Micro Swarming Robots

Nothing to see here folks.

ARTICLEOnline now100626December 18, 2024Open Access

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Magnetic swarm intelligence of mass-produced, programmable microrobot assemblies for versatile task execution

Kijun Yang^1,2,9 ∙ Sukyoung Won^1,3,9 ∙ Jeong Eun Park^1,3 ∙ Jisoo Jeon⁴ ∙ Jeong Jae Wie^{1,2,3,5,6,7,8,10} jjwie@hanyang.ac.kr

The bigger picture

Swarm robotics has emerged as a promising methodology for accomplishing complicated tasks through the collective behavior of multiple robots. Through inter-robot communications, robotic swarms can execute terrain reconnaissance, pattern formation, and cargo transportation. However, in miniaturized robotic systems, robots possess low kinetic energy to operate in various environments owing to their low body mass. Furthermore, battery- and sensor-free actuation of robots complicates inter-robot communication, limiting the extension of their functionalities. Herein, we present multifunctional swarm intelligence capable of versatile task execution via mass-produced magnetic microrobot swarms with programmed assembly configurations. The versatile task execution via microrobot swarms exhibits significant potential for various applications in robotic engineering, expanding foundational technology for developing advanced collective robot systems.

Highlights

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Magnetic microrobots are mass produced via in situ replica molding and magnetization

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Configurations of robot assembly are programmed by designing magnetization profiles

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Microrobot swarms exhibit distinct features depending on the assembly configurations

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Swarm intelligence enables microrobot swarms to execute versatile tasks

Summary

Battery- and sensor-free actuation of microrobots complicates heterarchical inter-robot communication. Herein, swarm intelligence of magnetically anisotropic microrobots is presented via the programming of magnetic interactions between the microrobots to self-assemble along their longitudinal, intermediate, or horizontal axis. Mass production is implemented through in situ replica molding and magnetization for hundreds of anisotropic microrobots on a single microarray mold. Under a rotating magnetic field, the anisotropic microrobots autonomously engage in local magnetic interactions, forming a swarm with a high aspect ratio, high packing density, or high assembly stiffness. The microrobot swarms are deployed to perform self-climbing, self-throwing over an obstacle, lifting of an obstacle, cargo transportation, wire connection and disconnection, liquid metal shape modification, tube unclogging, and organism guiding. The versatile task execution by mass-produced magnetic microrobots offers insights into high-throughput processing and swarm control of miniaturized robots, expanding the functional capabilities of robot collectives.

Graphical abstract

Keywords

1. microrobot swarms

2. swarm intelligences

3. magnetic actuations

4. mass productions

5. programming assembly configurations

6. magnetic microrobots

7. microrobots

Device Translation and Integration

1. DTI-2: Explore

Introduction

Swarm robotics has emerged as a promising methodology for accomplishing complicated tasks through collective behavior of multiple robots.^1,2,3 A key feature of swarm robotics is fault tolerance: while a single robot or multiple robots in a swarm may fail to complete an allocated mission, other robots continue to perform their programmed motions until the mission succeeds. The fault tolerance of swarms can be enhanced by organizing multiple robots through physical linkages or sensor-driven wireless communication.^4,5 Autonomous cooperation of multiple robots can improve fault tolerance and expand the functionality of swarms. Autonomous cooperation requires decentralized, heterarchical self-organization, in which individual robots in close proximity communicate with each other through local interactions. A decentralized organization of multiple robots can attain swarm intelligence, a phenomenon commonly observed in nature. For instance, ants can organize themselves into a high-aspect-ratio assembly to bridge disconnected paths by physically gripping each other.⁶ Through gripping, the organization of ants can be shaped into a raft-shaped assembly to survive floods.⁷ Moreover, ants can forage for feed via chemical communication using pheromone trails⁸ and transport feed through autonomous cooperation.⁹

The multifunctional swarm intelligence of social insects serves as an inspirational model for swarm robotics. However, in miniaturized robotics, battery- and sensor-free microrobots suffer from swarm control problems. To realize multifunctional swarm intelligence, microrobots require wireless communication without traditional onboard sensors. Actuation of microrobot swarms relies on external stimuli, such as lights, acoustic fields, and magnetic fields.^{10,11,12,13,14,15,16} Wireless communication and autonomous cooperation of microrobots are more intricate than those of traditional mechatronic robots. Furthermore, owing to their low body mass, miniaturized robots possess low inertial forces and kinetic energy to overcome viscous forces or interfacial energy from their surroundings.¹⁷ Although an increase in the number of miniaturized robots can ameliorate this constraint, the inherently low kinetic energy of individual robots limits the extension of their functionalities. Microrobot assemblies may offer a solution to this challenge. In particular, magnetic robots can communicate with each other via magnetic dipole-dipole interactions, allowing self-organization of magnetically assembled structures.^12,18,19 Multiple magnetic robots can reversibly assemble and disassemble based on competition between magnetic attraction and friction applied to the joints of assembled robots.²⁰ Through the magnetic assemblies, spherical magnetic robots have demonstrated functionalities, such as forming assembled patterns, navigating narrow channels, transporting cargo, and building conductive pathways between disconnected electrodes.^21,22,23 These tasks are impossible for a single spherical robot, showing the advantages of collective behavior through magnetic robot swarming.

