Mimicking the Structure and Function of Ant Bridges in a

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Mimicking the Structure and Function of Ant Bridge in Reconfigurable Microswarm for Electronic Applications Dongdong Jin, Jiangfan Yu, Ke Yuan, and Li Zhang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b02139 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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Mimicking the Structure and Function of Ant Bridge in Reconfigurable Microswarm for Electronic Applications Dongdong Jin, †, ‡, & Jiangfan Yu, †, & Ke Yuan, † Li Zhang†,‡,§, $, * † Department

of Mechanical and Automation Engineering, The Chinese University of Hong

Kong, Shatin, N.T., Hong Kong 999077, China. ‡ Department

of Biomedical Engineering, The Chinese University of Hong Kong, Shatin, N.T.,

Hong Kong 999077, China. § Chow

Yuk Ho Technology Centre for Innovative Medicine, The Chinese University of Hong

Kong, Shatin, N.T., Hong Kong 999077, China. $ T-Stone

Robotics Institute, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong

999077, China. ABSTRACT: In nature, social insects are capable of self-organizing into various sophisticated and functional structures through local communications, which facilitate them to cooperatively accomplish complex tasks that are beyond the capabilities of individuals. Emulating this collective behavior in artificial robotic systems promises benefits in various engineering fields and has been partially realized through elaborate algorithm and physical designs. However, developing swarm

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robotic system with group-level functionality at small scales remains a challenge. Herein, a microswarm system that mimics the structure and function of ant bridge is realized by employing functionalized magnetite nanoparticles, which are paramagnetic and electrically conductive, as the building blocks. Through applying a programmed oscillating magnetic field, the building blocks are reconfigured into a ribbon-like microswarm, which can perform reversible elongation with a high aspect ratio, thus capable of constructing a conductive pathway for electrons between two disconnected electrodes with the bodies of functionalized nanoparticles. Furthermore, the microswarm is demonstrated to serve as a microswitch, repair broken microcircuits and constitute flexible circuits, exhibiting a promising future for the practical applications in electronic field. KEYWORDS: collective behaviors; magnetic microswarms; self-assembly; colloids; surface functionalization; microelectronics Swarm behavior of social insects in nature plays a vital role in their living activities including foraging, nest construction and survival in harsh environments.1,2 Self-assembly of ant colonies constitutes one of the most striking collective phenomena that forms various sophisticated structures, such as bridges, bivouacs, rafts and so on.3 For example, to traverse difficult terrain, ants grip the bodies of others with their mandibles, tarsal claws and the adhesive pads of tarsi, thus forming a living, flexible and robust chain-like ant bridge that facilitates ant colonies to march across gaps beyond the reach of individuals. This self-assembly behavior with group-level functionality unachievable by individuals in nature is thus fascinating and attractive to scientists, especially the researchers in robotic field. Inspired from the social insects, roboticists create artificial swarm robots with the collective abilities emerged from the local interactions between individual components. Similar to natural


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counterparts, swarm robots are able to collectively accomplish tasks that are beyond the capabilities of a single robot. Representative examples include task sequencing and allocation,4,5 construction of large-scale architectures,6,7 user-specified morphology transformation8,9 and so on. 10-12

Moreover, when the dimension of components is decreased to micro/nanoscale, microbotic

swarms are envisioned to perform tasks that are impossible for humans, and their applications can be further expanded to various engineering fields, such as targeted delivery,13 micromanipulation14 and microelectronics.15 Nevertheless, the physical and algorithmic designs of traditional robotic swarms are hardly applicable to those of microswarms due to the difficulties in miniaturizing and intergrating hardwares, and thus new strategies are required. In this respect, colloids emerge as a promising platform to understand the guiding principles of swarm behavior at small scales16 because of the ease of batch fabrication, excellent controllability in sizes and diverse range of interactions including dipole-dipole interactions,17-20 capillary forces,21 van der Waals forces22 and so on.23 Through elaborate design of components and agent-agent interactions, various complex systems can be programmed via static or dynamic self-assembly.24-28 However, it is still challenging to imitate the collective behavior of social insects with group-level functionality because the underlying principles and proper actuation strategies that trigger colloids to selforganize into specific structures with desired functionalities are not fully understood. Recently, we employed paramagnetic Fe3O4 nanoparticles to develop a reconfigurable ribbon-like microswarm with the capability of reversible elongating and shortening via applying a programmed oscillating magnetic field (Fig. 1A).29 Herein, we have further designed the components of microswarm through functionalizing Fe3O4 nanoparticles with a continuous gold surface coating that makes the building blocks both paramagnetic and electrically conductive. Under the wireless actuation of an external oscillating magnetic field, the functionalized magnetic


