Fabrication of Magnetic Microrobots by Assembly

Magnetic microrobots have gained significant attention in the biomedical field due to their wireless actuation, strong controllability, fast response, and minimal impact on the environment. As the task complexity keeps increasing in the clinical applications of magnetic microrobots, more geometric structures and magnetization profiles have been included in the designs of magnetic microrobots, posing significant challenges to the fabrication of magnetic microrobots. Microassembly is a fabrication method that can create convoluted structures with small‐scale modules. It can accurately control the position and orientation of each magnetic module, resulting in a magnetic microrobot with arbitrary 3D geometries and magnetization profiles. This article reviews recent advanced assembly‐based fabrication methods of magnetic microrobots, including microassembly driven by contact mechanical forces and noncontact field forces. The principles, fabrication processes, and the advantages and disadvantages of each assembly‐based fabrication method are summarized. The existing challenges and future development of fabricating magnetic microrobots by assembly are discussed in detail. It is believed that this review will provide a methodological reference and inspire new ideas for manufacturing powerful magnetic microrobots in future biomedical applications.


Introduction
Microrobots, due to their unique feature of minute size, [1][2][3] have the potential to perform tasks in hard-to-reach sites, such as microtubes, biochips, and blood vessels in living organisms.[24][25][26] However, the clinical applications of magnetic microrobots require more advanced capabilities based on arbitrary geometry design, flexible material selection, and programmable magnetization patterns, which pose significant challenges in the fabrication process.
In the past two decades, various magnetic microrobot fabrication methods including stereolithographic 3D printing, [27,28] extrusion printing, [29] heating, [30] mold casting, [3,31] laser cutting, [32][33][34][35] and chemical self-assembly or synthesis [36,37] have been proposed.However, in magnetic microrobot fabrication based on stereolithographic 3D printing, programming arbitrary magnetization of neighboring voxels is restricted by the strong local interaction of magnetic particles inside uncured liquid photoresists. [27]The demanding requirement of storage modulus, the deterioration of performance caused by the addition of magnetic particles, and the extrudate swell of the soft elastomers (SEs) all make it difficult for extrusion printing to fabricate finer magnetic soft fibers to wave complex structures. [29]The laser heating can only penetrate a limited depth, which hinders the magnetic (re)programming of 3D structures. [30]Mold casting is limited to 2D structures due to challenges in separating the cured microrobot from the mold with intricate 3D structures. [31]aser cutting is a technique solely utilized for cutting planarized patterns.The chemical self-assembly or synthesis process lacks controllability, making it difficult to achieve precise 3D geometries and magnetic programming. [32]Thus, magnetic microrobots with arbitrary geometry design, flexible material selection, and programmable magnetization patterns for conducting multiple complex tasks call for more advanced fabrication methods.
Microassembly is the process of constructing micromachines with multiple components using high-precision micromanipulation technology.By precisely assembling premagnetized micromodules, it can create magnetic microrobots capable of integrating multiple materials and forming sufficient local magnetic torque vectors in desired orientations.Through this method, the design space of the magnetic microrobots becomes larger and the machine functionality is enhanced substantially.This fabrication method can greatly extend possibilities toward microrobots with reprogrammable magnetization, shape reconfiguration, and multiple functions.By selecting multiple materials and finely controlling the position and orientation of the assembled modules with sizes ranging from microns to millimeters, various microassembly methods can create powerful microrobots tailored to specific biomedical applications.
In this review, focusing on the fabrication of functional magnetic microrobots, various microassembly methods driven by contact mechanical forces and noncontact field forces are reported (see Figure 1).The working principles, assembly processes, and specific applications of different microassembly methods are described.Focusing on the mechanical microassembly, the tool-assisted manual assembly and the assembly combined with 3D printing are introduced from the perspective of the dimension of processing and the magnetic properties of microrobot fabricated.The emerging biohybrid microrobots fabricated by cell-adhesion-driven self-assembly are also introduced.In addition, the field-driven microassembly, including magnetic, acoustic, electric, and optical forces and their combinations are discussed.The biomedical applications of magnetic microrobots fabricated by microassembly are highlighted, ranging from targeted delivery, minimally invasive surgery, and cellular measurement to intelligent sensing.Next, the limitations of the existing microassembly methods are briefly discussed in terms of accuracy, efficiency, and applicability.Finally, this article identifies the main challenges of magnetic microrobots fabricated by microassembly and provides the future directions of this emerging field.

Fabrication by Contact Assembly
As the name suggests, "contact assembly" refers to microassembly driven by contact mechanical forces.The fabrication strategy of this method is to assemble the premagnetized modules or assemble the modules while magnetizing.47][48][49][50][51][52] Fabrication by contact assembly includes fabrication by toolassisted manual assembly, fabrication by microassembly combined with 3D printing, and fabrication by cell-adhesion-driven self-assembly.Their principles, fabrication steps, and applications are described in the following sections.
Figure 1.Assembly methods for the fabrication of magnetic microrobots, including contact assembly and noncontact assembly.The former is divided into three types: tool-assisted manual assembly, assembly combined with 3D printing, and cell-adhesion-driven self-assembly, while the latter is divided into four types: magnetically driven assembly, acoustically driven assembly, electronically driven assembly, and optically driven assembly.

