Hydrodynamic tweezers: 1. Noncontact trapping of single cells using steady streaming microeddies

Programmable photoacoustic manipulation of microparticles in liquid

Particle manipulation through the transfer of light or sound momentum has emerged as a powerful technique with immense potential in various fields, including cell biology, microparticle assembly, and lab-on-chip technology. Here, we present a novel method called Programmable Photoacoustic Manipulation (PPAM) of microparticles in liquid, which enables rapid and precise arrangement and controllable transport of numerous silica particles in water. Our approach leverages the modulation of pulsed laser using digital micromirror devices (DMD) to generate localized Lamb waves in a stainless steel membrane and acoustic waves in water. The particles undergo a mechanical force of about several µN due to membrane vibrations and an acoustic radiation force of about tens of nN from the surrounding water. Consequently, this approach surpasses the efficiency of optical tweezers by effectively countering the viscous drag imposed by water and can be used to move thousands of particles on the membrane. The high power of the pulsed laser and the programmability of the DMD enhance the flexibility in particle manipulation. By integrating the benefits of optical and acoustic manipulation, this technique holds great promise for advancing large-scale manipulation, cell assembly, and drug delivery.

Enhanced particle separation through ultrasonically-induced microbubble streaming for automated size-selective particle depletion

In this study, we present an automated method for achieving Size-Selective Particle Depletion in microchannels. This technique is notable for its label-free, sheath-free, and cost-effective attributes. It combines continuous Poiseuille flow with microbubble streaming to enable the manipulation of particles in an automatic or semi-automatic manner at periodic intervals. Larger particles are retained in proximity to the microbubble, with the option for subsequent eviction through a designated waste exit or their accumulation within a collection chamber for future analysis or manipulation. Unlike many conventional methods, our approach keeps the target particles in the vortices near the microbubble while the primary fluid flows continuously through the microchannel. Subsequently, these particles are ejected in just a few milliseconds, preserving the primary fluid and significantly reducing fluid wastage. We conducted an analysis covering multiple critical facets of the study. This included a rigorous statistical examination, flow characterization using volumetric micro PTV, high-frequency micro PTV for observing flow field transitions, evaluating the system's particle trapping capabilities across different sizes with a proprietary algorithm, and investigating the z-axis distribution of both incoming and escaped particles using volumetric micro PTV. The invaluable insights gleaned from this data played a pivotal role in refining the system and optimizing its operational parameters to achieve peak efficiency across various conditions, encompassing varying particle sizes, flow rates, and seeding densities.

Pneumatic Microballoons for Active Control of the Vibration-Induced Flow

Vibration-induced flow (VIF), in which a mean flow is induced around a microstructure by applying periodic vibrations, is increasingly used as an active flow-control technique at the microscale. In this study, we have developed a microdevice that actively controls the VIF patterns using elastic membrane protrusions (microballoons) actuated by pneumatic pressure. This device enables on-demand spatial and temporal fluid manipulation using a single device that cannot be achieved using a conventional fixed-structure arrangement. We successfully demonstrated that the device achieved displacements of up to 38 µm using the device within a pressure range of 0 to 30 kPa, indicating the suitability of the device for microfluidic applications. Using this active microballoon array, we demonstrated that the device can actively manipulate the flow field and induce swirling flows. Furthermore, we achieved selective actuation of the microballoon using this system. By applying air pressure from a multi-input channel system through a connection tube, the microballoons corresponding to each air channel can be selectively actuated. This enabled precise control of the flow field and periodic switching of the flow patterns using a single chip. In summary, the proposed microdevice provides active control of VIF patterns and has potential applications in advanced microfluidics, such as fluid mixing and particle manipulation.

Controlled roll rotation of a microparticle in a hydro-thermophoretic trap

In recent years, there has been a growing interest in controlling the motion of microparticles inside and outside a focused laser beam. A hydro-thermophoretic trap was recently reported [Nalupurackal et al., Soft Matter 18, 6825 (2022)], which can trap and manipulate microparticles and living cells outside a laser beam. Briefly, a hydro-thermophoretic trap works by the competition between thermoplasmonic flows due to laser heating of a substrate and thermophoresis away from the hotspot of the laser. Here, we extend that work to demonstrate the controlled roll rotation of a microparticle in a hydro-thermophoretic trap using experiments and theory. We experimentally measure the roll angular velocity of the trapped particle. We predict this roll rotation from theoretical computation of the fluid flow. The expression for the angular velocity fits the experimental data. Our method has potential applications in microrheology by employing a different mode of rotation.

Characterisation of hydrodynamic trapping in microfluidic cross-slot devices for high strain rate applications

Hydrodynamic trapping of a particle or cluster of particles based on contact and non-contact approaches has brought prominent insights to micro-nano scale applications. Of the non-contact methods, image-based real-time control in cross-slot microfluidic devices is one of the most promising potential platform for single cellular assays. Here, we report results from experiments conducted in two cross-slot microfluidic channels of different widths, with varying real-time delay of the control algorithm and different magnification. Sustained trapping of 5 μm diameter particles was achieved with high strain rates, of order 10² s^-1, higher than in any previous studies. Our experiments show that the maximum attainable strain rate is a function of the real-time delay of the control algorithm and the particle resolution (pixel/μm). Therefore, we anticipate that with further reduced time delays and enhanced particle resolution, considerably higher strain rates can be attained, opening the platform to single cellular assay studies where very high strain rates are required.