Herein, we present multifunctional swarm intelligence of magnetically anisotropic cuboid microrobots, capable of self-climbing, self-throwing over an obstacle, lifting of an obstacle, cargo transportation, wire connection and disconnection, liquid metal (LM) shape modification, tube unclogging, and organism guiding. To obtain swarm intelligence, microrobots required autonomous cooperation via magnetic interactions among the microrobots alongside endurance to undesired disassembly. Hence, we considered inter-dipolar potential energy between microrobots. The cuboid microrobots can have magnetic anisotropy by encoding magnetization profiles. The programmed magnetization profiles allowed directional magnetic interactions among microrobots, the direction of which minimized the inter-dipolar potential energy. The microrobots enabled magnetic self-organization into the deterministic one-dimensional (1D) assemblies. With the respect of microrobot geometry, a cuboid—a three-dimensional (3D) shape with six rectangular faces— was introduced to endure microrobot disassembly by external shear force, termed assembly stiffness. While spherical microrobots assemble into a 1D chain through point-to-point contact, an interface between the assembled cuboid microrobots is generated through area-to-area contact. At a given characteristic length, cuboid microrobots possess a higher contact area compared to spherical microrobots, increasing inter-robot magnetic attractive force. The endurance to disassembly facilitates achieving multifunctionality in situations where a high shear force is applied to the microrobot assembly.

In addition to robot geometry, microrobot materials were considered for assembly stiffness. Epoxy with a high elastic modulus (∼5.2 GPa) was utilized as the matrix material of the microrobot, thereby enhancing assembly stiffness by minimizing dissipation of elastic energy. Within epoxy-based microrobots, ferromagnetic neodymium-iron-boron (NdFeB) microparticles were dispersed for encoding magnetization profiles of the microrobots. Owing to the inherent magnetic property of NdFeB particles as hard magnets with high residual magnetization, the microrobots maintained their programmed magnetization profiles even after removing the magnetic fields.²⁴ Assembly stiffness can be directionally enhanced by the programmed magnetization profiles. Microrobot swarms comprising microrobot assemblies with different assembly configurations were allocated to perform versatile tasks. Also, the deterministic assembly configuration facilitated the microrobot swarms to execute allocated tasks by empowering a high aspect ratio, high packing density, or high assembly stiffness of the respective microrobot assemblies, which will be discussed in the results and discussion section.

Background

Mass production of magnetically anisotropic microrobots

Mass production is a crucial requirement in preparing microrobot swarms for securing dimensional and magnetic uniformities across multiple microrobots. In particular, a uniform magnetization profile in the multiple microrobots is important to control microrobot assemblies. Random magnetization profile of the microrobots may result in unpredictable assembly, implying that the microrobots are unable to obtain a deterministic assembly configuration. Toward simultaneous production of hundreds of magnetically anisotropic yet dimensionally uniform microrobots, we introduced in situ replica molding and magnetization (Figure 1A), which drew inspiration from our previous studies on replica molding^25,26,27 of micropillar arrays. We designed a negative microarray mold composed of hundreds of cuboid structures with the dimension of 300 × 300 × 600 μm³ (width, length, and height). Using a single mold, magnetically anisotropic microrobots can be consecutively replicated, enabling mass production with time and cost efficiencies. This in situ process can ensure uniformity in both microrobot geometry and magnetization profile.

Results and discussion

Spinning motion of magnetically anisotropic microrobots

The assembly stiffness of microrobot assemblies is determined by the contact area-to-volume ratio (CA/V) between microrobots and the assembly thickness (Figures 2A and 2B). An increase in CA/V enhances the magnetic force among microrobots, resulting in higher assembly stiffness. Through experiments, the CA/Vs were measured to be 3.1, 4.6, and 6.2 mm⁻¹ for the HT, SC, and FF assemblies of 50 microrobots, respectively. These experimental results agreed with theoretical calculations, where the CA/Vs can be calculated as 𝑛−1𝑎⁢𝑛, 1.5⁢(𝑛−1)𝑎⁢𝑛, and 2⁢(𝑛−1)𝑎⁢𝑛 for HT, SC, and FF assemblies, respectively (Note S1). Here, a represents the short axial length of the robot, and n represents the number of assembled robots. Therefore, at a = 0.3 mm and n = 50, the CA/Vs of HT, SC, and FF assemblies are approximately 3.2, 4.9, and 6.5 mm⁻¹, respectively. Furthermore, the assembly stiffness of microrobot assemblies was primarily determined by their thickness, as the structural stiffness of an object is related to its thickness when the material modulus is identical. The randomly dispersed magnetic particles contributed to a consistent modulus across robots. As a result, the FF assembly with a high CA/V exhibited a larger assembly thickness, acquiring higher assembly stiffness compared to HT and SC assemblies. The longitudinally aligned HT assembly with a low CA/V could obtain longer assembly lengths compared to SC and FF assemblies. The long HT assembly length resulted in high-aspect-ratio assembly, which is defined as the ratio of the long axial length to the short axial length for microrobot assembly. The FF assembly length was the shortest one due to the transversal alignment of its assembly.

So should Stew get rhese?