Mechanical Properties Based Particle Separation via Traveling Surface Acoustic Wave

The impact of an ultrasonic standing wave on the sorption behavior of proteins: Investigation of the role of acoustically induced non-spherical bubble oscillations

HYPOTHESIS

Protein molecules adsorb on the air/liquid interface due to possessing a hydrophobic side. A full surface coverage is important in many processes such as in protein harvesting by foam fractionation. The adsorption of proteins in low concentration solutions is preceded by a relatively long time lag known as the induction period. This has been attributed to the formation of an adsorbed monolayer, which relies on the reorientation of the protein molecules. The reduction of the induction period can significantly facilitate the sorption process to reach full protein coverage. For this purpose acoustically induced non-spherical bubble oscillations can aid in the formation of the monolayer and enhance the sorption process.

EXPERIMENT

In this study, low frequency ultrasound was used to induce non-spherical oscillations on an air bubble attached to a capillary. Profile analysis tensiometry was deployed to examine the effect of these non-spherical oscillations on the sorption dynamics of different proteins.

FINDINGS

We observed that during the initial stages of adsorption, when the bubble surface is almost empty, non-spherical oscillations occur, which were found to significantly expedite the adsorption process. However, during later stages of the adsorption process, despite the continued presence of several sonication phenomena such as the primary radiation force and acoustic streaming, no change in adsorption behavior of the proteins could be noted. The occurrence, duration, and intensity of the non-spherical bubble oscillations appeared to be the sole contributing factors for the change of the sorption dynamics of proteins.

Evaluation of Acoustophoretic and Dielectrophoretic Forces for Droplet Injection in Droplet-Based Microfluidic Devices

Acoustophoretic forces have been successfully implemented into droplet-based microfluidic devices to manipulate droplets. These acoustophoretic forces in droplet microfluidic devices are typically generated as in acoustofluidic devices through transducer actuation of a piezoelectric substrate such as lithium niobate (LiNbO3), which is inherently accompanied by the emergence of electrical fields. Understanding acoustophoretic versus dielectrophoretic forces produced by electrodes and transducers within active microfluidic devices is important for the optimization of device performance during design iterations. In this case study, we design microfluidic devices with a droplet injection module and report an experimental strategy to deduce the respective contribution of the acoustophoretic versus dielectrophoretic forces for the observed droplet injection. Our PDMS-based devices comprise a standard oil-in-water droplet-generating module connected to a T-junction injection module featuring actuating electrodes. We use two different electrode geometries produced within the same PDMS slab as the droplet production/injection channels by filling low-melting-point metal alloy into channels that template the electrode geometries. When these electrodes are constructed on LiNbO3 as the substrate, they have a dual function as a piezoelectric transducer, which we call embedded liquid metal interdigitated transducers (elmIDTs). To decipher the contribution of acoustophoretic versus dielectrophoretic forces, we build the same devices on either piezoelectric LiNbO3 or nonpiezo active glass substrates with different combinations of physical device characteristics (i.e., elmIDT geometry and alignment) and operate in a range of phase spaces (i.e., frequency, voltage, and transducer polarity). We characterize devices using techniques such as laser Doppler vibrometry (LDV) and infrared imaging, along with evaluating droplet injection for our series of device designs, constructions, and operating parameters. Although we find that LiNbO3 device designs generate acoustic fields, we demonstrate that droplet injection occurs only due to dielectrophoretic forces. We deduce that droplet injection is caused by the coupled dielectrophoretic forces arising from the operation of elmIDTs rather than by acoustophoretic forces for this specific device design. We arrive at this conclusion because equivalent droplet injection occurs without the presence of an acoustic field using the same electrode designs on nonpiezo active glass substrate devices. This work establishes a methodology to pinpoint the major contributing force of droplet manipulation in droplet-based acoustomicrofluidics.

Acoustofluidic Actuation of Living Cells

Acoutofluidics is an increasingly developing and maturing technical discipline. With the advantages of being label-free, non-contact, bio-friendly, high-resolution, and remote-controllable, it is very suitable for the operation of living cells. After decades of fundamental laboratory research, its technical principles have become increasingly clear, and its manufacturing technology has gradually become popularized. Presently, various imaginative applications continue to emerge and are constantly being improved. Here, we introduce the development of acoustofluidic actuation technology from the perspective of related manipulation applications on living cells. Among them, we focus on the main development directions such as acoustofluidic sorting, acoustofluidic tissue engineering, acoustofluidic microscopy, and acoustofluidic biophysical therapy. This review aims to provide a concise summary of the current state of research and bridge past developments with future directions, offering researchers a comprehensive overview and sparking innovation in the field.

Bidimensional Dynamic Magnetic Levitation: Sequential Separation of Microplastics by Density and Size

There is a current gap in sample preparation techniques integrating the separation of microplastics according to their different material types and particle sizes. We describe herein the Bidimensional Dynamic Magnetic Levitation (2D-MagLev) technique, enabling the resolution of mixtures of microplastics sorting them by plastic type and particle size. Separations are carried out in a bespoke flow cell sandwiched between two ring magnets and connected to programmable pumps for flow control. The first separation dimension is based on sequential increases in the concentration of a paramagnetic salt (MnCl2), enabling magnetic levitation of microplastics with determined densities. The second dimension is based on increasing flow rate gradients and maintaining constant MnCl2 concentrations. This fractionates the magnetically levitating microplastics according to their different particle sizes. Microplastics are therefore collected by their increasing density, and the particles corresponding to each density are fractionated from smaller to larger size. Using polyethylene microspheres with defined density (1.03-1.13 g cm-3) and size (98-390 μm) as microplastic mimicking materials, we investigated their optimum threshold velocities for their size fractionation, potential effects of medium viscosity and sample loading, and types of flow rate gradients (linear, step). Performing a separation using a combination of step gradients in both MnCl2 concentration and flow rate, mixtures comprising microplastics of two different densities and three different particle sizes were separated. 2D-MagLev is simple, fast, versatile, and robust, opening new avenues to facilitate the study of the environmental presence and impact of microplastics.