Oscillating viscous flow past a streamwise linear array of circular cylinders

This paper addresses the viscous flow developing about an array of equally spaced identical circular cylinders aligned with an incompressible fluid stream whose velocity oscillates periodically in time. The focus of the analysis is on harmonically oscillating flows with stroke lengths that are comparable to or smaller than the cylinder radius, such that the flow remains two-dimensional, time-periodic and symmetric with respect to the centreline. Specific consideration is given to the limit of asymptotically small stroke lengths, in which the flow is harmonic at leading order, with the first-order corrections exhibiting a steady-streaming component, which is computed here along with the accompanying Stokes drift. As in the familiar case of oscillating flow over a single cylinder, for small stroke lengths, the associated time-averaged Lagrangian velocity field, given by the sum of the steady-streaming and Stokes-drift components, displays recirculating vortices, which are quantified for different values of the two relevant controlling parameters, namely, the Womersley number and the ratio of the inter-cylinder distance to the cylinder radius. Comparisons with results of direct numerical simulations indicate that the description of the Lagrangian mean flow for infinitesimally small values of the stroke length remains reasonably accurate even when the stroke length is comparable to the cylinder radius. The numerical integrations are also used to quantify the streamwise flow rate induced by the presence of the cylinder array in cases where the periodic surrounding motion is driven by an anharmonic pressure gradient, a problem of interest in connection with the oscillating flow of cerebrospinal fluid around the nerve roots located along the spinal canal.

A hydro-thermophoretic trap for microparticles near a gold-coated substrate

Optical tweezers have revolutionised micromanipulation from physics and biology to material science. However, the high laser power involved in optical trapping can damage biological samples. In this context, indirect trapping of microparticles and objects using fluid flow fields has assumed great importance. It has recently been shown that cells and particles can be turned in the pitch sense by opto-plasmonic heating of a gold surface constituting one side of a sample chamber. We extend that work to place two such hotspots in close proximity to each other to form a very unique configuration of flow fields forming an effective quasi-three-dimensional 'trap', assisted by thermophoresis. This is effectively a harmonic trap confining particles in all three dimensions without relying on other factors to confine the particles close to the surface. We use this to show indirect trapping of different types of upconverting particles and cells, and also show that we can approach a trap stiffness of 40 fN μm^-1 indicating a weak confinement regime without relying on feedback.

Multicurvature viscous streaming: Flow topology and particle manipulation

Viscous streaming refers to the rectified, steady flows that emerge when a liquid oscillates around an immersed microfeature. Relevant to microfluidics, the resulting local, strong inertial effects allow manipulation of fluid and particles effectively, within short time scales and compact footprints. Nonetheless, practically, viscous streaming has been stymied by a narrow set of achievable flow topologies, limiting scope and application. Here, by moving away from classically employed microfeatures of uniform curvature, we experimentally show how multicurvature designs, computationally obtained, give rise, instead, to rich flow repertoires. The potential utility of these flows is then illustrated in compact, robust, and tunable devices for enhanced manipulation, filtering, and separation of both synthetic and biological particles. Overall, our mixed computational/experimental approach expands the scope of viscous streaming application, with opportunities in manufacturing, environment, health, and medicine, from particle self-assembly to microplastics removal.

Advances in precise single-cell capture for analysis and biological applications

Cells are the basic structural and functional units of living organisms. However, conventional cell analysis only averages millions of cell populations, and some important information is lost. It is essential to quantitatively characterize the physiology and pathology of single-cell activities. Precise single-cell capture is an extremely challenging task during cell sample preparation. In this review, we summarize the category of technologies to capture single cells precisely with a focus on the latest development in the last five years. Each technology has its own set of benefits and specific challenges, which provide opportunities for researchers in different fields. Accordingly, we introduce the applications of captured single cells in cancer diagnosis, analysis of metabolism and secretion, and disease treatment. Finally, some perspectives are provided on the current development trends, future research directions, and challenges of single-cell capture.

Reversible single cell trapping of Paramecium caudatum to correlate swimming behavior and membrane state

Single cell measurements with living specimen like, for example, the ciliated protozoan Paramecium caudatum can be a challenging task. We present here a microfluidic trapping mechanism for measurements with these micro-organisms that can be used, e.g., for optical measurements to correlate cellular functions with the phase state of the lipid membrane. Here, we reversibly trap single cells in small compartments. Furthermore, we track and analyze the swimming behavior of single cells over several minutes. Before and after reversible trapping the swimming speed is comparable, suggesting that trapping does not have a large effect on cell behavior. Last, we demonstrate the feasibility of membrane order measurements on living cells using the fluorescent dye 6-lauryl-2-dimethylaminonaphthalene (Laurdan).