Tool-Assisted Manual Assembly
In general, the process of the tool-assisted manual assembly involves the steps illustrated in Figure 1.First, the SEs with embedded hard magnetic microparticles are filled into the photoresist molds.The molds bearing the composites are placed in a vacuum chamber and degassed to remove trapped air.After the magnetic elastomeric composites become partially cured, the electromagnets are employed to apply a uniform magnetic field to align the premagnetized particles in the formed modules.For more complex magnetization directions of the modules, assistant jigs can be used to provide mechanical fixing for controlled fabrication precision and repeatability. [32,53,54]Then small hand tools, such as tweezers, are used to pick up the cured modules and move them for positioning and orientation.Finally, modules are assembled with the bonding agent (e.g., liquid noncrosslinked SEs, universal glues, and optical glues) applied to connect neighboring modules.In addition, for the fabrication of planarization modules with small sizes and complex outlines, the magnetic elastomeric composites can be spin-coated on a clean substrate and baked to form a solid layer.Then, the laser cutting can be used to form arbitrary patterns with a high-quality edge on the solid layer.
Recently, tool-assisted manual assembly has been widely utilized in magnetic microrobot fabrication.Zhang et al. designed an environment-aware untethered 12-legged microrobot capable of locomotion and self-gripping, as shown in Figure 2A. [55]The 12 legs were made by casting uncured magnetic-responsive elastomers (MREs) into negative molds with predefined in-plane geometries.Then the cured legs were taken out and magnetized to program homogeneous magnetization profiles into them.Next, the legs were placed into a second negative mold with accommodating geometric features to fit the legs while leaving space for the connection pads.The central body of the microrobot was made of liquid crystal elastomers (LCEs), which were laser cut from a LCE film.Then the central body was aligned and placed on top of the uncured SE, which had been cast into the second mold.Finally, the legs were connected to the central body via the connection pads and the magnetic microrobot was fabricated.The flexible material selection provided by assembly offers enormous design freedom and abundant degree-of-freedoms (DoFs) due to LCE's programmable director field, the MRE's programmable magnetization profile, and diverse geometric configurations.This microrobot is thermally responsive, and it can be considered to use light as the control input to improve spatiotemporal resolution and actuation speed for better performance in the future.Giovanni Pittiglio et al. designed patient-specific magnetic catheters for atraumatic autonomous endoscopy. [56]As shown in Figure 2B, after cast from silicone doped with magnetic microparticles (NdFeB), each segment was subsequently fixed with a specific rotational alignment angle by using guide pins and bespoke printed trays for magnetization.Then magnetic segments were transferred to a second mold and arranged by using their indexing features, whereas undoped silicone was injected into the mold.Finally, the soft continuum magnetic robots (SCMRs) were fabricated after curing and demolding.To improve navigation and reduce contact with the environment, it is necessary to conduct nonplanar magnetization in the fabrication process in the future.Wu et al. fabricated magnetically controlled stretchable origami robotic arms, which can achieve multimodal deformations. [57]he Kresling units and magnetic plates are assembled using Sil-Poxy adhesive and the highly integrated motion of the robotic arms is attributed to inherent features of the reconfigurable Kresling unit and the fabrication method by assembly of multiple units.Wu et al. designed adhesive robot footpads bonded to both sides of the robot's soft body by Ecoflex 00-30 to fabricate a microrobot capable of climbing on 3D complex terrains. [58]Different adhesive robot footpad designs can be used for different 3D surfaces with complex geometries and different surface properties.This fabrication method by assembly provides more material selection and increases the reconfigurability and environment adaptability of the microrobots.For clinical trials that require immersion in a liquid environment within an organism, it is necessary to develop advanced underwater bioadhesives to complete the assembly of footpads, avoiding detachment during climbing tissue surfaces.
The existing magnetically activated soft robots cannot close the loop between sensing, signal processing, and actuation. [59]In this respect, the assembly-based fabrication method can simultaneously integrate programmable structural changes and various functional modules on a magnetic microrobot for the ability to perform multiple tasks.Dong et al. constructed an untethered magnetic soft microrobot with programmable magnetization and integrated multifunctional modules. [60]As shown in Figure 2C, the programmed magnetization modules were directly embedded into the adhesive sticker layers.Then the spatially distributed functional modules including temperature and ultraviolet (UV) light sensing particles, pH sensing sheets, oil sensing foams, positioning electronic components, circuit foils, and therapy patch films were integrated into a soft microrobot.The versatility and adaptability of magnetic microrobots can be adopted in engineering modular soft material systems.Moreover, smaller soft robots are required in biomedical applications such as surgeries in the gastrointestinal tract and nasal cavity.Therefore, on-chip fabrication technology and rolled-up technology can be employed in fabricating this microrobot with a much smaller size for future clinical applications.In addition, the magnetization profiles of this microrobot are fixed.In the future, it can be improved by locally changing the physical properties of the adhesive stickers and further reprograming the magnetization profiles.
To freely create microrobots with arbitrary 3D structures and magnetization profiles, Zhang et al. proposed a jig-assisted 3D heterogeneous bottom-up assembly approach. [61]It can assemble multimaterial heterogeneous microscale building blocks, which are called "voxels".As shown in Figure 2D, a 3D ring microrobot was fabricated by this jig-assisted assembly.First, two magnetized voxels were plugged into the jig and the long slit of the jig was to avoid the strong magnetic interaction between the voxels.Then the bonding agent was applied through the cap opening to form the edge bonding.Next, the magnetic and nonmagnetic voxels were sequentially plugged into a 2D and a 3D jig, forming a 3D ring by face bonding.These jigs can provide guidelines and references for controlled fabrication precision and repeatability, preventing the voxels from collapsing into each other or flying away during fabrication.This manual assembly method improves the achievable complexity of magnetic microrobots and boosts their capabilities in the biomedical field.To improve their biocompatibility, more biocompatible materials, such as gelatin and ferromagnetic FePt nanoparticles, can be used as voxels for assembly.
In recent years, more and more fabrication methods combining microassembly with 3D printing have been applied to the production process of magnetic microrobots.Hu et al. used two-photon polymerization (TPP) to selectively link Janus microparticle-based magnetic microactuators by 3D printing of soft or rigid polymer microstructures or links. [96]The general fabrication process is illustrated in Figure 3A.First, each magnetic microactuator is rolled to reach the target position under a rotating magnetic field, and its orientation is controlled by applying magnetic field-based torques.Subsequently, it is temporarily anchored on the glass substrate using TPP, with polyethylene glycol (PEG) diacrylate hydrogel as the anchoring sacrificial material.Next, 3D microprinting technology is used to create a ringshaped holding polymeric structure on the anchored microactuator for subsequent connection.After all the microactuators are fixed on the substance, 3D structures or links are 3D-printed between all fixed microactuators by a soft or hard polymer to connect each other.The linked 2D microactuator networks formed by this 1D microassembly method exhibit programmed 2D and 3D shape transformations.Due to the combination of the highresolution 3D printing, the overall sizes of the fabricated microrobots can be less than 100 μm.This fabrication method provides diverse programmed shape transformations and functions for future biomedical and organ-on-a-chip applications at cellular scales.
However, the fabrication speed of assembling the microactuator networks is limited by the one-by-one serial assembly procedure of each microactuator and multistep procedures for linking the microactuators.The fabrication method of 2D-patterned assembly is applied to solve this problem.This method does not target each magnetic particle, but orients all the magnetic particles in the desired region according to the designed pattern, to create a microrobot with arbitrary 2D geometries and magnetization profiles.Xu et al. proposed a fabrication strategy based on UV lithography to encode magnetic particles in planar materials with arbitrary 3D orientation at the submillimeter scale, as shown in Figure 3B. [27]The premagnetized permanent magnetic particles were reoriented precisely and then the UV resin was cured selectively to pattern the local magnetization.A magnetization feature size roughly one order of magnitude bigger than the particle size was recommended to maintain reasonably homogeneous magnetic moment density.This fabrication method by programing discrete 3D magnetization in flexible materials provides arbitrary distribution of magnetic torque for the planar actuators.The fabricated microrobots can have higher order and multiaxis bending, large-angle bending, and combined bending and torsion in one sheet of polymer through this method, which was unachievable previously.The method of creating double-layer structures is also demonstrated in their work.However, it is based on manually changing molds and adding composite materials during the fabrication process, which is not efficient enough and labor-intensive for producing 3D microrobots.In future clinical applications, due to the heavy metal toxicity of NdFeB magnetic particles, samarium cobalt or ferrite permanent magnetic particles can be considered as substitutes.Kim et al. proposed an in situ fabrication strategy that allows the free programming of heterogeneous magnetic anisotropy for unlimited shapes at the microscale, as shown in Figure 3C. [28]his fabrication process combines the self-assembling behavior of superparamagnetic nanoparticles with a spatially modulated photopatterning process.The microactuators for which all parts move in different directions under a homogeneous magnetic field were fabricated by repetitively tuning the nanoparticle assembly and fixing the assembled state using rapid photopolymerization.This microassembly method, combined with 3D printing, enables the creation of polymeric nanocomposite actuators capable of 2D and 3D complex actuation, which is difficult to achieve by conventional fabrication processes.Song et al. presented a shape-programming strategy that can program the magnetic moment in the soft matrix by printing diverse magnetic structural elements. [97]In this method, a 3D printer with FDM is employed for manufacturing the various oriented magnetic structural elements and encapsulated mold to form magneto-active soft materials (MASMs).Subsequently, the liquid silicone rubber (SR) is poured into the mold and cured at room temperature.The fabricated MASMs have fast, reversible, Figure 3. Microassembly combined with 3D printing.A) Schematic diagram of the two-particle chain fabrication process.Reproduced with permission. [96]Copyright 2021, AAAS.B) The system for patterning discrete 3D magnetization and a two-layer structure with horizontal and vertical magnetization components fabricated from it.Reproduced with permission. [27]Copyright 2019, AAAS.C) Schematic of the magnetic axis fixing process in the fabrication of polymeric magnetic microactuators.Reproduced with permission. [28]Copyright 2011, Springer Nature.D) Schematic of the scheme for 3D printing magnetic structures with custom anisotropy.Reproduced with permission. [98]Copyright 2015, IS&T.E) Schematic diagram of the fabrication system and multilayer printing process.Reproduced with permission. [99]Copyright 2023, Wiley-VCH.F) Schematic of the multistep DLW-molding process.Reproduced with permission. [100]Copyright 2022, Springer Nature.
programmable, and stable shape transformation properties, which are favorable in soft robotics, medical care, and bionics applications.
However, the above fabrication methods are limited to 2D microassembly and it is difficult to construct 3D complex structures.An approach proposed by Garrett Clay et al. involves 3D printing of magnetic materials with programmatically controlled anisotropy, as shown in Figure 3D. [98]The magnetic ink composed of magnetic nanoparticles in a UV-curable resin is jetted on the substance from a nozzle.For printing multiple layers to fabricate 3D magnetic structures, the ink is UV-curable and each layer is allowed to be hardened prior to printing the next.A controllable magnetic field is applied to precisely align the anisotropy as each droplet of ink containing magnetic nanoparticles is printed.This fabrication by layer-by-layer printing solves the problem of unsaturated magnetic particle orientation due to the difficulty of UV light penetrating too thick composites embedded with magnetic particles.In the future, the formulation of jetable and UV-curable inks with high magnetic nanoparticle loading and stability can be developed.However, this method of layer-by-layer magnetic ink printing is still limited in the flexibility of fabricated microrobots, and a method that can arbitrarily create 3D geometries and magnetic profiles in both 2D plane and vertical direction is required.Li et al. improved on their previous work [27] to achieve multilayer patterned 3D printing. [99]To solve the problem of manual adjustment during the fabrication process, a vertical motion stage was used to control the motion of the build plate, and multilayer printing was achieved by the increase of the small gap formed between the builder plate and the substrate, as shown in Figure 3E.The surrounding magnetic slurry flowed in and filled the gap due to its weight when the build plate was lifted, thus solving the problem of manual container replacement and material addition.All the premagnetized magnetic particles were first oriented by the magnetic field for alignment, and then UV light was projected to fix their direction mechanically by polymerization of the regions and freeze of the particles.Combined with the selective region curing in the 2D plane, [27] 3D microrobots can be fabricated by repeating the same steps.Compared with the single-layer magnetic microrobots, the microrobot fabricated by this method has a wider design spectrum, and the magnetic torques are enhanced greatly through the stack of multiple layers to obtain a higher strength for actuation.The maneuverability of the microrobot structure is also enhanced by the integration of various materials due to this assembly-based fabrication.However, the proposed fabrication method has limitations in terms of magnetization intensity and continuity.Due to the use of a medium-intensity alignment field to orient premagnetized NdFeB nanoparticles, the magnetization intensity is low and the structural deformation of the printed shape is limited.Moreover, it takes longer to form a smooth and continuous magnetization profile because the magnetization is discretely patterned for each individual segment.Liu et al. presented a facile fabrication strategy for creating 3D magnetic functional microdevices by molding-integrated DLW, as illustrated in Figure 3F. [100]For the positive photoresist solid layer, the femtosecond laser beam was focused and scanned to only expose the photoresist at the areas intended to be removed later.Then the photoreaction was completed by the postexposure bake, resulting in the exposed photoresist being soluble in the developer.The cavities were left as molds for filling the desired microrobot structure, and then the composites of magnetic particles and elastomers were poured into the molds under the external magnetic field to orient the particles along the desired direction.This whole exposure-molding cycle can be repeated with different programmed directions of magnetic materials because the unexposed part of the photoresist still exhibits photoreactivity.Multiple photoresist layers can also be prepared to mold 3D complex structures with heterogeneous materials for microrobot fabrication.It should be noted that this method is not applicable to those whose properties and geometries may be affected by repeated heating (at 100 °C) and developing steps or solvents.
In recent years, many fabrication methods combining microassembly and 3D printing have emerged, such as the transfer printing approach based on selective surface adhesion tuning, [101] modular 4D printing with great potential in future magnetic microrobot fabrication, [102] and so on.Compared to conventional fabrication by magnetizing after 3D printing, [103] this assembly-based 3D magnetic printing method makes it easier to create magnetic microrobots with heterogeneous materials, complex geometries, and arbitrary magnetic profiles.