Ultrasound robotics for precision therapy

In recent years, the application of microrobots in precision therapy has gained significant attention. The small size and maneuverability of these micromachines enable them to potentially access regions that are difficult to reach using traditional methods; thus, reducing off-target toxicities and maximizing treatment effectiveness. Specifically, acoustic actuation has emerged as a promising method to exert control. By harnessing the power of acoustic energy, these small machines potentially navigate the body, assemble at the desired sites, and deliver therapies with enhanced precision and effectiveness. Amidst the enthusiasm surrounding these miniature agents, their translation to clinical environments has proven difficult. The primary objectives of this review are threefold: firstly, to offer an overview of the fundamental acoustic principles employed in the field of microrobots; secondly, to assess their current applications in medical therapies, encompassing tissue targeting, drug delivery or even cell infiltration; and lastly, to delve into the continuous efforts aimed at integrating acoustic microrobots into in vivo applications.

Robust global arrangement by coherent enhancement in Huygens-Fresnel traveling surface acoustic wave interference field

The application of standing surface acoustic wave (SSAW) tweezers based on backpropagation superposition to achieve precise behavior manipulation of microscale cells and even nanoscale bacteria has been widely studied and industrialized. However, the structure requires multiple transducer components or full channel resonance. It is very challenging to design a simple structure for nano-control by complex acoustic field. In this study, a reflector-interdigital transducer (R-IDT) acoustofluidic device based on unilateral coherence enhancement is proposed to achieve SSAW definition features of periodic particle capture positions. The SAW device based on a unilateral transducer can not only generate leaky-SAW in water-filled microchannel, but also have a contribution of spherical waves in the vibration area of the substrate-liquid interface due to the Huygens-Fresnel diffractive principle. Both of them form a robust time-averaged spatial periodicity in the pressure potential gradient, accurately predicting the lateral spacing of these positions through acoustic patterning methods. Furthermore, a reflector based on Bragg-reflection is used to suppress backward transmitted SAW and enhance forward conducted SAW beams. By using a finite element model, R-IDT structure's amplitude enhances 60.78% compared to single IDT structure. The particle manipulation range of the diffractive acoustic field greatly improves, verified by experimental polystyrene microspheres. Besides, biocompatibility is conformed through red blood cells and Bacillus subtilis. We investigate the overall shift of periodic pressure field that can still occur when the phase changes. This work provides a simpler and low-cost solution for the application of acoustic tweezer in biological cell culture and filtering.

Acoustofluidic patterning in glass capillaries using travelling acoustic waves based on thin film flexible platform

Surface acoustic wave (SAW) technology has been widely used to manipulate microparticles and biological species, based on acoustic radiation force (ARF) and drag force induced by acoustic streaming, either by standing SAWs (SSAWs) or travelling SAWs (TSAWs). These acoustofluidic patterning functions can be achieved within a polymer chamber or a glass capillary with various cross-sections positioned along the wave propagating paths. In this paper, we demonstrated that microparticles can be aligned, patterned, and concentrated within both circular and rectangular glass capillaries using TSAWs based on a piezoelectric thin film acoustic wave platform. The glass capillary was placed at different angles along with the interdigital transducer directions. We systematically investigated effects of tilting angles and wave characteristics using numerical simulations in both circular and square shaped capillaries, and the patterning mechanisms were discussed and compared with those agitated under the SSAWs. We then experimentally verified the particle patterns within different glass capillaries using thin film ZnO SAW devices on aluminum (Al) sheets. Results show that the propagating SAWs can generate acoustic pressures and patterns in the fluid due to the diffractive effects, drag forces and ARF, as functions of the SAW device's resonant frequency and tilting angle. We demonstrated potential applications using this multiplexing, integrated, and flexible thin film-based platform, including patterning particles (1) inside multiple and successively positioned circular tubes; (2) inside a solidified hydrogel in the glass capillary; and (3) by wrapping a flexible ZnO/Al SAW device around the glass capillary.

A review of acoustofluidic separation of bioparticles

Acoustofluidics is an emerging interdisciplinary research field that involves the integration of acoustics and microfluidics to address challenges in various scientific areas. This technology has proven to be a powerful tool for separating biological targets from complex fluids due to its label-free, biocompatible, and contact-free nature. Considering a careful designing process and tuning the acoustic field particles can be separated with high yield. Recently the advancement of acoustofluidics led to the development of point-of-care devices for separations of micro particles which address many of the limitations of conventional separation tools. This review article discusses the working principles and different approaches of acoustofluidic separation and provides a synopsis of its traditional and emerging applications, including the theory and mechanism of acoustofluidic separation, blood component separation, cell washing, fluorescence-activated cell sorting, circulating tumor cell isolation, and exosome isolation. The technology offers great potential for solving clinical problems and advancing scientific research.

A surface treatment method for improving the attachment of PDMS: acoustofluidics as a case study

A method for a permanent surface modification of polydimethylsiloxane (PDMS) is presented. A case study on the attachment of PDMS and the lithium niobate (LiNbO3) wafer for acoustofluidics applications is presented as well. The method includes a protocol for chemically treating the surface of PDMS to strengthen its bond with the LiNbO3 surface. The PDMS surface is modified using the 3-(trimethoxysilyl) propyl methacrylate (TMSPMA) silane reagent. The effect of silane treatment on the hydrophilicity, morphology, adhesion strength to LiNbO3, and surface energy of PDMS is investigated. The results demonstrated that the silane treatment permanently increases the hydrophilicity of PDMS and significantly alters its morphology. The bonding strength between PDMS and LiNbO3increased with the duration of the silane treatment, reaching a maximum of approximately 500 kPa. To illustrate the effectiveness of this method, an acoustofluidic device was tested, and the device demonstrated very promising enhanced bonding and sealing capabilities with particle manipulation at a flow rate of up to 1 L/h by means of traveling surface acoustic waves (TSAW). The device was reused multiple times with no fluid leakage or detachment issues. The utility of the presented PDMS surface modification method is not limited to acoustofluidics applications; it has the potential to be further investigated for applications in various scientific fields in the future.

