The recent introduction of surface acoustic wave (SAW) technology onto lab-on-a-chip platforms has opened a new frontier in microfluidics. The advantages provided by such SAW microfluidics are numerous: simple fabrication, high biocompatibility, fast fluid actuation, versatility, compact and inexpensive devices and accessories, contact-free particle manipulation, and compatibility with other microfluidic components. We believe that these advantages enable SAW microfluidics to play a significant role in a variety of applications in biology, chemistry, engineering, and medicine. In this review article, we discuss the theory underpinning SAWs and their interactions with particles and the contacting fluids in which they are suspended. We then review the SAW-enabled microfluidic devices demonstrated to date, starting with devices that accomplish fluid mixing and transport through the use of travelling SAW; we follow that by reviewing the more recent innovations achieved with standing SAW that enable such actions as particle/cell focusing, sorting, and patterning. Finally, we look forward and appraise where the discipline of SAW microfluidics could go next.

The ability of surface acoustic waves to trap and manipulate micrometer-scale particles and biological cells has led to many applications involving "acoustic tweezers" in biology, chemistry, engineering, and medicine. Here, we present 3D acoustic tweezers, which use surface acoustic waves to create 3D trapping nodes for the capture and manipulation of microparticles and cells along three mutually orthogonal axes. In this method, we use standing-wave phase shifts to move particles or cells in-plane, whereas the amplitude of acoustic vibrations is used to control particle motion along an orthogonal plane. We demonstrate, through controlled experiments guided by simulations, how acoustic vibrations result in micromanipulations in a microfluidic chamber by invoking physical principles that underlie the formation and regulation of complex, volumetric trapping nodes of particles and biological cells. We further show how 3D acoustic tweezers can be used to pick up, translate, and print single cells and cell assemblies to create 2D and 3D structures in a precise, noninvasive, label-free, and contact-free manner.3D acoustic tweezers | cell printing | 3D cell manipulation | cell assembly | 3D particle manipulationT he ability to precisely manipulate living cells in three dimensions, one cell at a time, offers many possible applications in regenerative medicine, tissue engineering, neuroscience, and biophysics (1-3). However, current bioprinting methods are generally hampered by the need to reconstruct and mimic 3D cellto-cell communications and cell-environment interactions. Because of this constraint, bioprinting requires accurate reproduction of multicellular architecture (4, 5). Several approaches have been developed to produce complex cell patterns, clusters, assembled arrays, and even tissue structures. These approaches use many disparate technologies which include optics, magnetic and electrical fields, injection printing, physical or geometric constraints, or surface engineering (6-11). However, there is currently a paucity of a single method that can facilitate the formation of complex multicellular structures with high precision, high versatility, multiple dimensionality, and single-cell resolution, while maintaining cell viability, integrity, and function. As a result, there is a critical need to develop new methods that seek to overcome these limitations."Acoustic tweezers," which manipulate biological specimens using sound waves, offer several unique advantages (12, 13) in comparison with other techniques. First, the acoustic tweezers technology is the only active cell-manipulation method using gentle mechanical vibrations that do not alter cell characteristics. Acoustic vibrations create a pressure gradient in the medium to move suspended microobjects and cells, thereby resulting in a contaminationfree, contact-less, and label-free method for cell manipulation. Sound waves are preferred for cell manipulation for the following reasons: (i) cells maintain their native state (e.g., shape, size, reflective index,...

Acoustic tweezers are a versatile set of tools that use sound waves to manipulate bioparticles ranging from nanometer-sized extracellular vesicles to millimeter-sized multicellular organisms. Over the past several decades, the capabilities of acoustic tweezers have expanded from simplistic particle trapping to precise rotation and translation of cells and organisms in three dimensions. Recent advances have led to reconfigured acoustic tweezers that are capable of separating, enriching, and patterning bioparticles in complex solutions. Here, we review the history and fundamentals of acoustic-tweezer technology and summarize recent breakthroughs.