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nanoparticles can self-organize into a microswarm between two disconnected electrodes. As shown in Fig. 1B, through local communications between decentralized individuals, army ants use their bodies to constitute a bridge cross difficult terrain to aid their marching and food transportation. While for the microswarm in this work, it can transform into a chain-like structure with an ultrahigh aspect ratio through magnetic dipole-dipole interactions to form a conductive pathway for electrons with the bodies of functionalized nanoparticles. Although the interactions in microswarm and ant bridge are different, the structure and functionality of microswarm mimic those of ant bridge. Moreover, we demonstrate that the microswarm is capable of serving as a microswitch, repairing broken microcircuits and constituting flexible circuits with the advantages of customizable controllability, high precision and long-term stability, thus demonstrating promising potential in electronic field. RESULTS AND DISCUSSION Preparation of Fe3O4-Au (core-shell) nanoparticles. The design of building blocks that constitute the desired swarm structures and functions plays a key role. To realize the applications of microswarm in electronic field, the components are required to be both magnetic and electrically conductive, which is achieved by functionalizing Fe3O4 magnetic nanoparticles with a continuous gold surface layer in this work. The fabrication process consists of four steps as shown in Fig. 2A. Fe3O4 nanoparticles with a spherical structure are first prepared via solvothermal method.30 Then the nanoparticles are dispersed in an alkalescence buffer solution, followed by the addition of dopamine to form a polydopamine (PDA) layer on their surface (Fe3O4@PDA) via in situ polymerization process.31 The third step is accomplished by electrostatic immobilization of negatively-charged Au nanoparticles on positively-charged PDA layer which serve as the seeds of


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gold surface coating (Fe3O4@PDA@Seeds). Finally, electroless plating is employed to grow the Au seeds and form a continuous gold layer on the nanoparticles (Fe3O4@PDA@Au).32 The scanning electron microscope (SEM) images in Fig. 2B show that batch fabrication of nanoparticles with regular shapes and narrow size distribution can be achieved in a single fabrication process. From the quantitative statistics in Fig. S1, Fe3O4, Fe3O4@PDA, Fe3O4@PDA@Seeds and Fe3O4@PDA@Au nanoparticles have average diameters of ~660, 675, 670 and 675 nm, respectively, suggesting that surface functionalization does not induce significant variation in their sizes. In contrast, the surface morphology changes obviously as the process goes on, which can be confirmed by the transmission electron microscope (TEM) images in Fig. 2C. Compared to the surface of bare Fe3O4, a smooth PDA layer as thin as 8 nm can be found to be uniformly coated for Fe3O4@PDA. When dispersed in acidic solution with a pH value of 2.0, the PDA layer exhibits strong positive charge with a zeta potential of 19.3 mV (Fig. S2), while the prepared Au seeds with an average diameter of 7 nm have a zeta potential of –36.4 mV (Fig. S3), thus facilitating the adhesion of Au seeds via efficient electrostatic self-assembly. This can be verified by the rough surface morphology of Fe3O4@PDA@Seeds with discretely distributed goose pimples. Then with the assistance of reductive agent, immersing the resultant nanoparticles in gold electroless plating solution facilitates the growth of Au seeds and formation of a compact gold layer, thus making Fe3O4@PDA@Au totally opaque in TEM images. Moreover, energy dispersive X-ray (EDX) analysis is carried out and the corresponding results shown in Fig. 2D indicate the elemental distribution of Au, Fe and O in the obtained Fe3O4@PDA@Au, further certifying the existence of a continuous gold layer on the surface of nanoparticle. The magnetic properties of nanoparticles at each fabrication stage are evaluated using vibrating sample magnetometer (VSM) in Fig. 2E. It can be found that they are all paramagnetic