Cell-Adhesion-Driven Self-Assembly
Cell-adhesion-driven self-assembly is a novel microassembly method for creating biohybrid microrobots through the external and internal adhesion forces of cells.[106] The cell-adhesion-driven self-assembly methods for biohybrid microrobots fabrication are to use external adhesion force during cell growth to assemble cells with magnetic scaffolds, or to use the internal adhesion force of the cell matrix to trap magnetic particles that are swallowed into cells.The magnetic microrobots fabricated by these methods have natural biocompatibility, and the fabrication process does not require external intervention.Li et al. presented a magnetic microrobot capable of carrying and delivering targeted cells, as shown in Figure 4A. [107]This microrobot has a burr-like porous spherical structure fabricated by 3D laser lithography, with a Ni-coated surface for magnetic actuation and a Ti-coated surface to improve biocompatibility.After the structure was fabricated, it was assembled with MC3T3-E1 cells and mesenchymal stem cells through coculture for 12 h.The microrobots fabricated by this method can achieve the transport and delivery of targeted cells in vivo.Go et al. also proposed a magnetic microscaffold that was assembled with targeted mesenchymal stem cells through coculture for potential applications in articular cartilage repair. [108]To achieve the clinical application of this method, two key issues need to be addressed.On the one hand, effective in vivo imaging technology needs to be applied to deep tissues for automated real-time tracking control.On the other hand, 3D biodegradable structures with high mechanical strength need to be developed.Another method of utilizing internal cell adhesion for assembly relies on the phagocytic function of specific cells such as phagocytes.Lin Feng et al. designed magnetized macrophage cell microrobots for targeted drug delivery, as shown in Figure 4B. [109]The carriers of the biohybrid microrobots were mouse macrophages, which can swallow Fe 2 O 3 particles with a diameter of 10 nm.During the coculture process, macrophage cells were magnetized through endocytosis of macrophages.Due to the natural biocompatibility and biological properties of macrophages, these assembled microrobots are considered very promising platforms for intelligent drug delivery.Dogan et al. presented live immune cell-derived antitumorigenic microrobots, which were assembled through macrophages engulfing the engineered magnetic decoy bacteria, [110] similar to the work by Feng et al. [109] This method of constructing medical microrobots by freshly isolating living cells from the biological body has great potential in clinical applications.When they are injected into the biological body, the body recognizes them as its own. [6,111]More importantly, living cellbased therapies can perform sustainable long-term therapeutic functions that conventional drugs and biologics cannot.Moreover, due to the small size of cells and the limited number of engulfed magnetic nanoparticles, as well as the individual differences and biological properties of cells, the movement of cellular robots is quite slow.Future work will focus on improving the magnetic field strength and magnetization of cell robots.

