Non-contact acoustic cell trapping in disposable glass capillaries

An acoustofluidic device for the automated separation of platelet-reduced plasma from whole blood

Separating plasma from whole blood is an important sample processing technique required for fundamental biomedical research, medical diagnostics, and therapeutic applications. Traditional protocols for plasma isolation require multiple centrifugation steps or multiunit microfluidic processing to sequentially remove large red blood cells (RBCs) and white blood cells (WBCs), followed by the removal of small platelets. Here, we present an acoustofluidic platform capable of efficiently removing RBCs, WBCs, and platelets from whole blood in a single step. By leveraging differences in the acoustic impedances of fluids, our device generates significantly greater forces on suspended particles than conventional microfluidic approaches, enabling the removal of both large blood cells and smaller platelets in a single unit. As a result, undiluted human whole blood can be processed by our device to remove both blood cells and platelets (>90%) at low voltages (25 Vpp). The ability to successfully remove blood cells and platelets from plasma without altering the properties of the proteins and antibodies present creates numerous potential applications for our platform in biomedical research, as well as plasma-based diagnostics and therapeutics. Furthermore, the microfluidic nature of our device offers advantages such as portability, cost efficiency, and the ability to process small-volume samples.

Microfabricated acoustofluidic membrane acoustic waveguide actuator for highly localized in-droplet dynamic particle manipulation

Precision manipulation techniques in microfluidics often rely on ultrasonic actuators to generate displacement and pressure fields in a liquid. However, strategies to enhance and confine the acoustofluidic forces often work against miniaturization and reproducibility in fabrication. This study presents microfabricated piezoelectric thin film membranes made via silicon diffusion for guided flexural wave generation as promising acoustofluidic actuators with low frequency, voltage, and power requirements. The guided wave propagation can be dynamically controlled to tune and confine the induced acoustofluidic radiation force and streaming. This provides for highly localized dynamic particle manipulation functionalities such as multidirectional transport, patterning, and trapping. The device combines the advantages of microfabrication and advanced acoustofluidic capabilities into a miniature "drop-and-actuate" chip that is mechanically robust and features a high degree of reproducibility for large-scale production. The membrane acoustic waveguide actuators offer a promising pathway for acoustofluidic applications such as biosensing, organoid production, and in situ analyte transport.

Methods to mechanically perturb and characterize GUV-based minimal cell models

Cells shield organelles and the cytosol via an active boundary predominantly made of phospholipids and membrane proteins, yet allowing communication between the intracellular and extracellular environment. Micron-sized liposome compartments commonly known as giant unilamellar vesicles (GUVs) are used to model the cell membrane and encapsulate biological materials and processes in a cell-like confinement. In the field of bottom-up synthetic biology, many have utilized GUVs as substrates to study various biological processes such as protein-lipid interactions, cytoskeletal assembly, and dynamics of protein synthesis. Like cells, it is ideal that GUVs are also mechanically durable and able to stay intact when the inner and outer environment changes. As a result, studies have demonstrated approaches to tune the mechanical properties of GUVs by modulating membrane composition and lumenal material property. In this context, there have been many different methods developed to test the mechanical properties of GUVs. In this review, we will survey various perturbation techniques employed to mechanically characterize GUVs.

Ultra-trace enriching biosensing in nanoliter sample

Rapid detection and accurate analysis of trace samples is an important prerequisite for precision medicine. Here we integrated capillary with ultrasound to induce biomarkers enrichment in nanoliter samples, and developed a nanoliter sample enrichment analysis method for ultra-trace miRNA biosensing. The interaction between ultrasonic field and capillary provides a gradient ultrasound field, which is essential for the aggregation of functionalized microspheres along with the enrichment of specific biomarkers. The results indicated that the enrichment of the biomarkers effectively enhanced the fluorescence intensity, and the limit of detection reaches 7.8✕10^-12 M in 100 nL. Such integrated device can realize ultrasonic enrichment and visual analysis of target samples, and provides a new idea for rapid and highly sensitive detection of ultra-trace biomarkers in clinical diagnosis.

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.

Manipulation of single cells via a Stereo Acoustic Streaming Tunnel (SteAST)

At the single-cell level, cellular parameters, gene expression and cellular function are assayed on an individual but not population-average basis. Essential to observing and analyzing the heterogeneity and behavior of these cells/clusters is the ability to prepare and manipulate individuals. Here, we demonstrate a versatile microsystem, a stereo acoustic streaming tunnel, which is triggered by ultrahigh-frequency bulk acoustic waves and highly confined by a microchannel. We thoroughly analyze the generation and features of stereo acoustic streaming to develop a virtual tunnel for observation, pretreatment and analysis of cells for different single-cell applications. 3D reconstruction, dissociation of clusters, selective trapping/release, in situ analysis and pairing of single cells with barcode gel beads were demonstrated. To further verify the reliability and robustness of this technology in complex biosamples, the separation of circulating tumor cells from undiluted blood based on properties of both physics and immunity was achieved. With the rich selection of handling modes, the platform has the potential to be a full-process microsystem, from pretreatment to analysis, and used in numerous fields, such as in vitro diagnosis, high-throughput single-cell sequencing and drug development.

