Lamb Wave-Based Acoustic Radiation Force-Driven Particle Ring Formation Inside a Sessile Droplet

Sonoprinting nanoparticles on cellular spheroids via surface acoustic waves for enhanced nanotherapeutics delivery

Nanotherapeutics, on their path to the target tissues, face numerous physicochemical hindrances that affect their therapeutic efficacy. Physical barriers become more pronounced in pathological tissues, such as solid tumors, where they limit the penetration of nanocarriers into deeper regions, thereby preventing the efficient delivery of drug cargo. To address this challenge, we introduce a novel approach that employs surface acoustic wave (SAW) technology to sonoprint and enhance the delivery of nanoparticles onto and into cell spheroids. Our SAW platform is designed to generate focused and unidirectional acoustic waves for creating vigorous acoustic streaming while promoting Bjerknes forces. The effect of SAW excitation on cell viability, as well as the accumulation and penetration of nanoparticles on human breast cancer (MCF 7) and mouse melanoma (YUMM 1.7) cell spheroids were investigated. The high frequency, low input voltage, and contact-free nature of the proposed SAW system ensured over 92% cell viability for both cell lines after SAW exposure. SAW sonoprinting enhanced the accumulation of 100 nm polystyrene particles on the periphery of the spheroids to near four-fold, while the penetration of nanoparticles into the core regions of the spheroids was improved up to three times. To demonstrate the effectiveness of our SAW platform on the efficacy of nanotherapeutics, the platform was used to deliver nanoliposomes encapsulated with the anti-cancer metal compound copper diethyldithiocarbamate (CuET) to MCF 7 and YUMM 1.7 cell spheroids. A three-fold increase in the cytotoxic activity of the drug was observed in spheroids under the effect of SAW, compared to controls. The capacity of SAW-based devices to be manufactured as minuscule wearable patches can offer highly controllable, localized, and continuous acoustic waves to enhance drug delivery efficiency to target tissues.

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

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

Enhanced Detection in Droplet Microfluidics by Acoustic Vortex Modulation of Particle Rings and Particle Clusters via Asymmetric Propagation of Surface Acoustic Waves

As a basis for biometric and chemical analysis, issues of how to dilute or concentrate substances such as particles or cells to specific concentrations have long been of interest to researchers. In this study, travelling surface acoustic wave (TSAW)-based devices with three frequencies (99.1, 48.8, 20.4 MHz) have been used to capture the suspended Polystyrene (PS) microspheres of various sizes (5, 20, 40 μm) in sessile droplets, which are controlled by acoustic field-induced fluid vortex (acoustic vortex) and aggregate into clusters or rings with particles. These phenomena can be explained by the interaction of three forces, which are drag force caused by ASF, ARF caused by Leaky-SAW and varying centrifugal force. Eventually, a novel approach of free transition between the particle ring and cluster was approached via modulating the acoustic amplitude of TSAW. By this method, multilayer particles agglomerate with 20 μm wrapped around 40 μm and 20 μm wrapped around 5 μm can be obtained, which provides the possibility to dilute or concentrate the particles to a specific concentration.

Non-contact ultrasound oocyte denudation

Cumulus removal (CR) is a central prerequisite step for many protocols involved in the assisted reproductive technology (ART) such as intracytoplasmic sperm injection (ICSI) and preimplantation genetic testing (PGT). The most prevalent CR technique is based upon laborious manual pipetting, which suffers from inter-operator variability and therefore a lack of standardization. Automating CR procedures would alleviate many of these challenges, improving the odds of a successful ART or PGT outcome. In this study, a chip-scale ultrasonic device consisting of four interdigitated transducers (IDT) on a lithium niobate substrate has been engineered to deliver megahertz (MHz) range ultrasound to perform denudation. The acoustic streaming and acoustic radiation force agitate COCs inside a microwell placed on top of the LiNbO₃ substrate to remove the cumulus cells from the oocytes. This paper demonstrates the capability and safety of the denudation procedure utilizing surface acoustic wave (SAW), achieving automation of this delicate manual procedure and paving the steps toward improved and standardized oocyte manipulation.