Biophysical Approaches for Applying and Measuring Biological Forces

Over the past decades, increasing evidence has indicated that mechanical loads can regulate the morphogenesis, proliferation, migration, and apoptosis of living cells. Investigations of how cells sense mechanical stimuli or the mechanotransduction mechanism is an active field of biomaterials and biophysics. Gaining a further understanding of mechanical regulation and depicting the mechanotransduction network inside cells require advanced experimental techniques and new theories. In this review, the fundamental principles of various experimental approaches that have been developed to characterize various types and magnitudes of forces experienced at the cellular and subcellular levels are summarized. The broad applications of these techniques are introduced with an emphasis on the difficulties in implementing these techniques in special biological systems. The advantages and disadvantages of each technique are discussed, which can guide readers to choose the most suitable technique for their questions. A perspective on future directions in this field is also provided. It is anticipated that technical advancement can be a driving force for the development of mechanobiology.

Force measurement goes to femto-Newton sensitivity of single microscopic particle

Highly sensitive force measurements of a single microscopic particle with femto-Newton sensitivity have remained elusive owing to the existence of fundamental thermal noise. Now, researchers have proposed an optically controlled hydrodynamic manipulation method, which can measure the weak force of a single microscopic particle with femto-Newton sensitivity.

Soft-Contact Acoustic Microgripper Based on a Controllable Gas-Liquid Interface for Biomicromanipulations

The manipulation of microscale bioentities is desired in many biological and biomedical applications. However, the potential unobservable damage to bioparticles due to rigid contact has always been a source of concern. Herein, a soft-contact acoustic microgripper to handle microparticles to improve the interaction safety is introduced. The system takes advantage of the acoustic-enhanced adhesion of flexible gas-liquid interfaces to capture-release, transport, and rotate the target, such as microbeads (20-65 µm) and zebrafish embryos (from 950 µm to 1.4 mm). The gas-liquid interface generated at the tip of a microcapillary can be precisely controlled by a pneumatic pressure source. The gas-liquid interface oscillation excited by acoustic energy imposes coupled radiation force and drag force on the microparticles, enabling multidimensional movements. Experiments with the microbeads are conducted to evaluate the claimed function and quantify the key parameters that influence the manipulation result. Additionally, 250 zebrafish embryos are captured, transported, and rotated. The hatching rate of the 250 manipulated embryos is approximately 98% similar to that of the nonmanipulated group, which proves the noninvasiveness of the method. The derived theories and experimental data indicate that the developed soft-contact microgripper is functional and beneficial for biological and medical applications.

3-D Acoustic Tweezers Using a 2-D Matrix Array With Time-Multiplexed Traps

The use of acoustic tweezers for precise manipulation of microparticles in the aqueous environment is essential and challenging for biomechanical applications in vivo. A 3-D acoustic tweezer is developed in this study for 3-D manipulation by using a two-dimensional (2-D) phased array consisting of 256 elements operating at 1.04 MHz. The emission phases of each element are iteratively determined by a backpropagation algorithm to generate multiple acoustic traps. Different traps are multiplexed in time, thus forming synthesized acoustic fields. We demonstrate the 3-D levitation and translation of positive acoustic contrast particles, a major class of bioparticles, in water by different acoustic traps, and compare the positional deviation along the intended path via experimentally measured trajectories. Improved manipulating stability was achieved by multiplexed acoustic traps. The 3-D acoustic tweezers proposed in this study provide a versatile approach of contactless bioparticle trapping and translation, paving the way toward future application of nanodroplet and microbubble manipulations.

Potential of the acoustic micromanipulation technologies for biomedical research

Acoustic micromanipulation technologies are a set of versatile tools enabling unparalleled micromanipulation capabilities. Several characteristics put the acoustic micromanipulation technologies ahead of most of the other tweezing methods. For example, acoustic tweezers can be adapted as non-invasive platforms to handle single cells gently or as probes to stimulate or damage tissues. Besides, the nature of the interactions of acoustic waves with solids and liquids eliminates labeling requirements. Considering the importance of highly functional tools in biomedical research for empowering important discoveries, acoustic micromanipulation can be valuable for researchers in biology and medicine. Herein, we discuss the potential of acoustic micromanipulation technologies from technical and application points of view in biomedical research.

Feedback-based positioning and diffusion suppression of particles via optical control of thermoviscous flows

The ability to control the position of micron-size particles with high precision using tools such as optical tweezers has led to major advances in fields such as biology, physics and material science. In this paper, we present a novel optical strategy to confine particles in solution with high spatial control using feedback-controlled thermoviscous flows. We show that this technique allows micron-size particles to be positioned and confined with subdiffraction precision (24 nm), effectively suppressing their diffusion. Due to its physical characteristics, our approach might be particular attractive where laser exposure is of concern or materials are inherently incompatible with optical tweezing since it does not rely on contrast in the refractive index.

An unrecognized inertial force induced by flow curvature in microfluidics

Modern inertial microfluidics routinely employs oscillatory flows around localized solid features or microbubbles for controlled, specific manipulation of particles, droplets, and cells. It is shown that theories of inertial effects that have been state of the art for decades miss major contributions and strongly underestimate forces on small suspended objects in a range of practically relevant conditions. An analytical approach is presented that derives a complete set of inertial forces and quantifies them in closed form as easy-to-use equations of motion, spanning the entire range from viscous to inviscid flows. The theory predicts additional attractive contributions toward oscillating boundaries, even for density-matched particles, a previously unexplained experimental observation. The accuracy of the theory is demonstrated against full-scale, three-dimensional direct numerical simulations throughout its range.

