Research ArticleSOFT ROBOTS

Voxelated three-dimensional miniature magnetic soft machines via multimaterial heterogeneous assembly

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Science Robotics  28 Apr 2021:
Vol. 6, Issue 53, eabf0112
DOI: 10.1126/scirobotics.abf0112
  • Fig. 1 The proposed jig-assisted heterogeneous integration approach for 3D fabrication of small-scale magnetic soft machines.

    (A) Voxels with different materials, shapes, and magnetization profiles mv were integrated together by a bonding agent to build soft machines. (B) Bonding agent was applied to connect neighboring voxels in two different ways: face bonding and edge bonding, which respond to the external magnetic field Bc differently. (C) An example soft machine with two interconnected circular rings was fabricated by assembling several heterogeneous voxels at designated 3D positions and orientations. The insets show the places where two bonding options were used. The bonding agent in face bonding connections is omitted in all schematics for easier visibility. (D) The fabricated soft machine experimentally showed 3D mechanical metamaterial characteristic of having a negative Poisson’s ratio, consistent with the designed shape-morphing behavior.

  • Fig. 2 Illustration of the jig-assisted fabrication of the exemplar 3D ring.

    (A) Designs of the ring and its voxels. (B) Designs of the assistant jigs and schematics of the fabrication approach with experimental photographs. A VSM was used to magnetize the voxels in desired directions and magnitudes with a controlled uniform magnetic field up to 1.8 T. The jig for edge bonding has a long slit, to avoid the strong magnetic interaction between the voxels when plugging them into the jig. Bonding agent was applied through the cap opening to form the edge bonding. Next, magnetic and nonmagnetic voxels were plugged into a 2D jig fabricated by TPP or mold casting. They were assembled by face bonding to form a (half) ring. (C) The ring and two half rings were plugged into a 3D jig (fabricated by TPP-based 3D microprinting) and assembled to create a 3D soft machine by face bonding.

  • Fig. 3 Resolution characterization and creation of 3D magnetization profiles.

    (A) Schematic and SEM image of a checkerboard assembly. The checkerboard has a heterogeneous magnetization profile that was verified by the measurement of the normal component of its near-surface magnetic field Bv. (B) Three 100-μm side-length cubic voxels were magnetized to have mv with different 3D directions. Their respective mv were verified by the measurement of their respective Bv. (C) The measured and theoretical magnetization values of 100-μm side-length cubic magnetic voxels with different MMP concentrations and various magnetizing magnetic field strengths. The columns and bars represent the mean and the SD of the measurement of three samples in the same set, respectively. “*” marks the undistinguishable measurements from background noise. (D) A hollow cube has letters and symbols programmed into the magnetization profiles of its six faces. The invisible letters and symbols were revealed by imaging each face using a magnetic field imaging (magneto-optical sensor) instrument (MagView S with type B sensor, Matesy GmbH).

  • Fig. 4 Miniature magnetic soft machines demonstrating diverse characteristics.

    (A) A hollow cubic frame machine exhibiting 3D mechanical metamaterial characteristics with a negative Poisson’s ratio: It shrinks in all three dimensions in the presence of Bc. (B) A flower-shaped soft machine with complex stiffness distribution. Three of its petals bloomed first in lower field strength, and the rest three bloomed later as Bc increased. (C) A starfish-shaped machine with directional joint bending was made by two layers of magnetic voxels. Its flat body expanded vertically and shrunk horizontally in the presence of Bc. (D) A capsule-shaped machine with two cylindrical parts embedded with MMPs of different coercivity for reconfigurable magnetization. It was kept closed due to the attractive magnetic interactions between its two parts. After being exposed to a pulsing remagnetizing field, the part with lower coercivity MMPs reversed its magnetic moment direction, changing the magnetic interaction from attraction to repulsion, which opened the capsule. The bonding agent in face bonding connections is omitted in all schematics for easier visibility.

  • Fig. 5 Miniature wireless soft peristaltic pump with potential biomedical functionalities.

    (A) An intestine-inspired peristaltic soft pump. The tubular pump formed traveling wave-like deformation on its body under a rotating Bc. (B) When placed at the bottom of a reservoir, the machine pumped mouse whole blood when its peristaltic wave propagated downward. Comparison is given between the experimental pumping results of different pumping directions. (C) The pump transported a 1-mm diameter solid polystyrene sphere in air. The sphere location is marked out by red dashed circles. The videos of all tests are available in movie S3. (D) Characterization of the pumping performance at different magnetic field strengths and rotating frequencies.

  • Fig. 6 Miniature wireless soft capsule with potential biomedical functionalities.

