Acoustofluidics 5: Building microfluidic acoustic resonators

High-Throughput 3D Imaging Flow Cytometry of Suspended Adherent 3D Cell Cultures

3D cell cultures are indispensable in recapitulating in vivo environments. Among the many 3D culture methods, culturing adherent cells on hydrogel beads to form spheroid-like structures is a powerful strategy for maintaining high cell viability and functions in the adherent states. However, high-throughput, scalable technologies for 3D imaging of individual cells cultured on the hydrogel scaffolds are lacking. This study reports the development of a high throughput, scalable 3D imaging flow cytometry platform for analyzing spheroid models. This platform is realized by integrating a single objective fluorescence light-sheet microscopy with a microfluidic device that combines hydrodynamic and acoustofluidic focusing techniques. This integration enabled unprecedentedly high-throughput and scalable optofluidic 3D imaging, processing 1310 spheroids consisting of 28 117 cells min-1. The large dataset obtained enables precise quantification and comparison of the nuclear morphology of adhering and suspended cells, revealing that the adhering cells have smaller nuclei with less rounded surfaces. This platform's high throughput, robustness, and precision for analyzing the morphology of subcellular structures in 3D culture models hold promising potential for various biomedical analyses, including image-based phenotypic screening of drugs with spheroids or organoids.

Rapid Concentration and Detection of Bacteria in Milk Using a Microfluidic Surface Acoustic Wave Activated Nanosieve

Rapid detection of microbes is a key feature for monitoring food quality. Unfortunately, current detection systems rely on labor-intensive and time-consuming lab-based processes that are not suitable for point-of-interest applications and typically require several days before results are available. Here, we demonstrate a microfluidic system capable of rapidly concentrating, fluorescent staining, and detecting bacteria in unprocessed complex biological media such as milk. This concentration is done using a surface acoustic wave-driven microfluidic device which operates based on the Bjerknes force, a force generated on one particle by another in its close proximity. We exploit this effect by exciting a tightly packed bed of 50 μm polystyrene microparticles temporarily with surface acoustic waves within a microfluidic device to capture and release bacterial cells on demand. The bacterial cells are fluorescently stained during capture and then detected using fluorescence microscopy upon release. This device offers a high capturing efficiency (>80%) and a 34 Colony Forming Units (CFU)/mL limit of detection, which is 1 order of magnitude below that of plate counting at 30 CFU per standard 100 μL plate (or 300 CFU/mL). This can be attained in just 1 h of processing at 10 μL/min. With this system, we demonstrate that bacterial detection from extremely low concentration samples down to the order of ∼10 CFU/mL is possible without requiring any additional external pre- or postprocessing.

Exploring operational boundaries for acoustic concentration of cell suspensions

The development of a standardized, generic method for concentrating suspensions in continuous flow is challenging. In this study, we developed and tested a device capable of concentrating suspensions with an already high cell concentration to meet diverse industrial requirements. To address typical multitasking needs, we concentrated suspensions with high solid content under a variety of conditions. Cells from Saccharomyces cerevisiae, Escherichia coli, and Chinese hamster ovary cells were effectively focused in the center of the main channel of a microfluidic device using acoustophoresis. The main channel bifurcates into three outlets, allowing cells to exit through the central outlet, while the liquid evenly exits through all outlets. Consequently, the treatment separates cells from two-thirds of the surrounding liquid. We investigated the complex interactions between parameters. Increasing the channel depth results in a decrease in process efficiency, attributed to a decline in acoustic energy density. The study also revealed that different cell strains exhibit distinct acoustic contrast factors, originating from differences in dimensions, compressibility, and density values. Finally, a combination of high solid content and flow rate leads to an increase in diffusion through a phenomenon known as shear-induced diffusion. KEY POINTS: • Acoustic focusing in a microchannel was used to concentrate cell suspensions • The parameters influencing focusing at high concentrations were studied • Three different cell strains were successfully concentrated.

Micro-Acoustic Holograms for Detachable Microfluidic Devices

Acoustic microfluidic devices have advantages for diagnostic applications, therapeutic solutions, and fundamental research due to their contactless operation, simple design, and biocompatibility. However, most acoustofluidic approaches are limited to forming simple and fixed acoustic patterns, or have limited resolution. In this study,a detachable microfluidic device is demonstrated employing miniature acoustic holograms to create reconfigurable, flexible, and high-resolution acoustic fields in microfluidic channels, where the introduction of a solid coupling layer makes these holograms easy to fabricate and integrate. The application of this method to generate flexible acoustic fields, including shapes, characters, and arbitrarily rotated patterns, within microfluidic channels, is demonstrated.

Swarm Autonomy: From Agent Functionalization to Machine Intelligence

Swarm behaviors are common in nature, where individual organisms collaborate via perception, communication, and adaptation. Emulating these dynamics, large groups of active agents can self-organize through localized interactions, giving rise to complex swarm behaviors, which exhibit potential for applications across various domains. This review presents a comprehensive summary and perspective of synthetic swarms, to bridge the gap between the microscale individual agents and potential applications of synthetic swarms. It is begun by examining active agents, the fundamental units of synthetic swarms, to understand the origins of their motility and functionality in the presence of external stimuli. Then inter-agent communications and agent-environment communications that contribute to the swarm generation are summarized. Furthermore, the swarm behaviors reported to date and the emergence of machine intelligence within these behaviors are reviewed. Eventually, the applications enabled by distinct synthetic swarms are summarized. By discussing the emergent machine intelligence in swarm behaviors, insights are offered into the design and deployment of autonomous synthetic swarms for real-world applications.