Acoustofluidic lysis of cancer cells and Raman spectrum profiling

The lysis of cancer cells inside a sessile droplet was performed using traveling surface acoustic waves (SAWs) without any chemical reagents. Raman spectrum profiling was then carried out to explore detailed cell-derived data. The Rayleigh waves formed by an interdigital transducer were made to propagate along the surface of an LiNbO3 substrate. Polystyrene microparticles (PSMPs) were used to establish mechanical cell lysis effectively, and gold nanoparticles (AuNPs) were added to enhance the Raman signals from the lysed cells by SAWs. The lysis efficiency was evaluated according to the size and concentration of the PSMPs in experiments where the frequency was varied. Lysis occurred mainly by mechanical collision using PSMPs in a high-frequency domain, and the lysis efficiency was improved by increasing the application time and the energy density of the SAWs. Raman signals from the lysed cells were greatly enhanced by nanogaps formed by the AuNPs, which were evenly distributed irrespective of the SAWs through the frequency-independent behavior of the AuNPs. Finally, detailed Raman spectra of MDA-MB-231, malignant breast cancer cells, were acquired, and various organic matter-derived peaks were observed. The 95% confidence region for cells subjected to lysis was more widely distributed than that of cells not subjected to lysis. The proposed SAW platform is expected to facilitate the detection of small quantities and to be applied in biomedical applications.

Review of Ultrasonic Particle Manipulation Techniques: Applications and Research Advances

Ultrasonic particle manipulation technique is a non-contact label-free method for manipulating micro- and nano-scale particles using ultrasound, which has obvious advantages over traditional optical, magnetic, and electrical micro-manipulation techniques; it has gained extensive attention in micro-nano manipulation in recent years. This paper introduces the basic principles and manipulation methods of ultrasonic particle manipulation techniques, provides a detailed overview of the current mainstream acoustic field generation methods, and also highlights, in particular, the applicable scenarios for different numbers and arrangements of ultrasonic transducer devices. Ultrasonic transducer arrays have been used extensively in various particle manipulation applications, and many sound field reconstruction algorithms based on ultrasonic transducer arrays have been proposed one after another. In this paper, unlike most other previous reviews on ultrasonic particle manipulation, we analyze and summarize the current reconstruction algorithms for generating sound fields based on ultrasonic transducer arrays and compare these algorithms. Finally, we explore the applications of ultrasonic particle manipulation technology in engineering and biological fields and summarize and forecast the research progress of ultrasonic particle manipulation technology. We believe that this review will provide superior guidance for ultrasonic particle manipulation methods based on the study of micro and nano operations.

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.

Selectable encapsulated cell quantity in droplets via label-free electrical screening and impedance-activated sorting

Single-cell encapsulation in droplets has become a powerful tool in immunotherapy, medicine discovery, and single-cell analysis, thanks to its capability for cell confinement in picoliter volumes. However, the purity and throughput of single-cell droplets are limited by random encapsulation process, which resuts in a majority of empty and multi-cells droplets. Herein we introduce the first label-free selectable cell quantity encapsulation in droplets sorting system to overcome this problem. The system utilizes a simple and reliable electrical impedance based screening (98.9% of accuracy) integrated with biocompatible acoustic sorting to select single-cell droplets, achieving 90.3% of efficiency and up to 200 ​Hz of throughput, by removing multi-cells (∼60% of rejection) and empty droplets (∼90% of rejection). We demonstrate the use of the droplet sorting to improve the throughput of single-cell encapsulation by ∼9-fold compared to the conventional random encapsulation process.

Numerical Study of Particle Separation through Integrated Multi-Stage Surface Acoustic Waves and Modulated Driving Signals

The manipulation of biomedical particles, such as separating circulating tumor cells from blood, based on standing surface acoustic wave (SSAW) has been widely used due to its advantages of label-free approaches and good biocompatibility. However, most of the existing SSAW-based separation technologies are dedicated to isolate bioparticles in only two different sizes. It is still challenging to fractionate various particles in more than two different sizes with high efficiency and accuracy. In this work, to tackle the problems of low efficiency for multiple cell particle separation, integrated multi-stage SSAW devices with different wavelengths driven by modulated signals were designed and studied. A three-dimensional microfluidic device model was proposed and analyzed using the finite element method (FEM). In addition, the effect of the slanted angle, acoustic pressure, and the resonant frequency of the SAW device on the particle separation were systemically studied. From the theoretical results, the separation efficiency of three different size particles based on the multi-stage SSAW devices reached 99%, which was significantly improved compared with conventional single-stage SSAW devices.

Optimization Analysis of Particle Separation Parameters for a Standing Surface Acoustic Wave Acoustofluidic Chip

Microparticle separation technology is an important technology in many biomedical and chemical engineering applications from sample detection to disease diagnosis. Although a variety of microparticle separation techniques have been developed thus far, surface acoustic wave (SAW)-based microfluidic separation technology shows great potential because of its high throughput, high precision, and integration with polydimethylsiloxane (PDMS) microchannels. In this work, we demonstrate an acoustofluidic separation chip that includes a piezoelectric device that generates tilted-angle standing SAWs and a permanently bonded PDMS microchannel. We established a mathematical model of particle motion in the microchannel, simulated the particle trajectory through finite element simulation and numerical simulation, and then verified the validity of the model through acoustophoresis experiments. To improve the performance of the separation chip, the influences of particle size, flow rate, and input power on the particle deflection distance were studied. These parameters are closely related to the separation purity and separation efficiency. By optimizing the control parameters, the separation of micron and submicron particles under different throughput conditions was achieved. Moreover, the separation samples were quantitatively analyzed by digital light scattering technology and flow cytometry, and the results showed that the maximum purity of the separated particles was ∼95%, while the maximum efficiency was ∼97%.

