Acoustically powered surface-slipping mobile microrobots

Abstract

Untethered synthetic microrobots have significant potential to revolutionize minimally invasive medical interventions in the future. However, their relatively slow speed and low controllability near surfaces typically are some of the barriers standing in the way of their medical applications. Here, we introduce acoustically powered microrobots with a fast, unidirectional surface-slipping locomotion on both flat and curved surfaces. The proposed three-dimensionally printed, bullet-shaped microrobot contains a spherical air bubble trapped inside its internal body cavity, where the bubble is resonated using acoustic waves. The net fluidic flow due to the bubble oscillation orients the microrobot's axisymmetric axis perpendicular to the wall and then propels it laterally at very high speeds (up to 90 body lengths per second with a body length of 25 µm) while inducing an attractive force toward the wall. To achieve unidirectional locomotion, a small fin is added to the microrobot's cylindrical body surface, which biases the propulsion direction. For motion direction control, the microrobots are coated anisotropically with a soft magnetic nanofilm layer, allowing steering under a uniform magnetic field. Finally, surface locomotion capability of the microrobots is demonstrated inside a three-dimensional circular cross-sectional microchannel under acoustic actuation. Overall, the combination of acoustic powering and magnetic steering can be effectively utilized to actuate and navigate these microrobots in confined and hard-to-reach body location areas in a minimally invasive fashion.

Keywords: acoustic actuation; bubble oscillation; magnetic control; microrobots; microswimmers.

Fig. 1.

Fabrication and propulsion behavior of the proposed acoustic microrobots. (A) Three-dimensional nanoprinting of the microrobot on a glass slide using the two-photon lithography technique. (Inset) The fabricated microrobot immersed in water, where ${\text{d}}_{\text{B}}$ and ${\text{d}}_{\text{O}}$ are the diameters of the air bubble and orifice, respectively, and $l$ is the length. (B) Schematics of the robot propulsion, where under the acoustic waves the robot flips toward the substrate and slips forward due to the asymmetric microstreaming pattern generated by the pulsating microbubble and the designed “fin” structure. (C and D) Scanning electron microscopy images of the fully symmetric and anisotropic microrobot designs. (E) The trochoidal random propulsion path example of the symmetric microrobot under ultrasound actuation from (i) t = 0 s to (ii) t = 0.27 s, which results from the combined translation and rotation of the body. (F) The directional forward motion example of the microrobot with a fin structure from (i) t = 0 s to (ii) t = 1.34 s, under ultrasound actuation. The introduction of the fin is crucial for creating the flow asymmetry, which allows the unidirectional motion (Movie S1).

Fig. 2.

Characterization of the resonance frequency of the microbubble trapped inside the robot body cavity. (A) The experimental apparatus for exciting the microbubbles at different frequencies. (i) An array of microrobots with trapped microbubbles. (ii) The confocal image of the supporting polymeric shell. (B) The microstreaming pattern of the acoustically actuated microrobot at the resonance frequency of $f_{\text{res}}=237\hspace{0.5em}\text{kHz}$, while the swimmer’s body is fixed and the orifice is parallel to the substrate (Movie S2). (C) The simulated acoustic pressure map at the resonance frequency, when the microrobot is immersed in a fluidic medium. (D) The average speeds of the 2-µm tracer particles at different frequencies, normalized by the piezoelectric sensor voltage. The averages are taken over 20 particles and the error bar represents the SD.

Fig. 3.

Flipping and surface-slipping locomotion of the microrobots under acoustic actuation. (A) The released microrobot, actuated by the acoustic waves, flips in such a way that the oscillating free surface of the microbubble faces toward the solid substrate (Movie S3), where the driving voltage amplitude is 3.5 V_pp. (i–iii) Time-lapse images of three microrobots from starting position at t = 0 s to fully flipped orientation at t = 1.67 s. (B) The trajectory of 1-µm tracer particles following the jet streams around the microrobot after the flipping event; the blue arrows represent the streaming flow direction (Movie S5). (C) Two microrobots with different fin orientations slip forward (Movie S6). (i–iii) Time-lapse images of two oppositely moving microrobots from t = 0 s to t = 0.63 s. (D) The average speed of the robots under different acoustic voltage amplitudes, reaching up to 90 body lengths per second. Each data point represents the swimming speed analyzed from five microrobots. The error bar represents the SD. (E) The instantaneous speed associated with four different acoustic voltage levels (Movie S7). The rise time of the microrobots is around 0.05 s in order to reach their terminal speed.

Fig. 4.

Steering control of the microrobots under acoustic propulsion and magnetic field-based orientation control. (A) Sketch of the anisotropic magnetic nanofilm coating on the microrobots, where a 20-nm Ni nanofilm is directionally sputtered on the 3D-microprinted robots in the given specific orientation (i.e., the fin side is on the top). (B) The instantaneous speed of the magnetically steered microrobot under acoustic waves, plotted by the red dotted line, for eight turning events shown with time-lapse images in C, i–viii, where the red line indicates the trajectory of the microrobot (Movie S9). An in-plane magnetic field, with the direction indicated by B arrows, of 10-mT amplitude is applied to rotate the microrobots by 90°.

Fig. 5.

Microrobot slipping on 3D curved surfaces. (A) Schematics of the experimental setup for actuation of the microrobots inside a circular channel of 300-µm diameter. (B) Sketch of the swimming behavior video snapshots on the curved walls. (C) The fin is oriented toward the cylindrical wall and, upon acoustic actuation, slips on the curved boundary and escapes the microscope focal plane. (i–iv) Time-lapse images of the surface-slipping motion of the microrobot, where the blue arrow indicates the locomotion direction. (ii, Inset) Schematics of the microrobot climbing the curved wall of circular cross-sectional channel. (D) The microrobot moves forward in length of the circular channel, under the applied ultrasound (Movie S10). (i–iii) Time-lapse images of the surface-slipping microrobot from the starting location at t = 0 s to t = 4.35 s, where the blue arrow indicates the motion direction. The symbols ↓g and ⓧg represent the downward and into-the-page gravity direction, respectively.