Extracellular vesicles-based point-of-care testing for the diagnosis and monitoring of Alzheimer's disease

Alzheimer's disease (AD) is a debilitating condition that affects millions of people worldwide. One promising strategy for detecting and monitoring AD early on is using extracellular vesicles (EVs)-based point-of-care testing; however, diagnosing AD using EVs poses a challenge due to the low abundance of EV-biomarkers. Here, we present a fully integrated organic electrochemical transistor (OECT) that enables high accuracy, speed, and convenience in the detection of EVs from AD patients. We incorporated self-aligned acoustoelectric enhancement of EVs on a chip that rapidly propels, enriches, and specifically binds EVs to the OECT detection area. With our enhancement of pre-concentration, we increased the sensitivity to a limit of detection of 500 EV particles/μL and reduced the required detection time to just two minutes. We also tested the sensor on an AD mouse model to monitor AD progression, examined mouse Aβ EVs at different time courses, and compared them with intraneuronal Aβ cumulation using MRI. This innovative technology has the potential to diagnose Alzheimer's and other neurodegenerative diseases accurately and quickly, enabling monitoring of disease progression and treatment response.

Continuous Perfusion Experiments on 3D Cell Proliferation in Acoustic Levitation

An acoustofluidic trap is used for accurate 3D cell proliferation and cell function analysis in levitation. The prototype trap can be integrated with any microscope setup, allowing continuous perfusion experiments with temperature and flow control under optical inspection. To describe the trap function, we present a mathematical and FEM-based COMSOL model for the acoustic mode that defines the nodal position of trapped objects in the spherical cavity aligned with the microscope field of view and depth of field. Continuous perfusion experiments were conducted in sterile conditions over 55 h with a K562 cell line, allowing for deterministic monitoring. The acoustofluidic platform allows for rational in vitro cell testing imitating in vivo conditions such as cell function tests or cell-cell interactions.

Dynamics of a two-layer immiscible fluid system exposed to ultrasound

The relocation dynamics of a two-layer immiscible fluid system exposed to bulk acoustic waves using simulations and experiments are reported. A theoretical formulation of the acoustic radiation pressure (ARP) acting on the interface reveals that ARP is a nonlinear function of the impedance contrast. It has been shown that the force acting on the interface is the simple sum of the ARP and the interfacial tension, which is dependent on the angle of the interface. It was discovered that although the acoustic radiation force is directed from high-impedance fluid (HIF) to low-impedance fluid (LIF), the final steady-state configuration depends on the wall-fluid contact angle (CA). Our study reveals that the HIF and LIF would relocate to the channel center for CA>110°, and CA<70°, respectively, while complete flipping of the fluids is observed for intermediate angles. The forces relocate the fluids in the channel, generally, by a clockwise or anticlockwise rotation. Here, it is demonstrated that the direction of this twist can be determined by the relative densities and wettabilities of the two fluids.

The effect of microchannel height on the acoustophoretic motion of sub-micron particles

Acoustophoresis is an effective technique for particle manipulation. Acoustic radiation force scales with particle volume, enabling size separation. Yet, isolating sub-micron particles remains a challenge due to the acoustic streaming effect (ASE). While some studies confirmed the focusing ability of ASE, others reported continuous stirring effects. To investigate the parameters that influence ASE-induced particle motion in a microchannel, this study examined the effect of microchannel height and particle size. We employed standing surface acoustic wave (SSAW) to manipulate polystyrene particles suspended in the water-filled microchannel. The results show that ASE can direct particles as small as 0.31 µm in diameter to the centre of the streaming vortices, and increasing the channel height enhances the focusing effect. Smaller particles circulate in the streaming vortices continuously, with no movement towards the centres. We also discovered that when the channel height is at least 0.75 the fluid wavelength, particles transitioning from acoustic radiation-dominated to ASE-dominated share the same equilibrium position, which differs from the pressure nodes and the vortices' centres. The spatial distance between particles in different categories can lead to particle separation. Therefore, ASE is a potential alternative mechanism for sub-micron particle sorting when the channel height is accurately adjusted.

Surface Acoustic Wave-Based Microfluidic Device for Microparticles Manipulation: Effects of Microchannel Elasticity on the Device Performance

Size sorting, line focusing, and isolation of microparticles or cells are fundamental ingredients in the improvement of disease diagnostic tools adopted in biology and biomedicine. Microfluidic devices are exploited as a solution to transport and manipulate (bio)particles via a liquid flow. Use of acoustic waves traveling through the fluid provides non-contact solutions to the handling goal, by exploiting the acoustophoretic phenomenon. In this paper, a finite element model of a microfluidic surface acoustic wave-based device for the manipulation of microparticles is reported. Counter-propagating waves are designed to interfere inside a PDMS microchannel and generate a standing surface acoustic wave which is transmitted to the fluid as a standing pressure field. A model of the cross-section of the device is considered to perform a sensitivity analysis of such a standing pressure field to uncertainties related to the geometry of the microchannel, especially in terms of thickness and width of the fluid domain. To also assess the effects caused by possible secondary waves traveling in the microchannel, the PDMS is modeled as an elastic solid material. Remarkable effects and possible issues in microparticle actuation, as related to the size of the microchannel, are discussed by way of exemplary results.