A simple acoustofluidic device for on-chip fabrication of PLGA nanoparticles

Miniaturization of systems and processes provides numerous benefits in terms of cost, reproducibility, precision, minimized consumption of chemical reagents, and prevention of contamination. The field of microfluidics successfully finds a place in a plethora of applications, including on-chip nanoparticle synthesis. Compared with the bulk approaches, on-chip methods that are enabled by microfluidic devices offer better control of size and uniformity of fabricated nanoparticles. However, these microfluidic devices generally require complex and expensive fabrication facilities that are not readily available in low-resourced laboratories. Here, a low-cost and simple acoustic device is demonstrated by generating acoustic streaming flows inside glass capillaries through exciting different flexural modes. At distinct frequencies, the flexural modes of the capillary result in different oscillation profiles that can insert harmonic forcing into the fluid. We explored these flexural modes and identified the modes that can generate strong acoustic streaming vortices along the glass capillary. Then, we applied these modes for fluid mixing using an easy-to-fabricate acoustofluidic device architecture. This device is applied in the fabrication of poly(d,l-lactide-co-glycolide) (PLGA) nanoparticles. The acoustic device consists of a thin glass capillary and two polydimethylsiloxane adaptors that are formed using three-dimensional printed molds. By controlling the flow rates of the polymer and water solutions, PLGA nanoparticles with diameters between 65 and 96 nm are achieved with polydispersity index values ranging between 0.08 and 0.18. Owing to its simple design and minimal fabrication requirements, the proposed acoustofluidic mixer can be applied for microfluidic fluid mixing applications in limited resource settings.

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.

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 medium exchange for preparation of electrocompetent bacteria using channel wall trapping

Comprehensive integration of process steps into a miniaturised version of synthetic biology workflows remains a crucial task in automating the design of biosystems. However, each of these process steps has specific demands with respect to the environmental conditions, including in particular the composition of the surrounding fluid, which makes integration cumbersome. As a case in point, transformation, i.e. reprogramming of bacteria by delivering exogenous genetic material (such as DNA) into the cytoplasm, is a key process in molecular engineering and modern biotechnology in general. Transformation is often performed by electroporation, i.e. creating pores in the membrane using electric shocks in a low conductivity environment. However, cell preparation for electroporation can be cumbersome as it requires the exchange of growth medium (high-conductivity) for low-conductivity medium, typically performed via multiple time-intensive centrifugation steps. To simplify and miniaturise this step, we developed an acoustofluidic device capable of trapping the bacterium Escherichia coli non-invasively for subsequent exchange of medium, which is challenging in acoustofluidic devices due to detrimental acoustic streaming effects. With an improved etching process, we were able to produce a thin wall between two microfluidic channels, which, upon excitation, can generate streaming fields that complement the acoustic radiation force and therefore can be utilised for trapping of bacteria. Our novel design robustly traps Escherichia coli at a flow rate of 10 μL min^-1 and has a cell recovery performance of 47 ± 3% after washing the trapped cells. To verify that the performance of the medium exchange device is sufficient, we tested the electrocompetence of the recovered cells in a standard transformation procedure and found a transformation efficiency of 8 × 10⁵ CFU per μg of plasmid DNA. Our device is a low-volume alternative to centrifugation-based methods and opens the door for miniaturisation of a plethora of microbiological and molecular engineering protocols.

Acoustic Standing Wave Aided Multiparametric Photoacoustic Imaging Flow Cytometry

Photoacoustic imaging reveals great potential for the study of individual cells due to the rich imaging contrast for both label-free and labeled cells. However, previously reported photoacoustic imaging flow cytometry configuration suffers from inadequate imaging quality and challenge to distinguish multiple cells. In order to solve such issues, we propose a novel acoustic standing wave aided multiparametric photoacoustic imaging flow cytometry (MPAFC) system. The acoustic standing wave is introduced to improve the imaging quality and speed. Multispectral illumination along with cell geometry, photoacoustic amplitude, and acoustic frequency spectrum enables the proposed system to precisely identify multiple types of cells with one scanning. On the basis of the identification, elimination of melanoma cells, and targeted labeled glioma cells have been performed with an elimination efficiency of >95%. Additionally, the MPAFC system is able to image and capture melanoma cells at a lowest concentration of 100 cells mL^-1 in pure blood. Current results suggest that the proposed MPAFC may provide a precise and efficient tool for cell detection, manipulation, and elimination in both fundamental and clinical studies.

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.

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.

