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Mobile microrobots that maneuver in liquid environments and navigate inside the human body have drawn a great interest due to their possibility for medical uses serving as an in vivo cargo. For this system, the effective self-propelling method, which should be powered wirelessly and controllable in 3-D space, is of paramount importance. This article describes a bubble-powered swimming microdrone that can navigate in 3-D space in a controlled manner. To enable 3-D propulsion with steering capability, air bubbles of three lengths are trapped in microtubes that are embedded and three-dimensionally aligned inside the drone body using two-photon polymerization. These bubbles can generate on-demand 3-D propulsion through microstreaming when they are selectively excited at their individual resonance frequencies that depend on the bubble sizes. In order to equip the drone with highly stable maneuverability, a non-uniform mass distribution of the drone body is carefully designed to spontaneously restore the drone to the upright position from disturbances. A mathematical model of the restoration mechanism is developed to predict the restoration behavior showing a good agreement with the experimental data. The present swimming microdrone potentially lends itself to a robust 3-D maneuverable microscale mobile cargo navigating in vitro and in vivo for biomedical applications. 

Remotely activated drug release strategy with controllable dosage is the key factor of various targeting drug delivery methods as minimal invasive treatment. This article describes mass transport in liquid in microscale with a controllability of releasing amount, which is wirelessly excited by an external acoustic excitation. A liquid droplet (releasing agent, or drug in application) is trapped in the middle of a one-end open microtube which has a ratchet-structure on its inner wall. The droplet is trapped in the tube and neighbored by two gaseous air bubbles on both sides. In the presence of acoustic wave, the air bubbles oscillate and resonate. The air bubble near the tube opening segregates the liquid droplet into smaller ones and transport them on the ratchet-surface wall of the microtube. This mass transport occurs in both directions at similar rates: from the surrounding fluid to the trapped droplet and vice versa. As a result, the overall mass of droplet remains similar. Meanwhile, the other bubble positioned back in the tube sealing side enhances mixing between incoming mass from the surrounding and existing mass in the droplet. This mass transport is significant only when the inner wall of the tube has rachets. The exchanging mass between the surrounding and droplet is monotonically proportional to the excitation period, showing high controllability of mass transport. This mass transport phenomenon possibly provides a new mechanism of in vivo, on-demand, dose controllable drug delivery. 

Objective: The purpose of this paper is to demonstrate the ultrasound tracking strategy for the acoustically actuated bubble-based microswimmer. Methods: The ultrasound tracking performance is evaluated by comparing the tracking results with the camera tracking. A benchtop experiment is conducted to capture the motion of two types of microswimmers by synchronized ultrasound and camera systems. A laboratory developed tracking algorithm is utilized to estimate the trajectory for both tracking methods. Results: The trajectory reconstructed from ultrasound tracking method compares well with the conventional camera tracking, exhibiting a high accuracy and robustness for three different types of moving trajectories. Conclusion: Ultrasound tracking is an accurate and reliable approach to track the motion of the acoustically actuated microswimmers. Significance: Ultrasound imaging is a promising candidate for noninvasively tracking the motion of microswimmers inside body in biomedical applications and may further promote the real-time control strategy for the microswimmers. 

Wirelessly powered and controllable microscale propulsion in 3-D space is of critical importance to micro swimming drones serving as an active and maneuverable in vivo cargo for medical uses. This aritcle describes a 3-D micro swimming drone navigating in 3-D space, propelled by unidirectional microstreaming flow from acoutsically oscillating bubbles. 3-D propulsion is enabled by multiple bubbles with different lengths embedded in different orientations inside the drone body. Each bubble generats propulsion by applying acoustic field at its resonance frequency. Therefore, 3-D propulsion in any direction is achievable by resonating bubbles individually or jointly. The drone with such a complex design was fabricated by a two-photon polymerization 3-D printer. For stable maneuverability, a non-uniform mass distribution of the drone is designed to restore the drone to the designated posture under any disturbances. The restoration mechanism is formulated by a mathematical model, predicting the restoring time and shows an excellent agreemnt with the experimental results. This 3-D micro swimning drone proves its robustness as a manueverable microrobot navigating along programmble path in a 3-D space through selective and joint actuation of microbubbles. 

It has been previously reported that a gaseous bubble trapped in a one-end-open tube oscillates in the presence of acoustic wave and generates strong microstreaming flows and thus a propulsion force. The propulsion highly depends on the frequency and the voltage of the external acoustic wave. This paper presents a new discovery that the direction of this propulsion is dependent on the relative location of the bubble interface. The oscillating bubble propels forward when its interface stays deep inside the tube. On the contrary, the bubble propels in a reverse direction when its interface is at the exit of the tube. Learning from this phenomenon, we developed and introduced physical structures (necks) to precisely control the location of the bubble interface. As a result, the length and interface position of the bubble is more controllable, and the bubble oscillation and propulsion becomes more predictable and consistent.