Mixing enhancement in T-junction microchannel with acoustic streaming induced by triangular structure

The T-shaped microchannel system is used to mix similar or different fluids, and the laminar flow nature makes the mixing at the entrance junction region a challenging task. Acoustic streaming is a steady vortical flow phenomenon that can be produced in the microchannel by oscillating acoustic transducer around the sharp edge tip structure. In this study, the acoustic streaming is produced using a triangular structure with tip angles of 22.62°, 33.4°, and 61.91°, which is placed at the entrance junction region and mixes the inlets flow from two directions. The acoustic streaming flow patterns were investigated using micro-particle image velocimetry (μPIV) in various tip edge angles, flow rate, oscillation frequency, and amplitude. The velocity and vorticity profiles show that a pair of counter-rotating streaming vortices were created around the sharp triangle structure and raised the Z vorticity up to 10 times more than the case without acoustic streaming. The mixing experiments were performed by using fluorescent green dye solution and de-ionized water and evaluated its performance with the degree of mixing (M) at different amplitudes, flow rates, frequencies, and tip edge angles using the grayscale value of pixel intensity. The degree of mixing characterized was found significantly improved to 0.769 with acoustic streaming from 0.4017 without acoustic streaming, in the case of 0.008 μl/min flow rate and 38 V oscillation amplitude at y = 2.15 mm. The results suggested that the creation of acoustic streaming around the entrance junction region promotes the mixing of two fluids inside the microchannel, which is restricted by the laminar flow conditions.

Comparison of Acoustic Streaming Flow Patterns Induced by Solid, Liquid and Gas Obstructions

In this study, acoustic streaming flows inside micro-channels induced by three different types of obstruction—gaseous bubble, liquid droplet and solid bulge—are compared and investigated experimentally by particle tracking velocimetry (PTV) and numerically using the finite element method (FEM). The micro-channels are made by poly(dimethylsiloxane) (PDMS) using soft lithography with low-cost micro-machined mold. The characteristic dimensions of the media are 0.2 mm in diameter, and the oscillation generated by piezoelectric actuators has frequency of 12 kHz and input voltages of 40 V. The experimental results show that in all three obstruction types, a pair of counter-rotating vortical patterns were observed around the semi-circular obstructions. The gaseous bubble creates the strongest vortical streaming flow, which can reach a maximum of 21 mm/s, and the largest u component happens at Y/D = 0. The solid case is the weakest of the three, which can only reach 2 mm/s. The liquid droplet has the largest v components and speed at Y/D = 0.5 and Y/D = 0.6. Because of the higher density and incompressibility of liquid droplet compared to the gaseous bubble, the liquid droplet obstruction transfers the oscillation of the piezo plate most efficiently, and the induced streaming flow region and average speed are both the largest of the three. An investigation using numerical simulation shows that the differing interfacial conditions between the varying types of obstruction boundaries to the fluid may be the key factor to these differences. These results suggest that it might be more energy-efficient to design an acoustofluidic device using a liquid droplet obstruction to induce the stronger streaming flow.

Characterizing Extracellular Vesicles and Their Diverse RNA Contents

Cells release nanometer-scale, lipid bilayer-enclosed biomolecular packages (extracellular vesicles; EVs) into their surrounding environment. EVs are hypothesized to be intercellular communication agents that regulate physiological states by transporting biomolecules between near and distant cells. The research community has consistently advocated for the importance of RNA contents in EVs by demonstrating that: (1) EV-related RNA contents can be detected in a liquid biopsy, (2) disease states significantly alter EV-related RNA contents, and (3) sensitive and specific liquid biopsies can be implemented in precision medicine settings by measuring EV-derived RNA contents. Furthermore, EVs have medical potential beyond diagnostics. Both natural and engineered EVs are being investigated for therapeutic applications such as regenerative medicine and as drug delivery agents. This review focuses specifically on EV characterization, analysis of their RNA content, and their functional implications. The NIH extracellular RNA communication (ERC) program has catapulted human EV research from an RNA profiling standpoint by standardizing the pipeline for working with EV transcriptomics data, and creating a centralized database for the scientific community. There are currently thousands of RNA-sequencing profiles hosted on the Extracellular RNA Atlas alone (Murillo et al., 2019), encompassing a variety of human biofluid types and health conditions. While a number of significant discoveries have been made through these studies individually, integrative analyses of these data have thus far been limited. A primary focus of the ERC program over the next five years is to bring higher resolution tools to the EV research community so that investigators can isolate and analyze EV sub-populations, and ultimately single EVs sourced from discrete cell types, tissues, and complex biofluids. Higher resolution techniques will be essential for evaluating the roles of circulating EVs at a level which impacts clinical decision making. We expect that advances in microfluidic technologies will drive near-term innovation and discoveries about the diverse RNA contents of EVs. Long-term translation of EV-based RNA profiling into a mainstay medical diagnostic tool will depend upon identifying robust patterns of circulating genetic material that correlate with a change in health status.