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yet the saturation magnetization gradually decreases from 84.3 emu/g for bare Fe3O4 to 80.1, 75.0, and 57.1 emu/g for Fe3O4@PDA, Fe3O4@PDA@Seeds and Fe3O4@PDA@Au, respectively. We believe that the reduction of saturation magnetization is caused by the decreased proportion of magnetic matter per unit weight. However, the functionalized nanoparticles still exhibit strong response to a permanent magnet as shown in the inset, demonstrating the feasibility of generating microswarm via magnetic actuation. Collective behaviors of microswarm. The swarm behaviors of functionalized nanoparticles are wirelessly driven by an oscillating magnetic field which is schematically demonstrated in Fig. 3A. The magnetic field B consists of two parts: a sinusoidally alternating magnetic field BOSC = Asin(2πft) along the oscillating direction, where A is the amplitude as a constant, f represents the input oscillating frequency and t is time; and a uniform magnetic field BC applied perpendicularly to the oscillating direction with a constant field strength of C. An amplitude ratio (γ = A/C) is proposed. It should be noted that A in this work is always fixed as 10 mT and adjusting γ is achieved by changing the magnetic field strength of BC, i.e., the value of C. Moreover, the angle between the projection of BC on x-y plane and x axis is defined as the direction angle α, while the included angle between BC and x-y plane is named as pitch angle β. As a result, the magnetic field B has a time-varying direction and field strength. The input magnetic field parameters have a significant effect on the generation behaviors of microswarm. Amplitude ratio γ and oscillating frequency f determine whether microswarm can be generated, as shown in the phase diagram (Fig. 3B) and Movie S1. During the generation process, the functionalized nanoparticles are initially dispersed uniformly on substrate. If γ is too low (region I), massive chain-like structures are formed and then just oscillating on the plane with irregular pattern. However, excessively increasing γ beyond critical values (region III) induces the


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formation of several parallel ribbon-like patterns with uncontrollable elongation. Only the moderate collocation of γ and f in region II can generate a microswarm with a stable pattern. In this case, once the oscillating magnetic field is imposed, adjacent nanoparticles begin to selforganize themselves into several small subswarms, which are further gathered into a dynamicequilibrium ribbon-like microswarm by adjusting their orientation with direction angle. The mechanism of generation process has been well explained in our previous work.29 We assume that the magnetic field has the maximal oscillating amplitude (BOSC = A, BC = C) at the initial state. At this moment, the field strength is maximum while the oscillating angular velocity is minimum, so that the dispersed nanoparticles will form long chains and oscillate with the magnetic field. Then as time goes on, the strength and angular velocity will continuously decrease and increase, respectively, until the magnetic field reaches the minimum oscillating amplitude (BOSC = 0, BC = C). During this process, the long chains are broken into shorter ones due to the gradually smaller magnetic field strength and larger fluid drag. As the magnetic field further oscillates, the interactions between the short chains become stronger with the increasing field strength, which makes the chains attract each other. As a result, the distribution of nanoparticle chains become narrower. Repeating this cycle makes the nanoparticle chains more and more compact and finally a stable swarm pattern is formed when the magnetic and fluidic interactions are balanced. Through further zooming into region II, it can be found that altering γ and f triggers the morphology transformation of microswarm as shown in Fig. 3C, Fig. S4 and Movie S2. For example, when γ is 4 and f is 10 Hz, a relatively thick and short pattern is generated with an aspect ratio of 3.2. Raising the values of γ and f either separately or synchronously causes the elongation of microswarm, and a thin and long chain-like structure with an aspect ratio as high as 45.3 can be


