Acoustofluidics 23: acoustic manipulation combined with other force fields

Soft robotic patterning of liquids

Patterning of two or more liquids, either homogeneous in each phase or mixed with particles (including biological matter, such as cells and proteins), by controlling their flow dynamics, is relevant to several applications. Examples include dynamic spatial confinement of liquids in microfluidic systems (such as lab-on-a-chip and organ-on-a-chip devices) or structuring of polymers to modulate various properties (such as strength, conductivity, transparency and surface finishing). State-of-the-art strategies use various technologies, including positioners, shakers and acoustic actuators, which often combine limited versatility of mixing with significant inefficiency, energy consumption, and noise, as well as tendency to increase the temperature of the liquids. Here, we describe a new kind of robotic mixers of liquids, based on electro-responsive smart materials (dielectric elastomer actuators). We show for the first time how an efficient soft robotic device can be used to produce, via combinations of rotations and translations, various spatial patterns in liquids and maintain them stable for a few minutes. Moreover, we show that, as compared to a conventional orbital shaker, the new type of robotic device can mix liquids with a higher efficacy (~ 94% relative to ~ 80%, after 8 min of mixing) and with a significantly lower increase of the liquids' temperature (+ 1 °C relative to + 5 °C, after 6 h of mixing). This is especially beneficial when mixing should occur according to controllable spatial features and should involve temperature-sensitive matter (such as biological cells, proteins, pre-polymers and other thermolabile molecules).

Force-Induced Visualization of Nucleic Acid Functions with Single-Nucleotide Resolution

Nucleic acids are major targets for molecular sensing because of their wide involvement in biological functions. Determining their presence, movement, and binding specificity is thus well pursued. However, many current techniques are usually sophisticated, expensive, and often lack single-nucleotide resolution. In this paper, we report the force-induced visualization method that relies on the novel concept of mechanical force to determine the functional positions of nucleic acids with single-nucleotide resolution. The use of an adjustable mechanical force overcomes the variation of analyte concentration and differences in buffer conditions that are common in biological settings. Two examples are described to validate the method: one is probing the mRNA movement during ribosomal translocation, and the other is revealing the interacting sites and strengths of DNA-binding drugs based on the force amplitude. The flexibility of the method, simplicity of the associated device, and capability of multiplexed detection will potentially enable a broad range of biomedical applications.

Unconventional acoustic approaches for localized and designed micromanipulation

Acoustic fields are ideal for micromanipulation, being biocompatible and with force gradients approaching the scale of single cells. They have accordingly found use in a variety of microfluidic devices, including for microscale patterning, separation, and mixing. The bulk of work in acoustofluidics has been predicated on the formation of standing waves that form periodic nodal positions along which suspended particles and cells are aligned. An evolving range of applications, however, requires more targeted micromanipulation to create unique patterns and effects. To this end, recent work has made important advances in improving the flexibility with which acoustic fields can be applied, impressively demonstrating generating arbitrary arrangements of pressure fields, spatially localizing acoustic fields and selectively translating individual particles in ways that are not achievable via traditional approaches. In this critical review we categorize and examine these advances, each of which open the door to a wide range of applications in which single-cell fidelity and flexible micromanipulation are advantageous, including for tissue engineering, diagnostic devices, high-throughput sorting and microfabrication.

Rapid measurement of the local pressure amplitude in microchannel acoustophoresis using motile cells

Acoustic microfluidics (or acoustofluidics) provides a non-contact and label-free means to manipulate and interrogate bioparticles. Owing to their biocompatibility and precision, acoustofluidic approaches have enabled innovations in various areas of biomedical research. Future breakthroughs will rely on the translation of these techniques from academic labs to clinical and industrial settings. Here, accurate characterization and standardization of device performance are crucial. Versatile, rapid, and widely accessible performance quantification is needed. We propose a field quantification method using motile Chlamydomonas reinhardtii algae cells. We previously reported qualitative mapping of acoustic fields using living microswimmers as active probes. In the present study, we extend our approach to achieve the challenging quantitative in situ measurement of the acoustic energy density. C. reinhardtii cells continuously swim in an imposed force field and dynamically redistribute as the field changes. This behavior allows accurate and complete, real-time performance monitoring, which can be easily applied and adopted within the acoustofluidics and broader microfluidics research communities. Additionally, the approach relies only on standard bright-field microscopy to assess the field under numerous conditions within minutes. We benchmark the method against conventional passive-particle tracking, achieving agreement within 1% for field strengths from 0 to 70 J m^-3 (from 0 to ∼0.8 MPa).

