A sharp-edge-based acoustofluidic chemical signal generator

Enhanced acoustic streaming effects via sharp-edged 3D microstructures

Acoustofluidic micromanipulation is an important tool for biomedical research, where acoustic forces offer the ability to manipulate fluids, cells, and particles in a rapid, biocompatible, and contact-free manner. Of particular interest is the investigation of acoustically driven sharp edges, where high tip velocity magnitudes and strong acoustic potential gradients drive rapid motion. Whereas prior devices utilizing 2D sharp edges have demonstrated promise for micromanipulation activities, taking advantage of 3D structures has the potential to increase their performance and the range of manipulation activities. In this work, we investigate high-magnitude acoustic streaming fields in the vicinity of sharp-edged, sub-wavelength 3D microstructures. We numerically model and experimentally demonstrate this in fabricating parametrically configured 3D microstructures whose tip-angle and geometry influence acoustic streaming velocities and the complexity of streaming vortices, finding that the simulated and realized velocities and streaming patterns are both tunable and a function of microstructure shape. These sharp-edge interfaces hold promise for biomedical studies benefiting from precise and targeted micromanipulation.

Cavity-agnostic acoustofluidic manipulations enabled by guided flexural waves on a membrane acoustic waveguide actuator

This article presents an in-depth exploration of the acoustofluidic capabilities of guided flexural waves (GFWs) generated by a membrane acoustic waveguide actuator (MAWA). By harnessing the potential of GFWs, cavity-agnostic advanced particle manipulation functions are achieved, unlocking new avenues for microfluidic systems and lab-on-a-chip development. The localized acoustofluidic effects of GFWs arising from the evanescent nature of the acoustic fields they induce inside a liquid medium are numerically investigated to highlight their unique and promising characteristics. Unlike traditional acoustofluidic technologies, the GFWs propagating on the MAWA's membrane waveguide allow for cavity-agnostic particle manipulation, irrespective of the resonant properties of the fluidic chamber. Moreover, the acoustofluidic functions enabled by the device depend on the flexural mode populating the active region of the membrane waveguide. Experimental demonstrations using two types of particles include in-sessile-droplet particle transport, mixing, and spatial separation based on particle diameter, along with streaming-induced counter-flow virtual channel generation in microfluidic PDMS channels. These experiments emphasize the versatility and potential applications of the MAWA as a microfluidic platform targeted at lab-on-a-chip development and showcase the MAWA's compatibility with existing microfluidic systems.

Acoustophoresis around an elastic scatterer in a standing wave field

Acoustofluidic systems often employ prefabricated acoustic scatterers that perturb the imposed acoustic field to realize the acoustophoresis of immersed microparticles. We present a numerical study to investigate the time-averaged streaming and radiation force fields around a scatterer. Based on the streaming and radiation force field, we obtain the trajectories of the immersed microparticles with varying sizes and identify a critical transition size at which the motion of immersed microparticles in the vicinity of a prefabricated scatterer shifts from being streaming dominated to radiation dominated. We consider a range of acoustic frequencies to reveal that the critical transition size decreases with increasing frequency; this result explains the choice of acoustic frequencies in previously reported experimental studies. We also examine the impact of scatterer material and fluid properties on the streaming and radiation force fields, as well as on the critical transition size. Our results demonstrate that the critical transition size decreases with an increase in acoustic contrast factor: a nondimensional quantity that depends on material properties of the scatterer and the fluid. Our results provide a pathway to realize radiation force based manipulation of small particles by increasing the acoustic contrast factor of the scatterer, lowering the kinematic viscosity of the fluid, and increasing the acoustic frequency.

