Fig. 4. Comparison of high shear rate propulsion of the acoustic microrobots with the surface-rolling motion of the spherical magnetic microrobots inside mucus.

(A) The schematics of the surface-rolling mechanism of the Janus magnetic microrobots inside mucus medium under a rotational uniform magnetic field. (B) The time-lapse trajectories of 25-μm-diameter magnetic microrobots inside mucus, actuated by a rotational frequency of 50 Hz and a uniform magnetic field of 10 mT. The magnetic microrobots were spinning without considerable propulsion, as they were pushed back by the viscoelastic mucus network. Increasing the rotational frequency of the magnetic microrobots to 100 Hz slightly improved their locomotion inside mucus (C). (D) The schematics of the puller-type propulsion for the acoustic microrobots inside mucus. (E) The time-lapse trajectories of the acoustic microrobots penetrating through mucus under 3 V_pp driving voltage. The random trajectories are caused by the heterogeneous mucus microstructure. (F) Enhanced locomotion trajectories of the acoustic microrobots under 4 V_pp driving voltage. Scale bars, 50 μm. (G) The experimental ensemble mean square displacement (MSD) results as a function of the lag time for the acoustic and magnetic microrobots. The MSD values for the acoustic microrobots indicate superior locomotion over the magnetic microrobots inside mucus. The single magnetic microrobots showed back-and-forth motion due to mucus resistance, which is captured by the MSD values after around 1-s lag time.