Research ArticleAPPLIED SCIENCES AND ENGINEERING

3D steerable, acoustically powered microswimmers for single-particle manipulation

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Science Advances  25 Oct 2019:
Vol. 5, no. 10, eaax3084
DOI: 10.1126/sciadv.aax3084
  • Fig. 1 Schematic of the microswimmers and the acoustofluidic chamber.

    (A) Schematic and false-colored scanning electron microscopy image of microswimmers (with Au layers). (B) Schematic of the acoustofluidic chamber. The microswimmers with and without the Au layer sink to the bottom or float to the top of the chamber, respectively.

  • Fig. 2 Theoretical analysis and experimental demonstrations of the microswimmers’ behaviors responding to an acoustic field and a magnetic field.

    (A) Schematic of the primary Bjerknes force FPB (blue solid arrow), secondary Bjerknes force FSB (red solid arrow), and streaming propulsive force FSP (black solid arrow) on a microswimmer at the moment that an acoustic field is applied (T = 0). The yellow dot indicates the center of mass of the microswimmer, and black dashed arrows indicate the streaming flow pattern. The total force (yellow dashed arrow) generates a torque to rotate the axis of the microswimmer into the z direction. When the magnetic field is on (blue dashed arrows), the microswimmer is tilted at an angle α and starts to translate. (B) Experimental demonstrations (top view) of the self-rotation of microswimmers when the acoustic field is on and the translation of microswimmers over 2 s when both acoustic and magnetic fields are applied. (C) Dependence of the speed of the microswimmers upon the acoustic frequency in a constant magnetic field. (D) The relationship of the speed of microswimmers and applied acoustic pressure; the red line is a quadratic fit.

  • Fig. 3 Regulating the speed of microswimmers by the direction of the magnetic field and numerical simulation of the acoustic streaming pattern.

    (A) An individual microswimmer traveled different distances in 2 s under the same acoustic pressure but with different magnetic field directions. (B) Experimental measurement of the microswimmer’s speed (black squares) and simulated hydrodynamic stress in the x direction σx (blue line) when the microswimmer was tilted with different α values. The σx is normalized for comparison. (C) Numerical simulation of the acoustic pressure (surface) and streaming pattern (arrows) induced by the bubble oscillation at the water-air interface (red dashed lines) when the microswimmer is normal to the substrate (0°) and when it is tilted at α = 45°. A distance of 500 nm was chosen between the substrate and the lowest point of the microswimmer. (D) Schematic of microswimmers that were coated with a nickel layer in different orientations and their distinct alignments in the same magnetic field. In a magnetic field B12 = 0), the microswimmer M1 traveled along the red trajectory, while the M2 remained stationary. Then, the magnetic field was rotated (B2) to make α1 = 0, the motion of M1 was terminated, and M2 was propelled along the green trajectory.

  • Fig. 4 Manipulation of passive particles with a microswimmer in pushing and pulling modes.

    (A) Illustration of the pushing mode for particle manipulation. Black arrows indicate the motion direction of the microswimmer, and the green arrow indicates the direction of the pushing force. The experimental demonstration was conducted at a low acoustic pressure of 300 Pa. A microswimmer was steered to separate two adjacent particles and push the target particle (blue) away. The weak attractive force between the microswimmer and particles generated a very subtle effect on the green particle. (B) Illustration of the pulling mode of the microswimmer for particle manipulation. The black arrows indicate the direction of motion of the microswimmer, and the red arrows indicate the pulling force for loading and the streaming repulsive force for release. This experimental demonstration was conducted at an acoustic pressure of 1 kPa. The microswimmer moved to the target, attracted it, and dragged it to a new location. The particle was released by further tilting the microswimmer to increase the streaming propulsive force on the particle. (C) Silica particles (4 μm) were patterned into the letters PSU. (D) The microswimmer was propelled in a cell culture medium and pushed a HeLa cell (~20 μm in diameter) into contact with another cell.

  • Fig. 5 Manipulation of a microswimmer in 3D.