Manipulation with sound and vibration: A review on the micromanipulation system based on sub-MHz acoustic waves

Manipulation of micro-objects have been playing an essential role in biochemical analysis or clinical diagnostics. Among the diverse technologies for micromanipulation, acoustic methods show the advantages of good biocompatibility, wide tunability, a label-free and contactless manner. Thus, acoustic micromanipulations have been widely exploited in micro-analysis systems. In this article, we reviewed the acoustic micromanipulation systems that were actuated by sub-MHz acoustic waves. In contrast to the high-frequency range, the acoustic microsystems operating at sub-MHz acoustic frequency are more accessible, whose acoustic sources are at low cost and even available from daily acoustic devices (e.g. buzzers, speakers, piezoelectric plates). The broad availability, with the addition of the advantages of acoustic micromanipulation, make sub-MHz microsystems promising for a variety of biomedical applications. Here, we review recent progresses in sub-MHz acoustic micromanipulation technologies, focusing on their applications in biomedical fields. These technologies are based on the basic acoustic phenomenon, such as cavitation, acoustic radiation force, and acoustic streaming. And categorized by their applications, we introduce these systems for mixing, pumping and droplet generation, separation and enrichment, patterning, rotation, propulsion and actuation. The diverse applications of these systems hold great promise for a wide range of enhancements in biomedicines and attract increasing interest for further investigation.

Two-Layered Microfluidic Devices for High-Throughput Dynamic Analysis of Synthetic Gene Circuits in E. coli

Escherichia coli is a common chassis for synthetic gene circuit studies. In addition to the dose-response of synthetic gene circuits, the analysis of dynamic responses is also an important part of the future design of more complicated synthetic systems. Recently, microfluidic-based methods have been widely used for the analysis of gene expression dynamics. Here, we established a two-layered microfluidic platform for the systematic characterization of synthetic gene circuits (eight strains in eight different culture environments could be observed simultaneously with a 5 min time resolution). With this platform, both dose responses and dynamic responses with a high temporal resolution could be easily derived for further analysis. A controlled environment ensures the stability of the bacterial growth rate, excluding changes in gene expression dynamics caused by changes of the growth dilution rate. The precise environmental switch and automatic micrograph shooting ensured that there was nearly no time lag between the inducer addition and the data recording. We studied four four-node incoherent-feedforward-loop (IFFL) networks with different operators using this device. The experimental results showed that as the effect of inhibition increased, two of the IFFL networks generated pulselike dynamic gene expressions in the range of the inducer concentrations, which was different from the dynamics of the two other circuits with only a simple pattern of rising to the platform. Through fitting the dose-response curves and the dynamic response curves, corresponding parameters were derived and introduced to a simple model that could qualitatively explain the generation of pulse dynamics.

Microfluidic Continuous Flow DNA Fragmentation based on a Vibrating Sharp-tip

Fragmentation of DNA into short fragments is of great importance for detecting and studying DNAs. Current microfluidic methods of DNA fragmentation are either inefficient for generating small fragments or rely on microbubbles. Here, we report a DNA fragmentation method in a 3D-printed microfluidic device, which allows efficient continuous flow fragmentation of genomic DNAs without the need for microbubbles. This method is enabled by localized acoustic streaming induced by a single vibrating sharp-tip. Genomic DNAs were fragmented into 700 to 3000 bp fragments with a low power consumption of ~140 mW. The system demonstrated successful fragmentation under a wide range of flow rates from 1 to 50 μL/min without the need for air bubbles. Finally, the utility of the continuous DNA fragmentation method was demonstrated to accelerate the DNA hybridization process for biosensing. Due to the small footprint, continuous flow and bubble-free operation, and high fragmentation efficiency, this method demonstrated great potential for coupling with other functional microfluidic units to achieve an integrated DNA analysis platform.