Acoustic and Magnetic Stimuli-Based Three-Dimensional Cell Culture Platform for Tissue Engineering

In a conventional two-dimensional (2D) culture method, cells are attached to the bottom of the culture dish and grow into a monolayer. These 2D culture methods are easy to handle, cost-effective, reproducible, and adaptable to growing many different types of cells. However, monolayer 2D cell culture conditions are far from those of natural tissue, indicating the need for a three-dimensional (3D) culture system. Various methods, such as hanging drop, scaffolds, hydrogels, microfluid systems, and bioreactor systems, have been utilized for 3D cell culture. Recently, external physical stimulation-based 3D cell culture platforms, such as acoustic and magnetic forces, were introduced. Acoustic waves can establish acoustic radiation force, which can induce suspended objects to gather in the pressure node region and aggregate to form clusters. Magnetic targeting consists of two components, a magnetically responsive carrier and a magnetic field gradient source. In a magnetic-based 3D cell culture platform, cells are aggregated by changing the magnetic force. Magnetic fields can manipulate cells through two different methods: positive magnetophoresis and negative magnetophoresis. Positive magnetophoresis is a way of imparting magnetic properties to cells by labeling them with magnetic nanoparticles. Negative magnetophoresis is a label-free principle-based method. 3D cell structures, such as spheroids, 3D network structures, and cell sheets, have been successfully fabricated using this acoustic and magnetic stimuli-based 3D cell culture platform. Additionally, fabricated 3D cell structures showed enhanced cell behavior, such as differentiation potential and tissue regeneration. Therefore, physical stimuli-based 3D cell culture platforms could be promising tools for tissue engineering.

Low-cost acoustic force trap in a microfluidic channel

A low-cost glass-based microfluidic flow cell with a piezo actuator is built using off-the-shelf parts (total cost €9 per device) to apply acoustophoretic force on polystyrene micro-beads. The main challenge in the fabrication of these devices was to ensure their leak tightness, which we solved using double-sided tape and nail polish. Beads with 1.5 μm diameter flowing in a 100 μm deep channel were trapped at 7.5 MHz using a 23.7 peak-to-peak voltage (Vpp) sinusoidal input. The trap located at 50 ± 0.1 μm depth was measured to have a stiffness of approximately 0.6 pN/μm. With this simple device we can trap and control the axial position of micrometer scale objects, which allows for the manipulation of beads and cells. We intend to use the device for force spectroscopy on micro-bead tethered DNA. This can be combined with super-resolution imaging techniques to study mechanics and binding of protein structures along a DNA strand as a function of induced tension.

Acoustic radiation force on a spherical thermoviscous particle in a thermoviscous fluid including scattering and microstreaming

We derive general analytical expressions for the time-averaged acoustic radiation force on a small spherical particle suspended in a fluid and located in an axisymmetric incident acoustic wave. We treat the cases of the particle being either an elastic solid or a fluid particle. The effects of particle vibrations, acoustic scattering, acoustic microstreaming, heat conduction, and temperature-dependent fluid viscosity are all included in the theory. Acoustic streaming inside the particle is also taken into account for the case of a fluid particle. No restrictions are placed on the widths of the viscous and thermal boundary layers relative to the particle radius. We compare the resulting acoustic radiation force with that obtained from previous theories in the literature, and we identify limits, where the theories agree, and specific cases of particle and fluid materials, where qualitative or significant quantitative deviations between the theories arise.

Assessment of silicon, glass, FR4, PDMS and PMMA as a chip material for acoustic particle/cell manipulation in microfluidics

In the present study, the capabilities of different chip materials for acoustic particle manipulation have been assessed with the same microfluidic device architecture, under the same actuator and flow conditions. Silicon, glass, epoxy with fiberglass filling (FR4), polydimethylsiloxane (PDMS) and polymethyl methacrylate (PMMA) are considered as chip materials. The acoustophoretic chips in this study were manufactured with four different fabrication methods: plasma etching, chemical etching, micromachining and molding. A novel chip material, FR4, has been employed as a microfluidic chip material in acoustophoretic particle manipulation for the first time in literature, which combines the ease of manufacturing of polymer materials with improved acoustic performance. The acoustic particle manipulation performance is evaluated through acoustophoretic focusing experiments with 2μm and 12μm polystyrene microspheres and cultured breast cancer cell line (MDA-MB-231). Unlike the common approach in the literature, the piezoelectric materials were actuated with partitioned cross-polarized electrodes which allowed effective actuation of different family of chip materials. Different from previous studies, this study evaluates the performance of each acoustophoretic device through the perspective of synchronization of electrical, vibrational and acoustical resonances, considers the thermal performance of the chip materials with their effects on cell viability as well as manufacturability and scalability of their fabrication methods. We believe our study is an essential work towards the commercialization of acoustophoretic devices since it brings a critical understanding of the effect of chip material on device performance as well as the cost of achieving that performance.

Single-Cell Microarray Chip with Inverse-Tapered Wells to Maintain High Ratio of Cell Trapping

A single-cell microarray (SCM) influenced by gravitational force is expected to be one of the simple methods in various fields such as DNA analysis and antibody production. After trapping the cells in the SCM chip, it is necessary to remove the liquid from the SCM to wash away the un-trapped cells on the chip and treat the reagents for analysis. The flow generated during this liquid exchange causes the trapped cells to drop out of conventional vertical wells. In this study, we propose an inverse-tapered well to keep trapped cells from escaping from the SCM. The wells with tapered side walls have a reduced force of flow toward the opening, which prevents trapped cells from escaping. The proposed SCM chip was fabricated using 3D photolithography and polydimethylsiloxane molding techniques. In the trapping experiment using HeLa cells, the cell residual rate increased more than two-fold for the SCM chip with the inverse-tapered well with a taper angle of 30° compared to that for the conventional vertical SCM chip after multiple rounds of liquid exchanges. The proposed well structure increases the number of trapped cells and decreases the cell dropout rate to improve the efficiency of cellular analysis.