Ultrasonic Particle Manipulation in Glass Capillaries: A Concise Review

Ultrasonic particle manipulation (UPM), a non-contact and label-free method that uses ultrasonic waves to manipulate micro- or nano-scale particles, has recently gained significant attention in the microfluidics community. Moreover, glass is optically transparent and has dimensional stability, distinct acoustic impedance to water and a high acoustic quality factor, making it an excellent material for constructing chambers for ultrasonic resonators. Over the past several decades, glass capillaries are increasingly designed for a variety of UPMs, e.g., patterning, focusing, trapping and transporting of micron or submicron particles. Herein, we review established and emerging glass capillary-transducer devices, describing their underlying mechanisms of operation, with special emphasis on the application of glass capillaries with fluid channels of various cross-sections (i.e., rectangular, square and circular) on UPM. We believe that this review will provide a superior guidance for the design of glass capillary-based UPM devices for acoustic tweezers-based research.

Controlled orientation and sustained rotation of biological samples in a sono-optical microfluidic device

In cell biology, recently developed technologies for studying suspended cell clusters, such as organoids or cancer spheroids, hold great promise relative to traditional 2D cell cultures. There is, however, growing awareness that sample confinement, such as fixation on a surface or embedding in a gel, has substantial impact on cell clusters. This creates a need for contact-less tools for 3D manipulation and inspection. This work addresses this demand by presenting a reconfigurable, hybrid sono-optical system for contact-free 3D manipulation and imaging, which is suitable for biological samples up to a few hundreds of micrometers in liquid suspension. In our sono-optical device, three independently addressable MHz transducers, an optically transparent top-transducer for levitation and two side-transducers, provide ultrasound excitation from three orthogonal directions. Steerable holographic optical tweezers give us an additional means of manipulation of the acoustically trapped specimen with high spatial resolution. We demonstrate how to control the reorientation or the spinning of complex samples, for instance for 3D visual inspection or for volumetric reconstruction. Whether continuous rotation or transient reorientation takes place depends on the strength of the acoustic radiation torque, arising from pressure gradients, compared to the acoustic viscous torque, arising from the shear forces at the viscous boundary layer around the particle. Based on numerical simulations and experimental insights, we develop a strategy to achieve a desired alignment or continuous rotation around a chosen axis, by tuning the relative strengths of the transducers and thus adjusting the relative contributions of viscous and radiation torques. The approach is widely applicable, as we discuss in several generic examples, with limitations dictated by size and shape asymmetry of the samples.

Reversible Design of Dynamic Assemblies at Small Scales

Emerging bottom-up fabrication methods have enabled the assembly of synthetic colloids, microrobots, living cells, and organoids to create intricate structures with unique properties that transcend their individual components. This review provides an access point to the latest developments in externally driven assembly of synthetic and biological components. In particular, we emphasize reversibility, which enables the fabrication of multiscale systems that would not be possible under traditional techniques. Magnetic, acoustic, optical, and electric fields are the most promising methods for controlling the reversible assembly of biological and synthetic subunits since they can reprogram their assembly by switching on/off the external field or shaping these fields. We feature capabilities to dynamically actuate the assembly configuration by modulating the properties of the external stimuli, including frequency and amplitude. We describe the design principles which enable the assembly of reconfigurable structures. Finally, we foresee that the high degree of control capabilities offered by externally driven assembly will enable broad access to increasingly robust design principles towards building advanced dynamic intelligent systems.