Activation and Switching of Supramolecular Chemical Signals in Multi-Output Microfluidic Devices

In this study, we report on the developing of a continuous microfluidic reaction device that allows selective activation of polyelectrolyte-surfactant chemical signals in microflows and switches them between multiple outputs. A numerical model was developed for convection-diffusion reaction processes in reactive polymer-colloid microfluidic flows. Matlab scripts and scaling laws were developed for this model to predict reaction initiation and completion conditions in microfluidic devices and the location of the reaction front. The model allows the optimization of microfluidic device geometry and the setting of operation modes that provide release of the reaction product through specific outputs. Representing a chemical signal, polyelectrolyte-surfactant reaction products create various logic gate states at microfluidic chip outputs. Such systems may have potential as biochemical signal transmitters in organ-on-chip applications or chemical logic gates in cascaded microfluidic devices.

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.

A portable droplet generation system for ultra-wide dynamic range digital PCR based on a vibrating sharp-tip capillary

Monodisperse droplet has been widely used as a versatile tool in different disciplines including biosensing. Existing methods still struggle to balance the droplet generation performance with system simplicity. Here we introduce a novel droplet generation scheme based on the acoustic streaming generated from a vibrating sharp-tip capillary. The unique fluid pattern enables efficient droplet generation without any external pressure sources. This method achieved real-time modulation of droplet size over an ultra-wide range (6.77-661 μm), high throughput (up to 5000 droplets/s), and good monodispersity (<4%) with a power consumption below 60 mW. This method has enabled a multi-volume digital PCR with a dynamic range of ~6 orders of magnitude and multiplexing capability. It has also enabled a simple protocol to produce cell-laden alginate microcapsules in variable sizes with excellent biocompatibility. Overall, the present method combines high performance with small footprint and portability, which will be especially valuable for droplet applications requiring variable droplet size and performed in resource-limited settings.

A review on acoustic field-driven micromixers

A review on acoustic field-driven micromixers is given. This is supplemented by the governing equations, governing non-dimensional parameters, numerical simulation approaches, and fabrication techniques. Acoustically induced vibration is a kind of external energy input employed in active micromixers to improve the mixing performance. An air bubble energized by an acoustic field acts as an external energy source and induces friction forces at the interface between an air bubble and liquid, leading to the formation of circulatory flows. The current review (with 200 references) evaluates different characteristics of microfluidic devices working based on acoustic field shaking.

One-step enzyme kinetics measurement in 3D printed microfluidics devices based on a high-performance single vibrating sharp-tip mixer

Measuring enzyme kinetics is of great importance to understand many biological processes and improve biosensing and industrial applications. Conventional methods of measuring enzyme kinetics require to prepare a series of solutions with different substrate concentrations and measure the signal response over time with these solutions, leading to tedious sample preparation steps, high reagents/sample consumption, and difficulties in studying fast enzyme kinetics. Here we report a one-step assay to measure enzyme kinetics using a 3D-printed microfluidic device, which eliminates the steps of preparing and handling multiple solutions thereby simplifying the whole workflow significantly. The assay is enabled by a highly efficient vibrating sharp-tip mixing method that can mix multiple streams of fluids with minimal mixing length (∼300 μm) and time (as low as 3 ms), and a wide range of working flow rates from 1.5 μL/min to 750 μL/min. Owing to the high performance of the mixer, a series of experiments with different substrate concentrations are performed by simply adjusting the flow rates of reagents loaded from three inlets in one experiment run. The Michaelis-Menten kinetics of the horseradish peroxidase (HRP)-catalyzed reaction between H₂O₂ and amplex red is measured in this system. The calculated Michaelis constant is consistent with the values from literature and conventional analysis methods. Due to the simplicity in fabrication and operation, rapid analysis, low power consumption (1.4-45.0 mW), and high temporal resolution, this method will significantly facilitate enzyme kinetics measurement, and offers great potential for optimizing enzyme based biosensing experiments and probing many biochemical processes.