    (A) Schematic drawing of the orientation of a microswimmer when it is attracted to a vertical boundary. (B) A microswimmer moving on a vertical boundary. The microswimmer is initially oriented by acoustic force at the vertical boundary with its open end facing the boundary. The red line shows its trajectory when it was tilted to the right by a magnetic field. (C) Schematic of the staircase used to illustrate boundary climbing behavior. The length of each step gradually decreases as their height increases. (D) Time-lapse images show the microswimmer climbing the stairs from bottom to top. (E) Schematic of a tailed microswimmer. (F) Time-lapse images show that a tailed microswimmer detaches from the bottom surface of the cell and moves in free space under magnetic field control.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/10/eaax3084/DC1

    Fig. S1. Fabrication process of microswimmers.

    Fig. S2. Optical microscopic image of an uncoated microswimmer floating in water.

    Fig. S3. Experimental distribution of the resonant frequency of microswimmers and theoretical calculation.

    Fig. S4. Characterization of the resonant frequencies of a small microswimmer (a = 1 μm).

    Fig. S5. Schematic of the response of an uncoated microswimmer in the fluidic chamber.

    Fig. S6. HeLa cell viability assay.

    Fig. S7. Scanning electron microscopy image of tailed microswimmers.

    Fig. S8. Measurement of the acoustic pressure with a hydrophone at a distance of ~13 mm from the surface of the silicon wafer, the input voltage was 10 Vpp.

    Fig. S9. Simulated and experimental streaming patterns.

    Movie S1. The self-rotation of a microswimmer responding to an acoustic field.

    Movie S2. The magnetic field–initiated steerable translational motion of a microswimmer.

    Movie S3. An individual microswimmer traveled different distances in 2 s under the same acoustic pressure but with different tilt angles.

    Movie S4. Two microswimmers were propelled independently in the mixture by changing the direction of a magnetic field.

    Movie S5. Separation of two adjacent silica particles with the microswimmer in its pushing mode.

    Movie S6. Transport of particles with the microswimmer in its pulling mode.

    Movie S7. Patterning particles in the shapes of letters PSU by a microswimmer.

    Movie S8. A microswimmer climbing up and down a stairway.

    Movie S9. A tailed microswimmer free swimming in 3D.

  • Supplementary Materials

    The PDF file includes:

    • Fig. S1. Fabrication process of microswimmers.
    • Fig. S2. Optical microscopic image of an uncoated microswimmer floating in water.
    • Fig. S3. Experimental distribution of the resonant frequency of microswimmers and theoretical calculation.
    • Fig. S4. Characterization of the resonant frequencies of a small microswimmer (a = 1 μm).
    • Fig. S5. Schematic of the response of an uncoated microswimmer in the fluidic chamber.
    • Fig. S6. HeLa cell viability assay.
    • Fig. S7. Scanning electron microscopy image of tailed microswimmers.
    • Fig. S8. Measurement of the acoustic pressure with a hydrophone at a distance of ~13 mm from the surface of the silicon wafer, the input voltage was 10 Vpp.
    • Fig. S9. Simulated and experimental streaming patterns.

    Download PDF

    Other Supplementary Material for this manuscript includes the following:

    • Movie S1 (.mp4 format). The self-rotation of a microswimmer responding to an acoustic field.
    • Movie S2 (.mp4 format). The magnetic field–initiated steerable translational motion of a microswimmer.
    • Movie S3 (.mp4 format). An individual microswimmer traveled different distances in 2 s under the same acoustic pressure but with different tilt angles.
    • Movie S4 (.mp4 format). Two microswimmers were propelled independently in the mixture by changing the direction of a magnetic field.
    • Movie S5 (.mp4 format). Separation of two adjacent silica particles with the microswimmer in its pushing mode.
    • Movie S6 (.mp4 format). Transport of particles with the microswimmer in its pulling mode.
    • Movie S7 (.mp4 format). Patterning particles in the shapes of letters PSU by a microswimmer.
    • Movie S8 (.mp4 format). A microswimmer climbing up and down a stairway.
    • Movie S9 (.mp4 format). A tailed microswimmer free swimming in 3D.

    Files in this Data Supplement:

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