Acoustic Microfluidics

Acoustic microfluidic devices are powerful tools that use sound waves to manipulate micro- or nanoscale objects or fluids in analytical chemistry and biomedicine. Their simple device designs, biocompatible and contactless operation, and label-free nature are all characteristics that make acoustic microfluidic devices ideal platforms for fundamental research, diagnostics, and therapeutics. Herein, we summarize the physical principles underlying acoustic microfluidics and review their applications, with particular emphasis on the manipulation of macromolecules, cells, particles, model organisms, and fluidic flows. We also present future goals of this technology in analytical chemistry and biomedical research, as well as challenges and opportunities.

Advances in Micromanipulation Actuated by Vibration-Induced Acoustic Waves and Streaming Flow

The use of vibration and acoustic characteristics for micromanipulation has been prevalent in recent years. Due to high biocompatibility, non-contact operation, and relatively low cost, the micromanipulation actuated by the vibration-induced acoustic wave and streaming flow has been widely applied in the sorting, translating, rotating, and trapping of targets at the submicron and micron scales, especially particles and single cells. In this review, to facilitate subsequent research, we summarize the fundamental theories of manipulation driven by vibration-induced acoustic waves and streaming flow. These methods are divided into two types: actuated by the acoustic wave, and actuated by the steaming flow induced by vibrating geometric structures. Recently proposed representative vibroacoustic-driven micromanipulation methods are introduced and compared, and their advantages and disadvantages are summarized. Finally, prospects are presented based on our review of the recent advances and developing trends.

Acoustic trap-and-release for rapid assessment of cell motility

Functional cilia and flagella are crucial to the propulsion of physiological fluids, motile cells, and microorganisms. Motility assessment of individual cells allows discrimination of normal from dysfunctional behavior, but cell-scale analysis of individual trajectories to represent a population is laborious and impractical for clinical, industrial, and even research applications. We introduce an assay that quantifies swimming capability as a function of the variation in polar moment of inertia of cells released from an acoustic trap. Acoustic confinement eliminates the need to trace discrete trajectories and enables automated analysis of hundreds of cells in minutes. The approach closely approximates the average speed estimated from the mean squared displacement of individual cells for wild-type Chlamydomonas reinhardtii and two mutants (ida3 and oda5) that display aberrant swimming behaviors. Large-population acoustic trap-and-release rapidly differentiates these cell types based on intrinsic motility, which provides a highly sensitive and efficient alternative to conventional particle tracing.

A review of sorting, separation and isolation of cells and microbeads for biomedical applications: microfluidic approaches

We have reviewed the microfluidic approaches for cell/particle isolation and sorting, and extensively explained the mechanism behind each method.

Microparticle Manipulation by Standing Surface Acoustic Waves with Dual-frequency Excitations

We demonstrate a method for increasing the tuning ability of a standing surface acoustic wave (SSAW) for microparticles manipulation in a lab-on-a-chip (LOC) system. The simultaneous excitation of the fundamental frequency and its third harmonic, which is termed as dual-frequency excitation, to a pair of interdigital transducers (IDTs) could generate a new type of standing acoustic waves in a microfluidic channel. Varying the power and the phase in the dual-frequency excitation signals results in a reconfigurable field of the acoustic radiation force applied to the microparticles across the microchannel (e.g., the number and location of the pressure nodes and the microparticle concentrations at the corresponding pressure nodes). This article demonstrates that the motion time of the microparticle to only one pressure node can be reduced ~2-fold at the power ratio of the fundamental frequency greater than ~90%. In contrast, there are three pressure nodes in the microchannel if less than this threshold. Furthermore, adjusting the initial phase between the fundamental frequency and the third harmonic results in different motion rates of the three SSAW pressure nodes, as well as in the percentage of microparticles at each pressure node in the microchannel. There is a good agreement between the experimental observation and the numerical predictions. This novel excitation method can easily and non-invasively integrate into the LOC system, with a wide tenability and only a few changes to the experimental set-up.