Next-Generation Optical Nanotweezers for Dynamic Manipulation: From Surface to Bulk

Optical traps based on strongly confined electromagnetic fields at metal-dielectric interfaces are far more efficient than conventional optical tweezers. Specifically, these near-field nanotweezers allow the trapping of smaller particles at lower optical intensities, which can impact diverse research fields ranging from soft condensed matter physics to materials science and biology. A major thrust in the past decade has been focused on extending the capabilities of plasmonically enhanced nanotweezers beyond diffusion-limited trapping on surfaces such as to achieve dynamic control in the bulk of fluidic environments. Here, we review the recent efforts in optical nanotweezers, especially those involving hybrid forcing schemes, covering both surface and bulk-based techniques. We summarize the important capabilities demonstrated with this promising approach, with niche applications in reconfigurable nanopatterning and on-chip assembly as well as in sorting and separating colloidal nanoparticles.

Methods for Single-Cell Isolation and Preparation

Within the last decade, single-cell analysis has revolutionized our understanding of cellular processes and heterogeneity across all disciplines of life science. As the transcriptome, genome, or epigenome of individual cells can nowadays be analyzed at low cost and in high-throughput within a few days by modern techniques, tremendous improvements in disease diagnosis on the one hand and the investigation of disease-relevant mechanisms on the other were achieved so far. This relies on the parallel development of reliable cell capturing and single-cell sequencing approaches that have paved the way for comprehensive single-cell studies. Apart from single-cell isolation methods in high-throughput, a variety of methods with distinct specializations were developed, allowing for correlation of transcriptomics with cellular parameters like electrophysiology or morphology.For all single-cell-based approaches, accurate and reliable isolation with proper quality controls is prerequisite, whereby different options exist dependent on sample type and tissue properties. Careful consideration of an appropriate method is required to avoid incorrect or biased data that may lead to misinterpretations.In this chapter, we will provide a broad overview of the current state of the art in matters of single-cell isolation methods mostly applied for sequencing-based downstream analysis, and their respective advantages and drawbacks. Distinct technologies will be discussed in detail addressing key parameters like sample compatibility, viability, purity, throughput, and isolation efficiency.

Multifunctional Noncontact Micromanipulation Using Whirling Flow Generated by Vibrating a Single Piezo Actuator

A noncontact method that can achieve immobilization, transportation, and rotation in the microscale is desired in biological micromanipulation. A multifunctional noncontact micromanipulation method is proposed here based on a vibration-generated whirling flow. Resonance of a cantilever structure is utilized to extend the straight vibration of a single piezo actuator to the 2D circular vibration of a micropipette. The circular vibration in fluids can generate the whirling flow featured with low pressure in the core area and flow velocity gradient. The low pressure can immobilize the objects nearby and transport them together with the micropipette, and the flow velocity gradient is utilized to form a torque to rotate the immobilized object. Experiments of the microbeads are conducted to evaluate the claimed functions and quantify the key parameters that influence the rotation velocity. The cell spheroid is immobilized and rotated for 3D observation, and by assessing the viability of the cells containing in the spheroid, the proposed method is proved noninvasive to living cells. Finally, another important application in operations of mouse egg cells is shown, which indicates that the proposed method is a potential valuable tool in biological micromanipulation.

Microfluidic Single-Cell Manipulation and Analysis: Methods and Applications

In a forest of a hundred thousand trees, no two leaves are alike. Similarly, no two cells in a genetically identical group are the same. This heterogeneity at the single-cell level has been recognized to be vital for the correct interpretation of diagnostic and therapeutic results of diseases, but has been masked for a long time by studying average responses from a population. To comprehensively understand cell heterogeneity, diverse manipulation and comprehensive analysis of cells at the single-cell level are demanded. However, using traditional biological tools, such as petri-dishes and well-plates, is technically challengeable for manipulating and analyzing single-cells with small size and low concentration of target biomolecules. With the development of microfluidics, which is a technology of manipulating and controlling fluids in the range of micro- to pico-liters in networks of channels with dimensions from tens to hundreds of microns, single-cell study has been blooming for almost two decades. Comparing to conventional petri-dish or well-plate experiments, microfluidic single-cell analysis offers advantages of higher throughput, smaller sample volume, automatic sample processing, and lower contamination risk, etc., which made microfluidics an ideal technology for conducting statically meaningful single-cell research. In this review, we will summarize the advances of microfluidics for single-cell manipulation and analysis from the aspects of methods and applications. First, various methods, such as hydrodynamic and electrical approaches, for microfluidic single-cell manipulation will be summarized. Second, single-cell analysis ranging from cellular to genetic level by using microfluidic technology is summarized. Last, we will also discuss the advantages and disadvantages of various microfluidic methods for single-cell manipulation, and then outlook the trend of microfluidic single-cell analysis.

Single-cell assay on microfluidic devices

Advances in microfluidic techniques have prompted researchers to study the inherent heterogeneity of single cells in cell populations.

Acoustic tweezers for the life sciences

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.