Numerical study of the coupling layer between transducer and chip in acoustofluidic devices

By numerical simulation in two and three dimensions, the coupling layer between the transducer and microfluidic chip in ultrasound acoustofluidic devices is studied. The model includes the transducer with electrodes, microfluidic chip with a liquid-filled microchannel, and coupling layer between the transducer and chip. Two commonly used coupling materials, solid epoxy glue and viscous glycerol, as well as two commonly used device types, glass capillary tubes and silicon-glass chips, are considered. It is studied how acoustic resonances in ideal devices without a coupling layer are either sustained or attenuated as a coupling layer of increasing thickness is inserted. A simple criterion based on the phase of the acoustic wave for whether a given zero-layer resonance is sustained or attenuated by the addition of a coupling layer is established. Finally, by controlling the thickness and the material, it is shown that the coupling layer can be used as a design component for optimal and robust acoustofluidic resonances.

Nanofountain Probe Electroporation Enables Versatile Single-Cell Intracellular Delivery and Investigation of Postpulse Electropore Dynamics

Introducing exogenous molecules into cells with high efficiency and dosage control is a crucial step in basic research as well as clinical applications. Here, the capability of the nanofountain probe electroporation (NFP-E) system to deliver proteins and plasmids in a variety of continuous and primary cell types with appropriate dosage control is reported. It is shown that the NFP-E can achieve fine control over the relative expression of two cotransfected plasmids. Finally, the dynamics of electropore closure after the pulsing ends with the NFP-E is investigated. Localized electroporation has recently been utilized to demonstrate the converse process of delivery (sampling), in which a small volume of the cytosol is retrieved during electroporation without causing cell lysis. Single-cell temporal sampling confers the benefit of monitoring the same cell over time and can provide valuable insights into the mechanisms underlying processes such as stem cell differentiation and disease progression. NFP-E parameters that maximize the membrane resealing time, which is essential for increasing the sampled volume and in meeting the challenge of monitoring low copy number biomarkers, are identified. Its application in CRISPR/Cas9 gene editing, stem cell reprogramming, and single-cell sampling studies is envisioned.

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 Radiation Forces Produced by Sharp-Edge Structures in Microfluidic Systems

We study sharp-edge structures that are used in microfluidic systems for particle and cell manipulation. Experiments show that oscillating sharp edges can attract or repel particles suspended in a microfluidic channel. This effect is caused by acoustic radiation forces induced by sharp edges. We propose an analytical theory that allows one to evaluate the acoustic radiation force produced by a sharp-edge structure on elastic particles and to study which parameters govern the interaction of particles with a sharp-edge structure, forcing them to be attracted in one situation and to be repelled in another situation. The proposed theory gives foundations for the design of microfluidic systems making use of sharp edges for particle trapping. We also provide experimental data to validate the theory.

Theory of acoustic trapping of microparticles in capillary tubes

We present a semianalytical theory for the acoustic fields and particle-trapping forces in a viscous fluid inside a capillary tube with arbitrary cross section and ultrasound actuation at the walls. We find that the acoustic fields vary axially on a length scale proportional to the square root of the quality factor of the two-dimensional (2D) cross-section resonance mode. This axial variation is determined analytically based on the numerical solution to the eigenvalue problem in the 2D cross section. The analysis is developed in two steps: First, we generalize a recently published expression for the 2D standing-wave resonance modes in a rectangular cross section to arbitrary shapes, including the viscous boundary layer. Second, based on these 2D modes, we derive analytical expressions in three dimensions for the acoustic pressure, the acoustic radiation and trapping force, as well as the acoustic energy flux density. We validate the theory by comparison to three-dimensional numerical simulations.

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.

The acoustofluidic focusing and separation of rare tumor cells using transparent lithium niobate transducers

Research on circulating tumor cells (CTCs) is of great significance in cancer diagnosis, prognosis and even the development of personalized therapy. Here, we present a simple and transparent acoustofluidic device for CTC separation in a label-free and non-invasive manner, instead of traditional acoustic devices based on silicon substrates, which are not only expensive, but also not conducive to optical visualization. The device is based on cheaper glass fabrication and integrated with a 36°Y-rotated cut lithium niobate single crystal (36° LNO) coated indium tin oxide (ITO) acoustic actuator instead of piezoceramics. It could greatly reduce the generation of heat when the signal is excited by utilizing the thickness vibration mode of the 36° LNO single crystal material because of its super-linear performance. Moreover, pre-aligning the particles in the sample inlet in a two-dimensional (2D) mode served to improve the separation efficiency of the device. It was proved that the separation efficiency of polystyrene particles was 97.1 ± 1.0%. The average separation efficiency of cancer cell lines (MCF7 and HeLa) mixed with white bloods cells was about 91.5 ± 4.5%. Owing to the excellent light transmittance of this acoustofluidic device, it has great potential for application to related optical techniques for cell detection while simultaneously separating cells relying on an acoustic field.