Recent advances in non-optical microfluidic platforms for bioparticle detection

The effective analysis of the basic structure and functional information of bioparticles are of great significance for the early diagnosis of diseases. The synergism between microfluidics and particle manipulation/detection technologies offers enhanced system integration capability and test accuracy for the detection of various bioparticles. Most microfluidic detection platforms are based on optical strategies such as fluorescence, absorbance, and image recognition. Although optical microfluidic platforms have proven their capabilities in the practical clinical detection of bioparticles, shortcomings such as expensive components and whole bulky devices have limited their practicality in the development of point-of-care testing (POCT) systems to be used in remote and underdeveloped areas. Therefore, there is an urgent need to develop cost-effective non-optical microfluidic platforms for bioparticle detection that can act as alternatives to optical counterparts. In this review, we first briefly summarise passive and active methods for bioparticle manipulation in microfluidics. Then, we survey the latest progress in non-optical microfluidic strategies based on electrical, magnetic, and acoustic techniques for bioparticle detection. Finally, a perspective is offered, clarifying challenges faced by current non-optical platforms in developing practical POCT devices and clinical applications.

Tools for manipulation and positioning of microtissues

Complex three-dimensional (3D) in vitro models are emerging as a key technology to support research areas in personalised medicine, such as drug development and regenerative medicine. Tools for manipulation and positioning of microtissues play a crucial role in the microtissue life cycle from production to end-point analysis. The ability to precisely locate microtissues can improve the efficiency and reliability of processes and investigations by reducing experimental time and by providing more controlled parameters. To achieve this goal, standardisation of the techniques is of primary importance. Compared to microtissue production, the field of microtissue manipulation and positioning is still in its infancy but is gaining increasing attention in the last few years. Techniques to position microtissues have been classified into four main categories: hydrodynamic techniques, bioprinting, substrate modification, and non-contact active forces. In this paper, we provide a comprehensive review of the different tools for the manipulation and positioning of microtissues that have been reported to date. The working mechanism of each technique is described, and its merits and limitations are discussed. We conclude by evaluating the potential of the different approaches to support progress in personalised medicine.

Programmable Droplet Microfluidics Based on Machine Learning and Acoustic Manipulation

Typical microfluidic devices are application-specific and have to be carefully designed to implement the necessary functionalities for the targeted application. Programmable microfluidic chips try to overcome this by offering reconfigurable functionalities, allowing the same chip to be used in multiple different applications. In this work, we demonstrate a programmable microfluidic chip for the two-dimensional manipulation of droplets, based on ultrasonic bulk acoustic waves and a closed-loop machine-learning-based control algorithm. The algorithm has no prior knowledge of the acoustic fields but learns to control the droplets on the fly. The manipulation is based on switching the frequency of a single ultrasonic transducer. Using this method, we demonstrate 2D transportation and merging of water droplets in oil and oil droplets in water, and we performed the chemistry that underlies the basis of a colorimetric glucose assay. We show that we can manipulate drops with volumes ranging from ∼200 pL up to ∼30 nL with our setup. We also demonstrate that our method is robust, by changing the system parameters and showing that the machine learning algorithm can still complete the manipulation tasks. In short, our method uses ultrasonics to flexibly manipulate droplets, enabling programmable droplet microfluidic devices.

Recent advances in acoustofluidic separation technology in biology

Acoustofluidic separation of cells and particles is an emerging technology that integrates acoustics and microfluidics. In the last decade, this technology has attracted significant attention due to its biocompatible, contactless, and label-free nature. It has been widely validated in the separation of cells and submicron bioparticles and shows great potential in different biological and biomedical applications. This review first introduces the theories and mechanisms of acoustofluidic separation. Then, various applications of this technology in the separation of biological particles such as cells, viruses, biomolecules, and exosomes are summarized. Finally, we discuss the challenges and future prospects of this field.

Square microchannel enables to focus and orient ellipsoidal Euglena gracilis cells by two-dimensional acoustic standing wave

Flow cytometry has become an indispensable tool for counting, analyzing, and sorting large cell populations in biological research and medical practice. Unfortunately, it has limitations in the analysis of non-spherically shaped cells due to the variation of their alignment with respect to the flow direction and, hence, the optical interrogation axis, resulting in unreliable cell analysis. Here, we present a simple on-chip acoustofluidic method to fix the orientation of ellipsoidal cells and focus them into a single, aligned stream. Specifically, by generating acoustic standing waves inside a 100 ⋅ 100 µm square-shaped microchannel, we successfully aligned and focused up to 97.7% of a population of Euglena gracilis (an ellipsoidal shaped microalgal species) cells in the center of the microchannel with high precision at a volume rate of 25 to 200 µL min^-1. Uniform positioning of ellipsoidal cells is essential for making flow cytometry applicable to the investigation of a greater variety of cell populations and is expected to be beneficial for ecological studies and aquaculture.

Recent advances in acoustic microfluidics and its exemplary applications

Acoustic-based microfluidics has been widely used in recent years for fundamental research due to its simple device design, biocompatibility, and contactless operation. In this article, the basic theory, typical devices, and technical applications of acoustic microfluidics technology are summarized. First, the theory of acoustic microfluidics is introduced from the classification of acoustic waves, acoustic radiation force, and streaming flow. Then, various applications of acoustic microfluidics including sorting, mixing, atomization, trapping, patterning, and acoustothermal heating are reviewed. Finally, the development trends of acoustic microfluidics in the future were summarized and looked forward to.

