Microbubbles are used as ultrasound contrast agents, which enhance ultrasound imaging techniques. In addition, microbubbles currently show promise in disease therapeutics. Microfluidic devices have increased the ability to produce microbubbles with precise size, and high monodispersity compared to microbubbles created using traditional methods. This paper will review several variations in microfluidic device structures used to produce microbubbles as ultrasound contrast agents. Microfluidic device structures include T-junction, and axisymmetric and asymmetric flow-focusing. These devices have made it possible to produce microbubbles that can enter the vascular space; these microbubbles must be less than 10 μm in diameter and have high monodispersity. For different demands of microbubbles production rate, asymmetric flow-focusing devices were divided into individual and integrated devices. In addition, asymmetric flow-focusing devices can produce double layer and multilayer microbubbles loaded with drug or biological components. Details on the mechanisms of both bubble formation and device structures are provided. Finally, microfluidically produced microbubble acoustic responses, microbubble stability, and microbubble use in ultrasound imaging are discussed.

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The unique electrical, optical and biological properties of silver nanomaterials have attracted significant attention of many researchers. Since the size and shape of silver nanomaterials have significant effects on the properties of silver nanomaterials, extensive research has focused on synthesis and characterization of silver nanomaterials. However, almost all of the syntheses of silver nanomaterials were carried out in traditional batch reactors, which typically suffer from inhomogeneous mixing and corresponding spatial variations under reaction conditions, ultimately leading to poor quality of the final nanomaterials. Recently, the emerging microfluidic technology not only furnishes novel strategies for the synthesis of silver nanomaterials but also brings great opportunities and impetus to improve the quality and yield of silver nanomaterials due to enhanced mass and heat transfer. The current paper reviews recent achievements in the synthesis of silver nanomaterials in flow microreactors. Various strategies adopted for the synthesis of silver nanomaterials in microreactors are presented and compared, including synthesis in single-phase and multi-phase flow microreactors. In addition, the factors that affect the size and size distribution of silver nanomaterials in flow microreactors synthesis are also discussed briefly.