Dosage-controlled intracellular delivery mediated by acoustofluidics for lab on a chip applications

Biological research and many cell-based therapies rely on the successful delivery of cargo materials into cells. Intracellular delivery in an in vitro setting refers to a variety of physical and biochemical techniques developed for conducting rapid and efficient transport of materials across the plasma membrane. Generally, the techniques that are time-efficient (e.g., electroporation) suffer from heterogeneity and low cellular viability, and those that are precise (e.g., microinjection) suffer from low-throughput and are labor-intensive. Here, we present a novel in vitro microfluidic strategy for intracellular delivery, which is based on the acoustic excitation of adherent cells. Strong mechanical oscillations, mediated by Lamb waves, inside a microfluidic channel facilitate the cellular uptake of different size (e.g., 3-500 kDa, plasmid encoding EGFP) cargo materials through endocytic pathways. We demonstrate successful delivery of 500 kDa dextran to various adherent cell lines with unprecedented efficiency in the range of 65-85% above control. We also show that actuation voltage and treatment duration can be tuned to control the dosage of delivered substances. High viability (≥91%), versatility across different cargo materials and various adherent cell lines, scalability to hundreds of thousands of cells per treatment, portability, and ease-of-operation are among the unique features of this acoustofluidic strategy. Potential applications include targeting through endocytosis-dependant pathways in cellular disorders, such as lysosomal storage diseases, which other physical methods are unable to address. This novel acoustofluidic method achieves rapid, uniform, and scalable delivery of material into cells, and may find utility in lab-on-a-chip applications.

Modeling of Endothelial Calcium Responses within a Microfluidic Generator of Spatio-Temporal ATP and Shear Stress Signals

Intracellular calcium dynamics play essential roles in the proper functioning of cellular activities. It is a well known important chemosensing and mechanosensing process regulated by the spatio-temporal microenvironment. Nevertheless, how spatio-temporal biochemical and biomechanical stimuli affect calcium dynamics is not fully understood and the underlying regulation mechanism remains missing. Herein, based on a developed microfluidic generator of biochemical and biomechanical signals, we theoretically analyzed the generation of spatio-temporal ATP and shear stress signals within the microfluidic platform and investigated the effect of spatial combination of ATP and shear stress stimuli on the intracellular calcium dynamics. The simulation results demonstrate the capacity and flexibility of the microfluidic system in generating spatio-temporal ATP and shear stress. Along the transverse direction of the microchannel, dynamic ATP signals of distinct amplitudes coupled with identical shear stress are created, which induce the spatio-temporal diversity in calcium responses. Interestingly, to the multiple combinations of stimuli, the intracellular calcium dynamics reveal two main modes: unimodal and oscillatory modes, showing significant dependence on the features of the spatio-temporal ATP and shear stress stimuli. The present study provides essential information for controlling calcium dynamics by regulating spatio-temporal biochemical and biomechanical stimuli, which shows the potential in directing cellular activities and understanding the occurrence and development of disease.

Unveiling of the mechanisms of acoustic streaming induced by sharp edges

Acoustic waves can generate steady streaming within a fluid owing to the generation of viscous boundary layers near walls of typical thickness δ. In microchannels, the acoustic wavelength λ is adjusted to twice the channel width w to ensure a resonance condition, which implies the use of MHz transducers. Recently, though, intense acoustic streaming was generated by acoustic waves of a few kHz (hence with λ≫w), owing to the presence of sharp-tipped structures of curvature radius at the tip r_{c} smaller than δ. The present study quantitatively investigates this sharp-edge acoustic streaming via the direct resolution of the full Navier-Stokes equation using the finite element method. The influence of δ,r_{c}, and viscosity ν on the acoustic streaming performance is quantified. Our results suggest choices of operating conditions and geometrical parameters, in particular the dimensionless tip radius of curvature r_{c}/δ and the liquid viscosity.