Recent Advances in Microfluidic Platform for Physical and Immunological Detection and Capture of Circulating Tumor Cells

CTCs (circulating tumor cells) are well-known for their use in clinical trials for tumor diagnosis. Capturing and isolating these CTCs from whole blood samples has enormous benefits in cancer diagnosis and treatment. In general, various approaches are being used to separate malignant cells, including immunomagnets, macroscale filters, centrifuges, dielectrophoresis, and immunological approaches. These procedures, on the other hand, are time-consuming and necessitate multiple high-level operational protocols. In addition, considering their low efficiency and throughput, the processes of capturing and isolating CTCs face tremendous challenges. Meanwhile, recent advances in microfluidic devices promise unprecedented advantages for capturing and isolating CTCs with greater efficiency, sensitivity, selectivity and accuracy. In this regard, this review article focuses primarily on the various fabrication methodologies involved in microfluidic devices and techniques specifically used to capture and isolate CTCs using various physical and biological methods as well as their conceptual ideas, advantages and disadvantages.

Study on acoustic radiation force of an elastic sphere in an off-axial Gaussian beam using localized approximation

In this paper, the expansion coefficients of the off-axial Gaussian beam are obtained using the localized approximation and the translational addition theorem for spherical wave function. The three-dimensional acoustic radiation force of a sphere positioned in an off-axial Gaussian beam is derived. The axial acoustic radiation force of a rigid sphere is computed to verify the derived expressions. The effect of the position of a polystyrene sphere in an off-axial Gaussian beam on the transverse and axial acoustic radiation forces is studied to explore the changing law of particle acoustic manipulation using a Gaussian beam. The calculated results show that the axial force repels the polystyrene particle away from the center of the beam. However, for the transverse force, there is a negative acoustic radiation force at some positions, which is related to the position of the polystyrene sphere in the Gaussian beam, and the negative transverse forces usually pull the polystyrene particle toward the beam axis. In addition, the numerical simulations based on the finite element method are presented to validate the analytical theory, and the comparison results are in good agreement with each other. The study may provide a theoretical basis for the development of single-beam acoustic tweezers using a Gaussian beam.

Acoustic particle trapping driven by axial primary radiation force in shaped traps

We study particle trapping driven by the axial primary radiation force (A-PRF) in shaped traps exposed to standing bulk acoustic waves (S-BAW) using numerical simulations and experiments. The utilization of the stronger A-PRF as the main retention force is a consequence of standing-wave formation along the flow direction, instead of the orthogonal direction as in the case of traditionally used lateral-PRF S-BAW trapping setups. The study of particle dynamics reveals that the competition between A-PRF and viscous drag force governs particle trajectory. The ratio of the acoustic energy to the viscous work (β) provides a general criterion for particle trapping at a distinctive off-node site that is spatially controllable. Particles get trapped for β≥β_{cr} at some distance away from the nodal plane and the distance varies as β^{-c} (c=0.6-1.0). The use of A-PRF as the retention force could potentially allow traditional S-BAW trapping systems to envisage high-throughput advancements surpassing the current standards in cell-handling unit operations.

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.

Label-Free Isolation of Exosomes Using Microfluidic Technologies

Exosomes are cell-derived structures packaged with lipids, proteins, and nucleic acids. They exist in diverse bodily fluids and are involved in physiological and pathological processes. Although their potential for clinical application as diagnostic and therapeutic tools has been revealed, a huge bottleneck impeding the development of applications in the rapidly burgeoning field of exosome research is an inability to efficiently isolate pure exosomes from other unwanted components present in bodily fluids. To date, several approaches have been proposed and investigated for exosome separation, with the leading candidate being microfluidic technology due to its relative simplicity, cost-effectiveness, precise and fast processing at the microscale, and amenability to automation. Notably, avoiding the need for exosome labeling represents a significant advance in terms of process simplicity, time, and cost as well as protecting the biological activities of exosomes. Despite the exciting progress in microfluidic strategies for exosome isolation and the countless benefits of label-free approaches for clinical applications, current microfluidic platforms for isolation of exosomes are still facing a series of problems and challenges that prevent their use for clinical sample processing. This review focuses on the recent microfluidic platforms developed for label-free isolation of exosomes including those based on sieving, deterministic lateral displacement, field flow, and pinched flow fractionation as well as viscoelastic, acoustic, inertial, electrical, and centrifugal forces. Further, we discuss advantages and disadvantages of these strategies with highlights of current challenges and outlook of label-free microfluidics toward the clinical utility of exosomes.

Numerical and experimental analysis of a hybrid material acoustophoretic device for manipulation of microparticles

Acoustophoretic microfluidic devices have been developed for accurate, label-free, contactless, and non-invasive manipulation of bioparticles in different biofluids. However, their widespread application is limited due to the need for the use of high quality microchannels made of materials with high specific acoustic impedances relative to the fluid (e.g., silicon or glass with small damping coefficient), manufactured by complex and expensive microfabrication processes. Soft polymers with a lower fabrication cost have been introduced to address the challenges of silicon- or glass-based acoustophoretic microfluidic systems. However, due to their small acoustic impedance, their efficacy for particle manipulation is shown to be limited. Here, we developed a new acoustophoretic microfluid system fabricated by a hybrid sound-hard (aluminum) and sound-soft (polydimethylsiloxane polymer) material. The performance of this hybrid device for manipulation of bead particles and cells was compared to the acoustophoretic devices made of acoustically hard materials. The results show that particles and cells in the hybrid material microchannel travel to a nodal plane with a much smaller energy density than conventional acoustic-hard devices but greater than polymeric microfluidic chips. Against conventional acoustic-hard chips, the nodal line in the hybrid microchannel could be easily tuned to be placed in an off-center position by changing the frequency, effective for particle separation from a host fluid in parallel flow stream models. It is also shown that the hybrid acoustophoretic device deals with smaller temperature rise which is safer for the actuation of bioparticles. This new device eliminates the limitations of each sound-soft and sound-hard materials in terms of cost, adjusting the position of nodal plane, temperature rise, fragility, production cost and disposability, making it desirable for developing the next generation of economically viable acoustophoretic products for ultrasound particle manipulation in bioengineering applications.