Surface acoustic wave (SAW) techniques in tissue engineering

Recently, the introduction of surface acoustic wave (SAW) technique for microfluidics has drawn a lot of attention. The pattern and mutual communication in cell layers, tissues, and organs play a critical role in tissue homeostasis and regeneration and may contribute to disease occurrence and progression. Tissue engineering aims to repair and regenerate damaged organs, depending on biomimetic scaffolds and advanced fabrication technology. However, traditional bioengineering synthesis approaches are time-consuming, heterogeneous, and unmanageable. It is hard to pattern cells in scaffolds effectively with no impact on cell viability and function. Here, we summarize a biocompatible, easily available, label-free, and non-invasive tool, surface acoustic wave (SAW) technique, which is getting a lot of attention in tissue engineering. SAW technique can realize accurate sorting, manipulation, and cells' pattern and rapid formation of spheroids. By integrating several SAW devices onto lab-on-a-chip platforms, tissue engineering lab-on-a-chip system was proposed. To the best of our knowledge, this is the first report to summarize the application of this novel technique in the field of tissue engineering.

Acoustofluidic localization of sparse particles on a piezoelectric resonant sensor for nanogram-scale mass measurements

The ability to weigh microsubstances present in low concentrations is an important tool for environmental monitoring and chemical analysis. For instance, developing a rapid analysis platform that identifies the material type of microplastics in seawater would help evaluate the potential toxicity to marine organisms. In this study, we demonstrate the integration of two different techniques that bring together the functions of sparse particle localization and miniaturized mass sensing on a microelectromechanical system (MEMS) chip for enhanced detection and minimization of negative measurements. The droplet sample for analysis is loaded onto the MEMS chip containing a resonant mass sensor. Through the coupling of a surface acoustic wave (SAW) from a SAW transducer into the chip, the initially dispersed microparticles in the droplet are localized over the detection area of the MEMS sensor, which is only 200 µm wide. The accreted mass of the particles is then calibrated against the resulting shift in resonant frequency of the sensor. The SAW device and MEMS chip are detachable after use, allowing the reuse of the SAW device part of the setup instead of the disposal of both parts. Our platform maintains the strengths of noncontact and label-free dual-chip acoustofluidic devices, demonstrating for the first time an integrated microparticle manipulation and real-time mass measurement platform useful for the analysis of sparse microsubstances.

On-Chip Generation of Vortical Flows for Microfluidic Centrifugation

Microcentrifugation constitutes an important part of the microfluidic toolkit in a similar way that centrifugation is crucial to many macroscopic procedures, given that micromixing, sample preconcentration, particle separation, component fractionation, and cell agglomeration are essential operations in small scale processes. Yet, the dominance of capillary and viscous effects, which typically tend to retard flow, over inertial and gravitational forces, which are often useful for actuating flows and hence centrifugation, at microscopic scales makes it difficult to generate rotational flows at these dimensions, let alone with sufficient vorticity to support efficient mixing, separation, concentration, or aggregation. Herein, the various technologies-both passive and active-that have been developed to date for vortex generation in microfluidic devices are reviewed. Various advantages or limitations associated with each are outlined, in addition to highlighting the challenges that need to be overcome for their incorporation into integrated microfluidic devices.

Lamb wave-based molecular diagnosis using DNA hydrogel formation by rolling circle amplification (RCA) process

Recent developments in microfluidics enable the lab-on-a-chip-based molecular diagnosis. Rapid and accurate diagnosis of infectious diseases is critical for preventing the transmission of the disease. Here, we characterize a Lamb wave-based device using various parameters including the contact angle and viscosity of the sample droplet, the applied voltage, and the temperature increase. Additionally, we demonstrate the functionality of the Lamb wave-based device in clinical application. Optimal temperature for rolling circle amplification (RCA) process is 30 °C, and it was achieved by Lamb wave generation at 17 V. Gene amplification due to RCA process could be detected by viscosity increase due to DNA hydrogel formation in a sample droplet, which induced the acoustic streaming velocity of suspended particles to be decreased. In our Lamb wave-based device, isothermal amplification of target nucleic acids could be successfully detected within 30 min using 10 μL of sessile droplet, and was validated by comparing that of commercial real-time fluorescence analysis. Our device enables simple and low-cost molecular diagnosis, which can be applied to resource-limited clinical settings.