Separation of sub-micron particles from micron particles using acoustic fluid relocation combined with acoustophoresis

Acoustophoresis has gained increasing attention as a gentle, non-contact, and high-throughput cell and particle separation technique. It is conveniently used to isolate and enrich particles that are greater than 2 μm; however, its use in manipulating particles smaller than 2 μm is limited. In this work, we present an alternative way of using acoustic forces to manipulate sub-micrometer particles in continuous flow fashion. It has been shown that acoustic forces can be employed to relocate parallel laminar flow streams of two impedance-mismatched fluids. We demonstrate the separation of sub-micron particles from micron particles by the combination of acoustophoresis and acoustic fluid relocation. The micron particles are focused into the middle of the flow channel via primary acoustic forces while sub-micron particles are moved to the side via drag forces created by the relocating fluid. We demonstrate the proof of the concept using binary mixtures of particles comprised of sub-micron/micron particles, micron/micron particles, and bovine red blood cells with E. coli. The efficiency of the particle enrichment is determined via flow cytometry analysis of the collected streams. This study demonstrates that by combining acoustic fluid relocation with acoustophoresis, sub-micron particles can be effectively separated from micron particles at high flow rates and it can be further implemented to separate binary mixtures of micron particles if the volumetric ratio of two particles is greater than 10 and the larger particle diameter is about 10 μm. The combined method is more appropriate to use than acoustophoresis in situations where acoustic streaming and differences in acoustic impedance of fluids can be of concern. Graphical abstract In the presence of a resonance acoustic field, the clean high-density fluid (dark gray) and the low-density sample fluid are relocated. During this process, E. coli are separated from the red blood cells (RBCs).

Super-resolution force spectroscopy reveals ribosomal motion at sub-nucleotide steps

Probing biomolecular motion beyond a single nucleotide is technically challenging but fundamentally significant. We have developed super-resolution force spectroscopy (SURFS) with 0.5 pN force resolution and revealed that the ribosome moves by half a nucleotide upon the formation of the pre-translocation complex, which is beyond the resolution of other techniques.

Augmented longitudinal acoustic trap for scalable microparticle enrichment

We introduce an acoustic microfluidic device architecture that locally augments the pressure field for separation and enrichment of targeted microparticles in a longitudinal acoustic trap. Pairs of pillar arrays comprise "pseudo walls" that are oriented perpendicular to the inflow direction. Though sample flow is unimpeded, pillar arrays support half-wave resonances that correspond to the array gap width. Positive acoustic contrast particles of supracritical diameter focus to nodal locations of the acoustic field and are held against drag from the bulk fluid motion. Thus, the longitudinal standing bulk acoustic wave (LSBAW) device achieves size-selective and material-specific separation and enrichment of microparticles from a continuous sample flow. A finite element analysis model is used to predict eigenfrequencies of LSBAW architectures with two pillar geometries, slanted and lamellar. Corresponding pressure fields are used to identify longitudinal resonances that are suitable for microparticle enrichment. Optimal operating conditions exhibit maxima in the ratio of acoustic energy density in the LSBAW trap to that in inlet and outlet regions of the microchannel. Model results guide fabrication and experimental evaluation of realized LSBAW assemblies regarding enrichment capability. We demonstrate separation and isolation of 20 μm polystyrene and ∼10 μm antibody-decorated glass beads within both pillar geometries. The results also establish several practical attributes of our approach. The LSBAW device is inherently scalable and enables continuous enrichment at a prescribed location. These features benefit separations applications while also allowing concurrent observation and analysis of trap contents.