Acoustofluidic platforms for particle manipulation

There is an increasing interest in acoustics for microfluidic applications. This field, commonly known as acoustofluidics involves the interaction of ultrasonic standing waves with fluids and dispersed microparticles. The combination of microfluidics and the so-called acoustic standing waves (ASWs) led to the development of integrated systems for contact-less on-chip cell and particle manipulation where it is possible to move and spatially localize these particles based on the different acoustophysical properties. While it was initially suggested that the acoustic forces could be harmful to the cells and could impact cell viability, proliferation, or function via phenotypic or even genotypic changes, further studies disproved such claims. This review is summarizing some interesting applications of acoustofluidics in the manipulations of biomaterials, such as cells or subcellular vesicles, in works published mainly within the last 5 years.

Real-Time Detection and Control of Microchannel Resonance Frequency in Acoustic Trapping Systems by Monitoring Amplifier Supply Currents

The utilization of bulk acoustic waves from a piezoelectric transducer for selective particle trapping under flow in a microchannel is limited by the high sensitivity of the resonance frequency to tolerances in device geometry, drift during trapping, and variations in the local flow or sample conditions in each channel. This is addressed by detecting the resonance condition based on the impedance minimum obtained by monitoring the amplitude of the stimulation voltage across the piezo transducer and utilizing real-time feedback to control the stimulation frequency. However, this requires an overlap in the frequency bandwidth of the detection and the stimulation system and is also limited by the decline in the acoustic trapping power when the stimulation and resonance frequency measurement are conducted simultaneously. Instead, we present a novel circuit implementation for on-chip real-time resonance frequency measurement and feedback control based on monitoring the current drawn from the amplifier used to stimulate the piezo transducer, since the need for high measurement sensitivity in this mode does not lower the power available for stimulation of the transducer. The enhanced level of control of acoustic trapping utilizing this current mode is validated for various localized channel perturbations, including drift, wash steps, and buffer swaps, as well as for selective sperm cell trapping from a heterogeneous sample that includes lysed epithelial cells.

Focalization Performance Study of a Novel Bulk Acoustic Wave Device

This work illustrates focalization performances of a silicon-based bulk acoustic wave device applied for the separation of specimens owing to micrometric dimensions. Samples are separated in the microfluidic channel by the presence of an acoustic field, which focalizes particles or cells according to their mechanical properties compared to the surrounded medium ones. Design and fabrication processes are reported, followed by focalization performance tests conducted either with synthetic particles or cells. High focalization performances occurred at different microparticle concentrations. In addition, preliminary tests carried out with HL-60 cells highlighted an optimal separation performance at a high flow rate and when cells are mixed with micro and nanoparticles without affecting device focalization capabilities. These encouraging results showed how this bulk acoustic wave device could be exploited to develop a diagnostic tool for early diagnosis or some specific target therapies by separating different kinds of cells or biomarkers possessing different mechanical properties such as shapes, sizes and densities.

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.

Numerical study of bulk acoustofluidic devices driven by thin-film transducers and whole-system resonance modes

In bulk acoustofluidic devices, acoustic resonance modes for fluid and microparticle handling are traditionally excited by bulk piezoelectric (PZE) transducers. In this work, it is demonstrated by numerical simulations in three dimensions that integrated PZE thin-film transducers, constituting less than 0.1% of the bulk device, work equally well. The simulations are performed using a well-tested and experimentally validated numerical model. A water-filled straight channel embedded in a mm-sized bulk glass chip with a 1- μm-thick thin-film transducer made of Al_0.6Sc_0.4N is presented as a proof-of-concept example. The acoustic energy, radiation force, and microparticle focusing times are computed and shown to be comparable to those of a conventional bulk silicon-glass device actuated by a bulk lead-zirconate-titanate transducer. The ability of thin-film transducers to create the desired acoustofluidic effects in bulk acoustofluidic devices relies on three physical aspects: the in-plane-expansion of the thin-film transducer under the applied orthogonal electric field, the acoustic whole-system resonance of the device, and the high Q-factor of the elastic solid, constituting the bulk part of the device. Consequently, the thin-film device is remarkably insensitive to the Q-factor and resonance properties of the thin-film transducer.

Selective particle separation on centimeter scale using a dual frequency dynamic acoustic field

This study investigated the application of dual-frequency type dynamic acoustic fields for size-selective particle separation on centimeter scale in a continuous flow. The 3D-printed X-shaped prototype has two inlets and two outlets. The dynamic acoustic field is generated by two transducers positioned under an angle of 60^° and operating at slightly different frequencies. The acoustic reflections are eliminated by placing sound-absorbing material inside the prototype and the non-resonant operation is confirmed by the electrical admittance measurements. Numerical calculations suggested that pressure generated by each transducer does not need to have equal amplitude. Computer simulations and lab experiments were carried out for different frequency differences and flow rates. The results demonstrated the ability of dual-frequency dynamic acoustic fields for size-selective particle filtration on centimeter scale, with a total flow rate up to.1Lh^-1.

Acoustic resonance and atomization for gas-liquid systems in microreactors

It is shown that a liquid slug in gas-liquid segmented flow in microchannels can act as an acoustic resonator to disperse large amounts of small liquid droplets, commonly referred to as atomization, into the gas phase. We investigate the principles of acoustic resonance within a liquid slug through experimental analysis and numerical simulation. A mechanism of atomization in the confined channels and a hypothesis based on high-speed image analysis that links acoustic resonance within a liquid slug with the observed atomization is proposed. The observed phenomenon provides a novel source of confined micro sprays and could be an avenue, amongst others, to overcome mass transfer limitations for gas-liquid processes in flow.