Acoustic Streaming Generated by Sharp Edges: The Coupled Influences of Liquid Viscosity and Acoustic Frequency

Acoustic streaming can be generated around sharp structures, even when the acoustic wavelength is much larger than the vessel size. This sharp-edge streaming can be relatively intense, owing to the strongly focused inertial effect experienced by the acoustic flow near the tip. We conducted experiments with particle image velocimetry to quantify this streaming flow through the influence of liquid viscosity ν , from 1 mm 2 /s to 30 mm 2 /s, and acoustic frequency f from 500 Hz to 3500 Hz. Both quantities supposedly influence the thickness of the viscous boundary layer δ = ν π f 1 / 2 . For all situations, the streaming flow appears as a main central jet from the tip, generating two lateral vortices beside the tip and outside the boundary layer. As a characteristic streaming velocity, the maximal velocity is located at a distance of δ from the tip, and it increases as the square of the acoustic velocity. We then provide empirical scaling laws to quantify the influence of ν and f on the streaming velocity. Globally, the streaming velocity is dramatically weakened by a higher viscosity, whereas the flow pattern and the disturbance distance remain similar regardless of viscosity. Besides viscosity, the frequency also strongly influences the maximal streaming velocity.

A review on microfluidics manipulation of the extracellular chemical microenvironment and its emerging application to cell analysis

Spatiotemporal manipulation of extracellular chemical environments with simultaneous monitoring of cellular responses plays an essential role in exploring fundamental biological processes and expands our understanding of underlying mechanisms. Despite the rapid progress and promising successes in manipulation strategies, many challenges remain due to the small size of cells and the rapid diffusion of chemical molecules. Fortunately, emerging microfluidic technology has become a powerful approach for precisely controlling the extracellular chemical microenvironment, which benefits from its integration capacity, automation, and high-throughput capability, as well as its high resolution down to submicron. Here, we summarize recent advances in microfluidics manipulation of the extracellular chemical microenvironment, including the following aspects: i) Spatial manipulation of chemical microenvironments realized by convection flow-, diffusion-, and droplet-based microfluidics, and surface chemical modification; ii) Temporal manipulation of chemical microenvironments enabled by flow switching/shifting, moving/flowing cells across laminar flows, integrated microvalves/pumps, and droplet manipulation; iii) Spatiotemporal manipulation of chemical microenvironments implemented by a coupling strategy and open-space microfluidics; and iv) High-throughput manipulation of chemical microenvironments. Finally, we briefly present typical applications of the above-mentioned technical advances in cell-based analyses including cell migration, cell signaling, cell differentiation, multicellular analysis, and drug screening. We further discuss the future improvement of microfluidics manipulation of extracellular chemical microenvironments to fulfill the needs of biological and biomedical research and applications.

Acoustofluidics-Assisted Engineering of Multifunctional Three-Dimensional Zinc Oxide Nanoarrays

The integration of acoustics and microfluidics (termed acoustofluidics) presents a frontier in the engineering of functional micro-/nanomaterials. Acoustofluidic techniques enable active and precise spatiotemporal control of matter, providing great potential for the design of advanced nanosystems with tunable material properties. In this work, we introduce an acoustofluidic approach for engineering multifunctional three-dimensional nanostructure arrays and demonstrate their potential in enrichment and biosensing applications. In particular, our acoustofluidic device integrates an acoustic transducer with a sharp-edge-based acoustofluidic reactor that enables uniform patterning of zinc oxide (ZnO) nanoarrays with customizable lengths, densities, diameters, and other properties. The resulting ZnO nanoarray-coated glass capillaries can rapidly and efficiently capture and enrich biomolecules with sizes ranging from a few nanometers to several hundred nanometers. In order to enable the detection of these biomolecules, silver (Ag) nanoparticles are deposited onto the ZnO nanoarrays, and the integrated ZnO-Ag capillary device functions as a label-free plasmonic biosensing system for surface-enhanced Raman spectroscopy (SERS) based detection of exosomes, DNA oligonucleotides, and E. coli bacteria. The optical sensing enhancement of ZnO-Ag capillary is further validated through finite-difference time-domain (FDTD) simulations. These findings not only provide insights into the engineering of functional micro/nanomaterials using acoustofluidics but also shed light onto the development of portable microanalytical devices for point-of-care applications.