Characterizing the Process Physics of Ultrasound-Assisted Bioprinting

3D bioprinting has been evolving as an important strategy for the fabrication of engineered tissues for clinical, diagnostic, and research applications. A major advantage of bioprinting is the ability to recapitulate the patient-specific tissue macro-architecture using cellular bioinks. The effectiveness of bioprinting can be significantly enhanced by incorporating the ability to preferentially organize cellular constituents within 3D constructs to mimic the intrinsic micro-architectural characteristics of native tissues. Accordingly, this work focuses on a new non-contact and label-free approach called ultrasound-assisted bioprinting (UAB) that utilizes acoustophoresis principle to align cells within bioprinted constructs. We describe the underlying process physics and develop and validate computational models to determine the effects of ultrasound process parameters (excitation mode, excitation time, frequency, voltage amplitude) on the relevant temperature, pressure distribution, and alignment time characteristics. Using knowledge from the computational models, we experimentally investigate the effect of selected process parameters (frequency, voltage amplitude) on the critical quality attributes (cellular strand width, inter-strand spacing, and viability) of MG63 cells in alginate as a model bioink system. Finally, we demonstrate the UAB of bilayered constructs with parallel (0°-0°) and orthogonal (0°-90°) cellular alignment across layers. Results of this work highlight the key interplay between the UAB process design and characteristics of aligned cellular constructs, and represent an important next step in our ability to create biomimetic engineered tissues.

Acoustofluidic stick-and-play micropump built on foil for single-cell trapping

The majority of microfluidic devices nowadays are built on rigid or bulky substrates such as glass slides and polydimethylsiloxane (PDMS) slabs, and heavily rely on external equipment such as syringe pumps. Although a variety of micropumps have been developed in the past, few of them are suitable for flexible microfluidics or lab-on-a-foil systems. In this paper, stick-and-play acoustic micropump is built on thin and flexible plastic film by printing microstructures termed defended oscillating membrane equipped structures (DOMES) using two-photon polymerization. Specifically, this new micropump induces rectified flow upon the actuation of acoustic waves, and the flow patterns agree with simulation results very well. More importantly, the developed micropump has the capabilities to generate adjustable flow rates as high as 420 nL min-1, and does not suffer from problems such as bubble instability, gas dissolution, and undesired bubble-trapping that commonly occur in other forms of acoustic micropumps. Since the micropump works in stick-and-play mode, it is reusable after cleaning thanks to the easy separation of covers and substrates. Lastly, the developed micropump is applied for creating a self-pumped single-cell trapping device. The excellent trapping capability of the integrated device proves its potential for long-term studies of biological behaviors of individual cells for biomedical applications.

An acoustofluidic platform for non-contact trapping of cell-laden hydrogel droplets compatible with optical microscopy

Production of cell-laden hydrogel droplets as miniaturized niches for 3D cell culture provides a new route for cell-based assays. Such production can be enabled by droplet microfluidics and here we present a droplet trapping system based on bulk acoustic waves for handling hydrogel droplets in a continuous flow format. The droplet trapping system consists of a glass capillary equipped with a small piezoelectric transducer. By applying ultrasound (4 MHz), a localized acoustic standing wave field is generated in the capillary, trapping the droplets in a well-defined cluster above the transducer area. The results show that the droplet cluster can be retained at flow rates of up to 76 μl/min, corresponding to an average flow speed of 3.2 mm/s. The system allows for important operations such as continuous perfusion and/or addition of chemical reagents to the encapsulated cells with in situ optical access. This feature is demonstrated by performing on-chip staining of the cell nuclei. The key advantages of this trapping method are that it is label-free and gentle and thus well-suited for biological applications. Moreover, the droplets can easily be released on-demand, which facilitates downstream analysis. It is envisioned that the presented droplet trapping system will be a valuable tool for a wide range of multistep assays as well as long-term monitoring of cells encapsulated in gel-based droplets.

Proteomic profiling of extracellular vesicles reveals additional diagnostic biomarkers for myocardial infarction compared to plasma alone

Extracellular vesicles (EVs) are submicron, membrane-enclosed particles that are released from cells in various pathophysiological states. The molecular cargo of these vesicles is considered to reflect the composition of the cell of origin, and the EV proteome is therefore a potential source of biomarkers for various diseases. Our aim was to determine whether EVs isolated from plasma provide additional diagnostic value or improved pathophysiological understanding compared to plasma alone in the context of myocardial infarction (MI). A panel of proximity extension assays (n = 92) was employed to analyze EV lysates and plasma from patients with MI (n = 60) and healthy controls (n = 22). After adjustment for multiple comparisons, a total of 11 dysregulated proteins were identified in EVs of MI patients compared to the controls (q < 0.01). Three of these proteins: chymotrypsin C (CTRC), proto-oncogene tyrosine-protein kinase SRC (SRC) and C-C motif chemokine ligand 17 (CCL17) were unaltered in the corresponding plasma samples. As biomarkers for MI, rudimentary to no evidence exists for these proteins. In a separate group of patients with varying degrees of coronary artery disease, the decrease in EV-associated (but not plasma-related) SRC levels was confirmed by ELISA. Confirmation of the presence of SRC on EVs of different sizes and cellular origins was performed with ELISA, flow cytometry and nanoparticle tracking analysis. In conclusion, the data revealed that despite a similarity in the EV and plasma proteomes, analysis of isolated EVs does indeed provide additional diagnostic information that cannot be obtained from plasma alone.