Controlled rotation and translation of spherical particles or living cells by surface acoustic waves

We show experimental evidence of the acoustically-assisted micromanipulation of small objects like solid particles or blood cells, combining rotation and translation, using high frequency surface acoustic waves. This was obtained from the leakage in a microfluidic channel of two standing waves arranged perpendicularly in a LiNbO₃ piezoelectric substrate working at 36.3 MHz. By controlling the phase lag between the emitters, we could, in addition to translation, generate a swirling motion of the emitting surface which, in turn, led to the rapid rotation of spherical polystyrene Janus beads suspended in the channel and of human red and white blood cells up to several rounds per second. We show that these revolution velocities are compatible with a torque caused by the acoustic streaming that develops at the particles surface, like that first described by [F. Busse et al., J. Acoust. Soc. Am., 1981, 69(6), 1634-1638]. This device, based on standard interdigitated transducers (IDTs) adjusted to emit at equal frequencies, opens a way to a large range of applications since it allows the simultaneous control of the translation and rotation of hard objects, as well as the investigation of the response of cells to shear stress.

Formation of cell spheroids using Standing Surface Acoustic Wave (SSAW)

3D bioprinting becomes one of the popular approaches in the tissue engineering. In this emerging application, bioink is crucial for fabrication and functionality of constructed tissue. The use of cell spheroids as bioink can enhance the cell-cell interaction and subsequently the growth and differentiation of cells in the 3D printed construct with the minimum amount of other biomaterials. However, the conventional methods of preparing the cell spheroids have several limitations, such as long culture time, low-throughput, and medium modification. In this study, the formation of cell spheroids by SSAW was evaluated both numerically and experimentally in order to overcome the aforementioned limitations. The effects of excitation frequencies on the cell accumulation time, diameter of the formed cell spheroids, and subsequently, the growth and viability of cell spheroids in the culture medium over time were studied. Using the high-frequency (23.8 MHz) excitation, cell accumulation time to the pressure nodes could be reduced in comparison to that of the low-frequency (10.4 MHz) excitation, but in a smaller spheroid size. SSAW excitation at both frequencies does not affect the cell viability up to 7 days, > 90% with no statistical difference compared with the control group. In summary, SSAW can effectively prepare the cell spheroids as bioink for the future 3D bioprinting and various biotechnology applications (e.g., pharmaceutical drug screening and tissue engineering).

Particle Accumulation in a Microchannel and Its Reduction by a Standing Surface Acoustic Wave (SSAW)

Accumulation of particles in a high concentration on a microchannel wall is a common phenomenon in a colloidal fluid. Gradual accumulation/deposition of particles can eventually obstruct the fluid flow and lead to clogging, which seriously affects the accuracy and reliability of nozzle-based printing and causes damage to the nozzle. Particle accumulation in a 100 μm microchannel was investigated by light microscopy, and its area growth in an exponential format was used to quantify this phenomenon. The effects of the constriction angle and alginate concentration on particle accumulation were also studied. In order to reduce the clogging problem, an acoustic method was proposed and evaluated here. Numerical simulation was first conducted to predict the acoustic radiation force on the particles in the fluid with different viscosities. Interdigital transducers (IDTs) were fabricated on the LiNbO₃ wafer to produce standing surface acoustic waves (SSAW) in the microchannel. It was found that the actuation of SSAW can reduce the accumulation area in the microchannel by 2 to 3.7-fold. In summary, the particle accumulation becomes significant with the increase of the constriction angle and fluid viscosity. The SSAW can effectively reduce the particle accumulation and postpone clogging.

Dynamic-field devices for the ultrasonic manipulation of microparticles

The use of acoustic radiation forces in lab-on-a-chip environments has seen a rapid development in recent years. Operations such as particle sieving, sorting and characterisation are becoming increasingly common with a range of applications in the biomedical sciences. Traditionally, these applications rely on static patterns of ultrasonic pressure and are often collectively referred to as ultrasonic standing wave devices. Recent years have also seen the emergence of devices which capitalise on dynamic and reconfigurable ultrasonic fields and these are the subject of this review. Dynamic ultrasonic fields lead to acoustic radiation forces that change with time. They have opened up the possibility of performing a wide range of manipulations such as the transport and rotation of individual particles or agglomerates. In addition, they have led to device reconfigurability, i.e. the ability of a single lab-on-a-chip device to perform multiple functions. This opens up the possibility of channel-less microfluidic devices which would have many applications, for example in biosensing and microscale assembly. This paper reviews the current state of the field of dynamic and reconfigurable ultrasonic particle manipulation devices and then discusses the open problems and future possibilities.