Miniaturised acoustofluidic tactile haptic actuator

Tactile haptic feedback is an important consideration in the design of advanced human-machine interfaces, particularly in an age of increasing reliance on automation and artificial intelligence. In this work, we show that the typical nanometer-order surface displacement amplitudes of piezoelectric transducers-which are too small to be detectable by the human touch, and constitute a significant constraint in their use for tactile haptic surface actuation-can be circumvented by coupling the vibration into a liquid to drive the deflection of a thermoplastic membrane. In particular, transmission of the sound energy from the standing wave vibration generated along a piezoelectric transducer into a microfluidic chamber atop which the membrane is attached is observed to amplify the mechanical vibration signalling through both the acoustic radiation pressure and the viscous normal stress acting on the membrane-the latter arising due to the acoustic streaming generated as the sound wave propagates through the liquid-to produce 100 μm-order static deflections of the membrane, upon which approximately 0.5 μm dynamic vibrations at frequencies around 1 kHz are superimposed; both these static and dynamic responses are within the perception range for human finger sensation. The large static deformation, the relatively fast response time, and the ability to incorporate a dynamic vibrotactile response together with the small size and potential for integration of the device into large scale arrays make this mechanism well suited for driving actuation in devices which require tactile haptic responses.

Acoustophoretic Control of Microparticle Transport Using Dual-Wavelength Surface Acoustic Wave Devices

We present a numerical and experimental study of acoustophoretic manipulation in a microfluidic channel using dual-wavelength standing surface acoustic waves (SSAWs) to transport microparticles into different outlets. The SSAW fields were excited by interdigital transducers (IDTs) composed of two different pitches connected in parallel and series on a lithium niobate substrate such that it yielded spatially superimposed and separated dual-wavelength SSAWs, respectively. SSAWs of a singltablee target wavelength can be efficiently excited by giving an RF voltage of frequency determined by the ratio of the velocity of the SAW to the target IDT pitch (i.e., f = c_SAW/p). However, the two-pitch IDTs with similar pitches excite, less efficiently, non-target SSAWs with the wavelength associated with the non-target pitch in addition to target SSAWs by giving the target single-frequency RF voltage. As a result, dual-wavelength SSAWs can be formed. Simulated results revealed variations of acoustic pressure fields induced by the dual-wavelength SSAWs and corresponding influences on the particle motion. The acoustic radiation force in the acoustic pressure field was calculated to pinpoint zero-force positions and simulate particle motion trajectories. Then, dual-wavelength SSAW acoustofluidic devices were fabricated in accordance with the simulation results to experimentally demonstrate switching of SSAW fields as a means of transporting particles. The effects of non-target SSAWs on pre-actuating particles were predicted and observed. The study provides the design considerations needed for the fabrication of acoustofluidic devices with IDT-excited multi-wavelength SSAWs for acoustophoresis of microparticles.

Microparticle self-assembly induced by travelling surface acoustic waves

We present an acoustofluidic method based on travelling surface acoustic waves (TSAWs) to induce self-assembly of microparticles inside a microfluidic channel. The particles are trapped above an interdigitated transducer, placed directly beneath the microchannel, by the TSAW-based direct acoustic radiation force (ARF). This approach was applied to trap 10 μm polystyrene particles, which were pushed towards the ceiling of the microchannel by 72 MHz TSAWs to form single- and multiple-layer colloidal structures. The repair of cracks and defects within the crystal lattice occurs as part of the self-assembly process. The sample flow through the first inlet can be switched with a buffer flow through the second inlet to control the number of particles assembled in the crystalline structure. The constant flow-induced Stokes drag force on the particles is balanced by the opposing TSAW-based ARF. This force balance is essential for the acoustics-based self-assembly of microparticles inside the microchannel. Moreover, we studied the effects of varying input voltage and fluid flow rate on the position and shape of the colloidal structure. The active self-assembly of microparticles into crystals with multiple layers can be used in the bottom-up fabrication of colloidal structures with dimensions greater than 500 μm × 500 μm, which is expected to have important applications in various fields.

We present an acoustofluidic method based on travelling surface acoustic waves (TSAWs) for the self-assembly of microparticles inside a microfluidic channel.

The size dependant behaviour of particles driven by a travelling surface acoustic wave (TSAW)

The use of travelling surface acoustic waves (TSAW) in a microfluidic system provides a powerful tool for the manipulation of particles and cells. In a TSAW driven system, acoustophoretic effects can cause suspended micro-objects to display three distinct responses: (1) swirling, driven by acoustic streaming forces, (2) migration, driven by acoustic radiation forces and (3) patterning in a spatially periodic manner, resulting from diffraction effects. Whilst the first two phenomena have been widely discussed in the literature, the periodic patterning induced by TSAW has only recently been reported and is yet to be fully elucidated. In particular, more in-depth understanding of the size-dependant nature of this effect and the factors involved are required. Herein, we present an experimental and numerical study of the transition in acoustophoretic behaviour of particles influenced by relative dominance of these three mechanisms and characterise it based on particle diameter, channel height, frequency and intensity of the TSAW driven microfluidic system. This study will enable better understanding of the performance of TSAW sorters and allow the development of TSAW systems for particle collection and patterning.

