As a simple, clean and effective tool, micro bubbles have enabled advances in various lab on a chip (LOC) applications recently. In bubble-based microfluidic applications, techniques for capturing and controlling the bubbles play an important role. Here we review active and passive techniques for bubble trapping and control in microfluidic applications. The active techniques are categorized based on various types of external forces from optical, electric, acoustic, mechanical and thermal fields. The passive approaches depend on surface tension, focusing on optimization of microgeometry and modification of surface properties. We discuss control techniques of size, location and stability of microbubbles and show how these bubbles are employed in various applications. To finalize, by highlighting the advantages of these approaches along with the current challenges, we discuss the future prospects of bubble trapping and control in microfluidic applications.

Efficient transportation of fluids and microparticles is an important capability in many medical and biological applications. In this article, an efficient bi-directional micropump using microcavity-trapped acoustic bubbles is studied. With acoustic actuation, a controllable microstreaming net flow is generated inside a microchannel by the oscillating bubbles. Based on theory and experimental results, different sized microbubbles have different resonant frequencies. Thus, by oppositely placing the different sized microbubbles, the flow direction can be switched via altering the frequency. The pumping flow rate can be tuned by adjusting the input voltage and can achieve as high as 1600 nl/min with high stability. Furthermore, the bi-directional pumping ability is also proved using blood-mimicking fluid (BMF), allowing for on-chip high-viscosity fluid pumping. In the end, the proposed device is employed in pumping Escherichia coli bacteria, indicating that the micropump is capable of pumping cells without damaging them. This inexpensive, portable and biocompatible acoustic bubble-based bi-directional pump for transporting fluids and particles has great potential to integrate with other on-chip platforms for multiple biological and chemical applications, such as drug delivery, cell separation, and chemical analysis.

In this paper, off-the-shelf materials such as polyethylene terephthalate films and double-sided tapes are applied to create lab-on-a-foil microfluidic devices via a cutting plotter. Microstructures termed defended oscillating membrane equipped structures (DOMES) are integrated in the microchannels. These dome-shaped pore-containing DOMES are created above a through hole on the films using two-photon polymerization. As the bottom side of the air-liquid interfaces trapped in DOMES’ pores is always facing ambient air, bubble instability that compromises acoustofluidic performance in conventional cases is alleviated or avoided. The acoustically induced flow is observed to be stronger with increasing pore size on DOMES. An acoustofluidic micromixer is proposed to further investigate the capabilities of DOMES, and it is the first time active micromixer is achieved on lab-on-a-foil devices, with good performance competitive to reported microfluidic mixers.

The majority of microfluidic devices nowadays are built on rigid or bulky substrates such as glass slides and polydimethylsiloxane (PDMS) slabs, and heavily rely on external equipment such as syringe pumps. Although a variety of micropumps have been developed in the past, few of them are suitable for flexible microfluidics or lab-on-a-foil systems. In this paper, stick-and-play acoustic micropump is built on thin and flexible plastic film by printing microstructures termed defended oscillating membrane equipped structures (DOMES) using two-photon polymerization. Specifically, this new micropump induces rectified flow upon the actuation of acoustic waves, and the flow patterns agree with simulation results very well. More importantly, the developed micropump has the capabilities to generate adjustable flow rates as high as 420 nL min−1, and does not suffer from problems such as bubble instability, gas dissolution, and undesired bubble-trapping that commonly occur in other forms of acoustic micropumps. Since the micropump works in stick-and-play mode, it is reusable after cleaning thanks to the easy separation of covers and substrates. Lastly, the developed micropump is applied for creating a self-pumped single-cell trapping device. The excellent trapping capability of the integrated device proves its potential for long-term studies of biological behaviors of individual cells for biomedical applications.