The interactions between pairs of cells and within multicellular assemblies are critical to many biological processes such as intercellular communication, tissue and organ formation, immunological reactions, and cancer metastasis. The ability to precisely control the position of cells relative to one another and within larger cellular assemblies will enable the investigation and characterization of phenomena not currently accessible by conventional in vitro methods. We present a versatile surface acoustic wave technique that is capable of controlling the intercellular distance and spatial arrangement of cells with micrometer level resolution. This technique is, to our knowledge, among the first of its kind to marry high precision and high throughput into a single extremely versatile and wholly biocompatible technology. We demonstrated the capabilities of the system to precisely control intercellular distance, assemble cells with defined geometries, maintain cellular assemblies in suspension, and translate these suspended assemblies to adherent states, all in a contactless, biocompatible manner. As an example of the power of this system, this technology was used to quantitatively investigate the gap junctional intercellular communication in several homotypic and heterotypic populations by visualizing the transfer of fluorescent dye between cells.cell-cell interaction | intercellular communication | surface acoustic waves | acoustic tweezers | acoustofluidics M ulticellular systems rely on the interaction between cells to coordinate cell signaling and regulate cell functions. Understanding the mechanism and process of cell-cell interaction is critical to many physiological and pathological processes, such as embryogenesis, differentiation, cancer metastasis, immunological interactions, and diabetes (1-3). Despite significant advances in this field, to further understand how cells interact and communicate with each other, a robust, biocompatible method to precisely control the spatial and temporal association of cells and to create defined cellular assemblies is urgently needed (4). Although several methods have been used to pattern cells, limitations still exist for the demonstrated methods including those that make use of optical, electrical, magnetic, hydrodynamic, and contact printing technologies (5-9). Firstly, most of the methods require modification of the cell's native state. The magnetic assembly method, for example, requires cells to be labeled with magnetic probes. Dielectrophoresis typically requires the use of a special medium (e.g., nonconductive) which may lack essential nutrients or have biophysical properties (such as the osmolality) that may adversely affect cell growth or physiology (6). Optical tweezers provide a label-free and contactless approach, but typically require high laser power to manipulate cells, leading to a high risk of cell damage (5). Secondly, the working principles of the existing technologies mostly preclude the combination of high precision and high throughput into a single ...

We present a theoretical analysis and experimental demonstration of particle trapping and manipulation around optothermally generated bubbles. We show that a particle located within 500 μm of a surface bubble can be attracted towards a bubble by the drag force resulting from a convective flow. Once the particle comes in contact with the bubble’s surface, a balance between surface tension forces and pressure forces traps the particle on the bubble surface, allowing the particle to move with the bubble without detaching. The proposed mechanism is confirmed by computational fluid dynamics simulations, force calculations, and experiments. Based on this mechanism, we experimentally demonstrated a novel approach for manipulating microparticles via optothermally generated bubbles. Using this approach, randomly distributed microparticles were effectively collected and carried to predefined locations. Single particles were also manipulated along prescribed trajectories. This bubble-based particle trapping and manipulation technique can be useful in applications such as micro assembly, particle concentration, and high-precision particle separation.

Multifunctional Janus particles have a variety of applications in a wide range of fields. However, to achieve many of these applications, high-throughput, low-cost techniques are needed to synthesize these particles with precise control of the various structural/physical/chemical properties. Microfluidics provides a unique platform to fabricate Janus particles using carefully controlled liquid flow in microfluidic channels to form Janus droplets and various types of solidification methods to solidify them into Janus particles. In this Focus article, we summarize the most recent representative works on Janus particle fabrication in microfluidics. The applications of Janus particles in biomedical areas are emphasized. We believe that microfluidics-enabled multifunctional Janus particles could resolve multiple prevalent issues in biomedicine (e.g., disease monitoring at an early stage, high-throughput bioassays, therapeutic delivery) if persistent effort and collaboration are devoted to this direction.

Nanoparticles are regarded as promising transfection reagents for effective and safe delivery of nucleic acids into specific type of cells or tissues providing an alternative manipulation/therapy strategy to viral gene delivery. However, the current process of searching novel delivery materials is limited due to conventional low-throughput and time-consuming multistep synthetic approaches. Additionally, conventional approaches are frequently accompanied with unpredictability and continual optimization refinements, impeding flexible generation of material diversity creating a major obstacle to achieving high transfection performance. Here we have demonstrated a rapid developmental pathway toward highly efficient gene delivery systems by leveraging the powers of a supramolecular synthetic approach and a custom-designed digital microreactor. Using the digital microreactor, broad structural/functional diversity can be programmed into a library of DNA-encapsulated supramolecular nanoparticles (DNA⊂SNPs) by systematically altering the mixing ratios of molecular building blocks and a DNA plasmid. In vitro transfection studies with DNA⊂SNPs library identified the DNA⊂SNPs with the highest gene transfection efficiency, which can be attributed to cooperative effects of structures and surface chemistry of DNA⊂SNPs. We envision such a rapid developmental pathway can be adopted for generating nanoparticle-based vectors for delivery of a variety of loads.

Microfluidics expands the synthetic space such as heat transfer, mass transport, and reagent consumption to conditions not easily achievable in conventional batch processes. Hydrodynamic focusing in particular enables the generation and study of complex engineered nanostructures and new materials systems. In this review, we present an overview of recent progress in the synthesis of nanostructures and microfibers using microfluidic hydrodynamic focusing techniques. Emphasis is placed on distinct designs of flow focusing methods and their associated mechanisms, as well as their applications in material synthesis, determination of reaction kinetics, and study of synthetic mechanisms.