    (A) Schematics of the capsule, ejecting its internal liquid cargo on-demand through its designed top opening. (B) The capsule rolled on a stomach phantom surface and released a food dye on-demand. Five capsules were moved and activated together to release the internal dummy drug liquids. (C) The capsule took fluid samples for future potential in situ liquid biopsy applications. (D) The capsule was initially filled with red food dye and then submerged in clear water. It rolled on a surface back and forth for 30 min. Then, the capsule was activated to eject the content in its chamber. (E) The capsule was filled with both liquid food dye and air. It was submerged in clear water for 30 min to observe diffusion. Next, it was activated to eject the air in its chamber and then the liquid food dye.

  • Fig. 7 Wireless miniature soft anchoring machine with potential biomedical functionalities.

    (A) A surface-anchoring machine anchored and released on-demand inside a tubular phantom with a fluid (water) flow: (i) relaxed state and (ii) activated state. (B) The machine adapted to a working environment with a dynamically changing diameter. (C) Anchoring and cargo delivery demonstration in a synthetic tube. (i) The machine entered the workspace. During the locomotion, the anchoring machine was at the activated state all the time. (ii) The anchoring machine restored to the relaxed state and anchored at the desired location. (iii) The soft capsule machine attached to the anchoring machine was activated to release the liquid cargo (dye here) at the targeted location. (iv) The anchoring machine was activated again and pulled away from the workspace. (D) Live stem cell integrated machine demonstration toward future vascular regeneration applications. (i) Microcages that entrap MSCs were heterogeneously integrated to the anchoring machine’s surface-contacting regions as a cell scaffold. (ii) The machine integrated live stem cells into its body. The zoomed fluorescence optical images, taken 24 hours after the cell culturing, showed that the cells lived and spread out on the cage surface with no noticeable toxic effect. The bonding agent in face bonding connections is omitted in all schematics for easier visibility.

  • Table 1 Fabrication durations and voxel types and numbers of all prototyped soft devices.

    The shown values refer to the time needed to magnetize and assemble voxels into each machine prototype. Curing of the bonding agents happens simultaneously during the assembly. A post-assembly curing of the machine on a hot plate or in an oven is performed to maximize the performance. This post-assembly curing does not require human intervention and is not counted in the durations here.

    Prototyped devicesFabrication
    duration (hours)
    Number of voxels
    TotalMagneticNonmagnetic
    3D ring5503218
    Hollow cubic frame668680
    Flower-shaped machine61271270
    Starfish-shaped machine424240
    Capsule-shaped machine2422
    Peristaltic pump82962960
    Capsule1990
    Anchoring device101849688
  • Movie 1. Overview of the jig-assisted 3D heterogeneous integration approach for small-scale wireless magnetic soft-bodied machines.
  • robotics.sciencemag.org/cgi/content/full/6/53/eabf0112/DC1

    Materials and Methods

    Section S1. Measurement of voxel magnetization

    Section S2. Evaluation of the fabrication precision and tolerance to fabrication variances

    Section S3. Characterization of the demonstrated functional soft machines

    Section S4. Design, assembly process, and experimental details of the reported devices

    Section S5. Compatibility of the proposed method with multiple kinds of materials

    Section S6. Comparison between assembled and cast samples

    Section S7. Extended discussion on mass production

    Fig. S1. Illustrations of the mold casting and magnetizing steps of the proposed fabrication approach and its versatility.

    Fig. S2. Magnetic characterization of the MMPs used in this work.

    Fig. S3. Schematic illustrations of the face and edge bonding methods.

    Fig. S4. Loading live stem cells to the microcages heterogeneously integrated to the anchoring soft machine top surface.

    Fig. S5. Quantitative characterization of the assembly precision.

    Fig. S6. Mechanical characterizations of the materials and comparisons against the neo-Hookean model.

    Fig. S7. Finite element simulation–based investigation of the effect of the fabrication imperfections on the two-ring anchoring machine performance.

    Fig. S8. Observations of the shape morphing of the peristaltic pump.

    Fig. S9. Liquid biopsy of the capsule.

    Fig. S10. Analysis of the magnetic attraction force on device deformation.

    Fig. S11. Illustration of the jig-assisted fabrication of the exemplar 3D ring.

    Fig. S12. Designs of the hollow cubic frame, simulation verification, fabrication process, and cargo transport demonstration.

    Fig. S13. Designs of the flower-shaped machine, simulation verification, and fabrication process.

    Fig. S14. Designs of the starfish-shaped machine, simulation verification, and fabrication process.

    Fig. S15. Designs of the capsule-shaped machine and its fabrication process.

    Fig. S16. Schematic illustrations of the electrical setup for the pulsing magnetic field.

    Fig. S17. Designs of the peristaltic pump and illustrations of its fabrication process.

    Fig. S18. Experimental setup to make a rotating magnetic field for the peristaltic pump.