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formed when γ is 9 and f is 30 Hz (Fig. 3D). It is worth noting that decreasing γ and f to their initial values once again makes the microswarm recover to its original shape, demonstrating that the morphology transformation of microswarm is reversible and fully controllable. Besides, except for the input parameters of magnetic field, the properties of building blocks, including concentration, diameter and Au coating thickness, also have nonnegligible effects on the generation and morphology transformation of microswarm, and the details can be found in Fig. S5-10. After a stable pattern is generated, pitch angle is tuned to actuate the locomotion of microswarm. The relationship between the translational velocity v and pitch angle β under different oscillating frequencies is presented in Fig. 3E. When β is 0°, microswarm keeps itself at the almost identical place for a long time. Gradually increasing β initiates and then accelerates the motion of microswarm, and moreover, the amplification in speed is directly proportional to frequency f. A velocity up to ~180 μm/s can be achieved when setting β and f as 4° and 30 Hz, respectively, demonstrating the efficient movement of microswarm. Besides, decreasing pitch angle from 0° to -4° will induce locomotion in the opposite direction yet with comparable speed. Magnetic field with absolute value of β beyond 4° is not investigated, because microswarm cannot maintain the stable pattern under this condition as shown in Fig. S11. Finally, combining direction angle and pitch angle, we are able to steer the movement along any desired trajectories. Fig. 3F and Movie S3 exemplify that a square route is accomplished by microswarm, demonstrating its highperformance motion controllability. Mimicking the structure and function of ant bridge for electrical connection. After confirming the compositions of building blocks and collective behaviors under programmed oscillating magnetic field, we apply the microswarm to self-assemble a conductive pathway for electrons between two isolated electrodes, which emulates both the structure and collective


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functionality of ant bridge. As shown in Fig. 4A and Movie S4, 3 μL Fe3O4@PDA@Au nanoparticle solution (2 mg/mL) together with 1.5 mL 0.1 wt.% polyvinylpyrrolidone aqueous solution is dropped on substrate and the nanoparticles are initially gathered around electrodes via a permanent magnet. Then an oscillating magnetic field with amplitude ratio γ = 4 and frequency f = 20 Hz is applied to generate a dynamically stable ribbon-like microswarm. Subsequently, microswarm is navigated to the middle between two isolated electrodes and align itself with the electrodes by setting pitch angle β = 2° and gradually adjusting direction angle α. Next, pitch angle is reset and amplitude ratio is slowly increased to form a chain-like pattern with its two terminals contacting with two electrodes, respectively, followed by cease of magnetic field. Finally, excess solution is carefully removed and the substrate is dried in either air or oven, generating a microswarm based wire for electrical connection. The change of electrical resistance between two isolated electrodes during the whole process is presented in Fig. 4B. At the beginning of experiment, the resistance is almost infinite which then levels out at approximately 110 kΩ after nanoparticles are added due to the trace conductivity of aqueous solution. When a chain-like microswarm is formed to connect electrodes, the resistance only slightly decreases to ~90 kΩ, which may result from the presence of teeny water droplets between the building blocks of microswarm.33 As the natural drying process in air goes on, the resistance continuously diminishes over a period of ~20 min, and finally a minimal value as low as 50 Ω is obtained after complete drying, suggesting the successful electrical connection between electrodes. At this moment, functionalized nanoparticles are densely packed in microswarm with direct physical contact (Fig. S12) and thus constitute an electrically conductive network via their gold surface coating. It is worth noting that placing the microswarm in an oven with a temperature


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of 60 °C will shorten the drying time to less than 10 min, and moreover, lower resistance, that is, higher conductivity can be achieved. Moreover, several control groups employing Fe3O4, Fe3O4@PDA, Fe3O4@PDA@Seeds nanoparticles as the building blocks of microswarm are conducted. As shown in Fig. 4C, both Fe3O4 and Fe3O4@PDA based microswarm exhibit substantial resistance larger than 0.7 MΩ due to the inferior conductivity of components. Besides, Fe3O4@PDA@Seeds also cannot selfassemble an electrically conductive pathway with acceptable resistance, that is, merely coating the nanoparticles with discretely distributed Au seeds is insufficient to connect electrodes, thus demonstrating the necessity of functionalizing Fe3O4 with a continuous gold surface layer. Furthermore, by zooming into the resistance of Fe3O4@PDA@Au based microswarm as a function of aspect ratio (inset), we can find that there exists an approximate linear relationship between these two parameters, which is similar to the electrical property of metallic conductor. Considering that the aspect ratio of pattern can be finely tuned by the input oscillating magnetic field, it can be concluded that we are able to not only realize electrical connection using microswarm, but also actively adjust the corresponding resistance. Applications of microswarm in electronic field. With the high-performance controllability and the ability of electrical connection, microswarm emerges as a promising tool that can be employed in various microelectronic devices. For example, it can serve as a microswitch with the corresponding schematics shown in Fig. 5A. A four-way microcircuit is fabricated and processing each circuit will lead to the lightening of “C”, “U”, “H” or “K” LED arrays. Due to the dexterity of microswarm, specific isolated electrodes can be connected on demand in a facile and intuitive manner as shown in Fig. 5B and Movie S5. Besides, the formed chain-like structures can be