Additive manufacturing of three-dimensional (3D) microfluidic-based microelectromechanical systems (MEMS) for acoustofluidic applications

Three-dimensional (3D) printing now enables the fabrication of 3D structural electronics and microfluidics. Further, conventional subtractive manufacturing processes for microelectromechanical systems (MEMS) relatively limit device structure to two dimensions and require post-processing steps for interface with microfluidics. Thus, the objective of this work is to create an additive manufacturing approach for fabrication of 3D microfluidic-based MEMS devices that enables 3D configurations of electromechanical systems and simultaneous integration of microfluidics. Here, we demonstrate the ability to fabricate microfluidic-based acoustofluidic devices that contain orthogonal out-of-plane piezoelectric sensors and actuators using additive manufacturing. The devices were fabricated using a microextrusion 3D printing system that contained integrated pick-and-place functionality. Additively assembled materials and components included 3D printed epoxy, polydimethylsiloxane (PDMS), silver nanoparticles, and eutectic gallium-indium as well as robotically embedded piezoelectric chips (lead zirconate titanate (PZT)). Electrical impedance spectroscopy and finite element modeling studies showed the embedded PZT chips exhibited multiple resonant modes of varying mode shape over the 0-20 MHz frequency range. Flow visualization studies using neutrally buoyant particles (diameter = 0.8-70 μm) confirmed the 3D printed devices generated bulk acoustic waves (BAWs) capable of size-selective manipulation, trapping, and separation of suspended particles in droplets and microchannels. Flow visualization studies in a continuous flow format showed suspended particles could be moved toward or away from the walls of microfluidic channels based on selective actuation of in-plane or out-of-plane PZT chips. This work suggests additive manufacturing potentially provides new opportunities for the design and fabrication of acoustofluidic and microfluidic devices.

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.

Plug-and-actuate on demand: multimodal individual addressability of microarray plates using modular hybrid acoustic wave technology

The microarray titre plate remains a fundamental workhorse in genomic, proteomic and cellomic analyses that underpin the drug discovery process. Nevertheless, liquid handling technologies for sample dispensing, processing and transfer have not progressed significantly beyond conventional robotic micropipetting techniques, which are not only at their fundamental sample size limit, but are also prone to mechanical failure and contamination. This is because alternative technologies to date suffer from a number of constraints, mainly their limitation to carry out only a single liquid operation such as dispensing or mixing at a given time, and their inability to address individual wells, particularly at high throughput. Here, we demonstrate the possibility for true sequential or simultaneous single- and multi-well addressability in a 96-well plate using a reconfigurable modular platform from which MHz-order hybrid surface and bulk acoustic waves can be coupled to drive a variety of microfluidic modes including mixing, sample preconcentration and droplet jetting/ejection in individual or multiple wells on demand, thus constituting a highly versatile yet simple setup capable of improving the functionality of existing laboratory protocols and processes.

On-demand acoustic droplet splitting and steering in a disposable microfluidic chip

On-chip droplet splitting is one of the fundamental droplet-based microfluidic unit operations to control droplet volume after production and increase operational capability, flexibility, and throughput. Various droplet splitting methods have been proposed, and among them the acoustic droplet splitting method is promising because of its label-free operation without any physical or thermal damage to droplets. Previous acoustic droplet splitting methods faced several limitations: first, they employed a cross-type acoustofluidic device that precluded multichannel droplet splitting; second, they required irreversible bonding between a piezoelectric substrate and a microfluidic chip, such that the fluidic chip was not replaceable. Here, we present a parallel-type acoustofluidic device with a disposable microfluidic chip to address the limitations of previous acoustic droplet splitting devices. In the proposed device, an acoustic field is applied in the direction opposite to the flow direction to achieve multichannel droplet splitting and steering. A disposable polydimethylsiloxane microfluidic chip is employed in the developed device, thereby removing the need for permanent bonding and improving the flexibility of the droplet microfluidic device. We experimentally demonstrated on-demand acoustic droplet bi-splitting and steering with precise control over the droplet splitting ratio, and we investigated the underlying physical mechanisms of droplet splitting and steering based on Laplace pressure and ray acoustics analyses, respectively. We also demonstrated droplet tri-splitting to prove the feasibility of multichannel droplet splitting. The proposed on-demand acoustic droplet splitting device enables on-chip droplet volume control in various droplet-based microfluidic applications.

