Research ArticleMICROROBOTS

Reconfigurable magnetic microrobot swarm: Multimode transformation, locomotion, and manipulation

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Science Robotics  20 Mar 2019:
Vol. 4, Issue 28, eaav8006
DOI: 10.1126/scirobotics.aav8006
  • Fig. 1 Multimode transformations and collective manipulation.

    (A) Schematic of four programmable collective formations and the transformation between them. The hematite colloidal microrobots (inset) with the magnetic moment m were energized by alternating magnetic fields. The collective formations—liquid, chain, vortex, and ribbon—were programmatically triggered by alternating magnetic fields of Hl(t), Hc(t), Hv(t), and Hr(t), respectively. Moreover, it is possible to program the fast, reversible transition between these four formations. (B) Schematic showing collective manipulation capabilities of microrobotic swarms that emulate biological swarms: passing through a confined channel (C), handing large loads (V), and large-area synchronized manipulation (R) by reconfiguring into narrow chains, ant colony–like vortices, and a herring school–like ribbon procession, respectively.

  • Fig. 2 Individual microrobot dynamics and corresponding collective formations.

    (A) Average velocity v versus the driving frequency f of a rolling microrobot (insets) subjected to a circularly polarized rotating field in the y-z plane (the bottom inset) with an amplitude of Hc = 4000 A m−1. The yellow area indicates the formation interval of the chain (about 1 to 20 Hz). Error bars indicate SD. (B) The relationship between the projected area of a spinning microrobot (insets) and the input frequency. The orange area indicates the formation interval of a vortex (about 15 to 75 Hz) under the magnetic field Hv(t), which is characterized by Hv = 2000 A m−1 circularly polarized in the x-y plane. (C) Averaged velocity v as a function of driving frequency f of a tumbling microrobot (insets) subjected to a conical field (Hr = Hx = 4000 A m−1). The green area indicates the formation interval of the ribbon (about 25 to 250 Hz). (D) Snapshots (20×) from the left to right showing swarming patterns of the liquid (Hl = 4000 A m−1, f = 10 Hz), chain (Hc = 4000 A m−1, f = 5 Hz), vortex (Hv =2000 A m−1, f = 30 Hz), and ribbon (Hr = Hx = 4000 A m−1, f = 150 Hz) in the microrobotic system. Scale bars, 50 μm.

  • Fig. 3 Emergence of the programmable chain.

    (A) Schematic showing a typical tank-treading motion of a tri-microrobot chain subjected to a circularly polarized rotating magnetic field in the (y, z) plane. (B) Snapshots (40×) of the generation process of a five-microrobot chain under a rotating magnetic field (Hc = 8000 A m−1, f = 5 Hz) (movie S2). Arrows indicate the motion direction. (C) Simulation results of the spatial trajectory of a tri-microrobot chain using LabVIEW. (D) Spatial positions of the microrobots on the x and z axes in one cycle. The seven typical phases presented in the top and side views depict the motion of the chain (movie S2). (E) Average velocity versus Ns (f = 4 Hz). (F) The chain’s average velocity shows a linear relationship with the driving frequency (Ns = 1, 3, and 5).

  • Fig. 4 Emergence of the programmable vortex.

    (A) Snapshots (20×) of the growth process from dispersion (t = 0 s) to the vortex state (t = 52 s) (f = 30 Hz, Hv = 6000 A m−1). (B) Experimental results showing the average size of the vortex as a function of the critical surface fraction φ (f = 30 Hz and Hv = 6000 A m−1). Error bars represent the SD of the averaged values from three measurements. (C) Simulation results showing a cross section of the velocity and vorticity distributions of a single particle as functions of the radius and the velocity and vorticity maps of two, four, and six microrobots at the beginning of the vortex formation. (D) The linear azimuthal velocity (Vt) profile of the vortex. r is the diameter. (E) Simulation result of the vortex generation process with the developed discrete particle simulation method and snapshots (40×) of the experimental result from dispersion (t = 0 s) to final vortex state (t = 7.8 s) (f = 30 Hz, Hv = 6000 A m−1). (F) Snapshots (20×) that display the chirality switch of a vortex from CW to CCW rotation (f = 30 Hz and Hv = 6000 A m−1). Rapid evolution of the polar order parameter φR verifies the controllability of the vortex chirality switching. (G) Simulation of the merging process of two neighboring vortices using the developed particle simulation approach (f = 30 Hz) and snapshots (40×) of the experimental result (f = 30 Hz, Hv = 8000 A m−1).

  • Fig. 5 Emergence of the programmable ribbon.

    (A) Schematic showing a generation process of a ribbon subjected to a circularly polarized rotating magnetic field in the (y, z) plane. (B) Snapshots (40×) showing the emergence of a six-microrobot ribbon under a precessing field (Hr = 4000 A m−1, Hx = 4000 A m−1, and f = 200 Hz) (movie S6). (C) Simulated flow streamlines on the (x, y) plane generated by a ribbon (Ns = 6) translating from left to right. (D) Simulation results of the 3D motion trajectory of the tracer particles. (E) Snapshots (40×) showing the motion of a tracer particle (diameter, 1 μm). (F) Average velocity versus Ns at different frequencies (f = 100 Hz, f = 200 Hz) and the same magnitude Hr = Hx = 4000 A m−1. (G) Average velocity of a ribbon versus driving frequency under two input fields of different strengths, where Hr1 = Hx1 = 6000 A m−1 and Hr2 = Hx2 = 8000 A m−1.

  • Fig. 6 Programmable multimode transformations.