Acoustic manipulation of particles in a cylindrical cavity: Theoretical and experimental study on the effects of boundary conditions

Precise manipulation of microparticles in microchannels is a primary technique for numerous lab-on-a-chip bioengineering research and applications, as it determines the chip's functions and analytical results. Acoustic manipulation, using the acoustic radiation force, is a compact, versatile and contactless manipulation technique, which can be easily integrated with other microfluidic components. It is our main purpose to report the effect of boundary condition of a cylindrical microfluidic cavity on the acoustic particles' manipulation. A device consisting of a cylindrical cavity in a silicon wafer with three kinds of top boundary conditions (rigid, soft, and imperfect rigid boundary) has been built. The corresponding distributions of acoustic radiation force are analyzed analytically and numerically. Experiments are performed with 2.5 μm radius polystyrene microspheres in the cavity covered by three reflective layers (340 μm-thick glass, 400 μm-thick PDMS, and 660 μm-thick glass film), respectively, which specify the three different boundary conditions at the top of the cavity. It is demonstrated that the boundary condition of a cavity influences the acoustic radiation force and the stable positions of particles, and this is in agreement with the theoretical predictions. Thus, the effects of boundary conditions need to be considered for precise acoustic manipulation.

Quantification of Cell Death Using an Impedance-Based Microfluidic Device

Dielectric spectroscopy is a nondestructive method to characterize dielectric properties by measuring impedance data over a frequency spectrum. This method has been widely used for various applications such as counting, sizing, and monitoring biological cells and particles. Recently, utilization of this method has been suggested in various stages of the drug discovery process due to low sample consumption and fast analysis time. In this study, we used a previously developed microfluidic system to confine single PC-3 cells in microwells using dielectrophoretic forces and perform the impedance measurements. PC-3 cells are treated with 100 μM Enzalutamide drug, and their impedance response is recorded until the cells are totally dead as predicted with viability tests. Four different approaches are used to analyze the impedance spectrum. Equivalent circuit modeling is used to extract the cell electrical properties as a function of time. Principal component analysis (PCA) is used to quantify cellular response to drug as a function of time. Single frequency measurements are conducted to observe how the cells respond over time. Finally, opacity ratio is defined as an additional quantification method. This device is capable of quantitatively measuring drug effects on biological cells and detecting cell death. The results show that the proposed microfluidic system has the potential to be used in early stages of the drug discovery process.

Spatiotemporal quantification of acoustic cell patterning using Voronoï tessellation

Acoustic patterning using ultrasound standing waves has recently emerged as a potent biotechnology enabling the remote generation of ordered cell systems. This capability has opened up exciting opportunities, for example, in guiding the development of organoid cultures or the organization of complex tissues. The success of these studies is often contingent on the formation of tightly-packed and uniform cell arrays; however, a number of factors can act to disrupt or prevent acoustic patterning. Yet, to the best of our knowledge, there has been no comprehensive assessment of the quality of acoustically-patterned cell populations. In this report we use a mathematical approach, known as Voronoï tessellation, to generate a series of metrics that can be used to measure the effect of cell concentration, pressure amplitude, ultrasound frequency and biomaterial viscosity upon the quality of acoustically-patterned cell systems. Moreover, we extend this approach towards the characterization of spatiotemporal processes, namely, the acoustic patterning of cell suspensions and the migration of patterned, adherent cell clusters. This strategy is simple, unbiased and highly informative, and we anticipate that the methods described here will provide a systematic framework for all stages of acoustic patterning, including the robust quality control of devices, statistical comparison of patterning conditions, the quantitative exploration of parameter limits and the ability to track patterned tissue formation over time.

Acoustofluidic particle steering

Steering micro-objects using acoustic radiation forces is challenging for several reasons: resonators tend to create fixed force distributions that depend primarily on device geometry, and even when using switching schemes, the forces are hard to predict a priori. In this paper an active approach is developed that measures forces from a range of acoustic resonances during manipulation using a computer controlled feedback loop based in matlab, with a microscope camera for particle imaging. The arrangement uses a planar resonator where the axial radiation force is used to hold particles within a levitation plane. Manipulation is achieved by summing the levitation frequency with an algorithmically chosen second resonance frequency, which creates lateral forces derived from gradients in the kinetic energy density of the acoustic field. Apart from identifying likely resonances, the system does not require a priori knowledge of the structure of the acoustic force field created by each resonance. Manipulation of 10 μm microbeads is demonstrated over 100 s μm. Manipulation times are of order 10 s for paths of 200 μm length. The microfluidic device used in this work is a rectangular glass capillary with a 6 mm wide and 300 μm high fluid chamber.

