Selectively manipulable acoustic-powered microswimmers

Abstract

Selective actuation of a single microswimmer from within a diverse group would be a first step toward collaborative guided action by a group of swimmers. Here we describe a new class of microswimmer that accomplishes this goal. Our swimmer design overcomes the commonly-held design paradigm that microswimmers must use non-reciprocal motion to achieve propulsion; instead, the swimmer is propelled by oscillatory motion of an air bubble trapped within the swimmer's polymer body. This oscillatory motion is driven by the application of a low-power acoustic field, which is biocompatible with biological samples and with the ambient liquid. This acoustically-powered microswimmer accomplishes controllable and rapid translational and rotational motion, even in highly viscous liquids (with viscosity 6,000 times higher than that of water). And by using a group of swimmers each with a unique bubble size (and resulting unique resonance frequencies), selective actuation of a single swimmer from among the group can be readily achieved.

Figure 1. Fabrication and design of microswimmers.

(a), Schematic of the fabrication setup. PEG solution containing photosensitive initiator is sandwiched between glass slides. The swimmers' geometries and the conical shaped indents were created by exposing the oligomer solution to UV light passing through a mask containing the blueprint of the swimmers. (b), Indentation diameter versus UV exposure time. (c), Indentation depth versus UV exposure time. (d), Images showing the decrease in indentation depth for increasing UV exposure time.

Figure 2. Geometry and experimental design of the acoustic microswimmers.

(a) Fluorescent images of four types of swimmer: linear microswimmers with a single (false-coloured yellow) or double (red) indent that is symmetric about the central axis, rotational microswimmers with off-centred (purple) indent and directional microswimmers with (green) indents of different diameter. (b), A piezoelectric transducer injects acoustic energy into a chamber that is filled with fluid, lined with acoustically-absorbent putty, and enclosed on top and bottom by glass slides. (c), An image sequence recorded at 360,000 frames per second showing bubble oscillation within the conical indentation, fitted to a sine function. (d), Acoustic oscillation of the microswimmer bubbles generates substantial acoustic microstreaming in water. Both ends of indentations are open.

Figure 3. High-speed imaging captures the translational and rotation motion of acoustic microswimmers moving through either a water/microbead mixture or hydrogel.

(a), A single on-centre bubble generates linear motion in water, as does (b), a pair of bubbles of equal size symmetrically placed. An off-centre indentation generates either (c), clockwise or (d), counterclockwise motion. (e), The same rotary motion (or linear motion, not shown) can also be achieved in viscous shear-thinning hydrogel.

Figure 4. Frequency dependence of bubble oscillation amplitude.

The bubble oscillation is largest when the acoustic driving field is resonant with the fundamental natural frequency of the bubble. The resonance peak for a bubble of diameter 45 µm in water is reasonably narrow. Corresponding to a quality factor Q ~ 25.

Figure 5. Characterization of the acoustic microswimmers.

(a), An acoustic microswimmer immersed in water moves at a speed nearly proportional to the square of the amplitude of the drive voltage, i.e., the square of the amplitude of the incident acoustic field. This dependence is consistent with the acoustic coupling to motility. (b), An acoustic mciroswimmer immersed in a more viscous solution, 50% glycerol, exhibits similar scaling, with a slightly higher slope. (c), Within the shear-thinning hydrogel, the microswimmer speed varies as the fourth power of the bubble oscillation amplitude (measured by direct high-speed imaging). This result is consistent with the shear-thinning behaviour and an acoustic propulsion that scales with the square of the oscillation amplitude. Similar results are obtained for two swimmers with bubble diameters of 30 µm (driven at 94.4 kHz, in red) and 67 µm (driven at 70.4 kHz, in blue).

Figure 6. Superimposed time-lapse images of selective actuation of an acoustic microswimmer from within a group.

Two swimmers with bubbles of different size were immersed in an acoustic field of variable frequency. (a), Swimmer A, with the larger bubble, begins acoustically-driven motion at 74 kHz, with little simultaneous motion of swimmer B. A video of this behaviour is available as Supplementary Video 6. (b), With further increase in frequency, swimmer A stops; swimmer B begins to move at 91 kHz, with swimmer A remaining essentially stationary. A video of this behaviour is available as Supplementary Video 7.

Figure 7. Superimposed time-lapse images of controlled two-dimensional motion of different microswimmers with bubbles of different sizes.

(a), When a two-bubble swimmer is driven at the resonance of just one bubble, it rotates in a wide orbit. (b), The orbit of an asymmetric one-bubble swimmer is much tighter due to its stronger asymmetry. (c), At a frequency intermediate between the resonances of the two constituent bubbles, a two-bubble swimmer can move in a straight line.

Figure 8. Interaction of two swimmers in water and hydrogel.

(a), Two swimmers undergoing clockwise motions in water firmly lock themselves together after snapping into contact, as shown in Supplementary Video 10. (b), Two swimmers in hydrogel, a medium that suppresses acoustic streaming, come apart after collision. The swimmer trajectories across the collision are traced in magenta and green. This event is also shown in Supplementary Video 11.