Imaging the position-dependent 3D force on microbeads subjected to acoustic radiation forces and streaming

Acoustic particle manipulation in microfluidic channels is becoming a powerful tool in microfluidics to control micrometer sized objects in medical, chemical and biological applications. By creating a standing acoustic wave in the channel, the resulting pressure field can be employed to trap or sort particles. To design efficient and reproducible devices, it is important to characterize the pressure field throughout the volume of the microfluidic device. Here, we used an optically trapped particle as probe to measure the forces in all three dimensions. By moving the probe through the volume of the channel, we imaged spatial variations in the pressure field. In the direction of the standing wave this revealed a periodic energy landscape for 2 μm beads, resulting in an effective stiffness of 2.6 nN m(-1) for the acoustic trap. We found that multiple fabricated devices showed consistent pressure fields. Surprisingly, forces perpendicular to the direction of the standing wave reached values of up to 20% of the main-axis-values. To separate the direct acoustic force from secondary effects, we performed experiments with different bead sizes, which attributed some of the perpendicular forces to acoustic streaming. This method to image acoustically generated forces in 3D can be used to either minimize perpendicular forces or to employ them for specific applications in novel acoustofluidic designs.

Chladni Patterns in a Liquid at Microscale

By means of ultrathin silicon membranes excited in the low ultrasound range, we show for the first time that it is possible to form two-dimensional Chladni patterns of microbeads in liquid. Unlike the well-known effect in a gaseous environment at the macroscale, where gravity effects are generally dominant, leading particles towards the nodal regions of displacement, we show that the combined effects of an ultrathin plate excited at low frequency (yielding to subsonic waves) together with reduced gravity (arising from buoyancy) will enhance the importance of microstreaming in the Chladni problem. Here, we report that for micrometric beads larger than the inner streaming layer, the microscale streaming in the vicinity of the plate tends to gather particles in antinodal regions of vibrations yielding to patterns in good agreement with the predicted modes for a liquid-loaded plate. Interestingly, a symmetry breaking phenomenon together with the streaming can trigger movements of beads departing from one cluster to another. We show that, for higher modes, this movement can appear as a collective rotation of the beads in the manner of a "farandole."

Dispersion of overdamped diffusing particles in channel flows coupled to transverse acoustophoretic potentials: transport regimes and scaling anomalies

We address the dispersion properties of overdamped Brownian particles migrating in a two-dimensional acoustophoretic microchannel, where a pressure-driven axial Stokes flow coexists with a transverse acoustophoretic potential. Depending on the number and symmetries of the stable nodal points of the acoustophoretic force with respect to the axial velocity profile, different convection-enhanced dispersion regimes can be observed. Among these regimes, an anomalous scaling, for which the axial dispersion increases exponentially with the particle Peclét number, is observed whenever two or more stable acoustophoretic nodes are associated with different axial velocities. A theoretical explanation of this regime is derived, based on exact moment homogenization. Attention is also focused on transient dispersion, which can exhibit superballistic behavior 〈(x-〈x〉)^{2}〉∼t^{3},x being the axial coordinate.

Screen-printed ultrasonic 2-D matrix array transducers for microparticle manipulation

This paper reports the development of a two-dimensional thick film lead zirconate titanate (PZT) ultrasonic transducer array, operating at frequency approximately 7.5MHz, to demonstrate the potential of this fabrication technique for microparticle manipulation. All layers of the array are screen-printed then sintered on an alumina substrate without any subsequent patterning processes. The thickness of the thick film PZT is 139±2μm, the element pitch of the array is 2.3mm, and the dimension of each individual PZT element is 2×2mm(2) with top electrode 1.7×1.7mm(2). The measured relative dielectric constant of the PZT is 2250±100 and the dielectric loss is 0.09±0.005 at 10kHz. Finite element analysis was used to predict the behaviour of the array and to optimise its configuration. Electrical impedance spectroscopy and laser vibrometry were used to characterise the array experimentally. The measured surface motion of a single element is on the order of tens of nanometres with a 10Vpeak continuous sinusoidal excitation. Particle manipulation experiments have been demonstrated with the array by manipulating Ø10μm polystyrene microspheres in degassed water. The simplified array fabrication process and the bulk production capability of screen-printing suggest potential for the commercialisation of multilayer planar resonant devices for ultrasonic particle manipulation.