Developments in label-free microfluidic methods for single-cell analysis and sorting

Advancements in microfluidic technologies have led to the development of many new tools for both the characterization and sorting of single cells without the need for exogenous labels. Label-free microfluidics reduce the preparation time, reagents needed, and cost of conventional methods based on fluorescent or magnetic labels. Furthermore, these devices enable analysis of cell properties such as mechanical phenotype and dielectric parameters that cannot be characterized with traditional labels. Some of the most promising technologies for current and future development toward label-free, single-cell analysis and sorting include electronic sensors such as Coulter counters and electrical impedance cytometry; deformation analysis using optical traps and deformation cytometry; hydrodynamic sorting such as deterministic lateral displacement, inertial focusing, and microvortex trapping; and acoustic sorting using traveling or standing surface acoustic waves. These label-free microfluidic methods have been used to screen, sort, and analyze cells for a wide range of biomedical and clinical applications, including cell cycle monitoring, rapid complete blood counts, cancer diagnosis, metastatic progression monitoring, HIV and parasite detection, circulating tumor cell isolation, and point-of-care diagnostics. Because of the versatility of label-free methods for characterization and sorting, the low-cost nature of microfluidics, and the rapid prototyping capabilities of modern microfabrication, we expect this class of technology to continue to be an area of high research interest going forward. New developments in this field will contribute to the ongoing paradigm shift in cell analysis and sorting technologies toward label-free microfluidic devices, enabling new capabilities in biomedical research tools as well as clinical diagnostics. This article is categorized under: Diagnostic Tools > Biosensing Diagnostic Tools > Diagnostic Nanodevices.

Theory of pressure acoustics with viscous boundary layers and streaming in curved elastic cavities

The acoustic fields and streaming in a confined fluid depend strongly on the viscous boundary layer forming near the wall. The width of this layer is typically much smaller than the bulk length scale set by the geometry or the acoustic wavelength, which makes direct numerical simulations challenging. Based on this separation in length scales, the classical theory of pressure acoustics is extended by deriving a boundary condition for the acoustic pressure that takes viscous boundary-layer effects fully into account. Using the same length-scale separation for the steady second-order streaming, and combining it with time-averaged short-range products of first-order fields, the usual limiting-velocity theory is replaced with an analytical slip-velocity condition on the long-range streaming field at the wall. The derived boundary conditions are valid for oscillating cavities of arbitrary shape and wall motion, as long as both the wall curvature and displacement amplitude are sufficiently small. Finally, the theory is validated by comparison with direct numerical simulation in two examples of two-dimensional water-filled cavities: The well-studied rectangular cavity with prescribed wall actuation, and a more generic elliptical cavity embedded in an externally actuated rectangular elastic glass block.

Spectral Optical Readout of Rectangular-Miniature Hollow Glass Tubing for Refractive Index Sensing

For answering the growing demand of innovative micro-fluidic devices able to measure the refractive index of samples in extremely low volumes, this paper presents an overview of the performances of a micro-opto-fluidic sensing platform that employs rectangular, miniature hollow glass tubings. The operating principle is described by showing the analytical model of the tubing, obtained as superposition of different optical cavities, and the optical readout method based on spectral reflectivity detection. We have analyzed, in particular, the theoretical and experimental optical features of rectangular tubings with asymmetrical geometry, thus with channel depth larger than the thickness of the glass walls, though all of them in the range of a few tens of micrometers. The origins of the complex line-shape of the spectral response in reflection, due to the different cavities formed by the tubing flat walls and channel, have been investigated using a Fourier transform analysis. The implemented instrumental configuration, based on standard telecom fiberoptic components and a semiconductor broadband optical source emitting in the near infrared wavelength region centered at 1.55 µm, has allowed acquisition of reflectivity spectra for experimental verification of the expected theoretical behavior. We have achieved detection of refractive index variations related to the change of concentration of glucose-water solutions flowing through the tubing by monitoring the spectral shift of the optical resonances.