Fabrication by Noncontact Assembly
Noncontact assembly refers to the self-assembly of magnetic microrobots by approaches of indirect contact, that is, the use of magnetic, acoustic, electrical, optical, fluidic, aerodynamic, [112] and other field forces.It can achieve the organization of diverse microcomponents into a desired structure, which is driven by physical interactions between the components spontaneously.The external fields can supply energy for powering the locomotion of the microrobots and direct physical interactions for assembling the subunits.The microrobots comprising these modular units enable the incorporation of reconfigurable and multiple functionalities and the ability of multimodal locomotion by the relative configuration of the components.The understanding of operational dynamics between the different components and the engineering of the assembly pathways through physical interactions are required for microrobot fabrication by noncontact assembly.In the last decade, various fabrication methods driven by noncontact field forces have emerged and the programming of physical interactions in individual components is achievable by taking advantage of the shape-and materialspecific force response under the external fields.Their principles, fabrication processes, and applications are described in the following sections.

Fabrication by Magnetically Driven Assembly
A commonly used approach for fabricating magnetic microrobots is magnetically driven assembly, which will be introduced in this section.In the fabrication process, the magnetic force is used for positioning magnetic components to specific positions driven by magnetic field gradients, [113] and the magnetic torque is used for orienting the direction of each magnetic component under a rotating magnetic field.Herein, magnetically driven microassembly is divided into two main categories according to the types of assembly objects.The first involves using micromagnetic modules with complex geometric structures to build microrobots.The second category involves magnetic microrobotic swarms formed by self-assembly.