Direct 2D measurement of time-averaged forces and pressure amplitudes in acoustophoretic devices using optical trapping

Ultrasonic standing waves are increasingly applied in the manipulation and sorting of micrometer-sized particles in microfluidic cells. To optimize the performance of such devices, it is essential to know the exact forces that the particles experience in the acoustic wave. Although much progress has been made via analytical and numerical modeling, the reliability of these methods relies strongly on the assumptions used, e.g. the boundary conditions. Here, we have combined an acoustic flow cell with an optical laser trap to directly measure the force on a single spherical particle in two dimensions. While performing ultrasonic frequency scans, we measured the time-averaged forces on single particles that were moved with the laser trap through the microfluidic cell. The cell including piezoelectric transducers was modeled with finite element methods. We found that the experimentally obtained forces and the derived pressure fields confirm the predictions from theory and modeling. This novel approach can now be readily expanded to other particle, chamber, and fluid regimes and opens up the possibility of studying the effects of the presence of boundaries, acoustic streaming, and non-linear fluids.

The intersection of flow cytometry with microfluidics and microfabrication

A modern flow cytometer can analyze and sort particles on a one by one basis at rates of 50,000 particles per second. Flow cytometers can also measure as many as 17 channels of fluorescence, several angles of scattered light, and other non-optical parameters such as particle impedance. More specialized flow cytometers can provide even greater analysis power, such as single molecule detection, imaging, and full spectral collection, at reduced rates. These capabilities have made flow cytometers an invaluable tool for numerous applications including cellular immunophenotyping, CD4+ T-cell counting, multiplex microsphere analysis, high-throughput screening, and rare cell analysis and sorting. Many bio-analytical techniques have been influenced by the advent of microfluidics as a component in analytical tools and flow cytometry is no exception. Here we detail the functions and uses of a modern flow cytometer, review the recent and historical contributions of microfluidics and microfabricated devices to field of flow cytometry, examine current application areas, and suggest opportunities for the synergistic application of microfabrication approaches to modern flow cytometry.

Theory and experiment on resonant frequencies of liquid-air interfaces trapped in microfluidic devices

Bubble-based microfluidic devices have been proven to be useful for many biological and chemical studies. These bubble-based microdevices are particularly useful when operated at the trapped bubbles' resonance frequencies. In this work, we present an analytical expression that can be used to predict the resonant frequency of a bubble trapped over an arbitrary shape. Also, the effect of viscosity on the dispersion characteristics of trapped bubbles is determined. A good agreement between experimental data and theoretical results is observed for resonant frequency of bubbles trapped over different-sized rectangular-shaped structures, indicating that our expression can be valuable in determining optimized operational parameters for many bubble-based microfluidic devices. Furthermore, we provide a close estimate for the harmonics and a method to determine the dispersion characteristics of a bubble trapped over circular shapes. Finally, we present a new method to predict fluid properties in microfluidic devices and complement the explanation of acoustic microstreaming.

Ultrasound assisted particle and cell manipulation on-chip

Ultrasonic fields are able to exert forces on cells and other micron-scale particles, including microbubbles. The technology is compatible with existing lab-on-chip techniques and is complementary to many alternative manipulation approaches due to its ability to handle many cells simultaneously over extended length scales. This paper provides an overview of the physical principles underlying ultrasonic manipulation, discusses the biological effects relevant to its use with cells, and describes emerging applications that are of interest in the field of drug development and delivery on-chip.