Active cell capturing for organ-on-a-chip systems: a review

Organ-on-a-chip (OOC) is an emerging technology that has been proposed as a new powerful cell-based tool to imitate the pathophysiological environment of human organs. For most OOC systems, a pivotal step is to culture cells in microfluidic devices. In active cell capturing techniques, external actuators, such as electrokinetic, magnetic, acoustic, and optical forces, or a combination of these forces, can be applied to trap cells after ejecting cell suspension into the microchannel inlet. This review paper distinguishes the characteristics of biomaterials and evaluates microfluidic technology. Besides, various types of OOC and their fabrication techniques are reported and various active cell capture microstructures are analyzed. Furthermore, their constraints, challenges, and future perspectives are provided.

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.

Direct separation and enumeration of CTCs in viscous blood based on co-flow microchannel with tunable shear rate: a proof-of-principle study

Circulating tumor cells (CTCs), which have extremely low density in whole blood, are an important indicator of primary tumor metastasis. Isolation and enumeration of these cells are critical for clinical applications. Separation of CTCs from massive blood cells without labeling and addition of synthetic polymers is challenging. Herein, a novel well-defined co-flow microfluidic device is presented and used to separate CTCs in viscous blood by applying both inertial and viscoelastic forces. Diluted blood without any synthetic polymer and buffer solution were used as viscoelastic fluid and Newtonian fluid, respectively, and they were co-flowed in the designed chip to form a sheath flow. The co-flow system provides the function of particle pre-focusing and creates a tunable shear rate region at the interface to adjust the migration of particles or cells from the sample solution to the buffer solution. Successful separation of CTCs from viscous blood was demonstrated and enumeration was also conducted by image recognition after separation. The statistical results indicated that a recovery rate of cancer cells greater than 87% was obtained using the developed method, which proved that the direct separation of CTCs from diluted blood can be achieved without the addition of any synthetic polymer to prepare viscoelastic fluid. This method holds great promise for the separation of cells in viscous biological fluid without either complicated channel structures or the addition of synthetic polymers.

Acoustophoresis of a resonant elastic microparticle in a viscous fluid medium

This work presents three-dimensional (3D) numerical analysis of acoustic radiation force on an elastic microsphere suspended in a viscous fluid. Acoustophoresis of finite-sized, neutrally buoyant, nearly incompressible soft particles may improve by orders of magnitude and change directions when going through resonant vibrations. These findings offer the potential to manipulate and separate microparticles based on their resonance frequency. This concept has profound implications in cell and microparticle handling, 3D printing, and enrichment in lab-on-chip applications. The existing analytical body of work can predict spheroidal harmonics of an elastic sphere and acoustic radiation force based on monopole and dipole scatter in an ideal fluid. However, little attention is given to the complex interplay of resonant fluid and solid bodies that generate acoustic radiation. The finite element method is used to find resonant modes, damping factors, and acoustic forces of an elastic sphere subject to a standing acoustic wave. Under fundamental spheroidal modes, the radiation force fluctuates significantly around analytical values due to constructive or destructive scatter-incident wave interference. This suggests that for certain materials, relevant to acoustofluidic applications, particle resonances are an important scattering mechanism and design parameter. The 3D model may be applied to any number of particles regardless of geometry or background acoustic field.

Acoustics-Actuated Microrobots

Microrobots can operate in tiny areas that traditional bulk robots cannot reach. The combination of acoustic actuation with microrobots extensively expands the application areas of microrobots due to their desirable miniaturization, flexibility, and biocompatibility features. Herein, an overview of the research and development of acoustics-actuated microrobots is provided. We first introduce the currently established manufacturing methods (3D printing and photolithography). Then, according to their different working principles, we divide acoustics-actuated microrobots into three categories including bubble propulsion, sharp-edge propulsion, and in-situ microrotor. Next, we summarize their established applications from targeted drug delivery to microfluidics operation to microsurgery. Finally, we illustrate current challenges and future perspectives to guide research in this field. This work not only gives a comprehensive overview of the latest technology of acoustics-actuated microrobots, but also provides an in-depth understanding of acoustic actuation for inspiring the next generation of advanced robotic devices.

Integrated microfluidic single-cell immunoblotting chip enables high-throughput isolation, enrichment and direct protein analysis of circulating tumor cells

Effective capture and analysis of a single circulating tumor cell (CTC) is instrumental for early diagnosis and personalized therapy of tumors. However, due to their extremely low abundance and susceptibility to interference from other cells, high-throughput isolation, enrichment, and single-cell-level functional protein analysis of CTCs within one integrated system remains a major challenge. Herein, we present an integrated multifunctional microfluidic system for highly efficient and label-free CTC isolation, CTC enrichment, and single-cell immunoblotting (ieSCI). The ieSCI-chip is a multilayer microfluidic system that combines an inertia force-based cell sorter with a membrane filter for label-free CTC separation and enrichment and a thin layer of a photoactive polyacrylamide gel with microwell arrays at the bottom of the chamber for single-cell immunoblotting. The ieSCI-chip successfully identified a subgroup of apoptosis-negative (Bax-negative) cells, which traditional bulk analysis did not detect, from cisplatin-treated cells. Furthermore, we demonstrated the clinical application of the ieSCI-chip with blood samples from breast cancer patients for personalized CTC epithelial-to-mesenchymal transition (EMT) analysis. The expression level of a tumor cell marker (EpCAM) can be directly determined in isolated CTCs at the single-cell level, and the therapeutic response to anticancer drugs can be simultaneously monitored. Therefore, the ieSCI-chip provides a promising clinical translational tool for clinical drug response monitoring and personalized regimen development.

