drug under the action of the inherent elasticity of the material. The above process of clamping, transporting, and releasing drug shows that the MOR has good motility and long-range maneuverability (Supplementary Movie S3). This demonstrates that MOR has the potential to be applied in the medical field for contactless remote drug transportation.

FIG. 7.

Optical image of MOR entrapping and releasing a drug in a closed tube. MOR, magnetic octopus robot.

Transportation of liquid drugs

In addition to solid drugs, the MOR can be retrofitted with silicone hollow spheres to transport liquid drugs. The silicone hollow sphere is prepared by rotational solidification of a mold, which is made by FDM 3D printing technology. First, the mold with cavities inside was printed by an FDM 3D printer (JG-Maker, A5-S) with PLA as the raw material, and this mold was printed in two parts, to facilitate demolding, Figure 8a, b. Then, the pre-prepared silica solution (with drops of highly active platinum water to accelerate its solidification) was injected into the fixed mold through the top opening, Figure 8c. The mold was rotated for 10 min at room temperature, using centrifugal force to distribute the silica gel solution uniformly on the inner wall of the mold, Figure 8d. Then, it was placed at room temperature for 30 min to be completely solidified, after which the mold was removed, Figure 8e, f. Finally, the silicone hollow sphere is glued to the bottom of the MOR and the black ink is injected with a syringe, Figure 8g, h. A small opening with a length of 2 mm at the end of the hollow ball facilitates the spraying of the ink inside. As shown in Figure 8i, the inkjet robot is placed in water, and the robot opening is upward due to the buoyancy of the hollow sphere. Makeing the N pole of the magnet close to the robot, which causes its opening downwards. Controlling the distance between the drive magnet and the robot to change the angle of its opening, the silicone hollow ball will eject ink from the small opening under the squeeze of the MOR’s 8 jaws. Repeating the appeal process, the inkjet robot filled the container with ink at 17.3 s (Supplementary Movie S4).

FIG. 8.

The preparation process of the inkjet robot includes (a, b) 3D printing to manufacture the mold, (c, d) silicone hollow sphere molding process, (e, f) demolding to obtain silicone spheres with cavities inside, (g) silicone hollow spheres bonded to MOR to form the inkjet robot, (h) optical image of the inkjet robot (with ink), (i) inkjet process of the robot. MOR, magnetic octopus robot.

The process is similar to the biological behavior of an octopus, which ejects ink by driving a magnetic field to control the MOR to squeeze the silicone hollow sphere. The stronger the applied magnetic field, the smaller the distance between the MOR grippers and the more ink is ejected. In practice, the amount of inkjet can be controlled by calculating the strength of the driving magnetic field. The total ink jet volume depends on the size of the ink bladder. This process demonstrates that MOR can transport not only solids but also liquids by adding hollow spheres, and also has the potential for contactless remote transport of liquid drugs and targeted release.

Conclusion

In summary, we report a magnetically controlled bionic robot with excellent grasping and transport capabilities. Due to the use of PA6 as matrix and NdFeB as filler, this material can reach a tensile strength of 20.31 MPa, which means that it has the stiffness and structural stability to cope with complex environments (compared to soft material magnetron robots). The MOR printed with this material has great magnetic responsiveness and can deform up to 248° under the action of magnetic field. By controlling the distance and direction of the drive field, MOR can achieve fast and complex movements. In addition, the MOR’s eight-jaw structure allows it to roll and rotate to cope with complex terrain and access. MOR also has the ability to move amphibiously, allowing free movement between land and water. The MOR can change shape under the control of the driving magnetic field, which allows it to squeeze through slits whose height is only 37.5% of the original state of MOR. With its excellent movement, the MOR can not only grip, transport, and release solids but also transport liquids by adding silicone hollow ball. This study demonstrates the potential of MOR to be applied in the medical field to enable contactless remote transport of solid or liquid drugs. This research has the potential to be applied to surgery of the stomach and intestines. In addition, there is a need to further reduce the scale of the robot and improve its control accuracy.

Author Disclosure Statement

No competing financial interests exist.

Authors’ Contributions

F.C. (First Author): Conceptualization, Methodology, Software, Investigation, Formal Analysis, Writing—Original Draft; Z.X. (Corresponding Author): Conceptualization, Funding Acquisition, Resources, Supervision, Writing—Review & Editing; K.C.: Data Curation, Investigation; X.W.: Visualization, Investigation; S.J.: Resources, Supervision.

Footnotes

Funding Information

This research was funded by Key Project of Research and Development Plan of Hunan Province (grant no. 2021GK2025), Natural Science Foundation of Hunan Province (grant no. 2021JJ20009), and Scientific Research Foundation of Hunan Provincial Education Department (grant no. 22A0104). The authors are grateful for the support.

Supplementary Material

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