We report magnetic-field-driven on-demand manipulation of liquid metal in microfluidic channels filled with base or acid. The liquid metal was coated with iron (Fe) particles and treated with hydrochloric acid to have strong bonding strength with the Fe particles. The magnetic liquid metal slug inserted in the microchannel is manipulated, merged, and separated. In addition, corresponding to the repositioning of an external magnet, the liquid metal slug can be readily moved in microfluidic channels with different angles (>90°) and cross-linked channels in any direction. We demonstrated the functionality of the liquid metal in the microfluidic channel for electrical switching applications by manipulation of the liquid metal, resulting in the sequential turning on of light emitting diodes (LEDs).

Targeted drug delivery is a promising application of microrobots owing to the capability of the microrobots to access nearly every region of the human body through the circulatory system. Research on microrobots over the past few decades has enabled substantial advances in the design of both the untethered microrobots swimming in a biofluid and the related mechanisms to carry and release therapeutic agents in a controlled manner. This paper presents a comprehensive review of the technological state of the art in untethered microrobots for targeted drug delivery applications. First, the in vivo microrobot locomotion techniques are discussed with respect of the different types of actuation energy sources such as magnetic fields, motile microorganisms, acoustic waves, and chemical reaction, outlining the respective advantages and major limitations. Subsequently, recent progress in various technologies of microrobot-driven targeted drug delivery is surveyed deliberating on the corresponding drug manipulation mechanisms: magnetically driven, motile microorganisms-driven, acoustic-aided, and stimuli-responsive hydrogels-aided. Although most studies on microrobot-driven targeted drug delivery were carried out in vitro, few among them successfully demonstrated in vivo operations in living animals. In the concluding section, current challenges and future perspectives of the microrobot-driven targeted drug delivery technology are discussed.

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This paper describes a new on-chip manipulation method for handling millimeter-and micron-sized objects using oscillating mobile bubbles. It is found that acoustically excited oscillating bubbles can attract and capture neighboring objects. A variety of objects, including hydrophilic glass beads (80 μm), polystyrene beads (100 μm), a fish egg (∼1 mm) and a live water flea (∼1 mm), are successfully captured. The capturing performance is characterized using 80 μm hydrophilic glass particles while varying the acoustic excitation frequency and amplitude. The oscillation amplitude of the bubbles is quantified using high-speed images. At the natural frequencies of the bubbles the capturing range is highest. The capturing range increases as the oscillation amplitude increases. It is also found that while the bubbles are in lateral motion the capturing force is strong enough to hold the captured objects. By integrating acoustic excitation with electrowetting-on-dielectric (EWOD) bubble transportation, it is demonstrated that oscillating mobile bubbles can capture, carry and release neighboring objects on a chip. This new manipulation method may provide an efficient tool for handling millimeter-as well as micron-sized objects such as biological cells.

W hen a gaseous bubble in liquid is excited by acoustic waves, it oscillates (expands and shrinks) at the wave frequencies and generates strong vortical flows around it, the so-called cavitational microstreaming. This article describes the development of a micropumping principle using cavitational microstreaming. The key idea is to place a capillary tube vertically above an oscillating bubble to collect the upward microstreaming flow. When the bubble is excited at its resonance frequency, it oscillates with surface undulations (surface wave mode) and pumps water through the tube. The performance of this pumping mechanism is experimentally studied using millimeter and microscale bubbles. The flow rate and generated pressure are measured in a variety of conditions. The measured results indicate that the present pump falls into the category of moderate-flow-rate and low-pressure type pumps. The present pump operates without physical connections or electrical wiring to the bubbles, implicating potential applications as implantable micropumps in many lab-on-a-chip type systems.