Closed loop control of microscopic particles incorporating steady streaming and visual feedback

Automatic manipulation of microscopic particles is very important in biology, especially in new lab-on-chip systems for automatic testing and DNA manipulation. We suggest a particle manipulation system (PMS) based on vibrating piezoelectric beams creating steady streaming flow in a viscous liquid. The flow is nearly unidirectional and it is used to control the position and velocity of the particles in the workspace of the PMS. The particles position in the PMS are controlled by visual feedback. This study presents the manipulation method, the system's model describing its behavior and characterizes experimentally its performance. The PMS is capable moving a 2-200 μm particle in a workspace of 8x8 mm² with an absolute accuracy of 0.2 μm. The characteristic velocity in 500 cP Si oil, is 20 μm/s using an actuation voltage amplitude of 5 V and can reach 250 μm/s using 15 V respectively. We can also move a constellation of several particles in various sizes without changing the distance between them. The accuracy of the manipulation can be increased by enhancing the amplification of the microscope on the expanse of a smaller workspace field of view.

Cellphone-Enabled Microwell-Based Microbead Aggregation Assay for Portable Biomarker Detection

Quantitative biomarker detection methods featured with rapidity, high accuracy, and label-free are demonstrated for the development of point-of-care (POC) technologies or "beside" diagnostics. Microbead aggregation via protein-specific linkage provides an effective approach for selective capture of biomarkers from the samples, and can directly readout the presence and amount of the targets. However, sensors or microfluidic analyzers that can accurately quantify the microbead aggregation are scared. In this work, we demonstrate a microwell-based microbeads analyzing system, by which online manipulations of microbeads including trapping, arraying, and rotations can be realized, providing a series of microfluidic approaches to layout the aggregated microbeads for further convenient characterizations. Prostate specific antigen is detected using the proposed system, demonstrating the limit of detection as low as 0.125 ng/mL (3.67 pM). A two-step reaction kinetics model is proposed for the first time to explain the dynamic process of microbeads aggregation. The developed microbeads aggregation analysis system has the advantages of label-free detection, high throughput, and low cost, showing great potential for portable biomarker detection.

Comparing methods for the modelling of boundary-driven streaming in acoustofluidic devices

Numerical simulations of acoustic streaming flows can be used not only to explain the complex phenomena observed in acoustofluidic manipulation devices, but also to predict and optimise their performances. In this paper, two numerical methods based on perturbation theory are compared in order to demonstrate their viability and applicability for modelling boundary-driven streaming flows in acoustofluidic systems. It was found that the Reynolds stress method, which predicts the streaming fields from their driving terms, can effectively resolve both the inner and outer streaming fields and can be used to demonstrate the driving mechanisms of a broad range of boundary-driven streaming flows. However, computational efficiency typically limits its useful application to two-dimensional models. We highlight the close relationship between the classical boundary-driven streaming vortices and the rotationality of the Reynolds stress force field. The limiting velocity method, which ignores the acoustic boundary layer and solves the outer streaming fields by applying the 'limiting velocities' as boundary conditions, is more computationally efficient and can be used for predicting three-dimensional outer streaming fields and provide insight into their origins, provided that the radius of curvature of the channel surfaces is much greater than the acoustic boundary layer thickness ( δ v ). We also show that for the limiting velocity method to be valid the channel scales must exceed a value of approximately 100 δ v (for an error of ~5% on the streaming velocity magnitudes) for the case presented in this paper. Comparisons of these two numerical methods can provide effective guidance for researchers in the field of acoustofluidics on choosing appropriate methods to predict boundary-driven streaming fields in the design of acoustofluidic particle manipulation devices.

An on-chip imaging droplet-sorting system: a real-time shape recognition method to screen target cells in droplets with single cell resolution

A microfluidic on-chip imaging cell sorter has several advantages over conventional cell sorting methods, especially to identify cells with complex morphologies such as clusters. One of the remaining problems is how to efficiently discriminate targets at the species level without labelling. Hence, we developed a label-free microfluidic droplet-sorting system based on image recognition of cells in droplets. To test the applicability of this method, a mixture of two plankton species with different morphologies (Dunaliella tertiolecta and Phaeodactylum tricornutum) were successfully identified and discriminated at a rate of 10 Hz. We also examined the ability to detect the number of objects encapsulated in a droplet. Single cell droplets sorted into collection channels showed 91 ± 4.5% and 90 ± 3.8% accuracy for D. tertiolecta and P. tricornutum, respectively. Because we used image recognition to confirm single cell droplets, we achieved highly accurate single cell sorting. The results indicate that the integrated method of droplet imaging cell sorting can provide a complementary sorting approach capable of isolating single target cells from a mixture of cells with high accuracy without any staining.

Stagnation point flows in analytical chemistry and life sciences

Isolated microfluidic stagnation points – formed within microfluidic interfaces – have come a long way as a tool for characterizing materials, manipulating micro particles, and generating confined flows and localized chemistries.

Design of Fresnel Lens-Type Multi-Trapping Acoustic Tweezers

In this paper, acoustic tweezers which use beam forming performed by a Fresnel zone plate are proposed. The performance has been demonstrated by finite element analysis, including the acoustic intensity, acoustic pressure, acoustic potential energy, gradient force, and particle distribution. The acoustic tweezers use an ultrasound beam produced by a lead zirconate titanate (PZT) transducer operating at 2.4 MHz and 100 V_peak-to-peak in a water medium. The design of the Fresnel lens (zone plate) is based on air reflection, acoustic impedance matching, and the Fresnel half-wave band (FHWB) theory. This acoustic Fresnel lens can produce gradient force and acoustic potential wells that allow the capture and manipulation of single particles or clusters of particles. Simulation results strongly indicate a good trapping ability, for particles under 150 µm in diameter, in the minimum energy location. This can be useful for cell or microorganism manipulation.