Acoustofluidic device for acoustic capture of Bacillus anthracis spore analogues at low concentration

A portable device for the rapid concentration of Bacillus subtilis var niger spores, also known as Bacillus globigii (BG), using a thin-reflector acoustofluidic configuration is described. BG spores form an important laboratory analog for the Bacillus anthracis spores, a serious health and bioterrorism risk. Existing systems for spore detection have limitations on detection time and detection that will benefit from the combination with this technology. Thin-reflector acoustofluidic devices can be cheaply and robustly manufactured and provide a more reliable acoustic force than previously explored quarter-wave resonator systems. The system uses the acoustic forces to drive spores carried in sample flows of 30 ml/h toward an antibody functionalized surface, which captures and immobilizes them. In this implementation, spores were fluorescently labeled and imaged. Detection at concentrations of 100 CFU/ml were demonstrated in an assay time of 10 min with 60% capture. We envisage future systems to incorporate more advanced detection of the concentrated spores, leading to rapid, sensitive detection in the presence of significant noise.

Acoustophoresis in polymer-based microfluidic devices: Modeling and experimental validation

A finite-element model is presented for numerical simulation in three dimensions of acoustophoresis of suspended microparticles in a microchannel embedded in a polymer chip and driven by an attached piezoelectric transducer at MHz frequencies. In accordance with the recently introduced principle of whole-system ultrasound resonances, an optimal resonance mode is identified that is related to an acoustic resonance of the combined transducer-chip-channel system and not to the conventional pressure half-wave resonance of the microchannel. The acoustophoretic action in the microchannel is of comparable quality and strength to conventional silicon-glass or pure glass devices. The numerical predictions are validated by acoustic focusing experiments on 5-μm-diameter polystyrene particles suspended inside a microchannel, which was milled into a polymethylmethacrylate chip. The system was driven anti-symmetrically by a piezoelectric transducer, driven by a 30-V peak-to-peak alternating voltage in the range from 0.5 to 2.5 MHz, leading to acoustic energy densities of 13 J/m³ and particle focusing times of 6.6 s.

Controlled Manipulation and Active Sorting of Particles Inside Microfluidic Chips Using Bulk Acoustic Waves and Machine Learning

Manipulation of cells, droplets, and particles via ultrasound within microfluidic chips is a rapidly growing field, with applications in cell and particle sorting, blood fractionation, droplet transport, and enrichment of rare or cancerous cells, among others. However, current methods with a single ultrasonic transducer offer limited control of the position of single particles. In this paper, we demonstrate closed-loop two-dimensional manipulation of particles inside closed-channel microfluidic chips, by controlling the frequency of a single ultrasound transducer, based on machine-vision-measured positions of the particles. For the control task, we propose using algorithms derived from the family of multi-armed bandit algorithms. We show that these algorithms can achieve controlled manipulation with no prior information on the acoustic field shapes. The method learns as it goes: there is no need to restart the experiment at any point. Starting with no knowledge of the field shapes, the algorithms can (eventually) move a particle from one position inside the chamber to another. This makes the method very robust to changes in chip and particle properties. We demonstrate that the method can be used to manipulate a single particle, three particles simultaneously, and also a single particle in the presence of a bubble in the chip. Finally, we demonstrate the practical applications of this method in active sorting of particles, by guiding each particle to exit the chip through one of three different outlets at will. Because the method requires no model or calibration, the work paves the way toward the acoustic manipulation of microparticles inside unstructured environments.

An extended view for acoustofluidic particle manipulation: Scenarios for actuation modes and device resonance phenomenon for bulk-acoustic-wave devices

For the manipulation of microparticles, ultrasonic devices, which employ acoustophoretic forces, have become an essential tool. There exists a widely used analytical expression in the literature which does not account for the effect of the geometry and acoustic properties of the chip material to calculate the acoustophoretic force and resonance frequencies. In this study, we propose an analytical relationship that includes the effect of the chip material on the resonance frequencies of an acoustophoretic chip. Similar to the analytical equation in the literature, this approach also assumes plane wave propagation. The relationship is simplified to a form which introduces a correction term to the acoustophoretic force equation for the presence of the chip material. The proposed equations reveal that the effect of the chip material on the resonance frequency is significant-and is called the device resonance-for acoustically soft materials. The relationship between the actuation modes of the piezoelectric actuator(s) and position of the nodal lines inside the channel are discussed. Finite element simulations are performed to verify the proposed equations. Simulations showed that even if some of the assumptions in the derivations are removed, the general conclusions about the motion of the microparticles are still valid.

A portable sperm cell purification instrument based on continuous flow acoustophoretic separation of sperm cells for on-site forensic sample pretreatment

Obtaining a purified male sperm specimen from the original forensic sample mixtures is a critical procedure in identifying the criminal suspect in the forensic analysis of sexual assault crimes. Differential extraction (DE) has been used as the gold standard for extracting DNA preferentially from sexual assault samples, however it is laborious, time-consuming and inefficient. In this report a continuous flow two-step acoustic cell separation scheme is proposed for the removal of cell debris and cell-free genomic contaminants, followed by separation of sperm cells from epithelial cells. Based on the above method, a portable forensic sperm cell purification instrument has been constructed for on-site pretreatment of forensic samples in sexual assault crimes, where removal of contaminants and isolation of sperm cells could be automatically performed without the use of cell lysis buffer.