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removed by sonicating them in ethanol, which recovers the circuit to its open state and then facilitates the next operation, thus proving the function of microswarm as a microswitch. Another example is that microswarm can be used to repair broken microcircuit. If encountering long-term environmental corrosion or scratch by sharp objects, electronic devices may become invalid due to local broken circuits and one of the most common methods to repair them is manual soldering. However, this method usually suffers from relatively low processing precision and thus has a risk of short connection, especially for those devices with closely arranged wires. In contrast, microswarm emerges as an alternative solution. As shown in Fig. 5C and Movie S6, the mending process consists of three steps: dropping of a few nanoparticle solution, generation of microswarm via magnetic field, and drying, which is quite straightforward and effective, as it is capable of connecting targeted broken circuit without disturbing others in a wire array with interval distance as low as ~200 μm. Moreover, we find that after connected by microswarm, the circuit is able to perform proper functions for at least several months without significant degradation. Therefore, the ant-bridge mimicked microswarm is highly customizable with controllable, high-precision, erasable and robust characteristics, exhibiting great potential in microelectronic field. Moreover, the electrical connection ability of microswarm also takes effect in flexible devices. Flexible electronic devices are among the most intensively studied topics at current stage that can be applied in wearable devices, electronic skin, implantable electrodes and various other fields, as they allow intimate contact between devices and various rigid or soft nonplanar objects in a safe, comfortable and nondestructive way.34 Herein, a polyethylene terephthalate (PET) film widely used to fabricate flexible printed circuit board is selected as the substrate. Two isolated copper-based electrodes are patterned and then microswarm is generated on PET film in the same


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manner with that on rigid substrate. As the nanoparticle solution contains a trace amount of PVP as mentioned above, the formed chain-like structure is able to stick firmly to the substrate after drying and even bending the PET film repeatedly will not break the structure as shown in Fig. 5D and Movie S7. Besides, we try to quantificationally characterize the influence of bending on the resistance of microswarm. For example, when the radius of bending curvature is larger than 15 mm, the resistance is always ~24.8 Ω, almost constant as that in flat state. Further increasing deformation curvature leads to the gradual decay of conductivity, but the resistance is still as low as approximately 25.9 Ω when the bending radius is 1.0 mm, indicating that only a slight variation in resistance (4.4%) is induced even when bending the microswarm to a large degree (Fig. 5E). Moreover, the anti-fatigue performance is also evaluated by bending the microswarm to the state with a bending radius of 1.0 mm for 100 times. As shown in Fig. 5F, it can be found that only 5.9% increase in resistance is caused, demonstrating the excellent stability of electrical connection achieved by microswarm. CONCLUSION In summary, we develop a microswarm system that is capable of constituting a conductive pathway for electrons across a gap far beyond the reach of individual building blocks, emulating both the structure and function of the ant bridge. Through a facile fabrication process, we create hierarchically structured Fe3O4@PDA@Au nanoparticles which have a paramagnetic core and a compact gold surface layer, and employ them as the building blocks of microswarm. The microswarm is triggered wirelessly based on magnetic dipole-dipole interactions, and its collective behaviors including generation, morphological transformation and navigated locomotion are fully controllable by tuning the input magnetic field parameters. Such design of components and agentagent interactions enables us to employ microswarm to serve as microswitch, repair broken


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microcircuits and constitute flexible circuits with distinct advantages, which promises various practical applications in electronic field. In future, other attributes not limited to magnetic and electrical properties can also be customized by modifying the surface functionalization process, which may endow the microswarm with new swarm behaviors and functions, thus facilitating us to better understand and mimic the complex collective behaviors of living systems in nature. EXPERIMENTAL SECTION Materials: The chemicals of iron(III) chloride hexahydrate (FeCl3·6H2O, 99%), ethylene glycol (EG, 99%), sodium acetate (NaAc, 99%), dopamine hydrochloride (98%), citric acid monohydrate (C6H8O7·H2O, 99.5%), sodium citrate dihydrate (C6H5Na3O7·2H2O, 99%), gold chloride trihydrate (HAuCl4·3H2O, 99.9%), sodium borohydride (NaBH4, 98%), potassium carbonate (K2CO3, 99%), mercaptosuccinic acid (C4H6O4S, 98%) and polyvinylpyrrolidone (PVP, Mw = 1300000) were purchased from Aladdin Chemicals. Polyethylene glycol (PEG, Mw = 2000) was

obtained

from

Advanced

Technology

and

Industrial

Co.,

Ltd.