Steady streaming around a cylinder pair

The steady streaming motion that appears around a pair of circular cylinders placed in a small-amplitude oscillatory flow is considered. Attention is focused on the case where the Stokes layer thickness at the surface of the cylinders is much smaller than the cylinder radius, and the streaming Reynolds number is of order unity or larger. In that case, the steady streaming velocity that persists at the edge of the Stokes layer can be imposed as a boundary condition to numerically solve the outer streaming motion that it drives in the bulk of the fluid. It is investigated how the gap width between the cylinders and the streaming Reynolds number affect the flow topology. The results are compared against experimental observations.

Acoustic streaming produced by a cylindrical bubble undergoing volume and translational oscillations in a microfluidic channel

A theoretical model is developed for acoustic streaming generated by a cylindrical bubble confined in a fluid channel between two planar elastic walls. The bubble is assumed to undergo volume and translational oscillations. The volume oscillation is caused by an imposed acoustic pressure field and generates the bulk scattered wave in the fluid gap and Lamb-type surface waves propagating along the fluid-wall interfaces. The translational oscillation is induced by the velocity field of an external sound source such as another bubble or an oscillatory fluid flow. The acoustic streaming is assumed to result from the interaction of the volume and the translational modes of the bubble oscillations. The general solutions for the linear equations of fluid motion and the equations of acoustic streaming are calculated with no restrictions on the ratio between the viscous penetration depth and the bubble size. Approximate solutions for the limit of low viscosity are provided as well. Simulations of streamline patterns show that the geometry of the streaming resembles flows generated by a source dipole, while the vortex orientation is governed by the driving frequency, bubble size, and the distance of the bubble from the source of translational excitation. Experimental verification of the developed theory is performed using data for streaming generated by bubble pairs.

A Microfluidic Approach for Inducing Cell Rotation by Means of Hydrodynamic Forces

Microfluidic technology allows to realize devices in which cells can be imaged in their three-dimensional shape. However, there are still some limitations in the method, due to the fact that cells follow a straight path while they are flowing in a channel. This can result in a loss in information, since only one side of the cell will be visible. Our work has started from the consideration that if a cell rotates, it is possible to overcome this problem. Several approaches have been proposed for cell manipulation in microfluidics. In our approach, cells are controlled by only taking advantages of hydrodynamic forces. Two different devices have been designed, realized, and tested. The first device induces cell rotation in a plane that is parallel (in-plane) to the observation plane, while the second one induce rotation in a plane perpendicular (out-of-plane) to the observation plane.

A Method of Three-Dimensional Micro-Rotational Flow Generation for Biological Applications

We report a convenient method to create a three-dimensional micro-rotational fluidic platform for biological applications in the direction of a vertical plane (out-of-plane) without contact in an open space. Unlike our previous complex fluidic manipulation system, this method uses a micro-rotational flow generated near a single orifice when the solution is pushed from the orifice by using a single pump. The three-dimensional fluidic platform shows good potential for fluidic biological applications such as culturing, stimulating, sorting, and manipulating cells. The pattern and velocity of the micro-rotational flow can be controlled by tuning the parameters such as the flow rate and the liquid-air interface height. We found that bio-objects captured by the micro-rotational flow showed self-rotational motion and orbital motion. Furthermore, the path length and position, velocity, and pattern of the orbital motion of the bio-object could be controlled. To demonstrate our method, we used embryoid body cells. As a result, the orbital motion had a maximum length of 2.4 mm, a maximum acceleration of 0.63 m/s², a frequency of approximately 0.45 Hz, a maximum velocity of 15.4 mm/s, and a maximum rotation speed of 600 rpm. The capability to have bio-objects rotate or move orbitally in three dimensions without contact opens up new research opportunities in three-dimensional microfluidic technology.

Transport of inertial particles by viscous streaming in arrays of oscillating probes

A mechanism for the transport of microscale particles in viscous fluids is demonstrated. The mechanism exploits the trapping of such particles by rotational streaming cells established in the vicinity of an oscillating cylinder, recently analyzed in previous work. The present work explores a strategy of transporting particles between the trapping points established by multiple cylinders undergoing oscillations in sequential intervals. It is demonstrated that, by controlling the sequence of oscillation intervals, an inertial particle is effectively and predictably transported between the stable trapping points. Arrays of cylinders in various arrangements are investigated, revealing a technique for constructing arbitrary particle trajectories. It is found that the domain from which particles can be transported and trapped by an oscillator is extended, even to regions in which particles are shielded, by the presence of other stationary cylinders. The timescales for transport are examined, as are the mechanisms by which particles are drawn away from an obstacle toward the trapping point of an oscillator.