An integrated high-throughput microfluidic circulatory fluorescence-activated cell sorting system (μ-CFACS) for the enrichment of rare cells

There is an increasing need for the enrichment of rare cells in the clinical environments of precision medicine, personalized medicine, and regenerative medicine. With the possibility of becoming the next-generation cell sorters, microfluidic fluorescence-activated cell sorting (μ-FACS) devices have been developed to avoid cross-contamination, minimize device footprint, and eliminate bio-aerosols. However, due to highly precise flow control, the achievable throughput of the μ-FACS system is generally lower than the throughput of conventional FACS devices. Here, we report a fully integrated high-throughput microfluidic circulatory fluorescence-activated cell sorting (μ-CFACS) system for the enrichment of clinical rare cells. A microfluidic sorting cartridge has been developed for enriching samples through a sequential sorting process, which was further realized by the integration of both fast amplified piezoelectrically actuated on-chip valves and compact pneumatic cylinders actuated on-chip valves. At an equivalent throughput of ∼8000 events per second (eps), the purity of rare fluorescent microparticles has been significantly increased from ∼0.01% to ∼27.97%. An enrichment of ∼9400-fold from 0.009% to 81.86% has also been demonstrated for isolating fluorescently labelled MCF-7 breast cancer cells from Jurkat cells at an equivalent sorting throughput of ∼6400 eps. With the advantages of high throughput and contamination-free design, the proposed integrated μ-CFACS system provides a new option for the enrichment of clinical rare cells.

Unconventional acoustic approaches for localized and designed micromanipulation

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

Acoustofluidic Separation of Proteins Using Aptamer-Functionalized Microparticles

We propose an acoustofluidic method for the triseparation of proteins conjugated with aptamer-coated microparticles inside a microchannel. Traveling surface acoustic waves (TSAWs) produced from a slanted-finger interdigital transducer (SFIT) are used to separate the protein-loaded microparticles of different sizes via the TSAW-driven acoustic radiation force (ARF). The acoustofluidic device consists of an SFIT deposited onto a piezoelectric lithium niobate substrate and a polydimethylsiloxane (PDMS) microfluidic channel on top of the substrate. The TSAWs propagating on the substrate penetrate into the sample fluid flow, where the human protein-conjugated microparticles are suspended, inside the PDMS microchannel. The microparticles are subjected to the TSAW-driven ARF with varying magnitude depending on their size and thus flow along different streamlines, leading to triseparation of the proteins. In this work, we used two different-sized streptavidin-functionalized polystyrene (PS) microparticles to capture two kinds of aptamers (apt15 and aptD17.4), which were labeled with a respective biotin molecule at one end. The biotin ends of the aptamers were attached to the microparticles through streptavidin-biotin linkage, whereas the free ends of the aptamers were used to capture their target proteins of thrombin (th) and immunoglobulin E (IgE). The resultant PS-apt15-th and PS-aptD17.4-IgE complexes, as well as mCardinal2, were used for experimental demonstration of acoustofluidic triseparation of the human proteins. We achieved simultaneous separation of proteins of three kinds (th, IgE, and mCardinal2) for the first time via the TSAW-driven ARF in the proposed acoustofluidic device.

Microliter ultrafast centrifuge platform for size-based particle and cell separation and extraction using novel omnidirectional spiral surface acoustic waves

Asymmetric surface acoustic waves have been shown useful in separating particles and cells in many microfluidics designs, mostly notably sessile microdroplets. However, no one has successfully extracted target particles or cells for later use from such samples. We present a novel omnidirectional spiral surface acoustic wave (OSSAW) design that exploits a new cut of lithium niobate, 152 Y-rotated, to rapidly rotate a microliter sessile drop to ∼10 g, producing efficient multi-size particle separation. We further extract the separated particles for the first time, demonstrating the ability to target specific particles, for example, platelets from mouse blood for further integrated point-of-care diagnostics. Within ∼5 s of surface acoustic wave actuation, particles with diameter of 5 μm and 1 μm can be separated into two portions with a purity of 83% and 97%, respectively. Red blood cells and platelets within mouse blood are further demonstrated to be separated with a purity of 93% and 84%, respectively. These advancements potentially provide an effective platform for whole blood separation and point-of-care diagnostics without need for micro or nanoscale fluidic enclosures.

Label-free Separation of Circulating Tumor Cells Using a Self-Amplified Inertial Focusing (SAIF) Microfluidic Chip

Circulating tumor cells (CTCs) are rare cells existing in the bloodstream with a relatively low number, which facilitate as a predictor of cancer progress. However, it is difficult to obtain highly purified intact CTCs with desired viability due to the low percentage of CTCs among blood cells. In this work, we demonstrate a novel self-amplified inertial focused (SAIF) microfluidic chip that enables size-based, high-throughput, label-free separation of CTCs from a patient's blood. The SAIF chip introduced in this study demonstrated the feasibility of an extremely narrow zigzag channel (with 40 μm channel width) connected with two expansion regions to effectively separate different-sized cells with amplified separation distance. The chip performance was optimized with different-sized polystyrene (PS) particles and blood cells spiked with three different types of cancer cells. The separation efficiencies for blood cells and spiked cancer cells are higher than 80%. Recovery rates of cancer cells were tested by spiking 1500 lung cancer cells (A549), breast cancer cells (MCF-7), and cervical cancer cells (HeLa) separately to 3 mL 0.09% saline with 3 × 10⁶ white blood cells (WBCs). The recovery rates for larger cells (MCF-7 and HeLa) were 79.1 and 85.4%, respectively. Viabilities of the cells harvested from outlets were all higher than 97% after culturing for 24, 48, and 72 h. The SAIF chip performance was further confirmed using the real clinical patient blood samples from four lung cancer patients. Theoretical force balance analysis in physics, computational simulations, and experimental observations indicate that the SAIF chip is simple but effective, and high-throughput separation CTCs can be readily achieved without complex structures.