Assembly of Micromagnetic Modules
A microassembly method similar to building Lego blocks has been developed.The fabricated miniature magnetic microrobot modules interact with each other under a specific magnetic field, allowing them to be organized into a desired pattern.As the external magnetic field changes, the diverse real-time morphologies of microrobots can be assembled selectively for the unstructured environments, and the multimodal locomotion behaviors can also be configured based on the on-site task requirements.This assembly-based fabrication method offers flexibility and versatility in navigating and performing diverse tasks in perturbed and confined environments.
Since the concept of cellular robots in 1988, [114] modular microrobots have been receiving increasing attention due to their exciting potential for payoffs.Ji et al. proposed a magnetic miniature propeller assembled by two modules, allowing for 3D locomotion and upstream swimming. [115]The two modules have identical structures but opposite magnetism for further magnetic docking.As shown in Figure 5A, the minipropellers are assembled under self-guidance from magnetic attraction due to the helical design.The directional self-assembly and  [107] Copyright 2018, AAAS.B) Diagram and light microscopic graph of the fabrication process of magnetized macrophage microrobots.Reproduced with permission. [109]Copyright 2021, IEEE.
on-demand disassembly can be conducted by regulating programmable magnetic fields.It can greatly decrease cost, and save complex hardware arrangements, [116] endow microrobots with increasing adaptability in complex environments and the ability to travel through a 3D maze.Yang et al. reported a millimeterscale cellular robot that can reconfigure its morphologies and behaviors simultaneously, as shown in Figure 5B. [117]The microrobot consists of two types of units (long and short units), which can be magnetized and demagnetized by the application and removal of a magnetic field for on-demand assembly and separation of units.The multiple configurations of morphologies are achieved under effective path planning.The microrobots fabricated by this reversible, controllable, and multiplex heterogeneous assembly can configure multimodal locomotion behaviors, demonstrating the potential for applications in unstructured environments with narrow space and high barriers.Rogowski et al. also designed miniature modular cuboids for 2D locomotion and collaborative assembly with any applied magnetic field. [118]They can be developed to capture objects, transport them within the working space, and subsequently release the payload in a new location.
With the increase in the structural complexity and number of micromagnetic modules, the assembled microrobots can have arbitrary geometries and magnetic profiles.Bhattacharjee et al. presented reconfigurable modular cubes, as shown in Figure 5C. [119]Two types of modular cubes (blue and red cubes) were fabricated by embedding eight cylindrical permanent magnets in the holes of a 3D-printed subunit, and the orientation of the magnets' north/south poles was different for further magnetic interaction.The assembly process was conducted in a bounded workspace, where a uniform magnetic field was applied.Figure 5C shows the process of the assembly and disassembly of six modular cubes.These modular microrobots can reconfigure themselves to create different tools that could be useful for complex mesoscale fabrication.The complexity of self-assembly/disassembly can be increased in the future through the use of more cubes and 3D constructions.Gu et al. proposed a magnetic quadrupole module that can form stable and frustration-free assemblies with arbitrary shapes and magnetizations, as shown in Figure 5D. [120]The magnetization of overall assemblies at the single module level can be programmed because each module has a tunable dipole moment, and a combinatorial design method is proposed to form desired patterns for the design of minimally invasive medical devices.This assembly approach is similar to the work by Bhattacharjee et al., [119] but the modules in this work are not uniformly controlled by applying a magnetic field.The magnetic tweezers, positioned under the workbench, are used to move the individual quadrupole on a thin brass substrate, as shown in Figure 5D.The fabrication process of magnetic modules is limited by the resolution of 3D printers, commercially available permanent magnets, and the process of manually manipulating permanent magnets.Further efforts can focus on overcoming these fabrication limitations using similar quadrupole designs and 3D robotic hands to manipulate magnets.
The concept of magnetic module assembly has become a widely adopted strategy for microrobot fabrication.Han et al. implemented sequence-encoded colloidal-scale assemblies from patchy magnetic cubes, which can be used for transporting cells and colloidal origami. [121]Kuang et al. proposed a method of seamless welding after module assembly to create deformable magnetic responsive materials. [122]Cheng et al. proposed a self-healing supramolecular magnetic elastomer, which is modularizable for customized constructs and functions. [123]In addition, magnetic biohybrid microrobots fabricated by assembly have also been highly favored in recent years.Gong et al.Reproduced with permission. [115]Copyright 2021, IEEE.B) Selective assembly process of milli-scale cellular robots that can reconfigure morphologies.Reproduced with permission. [117]Copyright 2022, Springer Nature.C) Self-assembly and disassembly of magnetically controlled modular cubes.Reproduced with permission. [119]Copyright 2022, IEEE.D) View of the assembly stage and the assembled π-shaped microrobot.Reproduced with permission. [120]Copyright 2019, AAAS.
presented magnetic biohybrid microrobot multimers based on Chlorella cells, which are reconfigurable via attraction-induced self-assembly and repulsion-induced disassembly due to magnetic dipolar interactions. [124]The biohybrid microrobots exhibit excellent biocompatibility with minor cell toxicity and have significant potential for targeted drug delivery including anticancer therapy.