Tris(hydroxymethyl)aminomethane (Tris, 99.9%) was purchased from Acros Organics. Hydrochloric acid (HCl, 37%) and formaldehyde (36%) was from RCI Labscan and VWR BDH Prolabo® Chemicals Promotion, respectively. All the chemicals were used without further purification. Preparation of Fe3O4 nanoparticles: The magnetite nanoparticles were prepared via a solvothermal method.30 Typically, 1.35 g FeCl3·6H2O was first dissolved in 40 mL EG by magnetic stirring to form an orange clear solution, followed by addition of 3.6 g NaAc and 1 g PEG. After vigorous stirring overnight, a russet and turbid mixture was obtained. Subsequently, the mixture was sealed in a 50 mL autoclave, heated at 200 °C for 10 h, and then naturally cooled


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to room temperature. The black product was collected by a permanent magnet and washed with deionized (DI) water for 5 times to remove retained solvent. Finally, the nanoparticles were dispersed in DI water with a concentration of 50 mg/mL for further use. Preparation of Fe3O4@PDA nanoparticles: A thin and smooth PDA layer was coated on the surface of Fe3O4 via a modified in situ polymerization process.31 First, 0.12 g Tris was dissolved in 100 mL DI water under sonication. The pH value of obtained solution was carefully adjusted to 8.5 using 1 M HCl to form an alkalescence buffer solution. Then the buffer was poured into a 250 mL round-bottom flask, followed by addition of 2 mL Fe3O4 nanoparticle solution (50 mg/mL). Next, the mixture was mechanically stirred and sonicated for at least 15 min to completely disperse magnetite nanoparticles. Subsequently, 0.02 g dopamine hydrochloride was added into the mixture, which further underwent continuous mechanical stirring and sonication for 5 h to allow for complete polymerization reaction. During this process, it is worth noting that the temperature of mixture should be kept at room temperature. Finally, the product was collected by a permanent magnet, washed with deionized (DI) water for 5 times and then dispersed in DI water with a concentration of 10 mg/mL for further use. Preparation of Fe3O4@PDA@Seeds nanoparticles: The seeds of gold surface coating were immobilized on the surface of Fe3O4@PDA via an electrostatic self-assembly process.32 On one hand, the surface charge of Fe3O4@PDA was tuned to positive. Typically, 0.158 g C6H8O7·H2O was dissolved in 50 mL DI water under sonication. The pH value of obtained solution was carefully adjusted to 2.0 using 1 M HCl to form an acidic buffer solution. Then the buffer was poured into a 250 mL round-bottom flask, followed by addition of 1 mL Fe3O4@PDA nanoparticle solution (10 mg/mL). The mixture was mechanically stirred and sonicated for at least 15 min to allow for the equilibration of positive surface charge of Fe3O4@PDA. On the other hand, Au seeds with