Amplified piezoelectrically actuated on-chip flow switching for a rapid and stable microfluidic fluorescence activated cell sorter

With the potential to avoid cross-contamination, eliminate bio-aerosols, and minimize device footprints, microfluidic fluorescence-activated cell sorting (μ-FACS) devices could become the platform for the next generation cell sorter. Here, we report an on-chip flow switching based μ-FACS mechanism with piezoelectric actuation as a fast and robust sorting solution. A microfluidic chip with bifurcate configuration and displacement amplified piezoelectric microvalves has been developed to build the μ-FACS system. Rare fluorescent microparticles of different sizes have been significantly enriched from a purity of ∼0.5% to more than 90%. An enrichment of 150-fold from ∼0.6% to ∼91% has also been confirmed for fluorescently labeled MCF-7 breast cancer cells from Jurkat cells, while viability after sorting was maintained. Taking advantage of its simple structure, low cost, fast response, and reliable flow regulation, the proposed μ-FACS system delivers a new option that can meet the requirements of sorting performance, target selectivity, device lifetime, and cost-effectiveness of implementation.

Recent advances in microfluidic technologies for separation of biological cells

Cell separation has always been a key topic in academic research, especially in the fields of medicine and biology, due to its significance in diagnosis and treatment. Accurate, high-throughput and non-invasive separation of individual cells is key to driving the development of biomedicine and cellular biology. In recent years, a series of researches on the use of microfluidic technologies for cell separation have been conducted to solve bio-related problems. Hence, we present here a comprehensive review on the recent developments of microfluidic technologies for cell separation. In this review, we discuss several cell separation methods, mainly including: physical and biochemical method, their working principles as well as their practical applications. We also analyze the advantages and disadvantages of each method in detail. In addition, the current challenges and future prospects of microfluidic-based cell separation were discussed.

Diffraction-based acoustic manipulation in microchannels enables continuous particle and bacteria focusing

Acoustic fields have shown wide utility for micromanipulation, though their implementation in microfluidic devices often requires accurate alignment or highly precise channel dimensions, including in typical standing surface acoustic wave (SSAW) devices and resonant channels. In this work we investigate an approach that permits continuous microscale focusing based on diffractive acoustics, a phenomenon where a time-averaged spatially varying acoustic pressure landscape is produced by bounding a surface acoustic wave (SAW) transducer with a microchannel. By virtue of diffractive effects, this acoustic field is formed with the application of only a single travelling wave. As the field is dictated by the interplay between a propagating substrate-bound wave and a channel geometry, the pressure distribution will be identical for a given channel orientation regardless of its translation on a SAW substrate, and where small variations in channel size have no substantive effect on the pressure field magnitude or overall particle migration. Moreover, in the case of a channel with dimensions on the order of the diffractive fringe pattern spacing, the number of focusing positions will be identical for all channel orientations, with acoustic radiation forces pushing suspended particles to the channel edges. We explore this highly robust particle manipulation technique, determining two distinct sets of streaming and acoustic radiation dominant concentration positions, and show the continuous focusing of polystyrene 1 μm and 0.5 μm diameter particles and fluorescently labeled E. coli bacteria cells at flow rates exceeding those of previous microfluidic implementations for micron and submicron sized particles.

A two-chip acoustofluidic particle manipulation platform with a detachable and reusable surface acoustic wave device

A two-chip acoustofluidic particle manipulation platform with a detachable and reusable surface acoustic wave device enables manipulation of microparticles in 2D on a replaceable silicon superstrate.

Microfluidic impedance cytometry device with N-shaped electrodes for lateral position measurement of single cells/particles

Tracking the lateral position of single cells and particles plays an important role in evaluating the efficiency of microfluidic cell focusing, separation and sorting. In this work, we present an N-shaped electrode-based microfluidic impedance cytometry device for the measurement of the lateral position of single cells and particles in continuous flows. Specifically, a simple analytical expression for determining the particle lateral position is derived from the measured electrical signal and geometry relationship among the positions of the flowing particles, electrodes and microchannel. This microfluidic system is experimentally validated by measuring the lateral positions of 5, 7 and 10 μm diameter beads and human red blood cells (RBCs) flowing in a 200 μm wide channel at varying flow rates up to 59.3 μl min^-1. Statistical analyses show a good correlation (R² = 0.99) and agreement (Bland-Altman analysis) between our results and those obtained by a microscopy imaging method. The resolution of our system reflected by the root-mean-square deviation (RMSD) is 10.3 μm (5.15% of the channel width) for 5 and 10 μm beads, and 11.4 μm (5.7% of the channel width) for RBCs at a flow rate of 42.4 μl min^-1. Compared to the existing impedance-based methods for measuring the particle lateral position, we achieve the highest resolution, highest flow rate and smallest measured particle size (3.6 μm beads). The experimental results of the mixture with 5 and 10 μm beads demonstrate that our device does not merely measure the lateral position of single particles or cells, but also can characterize their physical properties (e.g., size) simultaneously. Furthermore, we demonstrate the position monitoring of sheath flow-induced particle focusing, which is in quantitative agreement with the results by imaging quantification. With the advantages of rapid and accurate processing of electrical signal and high throughput of the impedance flow cytometry, this novel N-shaped electrode-based system can be easily integrated with other microfluidic platforms as a downstream approach for the real-time measurement of the lateral position and physical properties of single cells and particles.