Magnetic Microrobotic Swarms Formed by Self-Assembly
[127][128] For a single microrobot, the loading capability for drugs cannot meet the requirements due to its small sizes and volumes, and real-time in vivo imaging is also challenging.Microrobotic swarms can maintain controllable patterns during locomotion with a proper global input and their patterns can be reconfigured by tuning the actuation parameters. [129,130]ompared with independent microrobots, microrobotic swarms are more adaptive to fit inside complex environments for a high rate of access to the target.The collective behaviors of magnetic microrobotic swarms can be triggered by magnetic field stimuli, [131][132][133][134] and have the potential for navigated locomotion, active drug delivery, and sensing.This section summarizes the recent remarkable progress in the self-assembly of magnetic microrobotic swarms.
The individual microrobot exhibited multiple dynamic modes of oscillating, rolling, tumbling, and spinning under the different input magnetic fields.These multiple modes trigger microrobots to self-organize into corresponding swarm formations of liquid.
Tasci et al. proposed a colloidal microwheel formed of superparamagnetic colloidal particles by isotropic interactions induced by the in-plane rotating magnetic field. [135]The magnetically induced assembly process is shown in Figure 6A.This microrobot has a fast motion speed, although it still lags behind the fastest microbes due to the limitation of the magnetic field magnitude.Yu et al. reported a ribbon-like magnetic microswarm formed of paramagnetic nanoparticles using oscillating magnetic fields, as shown in Figure 6B. [136]The microswarm can perform a reversible elongation with an extremely high aspect ratio, as well as splitting and merging.Xie et al. presented a method for programmable generation and motion control of a snake-like magnetic microrobot swarm. [137]As shown in Figure 6C, peanut-shaped hematite colloidal particles assemble into a snake-like structure under rotating magnetic fields.In the work of Xie et al. [25] the fast and reversible transformations between the above multiple collective modes are achieved by alternating magnetic fields.The microrobotic swarms can be programmed to steer in any direction with excellent maneuverability, providing versatile collective modes to address environmental variations or multitasking requirements.
In addition to the above collective modes, there have been many innovative researches on magnetic microrobotic swarms in recent years.These recent advancements in magnetic microrobotic swarms formed by self-assembly are summarized in Table 1.
The other self-assembly form is that the magnetic chains are self-assembled on the tanglesome microfiber surface.Lu et al. proposed a self-assembly magnetic chain unit method for actuating larger bulk biomaterial. [138]The magnetic nanoparticles are Figure 6.Magnetic microrobotic swarms formed by self-assembly.A) Colloids assemble via isotropic interactions under the rotating magnetic field.Reproduced with permission. [135]Copyright 2016, Springer Nature.B) The schematic depiction illustrates the applied oscillating magnetic field and the particle chain formed under the oscillating magnetic field.Reproduced with permission. [136]Copyright 2018, Springer Nature.C) Schematic showing the motion of a swarm subjected to a rotating magnetic field circularly and the locomotion of snake-like magnetic microrobots.Reproduced with permission. [137]Copyright 2019, IEEE.
not homogeneous self-assembled on the surface of the tanglesome microfiber, and it further attracts more particles to form the aggregated clusters as aggregations become larger.This new magnetically actuated self-assembly possesses several superiorities, including succinct, economical, structure unconstrained, and material unconstrained.In addition, Pauer et al. achieved the assembly of a microswimmer via the programmed shape and arrangement of superparamagnetic micromodules, [139] combining the modular assembly concept and the self-assembly for magnetic microrobotic swarms.Furthermore, it is worth noting that although the swarm has diverse achievable collective modes, its locomotion behaviors are limited and mostly confined to the liquid environment.

Fabrication by Acoustically Driven Assembly
Acoustically driven assembly is an assembly technique that assembles micro-objects using acoustic fields at ultrasonic frequencies.Micro-objects can be migrated or collected in certain regions by creating potential wells of acoustic radiation forces. [140]With the frequencies of acoustic waves having a wide range, particles with different sizes can be manipulated for assembly regardless of their properties.Acoustically driven assembly offers advantages such as high efficiency, real-time control capability, low energy consumption, and no pollution.It also allows for simultaneous control of multiple targets and has biocompatibility.However, it is difficult for acoustically driven assembly to perform high-precision operations on specific targets among multiple objects.
The acoustically driven assembly has a relatively wide range of applications for microrobot swarms, mainly consisting of two subtypes: standing bulk acoustic wave-based and standing surface acoustic wave-based. [141,142]In addition, it is also used to lift and suspend target objects, as shown in Figure 7A.Youssefi et al. proposed a contactless micromanipulation method based on a magnetoacoustic system that enables 3D assembly in the air. [143]The component to be assembled is first lifted off by the activation of the acoustic field and then oriented by the magnetic field.This is a combination of the acoustically driven assembly and the magnetically driven assembly.This noncontact assembly proves to be highly practical for microrobot fabrication as objects do not experience high surface adhesion forces.The limitation of this method lies in the inability to perform directional control on nonmagnetic objects, so adding a magnetic backpack or coating is of interest for future work.Shen et al. proposed a method for assembling microparticles based on local acoustic forces nearby microstructures, as shown in Figure 7B. [144]This method utilizes the local sound field near the V-shaped micropillars to assemble microspheres.Figure 7B shows a flexible finger acoustically assembled from 5 μm magnetic particles, and it can swing periodically after applying a rotating magnetic field.

Fabrication by Electronically Driven Assembly
Electronically driven assembly is also an approach to assemble micro-objects using electronic fields.Generally, microrobots are sealed between the two parallel electrode plates in a water solution, and an electric current is applied to the two plates.The dielectric particles are polarized under the actuation of high-frequency alternating current, and the dielectrophoretic (DEP) force induced by the interaction between particles and The 3D translation and rotation of programmed external magnets produce time-varying pattern formations and collective motions of ferromagnetic microrobots at the airwater interface.
Programmable self-assembly Modular robotics Swarm robotics Biomedicine [163]   Liquid microrobots composed of ferrofluid droplets (Fan et al. 2020)   By changing the excitation mode of the external magnetic field, the liquid-robot aggregate can exhibit different formations and switch between them.

Untethered micromanipulation
Targeted cargo delivery [164]   Wheel-like magnetic-driven microswarm (Yue et al. 2022)   Microswarms with variable aspect ratios can be fabricated by tuning the frequency and strength of the external magnetic field.
Patching up microscale intestinal perforation Topical medication and microsurgery [165]   Reconfigurable assembled magnetic droplets (Wang et al. 2022)   Reconfigurable assembled magnetic droplets are formed and affected by the locally induced field gradient near the magnetized iron needle controlled by a 3-DoF manipulator.