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negative charge were prepared. Briefly, 58.8 mL DI water was added into a 100 mL round-bottom flask, followed by injection of 600 μL HAuCl4·3H2O solution (1.0 wt.%) and 600 μL C6H5Na3O7·2H2O solution (25 mM). After vigorous stirring for 10 min, 900 μL freshly prepared, ice-cooled 200 mM NaBH4 was injected to form a claret-colored solution, indicating the formation of Au seeds. The Au seed solution was stirred for another 15 min and then quickly poured into the flask with positively charged Fe3O4@PDA solution. After further mechanical stirring and sonication for 30 min, the product was finally collected by a permanent magnet, washed with deionized (DI) water for 5 times and then dispersed in DI water with a concentration of 10 mg/mL for further use. Preparation of Fe3O4@PDA@Au nanoparticles: A continuous gold surface layer was grown from the Au seeds on nanoparticles via a modified electroless plating process.32 In a typical synthesis, the gold plating solution was first prepared by sequentially dissolving 0.0125 g K2CO3 and 750 μL HAuCl4·3H2O solution (1.0 wt.%) in 50 mL DI water under magnetic stirring. After aging for 2 h, the solution was transferred to a 100 mL round-bottom flask, followed by addition of 1 mL Fe3O4@PDA@Seeds nanoparticle solution (10 mg/mL). The mixture was mechanically stirred and sonicated for at least 15 min to completely disperse nanoparticles. Next, 250 μL formaldehyde was injected into the mixture to initiate the growth of Au seeds. After further mechanical stirring and sonication for 10 min, 500 μL mercaptosuccinic acid solution (25 mM) was added to prevent the aggregation of nanoparticles. Finally, the product was collected by a permanent magnet, washed with deionized (DI) water for 5 times and then dispersed in DI water with a concentration of 2 mg/mL. Magnetic actuation experiments: Normally, 3 μL Fe3O4@PDA@Au nanoparticle solution (2 mg/mL) together with 1.5 mL 0.1 wt.% PVP aqueous solution was added into an open tank and


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the nanoparticles were initially gathered via a permanent magnet. Then the tank was transferred into the workspace of a 3-axis Helmholtz electromagnetic coil setup which has been reported previously.29 For the electrical connection experiments, functionalized nanoparticles with PVP solution were directly dropped on either rigid or flexible substrate patterned with electrodes. Finally, an oscillating magnetic field was applied to actuate the swarm behaviors. Fabrication of electrodes on substrates: The electrodes were fabricated via a mask-assisted physical vapor deposition process. To fabricate the mask, an A4 paper was first loaded into a commercial laser processing platform (Universal Laser Systems PLS6.75, USA) and then a laser beam with a power of ~1 W was scanned into the paper with a speed of ~20 mm/s to engrave the designed geometry. Then the paper-based mask was adhered to the substrate, which was loaded into an e-beam evaporator (EB-600, Innovative Vacuum Solution Co., Ltd., Taiwan, China), followed by sequential deposition of 20 nm Ti and 100 nm Cu. Finally, the mask was teared off and the substrate with patterned electrodes was obtained. Characterization techniques: The morphology and composition of nanoparticles at each fabrication stage were characterized by scanning electron microscopy (SEM, JEOL 7800F) and transmission electron microscopy (TEM, Tecnai F20). The magnetic properties were evaluated at 300 K using a PPMS Model 6000 Quantum Design VSM. The zeta potential was measured by Zeta Sizer (Nano ZS, Malvern). ASSOCIATED CONTENT The authors declare no competing financial interest. Supporting Information.


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The Supporting Information is available free of charge on the ACS Publications website at DOI: . Supporting figures (PDF) Movie S1. Generation behaviors of microswarm under the oscillating magnetic field with different amplitude ratios and frequencies (AVI) Movie S2. Reversible elongation of microswarm (AVI) Movie S3. Controlled locomotion of microswarm along a square route under the oscillating magnetic field with an amplitude ratio of 4, a frequency of 20 Hz and a pitch angle of 2°(AVI) Movie S4. Electrical connection process between two isolated electrodes achieved by microswarm (AVI) Movie S5. The application of microswarm as a microswitch (AVI) Movie S6. The application of microswarm in repairing microcircuits (AVI) Movie S7. The application of microswarm in flexible devices (AVI) AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Author Contributions D. J., J. Y. and L.Z. conceived the project. L. Z. supervised the studies. D.J. prepared and characterized the functionalized nanoparticles and J.Y. developed the magnetic actuation strategy.


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D. J. and J. Y. carried out the magnetic actuation experiments. D. J. fabricated the electrodes and performed the electrical connection process. K. Y. provided assistance in the experiments. D. J. analyzed the data. D. J., J. Y. and L. Z. co-wrote the manuscript. All authors have given approval to the final version of the manuscript. & These authors contributed equally. ACKNOWLEDGMENT The research work is financially supported by the General Research Fund (GRF) with Project Nos. 14209514, 14203715, and 14218516 from the Research Grants Council (RGC) of Hong Kong, the ITF project with Project No. ITS/440/17FP funded by the HKSAR Innovation and Technology Commission (ITC), and the funding support from the CUHK T Stone Robotics Institute. We thank Ben Wang, Chi Ian Vong for their help in experiments and Qi Zhou for his support on the electromagnetic actuation setup. REFERENCES 1.