Numerical simulation of acoustofluidic manipulation by radiation forces and acoustic streaming for complex particles

The numerical prediction of acoustofluidic particle motion is of great help for the design, the analysis, and the physical understanding of acoustofluidic devices as it allows for a simple and direct comparison with experimental observations. However, such a numerical setup requires detailed modeling of the acoustofluidic device with all its components and thorough understanding of the acoustofluidic forces inducing the particle motion. In this work, we present a 3D trajectory simulation setup that covers the full spectrum, comprising a time-harmonic device model, an acoustic streaming model of the fluid cavity, a radiation force simulation, and the calculation of the hydrodynamic drag. In order to make quantitatively accurate predictions of the device vibration and the acoustic field, we include the viscous boundary layer damping. Using a semi-analytical method based on Nyborg's calculations, the boundary-driven acoustic streaming is derived directly from the device simulation and takes into account cavity wall vibrations which have often been neglected in the literature. The acoustic radiation forces and the hydrodynamic drag are calculated numerically to handle particles of arbitrary shape, structure, and size. In this way, complex 3D particle translation and rotation inside experimental microdevices can be predicted. We simulate the rotation of a microfiber in an amplitude-modulated 2D field and analyze the results with respect to experimental observations. For a quantitative verification, the motion of an alumina microdisk is compared to a simple experiment. Demonstrating the potential of the simulation setup, we compute the trajectory of a red blood cell inside a realistic microdevice under the simultaneous effects of acoustic streaming and radiation forces.

Asymmetric traps array for particle transport

The asymmetric trap composed of three obstacles shows flow direction-dependent trap/particle interaction.

Review of methods to probe single cell metabolism and bioenergetics

Single cell investigations have enabled unexpected discoveries, such as the existence of biological noise and phenotypic switching in infection, metabolism and treatment. Herein, we review methods that enable such single cell investigations specific to metabolism and bioenergetics. Firstly, we discuss how to isolate and immobilize individuals from a cell suspension, including both permanent and reversible approaches. We also highlight specific advances in microbiology for its implications in metabolic engineering. Methods for probing single cell physiology and metabolism are subsequently reviewed. The primary focus therein is on dynamic and high-content profiling strategies based on label-free and fluorescence microspectroscopy and microscopy. Non-dynamic approaches, such as mass spectrometry and nuclear magnetic resonance, are also briefly discussed.

Automated single cell microbioreactor for monitoring intracellular dynamics and cell growth in free solution

We report an automated microfluidic-based platform for single cell analysis that allows for cell culture in free solution with the ability to control the cell growth environment. Using this approach, cells are confined by the sole action of gentle fluid flow, thereby enabling non-perturbative analysis of cell growth away from solid boundaries. In addition, the single cell microbioreactor allows for precise and time-dependent control over cell culture media, with the combined ability to observe the dynamics of non-adherent cells over long time scales. As a proof-of-principle demonstration, we used the platform to observe dynamic cell growth, gene expression, and intracellular diffusion of repressor proteins while precisely tuning the cell growth environment. Overall, this microfluidic approach enables the direct observation of cellular dynamics with exquisite control over environmental conditions, which will be useful for quantifying the behaviour of single cells in well-defined media.

A Bio-Manipulation Method Based on the Hydrodynamic Force of Multiple Microfluidic Streams

This paper proposes a multiple microfluidic streambased manipulation (MMSM) system for bio-objects. It uses micro hydrodynamics and lab on chip (LOC) technology. Our method can implement the functions of micro manipulation and micro assembly of bio-objects in an open space without contact. Compared to other conventional bio-micro-manipulation and assembly methods, this system manipulates micro objects by controlling multiple microfluidic streams onto them from various directions. The advantages of this method are that it performs open space, multifunction, multi-scale, multi-degree-of-freedom, and non-invasive 3D manipulation. These microfluidic streams are generated simultaneously from multiple orifices. By regulating the parameters of the microfluidic stream, such as the position and number of operating orifices and the flow rate, the direction and velocity of the object can be controlled. To verify this principle, we design an open-space fluidic system for on-chip manipulation and calculated velocity and direction of the microfluidic stream using CFD simulation. Then the prototype microchip with an array of nine orifices is fabricated from glass. In experiments, demonstrations of rectilinear motion of a single cell andmicro particle are observed. The results presented in this paper show that this MMSM is capable of biomicromanipulation and micro assembly of bio-objects.

Manipulation and confinement of single particles using fluid flow

High precision control of micro- and nanoscale objects in aqueous media is an essential technology for nanoscience and engineering. Existing methods for particle trapping primarily depend on optical, magnetic, electrokinetic, and acoustic fields. In this work, we report a new hydrodynamic flow based approach that allows for fine-scale manipulation and positioning of single micro- and nanoscale particles using automated fluid flow. As a proof-of-concept, we demonstrate trapping and two-dimensional (2D) manipulation of 500 nm and 2.2 μm diameter particles with a positioning precision as small as 180 nm during confinement. By adjusting a single flow parameter, we further show that the shape of the effective trap potential can be efficiently controlled. Finally, we demonstrate two distinct features of the flow-based trapping method, including isolation of a single particle from a crowded particle solution and active control over the surrounding medium of a trapped object. The 2D flow-based trapping method described here further expands the micro/nanomanipulation toolbox for small particles and holds strong promise for applications in biology, chemistry, and materials research.