Tapered Microfluidic for Continuous Micro-Object Separation Based on Hydrodynamic Principle

Recent advances in microfluidic technologies have created a demand for a simple and efficient separation intended for various applications such as food industries, biological preparation, and medical diagnostic. In this paper, we report a tapered microfluidic device for passive continuous separation of microparticles by using hydrodynamic separation. By exploiting the hydrodynamic properties of the fluid flow and physical characteristics of micro particles, effective size based separation is demonstrated. The tapered microfluidic device has widening geometries with respect to specific taper angle which amplify the sedimentation effect experienced by particles of different sizes. A mixture of 3-μm and 10-μm polystyrene microbeads are successfully separated using 20° and 25° taper angles. The results obtained are in agreement with three-dimensional finite element simulation conducted using Abaqus 6.12. Moreover, the feasibility of this mechanism for biological separation is demonstrated by using polydisperse samples consists of 3-μm polystyrene microbeads and human epithelial cervical carcinoma (HeLa) cells. 98% of samples purity is recovered at outlet 1 and outlet 3 with flow rate of 0.5-3.0 μl/min. Our device is interesting despite adopting passive separation approach. This method enables straightforward, label-free, and continuous separation of multiparticles in a stand-alone device without the need for bulky apparatus. Therefore, this device may become an enabling technology for point of care diagnosis tools and may hold potential for micrototal analysis system applications.

Flow-through micro-capillary refractive index sensor based on T/R spectral shift monitoring

We present a flow-through refractive index sensor for measuring the concentration of glucose solutions based on the application of rectangular glass micro-capillaries, with channel depth of 50 µm and 30 µm. A custom designed and 3D printed polymeric shell protects the tiny capillaries, ensuring an easier handling and interconnection with the macro-fluidic path. By illuminating the capillary with broadband radiation centered at λ~1.55 µm, both the transmitted (T) and reflected (R) optical spectrum from the capillary are detected with an optical spectrum analyzer, exploiting an all-fiber setup. Monitoring the spectral shift of the ratio T/R in response to increasing concentration of glucose solutions in water we have obtained sensitivities up to 530.9 nm/RIU and limit of detection in the range of 10^-5-10^-4 RIU. Experimental results are in agreement with the theoretically predicted principle of operation. After the demonstration of amplitude detection at a single wavelength, we finally discuss the impact of the capillary parameters on the sensitivity.

Comparative Proteomic Analysis of Extracellular Vesicles Isolated by Acoustic Trapping or Differential Centrifugation

Extracellular vesicles (ECVs), including microparticles and exosomes, are submicrometer membrane vesicles released by diverse cell types upon activation or stress. Circulating ECVs are potential reservoirs of disease biomarkers, and the complexity of these vesicles is significantly lower compared to their source, blood plasma, which makes ECV-based biomarker studies more promising. Proteomic profiling of ECVs is important not only to discover new diagnostic or prognostic markers but also to understand their roles in biological function. In the current study, we investigated the protein composition of plasma-derived ECVs isolated by acoustic seed trapping. Additionally, the protein composition of ECVs isolated with acoustic trapping was compared to that isolated with a conventional differential centrifugation protocol. Finally, the proteome of ECVs originating from ST-elevation myocardial infarction patients was compared with that of healthy controls using label-free LC-MS quantification. The acoustic trapping platform allows rapid and automated preparation of ECVs from small sample volumes, which are therefore well-suited for biobank repositories. We found that the protein composition of trapped ECVs is very similar to that isolated by the conventional differential centrifugation method.

On-chip droplet production regimes using surface acoustic waves

Aqueous droplets suspended in an immiscible carrier fluid are a key tool in microfluidic chemical analysis platforms. The approaches for producing droplets in microfluidic devices can be divided into three general categories: batch emulsification, continuous production and tailored on-demand production. The major distinctions between each category are the rate of production and the degree of control over the droplet formation process in terms of the size and quantity. On-demand methods are highly desirable when, for example, small numbers or even single droplets of one sample type are required at a time. Here, we present a method for the on-demand production of femtolitre droplets, utilising a pressure source generated by high frequency surface acoustic waves (SAW). An increase in the continuous phase flow rate is enabled by a quasi-3D feature at the droplet production nozzle. A wide range of accessible flow rates permits the identification of different physical regimes in which droplets of different dimensions are produced. In the system investigated droplets measuring as little as 200 fl have been produced, ∼1/60th of the minimum volume previously reported. The experimental findings are supported by a numerical model which demonstrates the link between the number of droplets formed and the pulse length used.

Highly focused high-frequency travelling surface acoustic waves (SAW) for rapid single-particle sorting

High-speed sorting is an essential process in a number of clinical and research applications, where single cells, droplets and particles are segregated based on their properties in a continuous flow. With recent developments in the field of microscale actuation, there is increasing interest in replicating the functions available to conventional fluorescence activated cell sorting (FACS) flow cytometry in integrated on-chip systems, which have substantial advantages in cost and portability. Surface acoustic wave (SAW) devices are ideal for many acoustofluidic applications, and have been used to perform such sorting at rates on the order of kHz. Essential to the accuracy of this sorting, however, is the dimensions of the region over which sorting occurs, where a smaller sorting region can largely avoid inaccurate sorting across a range of sample concentrations. Here we demonstrate the use of flow focusing and a highly focused SAW generated by a high-frequency (386 MHz), 10 μm wavelength set of focused interdigital transducers (FIDTs) on a piezoelectric lithium niobate substrate, yielding an effective sorting region only ~25 μm wide, with sub-millisecond pulses generated at up to kHz rates. Furthermore, because of the use of high frequencies, actuation of particles as small as 2 μm can be realized. Such devices represent a substantial step forward in the evolution of highly localized forces for lab-on-a-chip microfluidic applications.