Microfluidic chip for graduated magnetic separation of circulating tumor cells by their epithelial cell adhesion molecule expression and magnetic nanoparticle binding

The enumeration of circulating tumor cells (CTCs) in the peripheral bloodstream of metastatic cancer patients has contributed to improvements in prognosis and therapeutics. There have been numerous approaches to capture and counting of CTCs. However, CTCs have potential information beyond simple enumeration and hold promise as a liquid biopsy for cancer and a pathway for personalized cancer therapy by detecting the subset of CTCs having the highest metastatic potential. There is evidence that epithelial cell adhesion molecule (EpCAM) expression level distinguishes these highly metastatic CTCs. The few previous approaches to selective CTC capture according to EpCAM expression level are reviewed. A new two-stage microfluidic device for separation, enrichment and release of CTCs into subpopulations sorted by EpCAM expression level is presented here. It relies upon immunospecific magnetic nanoparticle labeling of CTCs followed by their field- and flow-based separation in the first stage and capture as discrete subpopulations in the second stage. To fine tune the separation, the magnetic field profile across the first stage microfluidic channel may be modified by bonding small Vanadium Permendur strips to its outer walls. Mathematical modeling of magnetic fields and fluid flows supports the soundness of the design.

Acoustic Microfluidic Separation Techniques and Bioapplications: A Review

Microfluidic separation technology has garnered significant attention over the past decade where particles are being separated at a micro/nanoscale in a rapid, low-cost, and simple manner. Amongst a myriad of separation technologies that have emerged thus far, acoustic microfluidic separation techniques are extremely apt to applications involving biological samples attributed to various advantages, including high controllability, biocompatibility, and non-invasive, label-free features. With that being said, downsides such as low throughput and dependence on external equipment still impede successful commercialization from laboratory-based prototypes. Here, we present a comprehensive review of recent advances in acoustic microfluidic separation techniques, along with exemplary applications. Specifically, an inclusive overview of fundamental theory and background is presented, then two sets of mechanisms underlying acoustic separation, bulk acoustic wave and surface acoustic wave, are introduced and discussed. Upon these summaries, we present a variety of applications based on acoustic separation. The primary focus is given to those associated with biological samples such as blood cells, cancer cells, proteins, bacteria, viruses, and DNA/RNA. Finally, we highlight the benefits and challenges behind burgeoning developments in the field and discuss the future perspectives and an outlook towards robust, integrated, and commercialized devices based on acoustic microfluidic separation.

Toward optimal acoustophoretic microparticle manipulation by exploiting asymmetry

The performance of a micro-acousto-fluidic device designed for microparticle trapping is simulated using a three-dimensional (3D) numerical model. It is demonstrated by numerical simulations that geometrically asymmetric architecture and actuation can increase the acoustic radiation forces in a liquid-filled cavity by almost 2 orders of magnitude when setting up a standing pressure half wave in a microfluidic chamber. Similarly, experiments with silicon-glass devices show a noticeable improvement in acoustophoresis of 20-μm silica beads in water when asymmetric devices are used. Microparticle acoustophoresis has an extensive array of applications in applied science fields ranging from life sciences to 3D printing. A more efficient and powerful particle manipulation system can boost the overall effectiveness of an acoustofluidic device. The numerical simulations are developed in the COMSOL Multiphysics^® software package (COMSOL AB, Stockholm, Sweden). By monitoring the modes and magnitudes of simulated acoustophoretic fields in a relatively wide range of ultrasonic frequencies, a map of device performance is obtained. 3D resonant acoustophoretic fields are identified to quantify the improved performance of the chips with an asymmetric layout. Four different device designs are analyzed experimentally, and particle tracking experimental data qualitatively supports the numerical results.

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.

On-Chip Sonoporation-Based Flow Cytometric Magnetic Labeling

Tracing magnetically labeled cells with magnetic resonance imaging (MRI) is an emerging and promising approach to uncover in vivo behaviors of cells in cell therapy. Today, existing methods for the magnetic labeling of cells are cumbersome and time-consuming, which has greatly limited the progress of such studies on cell therapy. Thus, in this study, using the flow cytometric loading technology, we develop a sonoporation-based microfluidic chip (i.e., a microfluidic chip integrated with ultrasound; MCU), to achieve the safe, instant, convenient, and continuous magnetic labeling of cells. For the MCU we designed, a suitable group of operating conditions for safely and efficiently loading superparamagnetic iron oxide (SPIO) nanoparticles into DC2.4 cells was identified experimentally. Under the identified operating conditions, the DC2.4 cells could be labeled in approximately 2 min with high viability (94%) and a high labeling quantity of SPIO nanoparticles (19 pg of iron per cell). In addition, the proliferative functions of the cells were also well maintained after labeling. Furthermore, the in vivo imaging ability of the DC2.4 cells labeled using the MCU was verified by injecting the labeled cells into the leg muscle of the C57BL/6 mice. The results show that the excellent imaging outcome can be continuously achieved for 7 days at a density of 10⁶ cells/mL. This work can provide insight for the design of magnetic cell labeling devices and promote the MRI-based study of cell therapies.

Microparticle Acoustophoresis in Aluminum-Based Acoustofluidic Devices with PDMS Covers

We present a numerical model for the recently introduced simple and inexpensive micromachined aluminum devices with a polydimethylsiloxane (PDMS) cover for microparticle acoustophoresis. We validate the model experimentally for a basic design, where a microchannel is milled into the surface of an aluminum substrate, sealed with a PDMS cover, and driven at MHz frequencies by a piezoelectric lead-zirconate-titanate (PZT) transducer. Both experimentally and numerically we find that the soft PDMS cover suppresses the Rayleigh streaming rolls in the bulk. However, due to the low transverse speed of sound in PDMS, such devices are prone to exhibit acoustic streaming vortices in the corners with a relatively large velocity. We predict numerically that in devices, where the microchannel is milled all the way through the aluminum substrate and sealed with a PDMS cover on both the top and bottom, the Rayleigh streaming is suppressed in the bulk thus enabling focusing of sub-micrometer-sized particles.