Selective photothermal ablation of cancer cells by patterned gold nanocages using surface acoustic waves

The patterning of nanoparticles, which are promising photothermal agents, is of great importance to selectively and precisely ablate tissues by thermal effects. In this paper, we demonstrated that nano-sized gold particles (gold nanocages, AuNCS) with a hollow structure could be used to generate various wavefront patterns of surface acoustic waves (SAWs) and the aligned AuNC lines facilitated the destruction of cancer cells by the thermal effect with high spatial resolution. The hollow structure improved the acoustic sensitivity of AuNCs, making them more sensitive to the acoustic radiation force. Moreover, the multi-scale patterning of AuNCs could be achieved by the interference of multiple acoustic beams. Given the photothermal characteristics of AuNCs, selective temperature elevation within a micrometer-sized region could be realized when the patterned AuNCs were irradiated by a laser. The cancer cells where the patterned AuNCs were located were eliminated by thermal ablation, while other cells remained alive. In particular, the acoustic frequency used in this study was as low as 11. 35 MHz and was in the range of diagnostic ultrasound (less than 12 MHz), offering a potential to serve as a powerful tool in clinical applications.

On-chip cell mechanophenotyping using phase modulated surface acoustic wave

A surface acoustic wave (SAW) microfluidic chip was designed to measure the compressibility of cells and to differentiate cell mechanophenotypes. Polystyrene microbeads and poly(methylmethacrylate) (PMMA) microbeads were first tested in order to calibrate and validate the acoustic field. We observed the prefocused microbeads being pushed into the new pressure node upon phase shift. The captured trajectory matched well with the equation describing acoustic radiation force. The compressibility of polystyrene microbeads and that of PMMA microbeads was calculated, respectively, by fitting the trajectory from the experiment and that simulated by the equation across a range of compressibility values. Following, A549 human alveolar basal epithelial cells (A549 cells), human airway smooth muscle (HASM) cells, and MCF-7 breast cancer cells were tested using the same procedure. The compressibility of each cell from the three cell types was measured also by fitting trajectories between the experiment and that from the equation; the size was measured by image analysis. A549 cells were more compressible than HASM and MCF-7 cells; HASM cells could be further distinguished from MCF-7 cells by cell size. In addition, MCF-7 cells were treated by colchicine and 2-methoxyestradiol to disrupt the cell microtubules and were found to be more compressible. Computer simulation was also carried out to investigate the effect of cell compressibility and cell size due to acoustic radiation force to examine the sensitivity of the measurement. The SAW microfluidic method is capable of differentiating cell types or cells under different conditions based on the cell compressibility and the cell size.

A digital acoustofluidic device for on-demand and oil-free droplet generation

We report a digital acoustofluidic device for on-demand and oil-free droplet generation. By applying a programmed radio frequency signal to a circular interdigital transducer, the dynamic focused acoustic pressure profiles generated rise up and dispense sample liquids from a reservoir to dynamically eject the droplets into the air. Our device allows droplets to be dispensed on demand with precisely controlled generation time and sequence, and accurate droplet volume. Moreover, we also demonstrate the generation of a droplet with a volume of 24 pL within 10 ms, as well as the encapsulation of a single cell into droplets. This acoustofluidic droplet generation technique is simple, biocompatible, and enables the on-demand droplet generation and encapsulation of many different biological materials with precise control, which is promising for single cell sampling and analysis applications.

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

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

In-droplet microparticle washing and enrichment using surface acoustic wave-driven acoustic radiation force

Washing and enrichment of particles and cells are crucial sample preparation procedures in biomedical and biochemical assays. On-chip in-droplet microparticle washing and enrichment have been pursued but remained problematic due to technical difficulties, especially simultaneous and precise control over the droplet interface and in-droplet samples. Here, we have achieved a breakthrough in label-free, continuous, on-demand, in-droplet microparticle washing and enrichment using surface acoustic waves. When exposed to the acoustic field, the droplet and suspended particles experience acoustic radiation force arising from inhomogeneous wave scattering at the liquid/liquid and liquid/solid interfaces. Based on these acoustophoretic phenomena, we have demonstrated in-droplet microparticle washing and enrichment in an acoustofluidic device. We expect that the proposed acoustic method will offer new perspectives to sample washing and enrichment by performing the operation in microscale droplets.

Micro/nano acoustofluidics: materials, phenomena, design, devices, and applications

Acoustic actuation of fluids at small scales may finally enable a comprehensive lab-on-a-chip revolution in microfluidics, overcoming long-standing difficulties in fluid and particle manipulation on-chip. In this comprehensive review, we examine the fundamentals of piezoelectricity, piezoelectric materials, and transducers; revisit the basics of acoustofluidics; and give the reader a detailed look at recent technological advances and current scientific discussions in the discipline. Recent achievements are placed in the context of classic reports for the actuation of fluid and particles via acoustic waves, both within sessile drops and closed channels. Other aspects of micro/nano acoustofluidics are examined: atomization, translation, mixing, jetting, and particle manipulation in the context of sessile drops and fluid mixing and pumping, particle manipulation, and formation of droplets in the context of closed channels, plus the most recent results at the nanoscale. These achievements will enable applications across the disciplines of chemistry, biology, medicine, energy, manufacturing, and we suspect a number of others yet unimagined. Basic design concepts and illustrative applications are highlighted in each section, with an emphasis on lab-on-a-chip applications.

High-Throughput Isolation of Circulating Tumor Cells Using Cascaded Inertial Focusing Microfluidic Channel

Circulating tumor cells (CTCs) are rare cells that detach from a primary or metastasis tumor and flow into the bloodstream. Intact and viable tumor cells are needed for genetic characterization of CTCs, new drug development, and other research. Although separation of CTCs using spiral channel with two outlets has been reported, few literature demonstrated simultaneous isolation of different types of CTCs from human blood using cascaded inertial focusing microfluidic channel. Herein, we introduce a cascaded microfluidic device consisting of two spiral channels and one zigzag channel designed with different fluid fields, including lift force, Dean drag force, and centrifugal force. Both red blood cells (RBCs)-lysed human blood spiked with CTCs and 1:50 diluted human whole blood spiked with CTCs were tested on the presented chip. This chip successfully separated RBCs, white blood cells (WBCs), and two different types of tumor cells (human lung cancer cells (A549) and human breast cancer cells (MCF-7)) simultaneously based on their physical properties. A total of 80.75% of A549 and 73.75% of MCF-7 were faithfully separated from human whole blood. Furthermore, CTCs gathered from outlets could propagate and remained intact. The cell viability of A549 and MCF-7 were 95% and 98%, respectively. The entire separating process for CTCs from blood cells could be finished within 20 min. The cascaded microfluidic device introduced in this study serves as a novel platform for simultaneous isolation of multiple types of CTCs from patient blood.