    (A) Snapshots showing transformations to the chain from liquid, vortex, and ribbon states. (B) Sequence of images showing the transformations to the ribbon from liquid, vortex, and chain. (C) Image sequence showing the transformations to the vortex from liquid, ribbon, and chain. (D) Image sequence showing the transformations to the liquid from chain, ribbon, and vortex. Scale bars, 50 μm.

  • Fig. 7 Programmable swarm locomotion.

    (A) Schematic description of the swarm locomotions of the chain, vortex, and ribbon. Images synthesized by movie screenshots illustrate the locomotion results of the chain (B), vortex (C), and ribbon (D) in tracking planned paths of a pentagon, a circle, and a rectangle, respectively. (E) Corresponding distribution histograms showing tracking errors (with SD) of 4.3 ± 3.1 μm, 5.5 ± 3.8 μm, and 2.0 ± 1.4 μm for the chain, vortex, and ribbon, respectively. Scale bars, 20 μm.

  • Fig. 8 Environmental adaptability and manipulation of the microrobot swarm.

    (A) Schematic description of the microfluidic device for experiments. Two microcells, named microcell 1 and microcell 2, were designed as sites for aggregating microrobots and performing the manipulation task, respectively. To complete the task, the microrobots must pass through a narrow channel (height, 6 μm; width, 6 μm; length, 260 μm) that links the microcells. (B) Microfluidics simulation results of the flow velocity field around the vortex (left), ribbon (middle), and chain (right). (C) Microfluidics simulation results of the flow velocity fields of the vortex and ribbon in micromanipulation of passive microparticles. (D) Experiments demonstrate that microrobots organized in a vortex (left) and ribbon (middle) were hard to pass through the narrow channel, whereas microrobots in the chain formation (right) were easy to pass. (E) Snapshots showing manipulation of PS microsphere (S1; diameter, 40 μm) by a vortex swarm (left) and synchronized manipulation of Ag microspheres (marked with s1 to s5; diameter, 8 μm) by the ribbon swarm (right).

Supplementary Materials

  • robotics.sciencemag.org/cgi/content/full/4/28/eaav8006/DC1

    Section S1. Experimental setup

    Section S2. Calculation of the pair correlation function

    Section S3. Hydrodynamic coupling in chain

    Section S4. Motion trajectories of a quat-microrobot chain

    Section S5. Giant number fluctuations in vortices

    Section S6. Velocity field of a vortex

    Section S7. Merging process of a giant vortex

    Section S8. Closed-loop controller for swarm locomotion

    Section S9. Simulation results of a microrobot that interacts with the microfluidics wall

    Fig. S1. Experimental setup system.

    Fig. S2. Calculation of the pair correlation function.

    Fig. S3. Simulation of hydrodynamic coupling in chain.

    Fig. S4. Motion trajectory simulation.

    Fig. S5. Magnitude of the velocity fields.

    Fig. S6. State of the vortex.

    Fig. S7. Merging process of a giant vortex.

    Fig. S8. Closed-loop controller.

    Fig. S9. Microfluidics simulation results of a rotating peanut-shaped particle.

    Movie S1. Four swarm formations of microrobots.

    Movie S2. Generation and motion trajectory simulation of the chain.

    Movie S3. Generation and chirality switch of the vortex.

    Movie S4. Vortex merging.

    Movie S5. Generation of a giant vortex.

    Movie S6. Generation and the flow field of a ribbon.

    Movie S7. Transformations of the chain from the other three formations.

    Movie S8. Transformations of the ribbon from the other three formations.

    Movie S9. Transformations of the vortex from the other three formations.

    Movie S10. Transformations of the liquid from the other three formations.

    Movie S11. Swarm locomotion.

    Movie S12. Environmental adaptability.

    Movie S13. Collective manipulation.

    Reference (49)

  • Supplementary Materials

    The PDF file includes:

    • Section S1. Experimental setup
    • Section S2. Calculation of the pair correlation function
    • Section S3. Hydrodynamic coupling in chain
    • Section S4. Motion trajectories of a quat-microrobot chain
    • Section S5. Giant number fluctuations in vortices
    • Section S6. Velocity field of a vortex
    • Section S7. Merging process of a giant vortex
    • Section S8. Closed-loop controller for swarm locomotion
    • Section S9. Simulation results of a microrobot that interacts with the microfluidics wall
    • Fig. S1. Experimental setup system.
    • Fig. S2. Calculation of the pair correlation function.
    • Fig. S3. Simulation of hydrodynamic coupling in chain.
    • Fig. S4. Motion trajectory simulation.
    • Fig. S5. Magnitude of the velocity fields.
    • Fig. S6. State of the vortex.
    • Fig. S7. Merging process of a giant vortex.
    • Fig. S8. Closed-loop controller.
    • Fig. S9. Microfluidics simulation results of a rotating peanut-shaped particle.
    • Reference (49)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Four swarm formations of microrobots.
    • Movie S2 (.mp4 format). Generation and motion trajectory simulation of the chain.
    • Movie S3 (.mp4 format). Generation and chirality switch of the vortex.
    • Movie S4 (.mp4 format). Vortex merging.
    • Movie S5 (.mp4 format). Generation of a giant vortex.
    • Movie S6 (.mp4 format). Generation and the flow field of a ribbon.
    • Movie S7 (.mp4 format). Transformations of the chain from the other three formations.
    • Movie S8 (.mp4 format). Transformations of the ribbon from the other three formations.
    • Movie S9 (.mp4 format). Transformations of the vortex from the other three formations.
    • Movie S10 (.mp4 format). Transformations of the liquid from the other three formations.
    • Movie S11 (.mp4 format). Swarm locomotion.
    • Movie S12 (.mp4 format). Environmental adaptability.
    • Movie S13 (.mp4 format). Collective manipulation.

    Files in this Data Supplement:

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