Cargo trapping Obstacle-avoidance transportation
Untethered micromanipulation [166]   Scale-reconfigurable miniature ferrofluidic robots (SMFR) (Fan et al. 2022) A custom-designed magnetic actuation system is used for deformation and scale reconfiguration of the SMFR: stretch deformation, scale-down through separation, and scale-up through recombination.
Laboratory on-a-chip applications Biomedical procedures [167]   Reprogrammable magnetically actuated self-assembled cilia array (RMS) (Sohn et al. 2022)   RMS cilia array is fabricated by a self-assembly method, which can be reprogrammed by changing the magnetization direction through additional magnetization.
Lab-on-a-chip or microfluidic channels for fluid mixing and pumping [168]   Vertical colloidal collectives (Law et al. 2022)   A unique dual-axis oscillating magnetic field induces timevarying interparticle interactions, assembling the magnetic particles against gravity into vertical collectives.

Gap and obstacle crossing under nonzero fluidic flow condition
Stair climbing [169]  the electric field can drive particles to move along the field gradient. [145]DEP allows for selective manipulation of micro-objects within a small region, but it is generally limited to the region near the electrodes and has low control accuracy.In addition to this DEP-based actuation, electrohydrodynamic (EHD)-based actuation is also an approach for assembly by inducing mobile charges to form the EHD flow. [146,147][150] In addition, the idea of assembling microdevices/machines using an electric field is very attractive.Sitti et al. described a shape-encoded dynamic assembly method for mobile micromachine fabrication using DEP. [151]The assembly is driven by DEP interactions, encoded in the 3D shape of individual components.The spatial encoding of DEP attraction sites is achieved by modulating the 3D geometry.The preprogrammed physical interactions between different components endow the micromachines with reconfigurable locomotion modes and additional rotational degrees of freedom, which is where fabrication is superior to monolithic fabrication.Four magnetic microactuators were assembled into a DLW-fabricated nonmagnetic body, as shown in Figure 7C.This work also demonstrated hierarchical assembly and 3D assembly of multiple microrobots, which will advance and inspire the development of more sophisticated, modular microrobots.However, the applications without electric fields, for example, in vivo biomedical applications, would require irreversible assembly of microcomponents.This process can be achieved by adding bonding sites to microcomponents during fabrication, such as surface functionalization.The microcomponents can be assembled using an electric field and then transferred to an applicationspecific environment for actuation by nonelectric means.

Fabrication by Optically Driven Assembly
Optically driven assembly uses focused laser beams to manipulate microscopic objects for assembly with pico Newton-level forces. [152]The process of using light to manipulate the target object, similar to tweezers, is also known as optical tweezers (OTs).It utilizes the mechanical effects of light, and light radiation can generate gradient and scattering forces, forming an optical trap.Particles within the optical trap can be restrained by light pressure and transported with high precision in 3D space as the plane moves.Due to its high accuracy and low potential for damage, this method has become increasingly popular in recent years. [153]As shown in Figure 7D, Decrop et al. used  OTs on a microfluidic platform to accurately manipulate individual magnetic beads in a microporous array, and this method has advantages such as high success rate, high accuracy, and recyclability. [154]Figure 7D also shows the letters KUL and smiley arrays assembled using OTs.At present, the assembly of magnetic  of magnetic microrobots by noncontact assembly.A) Conceptual schematic of the micromanipulation method using the magnetoacoustic system and demonstration of part lift-off activated by acoustic field.Reproduced with permission. [143]Copyright 2019, IEEE.B) Schematic illustration of a micromanipulation chip powered by the acoustic field, second-order acoustic streaming fields excited by the clockwise elliptical motions of the micropillar, and images showing the swing motion of a flexible finger.Reproduced with permission. [144]Copyright 2019, MDPI.C) A 3D microcar body with four-wheel pockets and directed assembly of the magnetic microactuators into the wheel pockets realized using DEP.Reproduced with permission. [151]Copyright 2019, Springer Nature.D) Step diagram of transferring beads to new microwell using OTs and assembly of KUL and smiley by specifically positioning beads in the microwell array using OTs.Reproduced with permission. [154]Copyright 2016, ACS.
objects based on OTs is limited to planar arrays.In the future, the 3D-level assembly can be explored by adding microporous array planes of different heights.
However, optically driven assembly also has its limitations.The target of OTs operation is usually small in size, ranging from nanometers to micrometers.This is due to its weak driving ability and limited output power, many large-scale microoperations cannot be achieved.In addition, prolonged exposure to the light field may cause thermal damage to the biological target object, which is also one of the drawbacks.The high light intensity and power requirements limit the application of OTs in the assembly of biohybrid microrobots.Optoelectronic tweezers, as a new operation technology that combines optics and electrodynamics, utilize DEP forces to manipulate particles. [155]This method offers benefits such as noncontact, low damage, and system closure, which is expected to be applied to microrobot assembly in the future.