A Review of Passive Micromixers with a Comparative Analysis

A wide range of existing passive micromixers are reviewed, and quantitative analyses of ten typical passive micromixers were performed to compare their mixing indices, pressure drops, and mixing costs under the same axial length and flow conditions across a wide Reynolds number range of 0.01–120. The tested micromixers were selected from five types of micromixer designs. The analyses of flow and mixing were performed using continuity, Navier-Stokes and convection-diffusion equations. The results of the comparative analysis were presented for three different Reynolds number ranges: low-Re (Re ≤ 1), intermediate-Re (1 < Re ≤ 40), and high-Re (Re > 40) ranges, where the mixing mechanisms are different. The results show a two-dimensional micromixer of Tesla structure is recommended in the intermediate- and high-Re ranges, while two three-dimensional micromixers with two layers are recommended in the low-Re range due to their excellent mixing performance.

Dancing with the Cells: Acoustic Microflows Generated by Oscillating Cells

The interaction of a sound or ultrasound wave with an elastic object, such as a microbubble, can give rise to a steady-state microstreaming flow in its surrounding liquid. Many microfluidic strategies for cell and particle manipulation, and analyte mixing, are based on this type of flow. In addition, there are reports that acoustic streaming can be generated in biological systems, for instance, in a mammalian inner ear. Here, new observations are reported that individual cells are able to induce microstreaming flow, when they are excited by controlled acoustic waves in vitro. Single adherent cells are exposed to an acoustic field inside a microfluidic device. The cell-induced microstreaming is then investigated by monitoring flow tracers around the cell, while the structure and extracellular environment of the cell are altered using different chemicals. The observations suggest that the maximum streaming flow induced by an MDA-MB-231 breast cancer cell can reach velocities on the order of mm s^-1 , and this maximum velocity is primarily governed by the overall cell stiffness. Therefore, such cell-induced microstreaming measurements, including flow pattern and velocity magnitude, may be used as label-free proxies of cellular mechanical properties, such as stiffness.

3D Printed Multiplexed Competitive Migration Assays with Spatially Programmable Release Sources

Here, a 3D printed multiplexed competitive migration assay is reported for characterizing a chemotactic response in the presence of multiple spatially distributed chemoattractants. The utility of the assay is demonstrated by examining the chemotactic response of human glioblastoma cells to spatially opposing chemotactic gradients of epidermal growth factor (EGF) and bradykinin (BK). Competitive migration assays involving spatially opposing gradients of EGF and BK that are optimized in the absence of the second chemoattractant show that 46% more glioblastoma cells migrate toward EGF sources. The migration velocities of human glioblastoma cells toward EGF and BK sources are reduced by 7.6 ± 2.2% and 11.6 ± 6.3% relative to those found in the absence of the spatially opposing chemoattractant. This work provides new insight to the chemotactic response associated with glioblastoma-vasculature interactions and a versatile, user-friendly platform for characterizing the chemotactic response of cells in the presence of multiple spatially distributed chemoattractants.

Cell lysis via acoustically oscillating sharp edges

In this article, we demonstrate an acoustofluidic device for cell lysis using the acoustic streaming effects induced by acoustically oscillating sharp-edged structures. The acoustic streaming locally generates high shear forces that can mechanically rupture cell membranes. With the acoustic-streaming-derived shear forces, our acoustofluidic device can perform cell lysis in a continuous, reagent-free manner, with a lysis efficiency of more than 90% over a range of sample flow rates. We demonstrate that our acoustofluidic lysis device works well on both adherent and non-adherent cells. We also validate it using clinically relevant samples such as red blood cells infected with malarial parasites. Additionally, the unique capability of our acoustofluidic device was demonstrated by performing downstream protein analysis and gene profiling without additional washing steps post-lysis. Our device is simple to fabricate and operate while consuming a relatively low volume of samples. These advantages and other features including the reagent-free nature and controllable lysis efficiency make our platform valuable for many biological and biomedical applications, particularly for the development of point-of-care platforms.