Reusable acoustic tweezers for disposable devices

We demonstrate acoustic tweezers used for disposable devices. Rather than forming an acoustic resonance, we locally transmitted standing surface acoustic waves into a removable, independent polydimethylsiloxane (PDMS)-glass hybridized microfluidic superstrate device for micromanipulation. By configuring and regulating the displacement nodes on a piezoelectric substrate, cells and particles were effectively patterned and transported into said superstrate, accordingly. With the label-free and contactless nature of acoustic waves, the presented technology could offer a simple, accurate, low-cost, biocompatible, and disposable method for applications in the fields of point-of-care diagnostics and fundamental biomedical studies.

Two-dimensional single-cell patterning with one cell per well driven by surface acoustic waves

In single-cell analysis, cellular activity and parameters are assayed on an individual, rather than population-average basis. Essential to observing the activity of these cells over time is the ability to trap, pattern and retain them, for which previous single-cell-patterning work has principally made use of mechanical methods. While successful as a long-term cell-patterning strategy, these devices remain essentially single use. Here we introduce a new method for the patterning of multiple spatially separated single particles and cells using high-frequency acoustic fields with one cell per acoustic well. We characterize and demonstrate patterning for both a range of particle sizes and the capture and patterning of cells, including human lymphocytes and red blood cells infected by the malarial parasite Plasmodium falciparum. This ability is made possible by a hitherto unexplored regime where the acoustic wavelength is on the same order as the cell dimensions.

Non-contact acoustic capture of microparticles from small plasma volumes

Microparticles (MP) are small (100-1000 nm) membrane vesicles shed by cells as a response to activation, stress or apoptosis. Platelet-derived MP (PMP) has been shown to reflect the pathophysiological processes of a range of cardiovascular diseases and there is a potential clinical value in using PMPs as biomarkers, as well as a need to better understand the biology of these vesicles. The current method for isolating MP depends on differential centrifugation steps, which require relatively large sample volumes and have been shown to compromise the integrity and composition of the MP population. We present a novel method for rapid, non-contact capture of PMP in minute sample volumes based on a microscale acoustic standing wave technology. Capture of PMPs from plasma is shown by scanning electron microscopy and flow cytometry. Furthermore, the system is characterized with regards to plasma sample concentration and flow rate. Finally, the technique is compared to a standard differential centrifugation protocol using samples from both healthy controls and ST-elevation myocardial infarction (STEMI) patient samples. The acoustic system is shown to offer a quick and automated setup for extracting microparticles from small sample volumes with higher recovery than a standard differential centrifugation protocol.

Advantages and Challenges of Relaxor-PbTiO3 Ferroelectric Crystals for Electroacoustic Transducers- A Review

Relaxor-PbTiO3 (PT) based ferroelectric crystals with the perovskite structure have been investigated over the last few decades due to their ultrahigh piezoelectric coefficients (d33 > 1500 pC/N) and electromechanical coupling factors (k33 > 90%), far outperforming state-of-the-art ferroelectric polycrystalline Pb(Zr,Ti)O3 ceramics, and are at the forefront of advanced electroacoustic applications. In this review, the performance merits of relaxor-PT crystals in various electroacoustic devices are presented from a piezoelectric material viewpoint. Opportunities come from not only the ultrahigh properties, specifically coupling and piezoelectric coefficients, but through novel vibration modes and crystallographic/domain engineering. Figure of merits (FOMs) of crystals with various compositions and phases were established for various applications, including medical ultrasonic transducers, underwater transducers, acoustic sensors and tweezers. For each device application, recent developments in relaxor-PT ferroelectric crystals were surveyed and compared with state-of-the-art polycrystalline piezoelectrics, with an emphasis on their strong anisotropic features and crystallographic uniqueness, including engineered domain - property relationships. This review starts with an introduction on electroacoustic transducers and the history of piezoelectric materials. The development of the high performance relaxor-PT single crystals, with a focus on their uniqueness in transducer applications, is then discussed. In the third part, various FOMs of piezoelectric materials for a wide range of ultrasound applications, including diagnostic ultrasound, therapeutic ultrasound, underwater acoustic and passive sensors, tactile sensors and acoustic tweezers, are evaluated to provide a thorough understanding of the materials' behavior under operational conditions. Structure-property-performance relationships are then established. Finally, the impacts and challenges of relaxor-PT crystals are summarized to guide on-going and future research in the development of relaxor-PT crystals for the next generation electroacoustic transducers.