Discussion
This review delves into the different assembly methods employed in the fabrication of magnetic microrobots.The advantage of noncontact assembly methods is that magnetic microrobots can be assembled by external physical fields without direct contact.On the one hand, it can avoid the potential damage caused by contact pressure during fabrication and reduce the possibility of polluting the fabrication environment.On the other hand, external fields can act as a switch to stimulate the reversible assembly and reconfiguration behaviors for the module-based microrobots or microrobotic swarms.In the future, in vivo microrobot assembly is expected to be achieved under remote control, making noncontact assembly highly promising for biomedical applications.However, the ambient noise will interfere with the physical signals of field-driven assembly and the generated driving force is also weak.The tool-assisted manual assembly has been widely adopted for the high flexibility and adjustability of assembly.However, it is hard to fabricate micrometer-scale robots by manual assembly, resulting in the combination of assembly and 3D printing.3D printing provides high resolution for microassembly, and the fabrication dimension has been expanded from 1D to 3D in recent years.Moreover, cell-adhesion-driven self-assembly is also introduced in this article.The biohybrid microrobots fabricated by this method have great natural biocompatibility in clinical applications, which is difficult to achieve through other methods.
The advantages of fabricating magnetic microrobots by microassembly include flexible material selection, arbitrary shapes, more integrated functions, and controllable programmable magnetization.First, more new intelligent materials can be added to the assembly process, such as stimuli-responsive polymers, shape memory alloys, nanomaterial-based composites, and biomaterials. [156]Second, smaller magnetic modules or particles can be rapidly printed or molded by higher-resolution 3D printing technology, which enables the creation of more arbitrary shapes and smoother magnetization profiles. [99]Third, more functional modules can be integrated into magnetic microrobots by assembly for higher physical intelligence, such as the ability to sense, interpret, control, and learn. [156]Fourth, by precisely orienting each magnetic module or particle within the region during the assembly process, controllable programmable magnetization can be achieved.
According to the reviewed recent advances in fabricating magnetic microrobots by assembly, there are the following four future directions, which could significantly accelerate their biomedical application.First, more high-permeability and low-loss magnetic materials can be added for assembly, which enable many basic components to be miniaturized, with the expectation of achieving higher effective permeability and lower hysteresis loss. [98]econd, the microassembly methods need further development to create more 3D structures instead of just 2D, which can be achieved by stacking multiple layers. [99][159] Fourth, it is crucial to explore the clinical applications of magnetic microrobot assembly.Since the electromagnetic actuation system and the permanent magnetic system have their limitations, [160] new magnetic actuation systems need to be further developed for better robot control.Moreover, high-resolution real-time imaging technologies are expected for multimodality monitoring of magnetic microrobots, such as magnetic resonance imaging and magnetic particle imaging.Fifth, developing new control methods for collective magnetic microrobots is also crucial, as many tasks typically require multiple microrobots to operate simultaneously. [161,162]

Conclusion
This article reviews state-of-the-art magnetic microrobot fabrication methods by microassembly, including contact and noncontact assembly methods.The tool-assisted manual assembly has high flexibility, which is suitable for fabricating millimeter robots with multiple materials, complex structures, and arbitrary magnetic profiles.The assembly method combined with 3D printing has higher resolution and can create microrobots at the micron scale, but the fabrication process is correspondingly more complex.The self-assembly driven by cell adhesion is spontaneous, and the fabricated biohybrid microrobots have good biocompatibility for clinical applications.Noncontact assembly methods based on multiple field forces are mostly for tasks that require remote reversible assembly.Magnetic microrobots fabricated by assembly are developing toward smaller sizes, more complex structures, more integrated functions, and clinical applications.It is convincing that with the development of assembly methods, more powerful and intelligent magnetic microrobots can be fabricated and make more progress for applications in robotics, biomedical engineering, and environmental governance.

Figure 4 .
Figure 4. Cell-adhesion-driven self-assembly.A) Schematic diagram and scanning electron microscope image of the coculture process of burr-like spherical structures with cells.Reproduced with permission.[107]Copyright 2018, AAAS.B) Diagram and light microscopic graph of the fabrication process of magnetized macrophage microrobots.Reproduced with permission.[109]Copyright 2021, IEEE.

Figure 5 .
Figure 5. Assembly of micromagnetic modules.A) Assembly and disassembly of the 3D swimming magnetic minipropeller with two modules.Reproduced with permission.[115]Copyright 2021, IEEE.B) Selective assembly process of milli-scale cellular robots that can reconfigure morphologies.Reproduced with permission.[117]Copyright 2022, Springer Nature.C) Self-assembly and disassembly of magnetically controlled modular cubes.Reproduced with permission.[119]Copyright 2022, IEEE.D) View of the assembly stage and the assembled π-shaped microrobot.Reproduced with permission.[120]Copyright 2019, AAAS.

Figure 7 .
Figure7.Fabrication of magnetic microrobots by noncontact assembly.A) Conceptual schematic of the micromanipulation method using the magnetoacoustic system and demonstration of part lift-off activated by acoustic field.Reproduced with permission.[143]Copyright 2019, IEEE.B) Schematic illustration of a micromanipulation chip powered by the acoustic field, second-order acoustic streaming fields excited by the clockwise elliptical motions of the micropillar, and images showing the swing motion of a flexible finger.Reproduced with permission.[144]Copyright 2019, MDPI.C) A 3D microcar body with four-wheel pockets and directed assembly of the magnetic microactuators into the wheel pockets realized using DEP.Reproduced with permission.[151]Copyright 2019, Springer Nature.D) Step diagram of transferring beads to new microwell using OTs and assembly of KUL and smiley by specifically positioning beads in the microwell array using OTs.Reproduced with permission.[154]Copyright 2016, ACS.
Figure7.Fabrication of magnetic microrobots by noncontact assembly.A) Conceptual schematic of the micromanipulation method using the magnetoacoustic system and demonstration of part lift-off activated by acoustic field.Reproduced with permission.[143]Copyright 2019, IEEE.B) Schematic illustration of a micromanipulation chip powered by the acoustic field, second-order acoustic streaming fields excited by the clockwise elliptical motions of the micropillar, and images showing the swing motion of a flexible finger.Reproduced with permission.[144]Copyright 2019, MDPI.C) A 3D microcar body with four-wheel pockets and directed assembly of the magnetic microactuators into the wheel pockets realized using DEP.Reproduced with permission.[151]Copyright 2019, Springer Nature.D) Step diagram of transferring beads to new microwell using OTs and assembly of KUL and smiley by specifically positioning beads in the microwell array using OTs.Reproduced with permission.[154]Copyright 2016, ACS.

Table 1 .
Recent advances in magnetic microrobotic swarms formed by self-assembly.