Acoustofluidic Synthesis of Particulate Nanomaterials

Synthesis of nanoparticles and particulate nanomaterials with tailored properties is a central step toward many applications ranging from energy conversion and imaging/display to biosensing and nanomedicine. While existing microfluidics-based synthesis methods offer precise control over the synthesis process, most of them rely on passive, partial mixing of reagents, which limits their applicability and potentially, adversely alter the properties of synthesized products. Here, an acoustofluidic (i.e., the fusion of acoustic and microfluidics) synthesis platform is reported to synthesize nanoparticles and nanomaterials in a controllable, reproducible manner through acoustic-streaming-based active mixing of reagents. The acoustofluidic strategy allows for the dynamic control of the reaction conditions simply by adjusting the strength of the acoustic streaming. With this platform, the synthesis of versatile nanoparticles/nanomaterials is demonstrated including the synthesis of polymeric nanoparticles, chitosan nanoparticles, organic-inorganic hybrid nanomaterials, metal-organic framework biocomposites, and lipid-DNA complexes. The acoustofluidic synthesis platform, when incorporated with varying flow rates, compositions, or concentrations of reagents, will lend itself unprecedented flexibility in establishing various reaction conditions and thus enable the synthesis of versatile nanoparticles and nanomaterials with prescribed properties.

Acoustofluidic methods in cell analysis

Cellular analysis is a central concept for both biology and medicine. Over the past two decades, acoustofluidic technologies, which marry acoustic waves with microfluidics, have significantly contributed to the development of innovative approaches for cellular analysis. Acoustofluidic technologies enable precise manipulations of cells and the fluids that confine them, and these capabilities have been utilized in many cell analysis applications. In this review article, we examine various applications where acoustofluidic methods have been implemented, including cell imaging, cell mechanotyping, circulating tumor cell phenotyping, sample preparation in clinics, and investigation of cell-cell interactions and cell-environment responses. We also provide our perspectives on the technological advantages, limitations, and potential future directions for this innovative field of methods.

On-chip stool liquefaction via acoustofluidics

Microfluidic-based portable devices for stool analysis are important for detecting established biomarkers for gastrointestinal disorders and understanding the relationship between gut microbiota imbalances and various health conditions, ranging from digestive disorders to neurodegenerative diseases. However, the challenge of processing stool samples in microfluidic devices hinders the development of a standalone platform. Here, we present the first microfluidic chip that can liquefy stool samples via acoustic streaming. With an acoustic transducer actively generating strong micro-vortex streaming, stool samples and buffers in microchannel can be homogenized at a flow rate up to 30 μL min-1. After homogenization, an array of 100 μm wide micropillars can further purify stool samples by filtering out large debris. A favorable biocompatibility was also demonstrated for our acoustofluidic-based stool liquefaction chip by examining bacteria morphology and viability. Moreover, stool samples with different consistencies were liquefied. Our acoustofluidic chip offers a miniaturized, robust, and biocompatible solution for stool sample preparation in a microfluidic environment and can be potentially integrated with stool analysis units for designing portable stool diagnostics platforms.

Ultrasonic Based Tissue Modelling and Engineering

Systems and devices for in vitro tissue modelling and engineering are valuable tools, which combine the strength between the controlled laboratory environment and the complex tissue organization and environment in vivo. Device-based tissue engineering is also a possible avenue for future explant culture in regenerative medicine. The most fundamental requirements on platforms intended for tissue modelling and engineering are their ability to shape and maintain cell aggregates over long-term culture. An emerging technology for tissue shaping and culture is ultrasonic standing wave (USW) particle manipulation, which offers label-free and gentle positioning and aggregation of cells. The pressure nodes defined by the USW, where cells are trapped in most cases, are stable over time and can be both static and dynamic depending on actuation schemes. In this review article, we highlight the potential of USW cell manipulation as a tool for tissue modelling and engineering.