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FIGURES AND TABLES

Figure 1. Schematics of ant bridge-mimicked reconfigurable microswarm. (A) The generation and reversible elongation process of microswarm under an oscillating magnetic field. (B) The formation steps of ant bridge and reconfigurable microswarm for ant colonies marching and electrical connection, respectively.


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Figure 2. Fabrication and characterization of Fe3O4 nanoparticles functionalized with gold surface coating. (A) Schematics of the fabrication process. (B) SEM images of nanoparticles at each fabrication stage. The scale bar is 1 μm. (C) TEM images of nanoparticles at each fabrication stage. The scale bar is 200 nm. The insets present the magnified surface morphology of Fe3O4@PDA and Fe3O4@PDA@Seeds, respectively. (D) EDX analysis of Fe3O4@PDA@Au showing the elemental distribution in a nanoparticle. The yellow and orange curves indicate the content variation of Au and Fe elements, respectively, along the black line. The scale bar is 200 nm. (E) Magnetic hysteresis loop of nanoparticles at each fabrication stage. The inset shows that Fe3O4@PDA@Au can be attracted by a permanent magnet.


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Figure 3. Actuation of reconfigurable microswarm using oscillating magnetic field. (A) The schematic illustration of applied magnetic field. The red arrow shows the magnetic field B, which is the superposition of a sinusoidally alternating field BOSC and a uniform field BC (blue arrows). The black double-sided arrow indicates the direction of BOSC. The grey area and red dotted arrow are the oscillating plane and direction of B, respectively. The direction angle of magnetic field is represented by α, while β is pitch angle. (B) Phase diagram showing the swarm patterns actuated by different input oscillating frequencies and amplitude ratios. (C) Reversible elongation of microswarm via changing amplitude ratio. The red arrows indicate the transformation direction. (D) Effects of input amplitude ratio and oscillating frequency on the aspect ratio of microswarm. (E) Effect of pitch angle and oscillating frequency on the translational velocity of microswarm when γ = 4. (F) Controllable navigation of microswarm along a square route with motion trajectory indicated by the blue line. All error bars represent the standard deviation (n=3) and all scale bars are 500 μm.


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Figure 4. Electrical connection achieved by microswarm. (A) The connection process between two isolated electrodes. The real-time input magnetic field parameters are also listed. The scale bar is 500 μm. (B) The change of resistance between electrodes as connection process goes on. All the events that cause the variation of resistance are marked by the black arrows. (C) Effect of aspect ratio on the resistance of microswarm with Fe3O4, Fe3O4@PDA, Fe3O4@PDA@Seeds and Fe3O4@PDA@Au as building blocks, respectively. The error bars represent the standard deviation (n=3).


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Figure 5. Applications of ant bridge-mimicked microswarm in electronic field. (A) Schematic of employing microswarm as a microswitch. At the initial state, all circuits are disconnected, making the “C”, “U”, “H” and “K” LED arrays turned off. Then microswarm is generated on the substrate and controlled to connect the vertically opposite electrodes, turning on the red “C” LED arrays. The microswitch can be reset to open state after erasing the microswarm by sonicating it in ethanol, thus achieving repeatable switching on and off. (B) Experimental demonstration of microswarm based switch. Each LED array can be lightened on demand. The scale bar is 500 μm. (C) Schematic and corresponding experimental results of repairing broken microcircuit with microswarm. The process can be achieved via dropping nanoparticle solution on broken site, generating microswarm and evaporating excess solution step by step. The scale bar is 250 μm. (D) Demonstration of applying microswarm in flexible devices. The microswarm is generated to electrically connect two isolated electrodes on a flexible PET film and then bending the film does not break the structure and function of microswarm. (E) Effect of bending curvature radius on the resistance of microswarm. The radius r is schematically explained in the inset. The error bars represent the standard deviation (n=3). (F) The anti-fatigue performance of microswarm after bending it to the state with a curvature radius of 1.0 mm for 100 times.


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GRAPHICAL TABLE OF CONTENTS


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Mimicking the Structure and Function of Ant Bridges in a

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