    Fig. S19. Designs of the capsule and its fabrication process.

    Fig. S20. Designs of the anchoring machine and illustrations of its fabrication process.

    Fig. S21. Exemplar cantilever beam made of voxels based on five different kinds of materials.

    Fig. S22. Shape morphing comparison between assembled and cast cantilever beams.

    Table S1. Comparison of the proposed fabrication method with the ones previously reported in the literature.

    Table S2. Material characterization of various kinds of materials used in the reported machines.

    Movie S1. Video recordings of the representative steps in the fabrication of the exemplar 3D ring machine.

    Movie S2. Video recordings of the representative steps in the fabrication of the functional peristaltic pump machine.

    Movie S3. Experimental results of a small-scale magnetic soft peristaltic pump that pumps mouse whole blood and transport a solid sphere in a rotating uniform magnetic field.

    Movie S4. Experimental results of the reported miniature magnetic soft capsules.

    Movie S5. Experimental results, including an experimental trial and a control trial, of a miniature magnetic soft capsule taking liquid samples and demonstrate potentials for future liquid biopsy applications.

    Movie S6. Experimental results of a small-scale magnetic soft anchoring machine.

    Reference (49)

  • Supplementary Materials

    The PDF file includes:

    • Materials and Methods
    • Section S1. Measurement of voxel magnetization
    • Section S2. Evaluation of the fabrication precision and tolerance to fabrication variances
    • Section S3. Characterization of the demonstrated functional soft machines
    • Section S4. Design, assembly process, and experimental details of the reported devices
    • Section S5. Compatibility of the proposed method with multiple kinds of materials
    • Section S6. Comparison between assembled and cast samples
    • Section S7. Extended discussion on mass production
    • Fig. S1. Illustrations of the mold casting and magnetizing steps of the proposed fabrication approach and its versatility.
    • Fig. S2. Magnetic characterization of the MMPs used in this work.
    • Fig. S3. Schematic illustrations of the face and edge bonding methods.
    • Fig. S4. Loading live stem cells to the microcages heterogeneously integrated to the anchoring soft machine top surface.
    • Fig. S5. Quantitative characterization of the assembly precision.
    • Fig. S6. Mechanical characterizations of the materials and comparisons against the neo-Hookean model.
    • Fig. S7. Finite element simulation–based investigation of the effect of the fabrication imperfections on the two-ring anchoring machine performance.
    • Fig. S8. Observations of the shape morphing of the peristaltic pump.
    • Fig. S9. Liquid biopsy of the capsule.
    • Fig. S10. Analysis of the magnetic attraction force on device deformation.
    • Fig. S11. Illustration of the jig-assisted fabrication of the exemplar 3D ring.
    • Fig. S12. Designs of the hollow cubic frame, simulation verification, fabrication process, and cargo transport demonstration.
    • Fig. S13. Designs of the flower-shaped machine, simulation verification, and fabrication process.
    • Fig. S14. Designs of the starfish-shaped machine, simulation verification, and fabrication process.
    • Fig. S15. Designs of the capsule-shaped machine and its fabrication process.
    • Fig. S16. Schematic illustrations of the electrical setup for the pulsing magnetic field.
    • Fig. S17. Designs of the peristaltic pump and illustrations of its fabrication process.
    • Fig. S18. Experimental setup to make a rotating magnetic field for the peristaltic pump.
    • Fig. S19. Designs of the capsule and its fabrication process.
    • Fig. S20. Designs of the anchoring machine and illustrations of its fabrication process.
    • Fig. S21. Exemplar cantilever beam made of voxels based on five different kinds of materials.
    • Fig. S22. Shape morphing comparison between assembled and cast cantilever beams.
    • Table S1. Comparison of the proposed fabrication method with the ones previously reported in the literature.
    • Table S2. Material characterization of various kinds of materials used in the reported machines.
    • Legends for movie S1 to S6
    • Reference (49)

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). Video recordings of the representative steps in the fabrication of the exemplar 3D ring machine.
    • Movie S2 (.mp4 format). Video recordings of the representative steps in the fabrication of the functional peristaltic pump machine.
    • Movie S3 (.mp4 format). Experimental results of a small-scale magnetic soft peristaltic pump that pumps mouse whole blood and transport a solid sphere in a rotating uniform magnetic field.
    • Movie S4 (.mp4 format). Experimental results of the reported miniature magnetic soft capsules.
    • Movie S5 (.mp4 format). Experimental results, including an experimental trial and a control trial, of a miniature magnetic soft capsule taking liquid samples and demonstrate potentials for future liquid biopsy applications.
    • Movie S6 (.mp4 format). Experimental results of a small-scale magnetic soft anchoring machine.

    Files in this Data Supplement:

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