A rapid and meshless analytical model of acoustofluidic pressure fields for waveguide design

Acoustofluidics has a strong pedigree in microscale manipulation, with particle and cell separation and patterning arising from acoustic pressure gradients. Acoustic waveguides are a promising candidate for localizing force fields in microfluidic devices, for which computational modelling is an important design tool. Meshed finite element analysis is a popular approach for this, yet its computation time increases rapidly when complex geometries are used, limiting its usefulness. Here, we present an analytical model of the acoustic pressure field in a microchannel arising from a surface acoustic wave (SAW) boundary condition that computes in milliseconds and provide the simulation code in the supplementary material. Unlike finite element analysis, the computation time of our model is independent of microchannel or waveguide shape, making it ideal for designing and optimising microscale waveguide structures. We provide experimental validation of our model with cases including near-field acoustic patterning of microparticles from a travelling SAW and two-dimensional patterning from a standing SAW and explore the design of waveguides for localised particle or cell capture.

Fluorescence activated cell sorting via a focused traveling surface acoustic beam

Fluorescence activated cell sorting (FACS) has become an essential technique widely exploited in biological studies and clinical applications. However, current FACS systems are quite complex, expensive, bulky, and pose potential sample contamination and biosafety issues due to the generation of aerosols in an open environment. Microfluidic technology capable of precise cell manipulation has great potential to reinvent and miniaturize conventional FACS systems. In this work, we demonstrate a benchtop scale FACS system that makes use of a highly focused traveling surface acoustic wave beam to sort out micron-sized particles and biological cells upon fluorescence interrogation at ∼kHz rates. The highly focused acoustic wave beam has a width of ∼50 μm that enables highly accurate sorting of individual particles and cells. We have applied our acoustic FACS system to isolate fluorescently labeled MCF-7 breast cancer cells from diluted whole blood samples with the purity of sorted MCF-7 cells higher than 86%. The cell viability before and after acoustic sorting is higher than 95%, indicating excellent biocompatibility that should enable a variety of cell sorting applications in biomedical research.

Particle separation by phase modulated surface acoustic waves

High efficiency isolation of cells or particles from a heterogeneous mixture is a critical processing step in lab-on-a-chip devices. Acoustic techniques offer contactless and label-free manipulation, preserve viability of biological cells, and provide versatility as the applied electrical signal can be adapted to various scenarios. Conventional acoustic separation methods use time-of-flight and achieve separation up to distances of quarter wavelength with limited separation power due to slow gradients in the force. The method proposed here allows separation by half of the wavelength and can be extended by repeating the modulation pattern and can ensure maximum force acting on the particles. In this work, we propose an optimised phase modulation scheme for particle separation in a surface acoustic wave microfluidic device. An expression for the acoustic radiation force arising from the interaction between acoustic waves in the fluid was derived. We demonstrated, for the first time, that the expression of the acoustic radiation force differs in surface acoustic wave and bulk devices, due to the presence of a geometric scaling factor. Two phase modulation schemes are investigated theoretically and experimentally. Theoretical findings were experimentally validated for different mixtures of polystyrene particles confirming that the method offers high selectivity. A Monte-Carlo simulation enabled us to assess performance in real situations, including the effects of particle size variation and non-uniform acoustic field on sorting efficiency and purity, validating the ability to separate particles with high purity and high resolution.

Surface acoustic wave devices for chemical sensing and microfluidics: A review and perspective

Surface acoustic waves (SAWs), are electro-mechanical waves that form on the surface of piezoelectric crystals. Because they are easy to construct and operate, SAW devices have proven to be versatile and powerful platforms for either direct chemical sensing or for upstream microfluidic processing and sample preparation. This review summarizes recent advances in the development of SAW devices for chemical sensing and analysis. The use of SAW techniques for chemical detection in both gaseous and liquid media is discussed, as well as recent fabrication advances that are pointing the way for the next generation of SAW sensors. Similarly, applications and progress in using SAW devices as microfluidic platforms are covered, ranging from atomization and mixing to new approaches to lysing and cell adhesion studies. Finally, potential new directions and perspectives on the field as it moves forward are offered, with a specific focus on potential strategies for making SAW technologies for bioanalytical applications.

Selective particle and cell capture in a continuous flow using micro-vortex acoustic streaming

Acoustic streaming has emerged as a promising technique for refined microscale manipulation, where strong rotational flow can give rise to particle and cell capture. In contrast to hydrodynamically generated vortices, acoustic streaming is rapidly tunable, highly scalable and requires no external pressure source. Though streaming is typically ignored or minimized in most acoustofluidic systems that utilize other acoustofluidic effects, we maximize the effect of acoustic streaming in a continuous flow using a high-frequency (381 MHz), narrow-beam focused surface acoustic wave. This results in rapid fluid streaming, with velocities orders of magnitude greater than that of the lateral flow, to generate fluid vortices that extend the entire width of a 400 μm wide microfluidic channel. We characterize the forces relevant for vortex formation in a combined streaming/lateral flow system, and use these acoustic streaming vortices to selectively capture 2 μm from a mixed suspension with 1 μm particles and human breast adenocarcinoma cells (MDA-231) from red blood cells.

Acoustic impedance-based manipulation of elastic microspheres using travelling surface acoustic waves

Size-independent separation of particles is performed using difference in acoustic impedances via travelling surface acoustic waves.