A Novel Acoustomicrofluidic Nebulization Technique Yielding New Crystallization Morphologies

A novel acoustic microfluidic nebulization platform is demonstrated, which, due to its unique ability to access intermediate evaporation rate regimes-significantly faster than that in slow solvent evaporation but considerably below that achieved in spray drying, is capable of producing novel crystal morphologies that have yet to be reported in both model inorganic and organic systems. In addition, the potential for simultaneously encapsulating single crystals within a biodegradable polymeric coating in a single simultaneous step together with the crystallization process as the solvent evaporates during nebulization is briefly shown. The platform not only has the potential to be highly scalable by employing a large number of these low-cost miniature devices in parallel to achieve industrially relevant particle production rates, but could also be advantageous over conventional spray drying in terms of energy utilization, given the tremendous efficiency associated with the high-frequency ultrasonic microdevice as well as its ambient temperature operation.

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.

Acoustially-mediated microfluidic nanofiltration through graphene films

We exploit the possibility of enhancing the molecular transport of liquids through graphene films using amplitude modulated surface acoustic waves (SAWs) to demonstrate effective and efficient nanoparticle filtration. The use of the SAW, which is an extremely efficient means for driving microfluidic transport, overcomes the need for the large mechanical pumps required to circumvent the large pressure drops encountered in conventional membranes for nanoparticle filtration. 100% filtration efficiency was obtained for micron-dimension particulates, decreasing to only 95% for the filtration of particles of tens of nanometers in dimension, which is comparable to that achieved with other methods. To circumvent clogging of the film, which is typical with all membrane filters, a backwash operation to flush the nanoparticles is incorporated simply by reversing the SAW-induced flow such that 98% recovery of the initial filtration rate is recovered. Given these efficiencies, together with the low cost and compact size of the chipscale SAW devices, we envisage the possibility of scaling out the process by operating a large number of devices in parallel to achieve typical industrial-scale throughputs with potential benefits in terms of substantially lower capital, operating and maintenance costs.

Compact SAW aerosol generator

In this work, we discuss and demonstrate the principle features of surface acoustic wave (SAW) aerosol generation, based on the properties of the fluid supply, the acoustic wave field and the acoustowetting phenomena. Furthermore, we demonstrate a compact SAW-based aerosol generator amenable to mass production fabricated using simple techniques including photolithography, computerized numerical control (CNC) milling and printed circuit board (PCB) manufacturing. Using this device, we present comprehensive experimental results exploring the complexity of the acoustic atomization process and the influence of fluid supply position and geometry, SAW power and fluid flow rate on the device functionality. These factors in turn influence the droplet size distribution, measured here, that is important for applications including liquid chromatography, pulmonary therapies, thin film deposition and olfactory displays.

Rainbow trapping of ultrasonic guided waves in chirped phononic crystal plates

The rainbow trapping effect has been demonstrated in electromagnetic and acoustic waves. In this study, rainbow trapping of ultrasonic guided waves is achieved in chirped phononic crystal plates that spatially modulate the dispersion, group velocity, and stopband. The rainbow trapping is related to the progressively slowing group velocity, and the extremely low group velocity near the lower boundary of a stopband that gradually varies in chirped phononic crystal plates. As guided waves propagate along the phononic crystal plate, waves gradually slow down and finally stop forward propagating. The energy of guided waves is concentrated at the low velocity region near the stopband. Moreover, the guided wave energy of different frequencies is concentrated at different locations, which manifests as rainbow guided waves. We believe implementing the rainbow trapping will open new paradigms for guiding and focusing of guided waves. Moreover, the rainbow guided waves with energy concentration and spatial separation of frequencies may have potential applications in nondestructive evaluation, spatial wave filtering, energy harvesting, and acoustofluidics.

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.

Continuous micro-vortex-based nanoparticle manipulation via focused surface acoustic waves

Despite increasing demand in the manipulation of nanoscale objects for next generation biological and industrial processes, there is a lack of methods for reliable separation, concentration and purification of nanoscale objects. Acoustic methods have proven their utility in contactless manipulation of microscale objects mainly relying on the acoustic radiation effect, though the influence of acoustic streaming has typically prevented manipulation at smaller length scales. In this work, however, we explicitly take advantage of the strong acoustic streaming in the vicinity of a highly focused, high frequency surface acoustic wave (SAW) beam emanating from a series of focused 6 μm substrate wavelength interdigital transducers patterned on a piezoelectric lithium niobate substrate and actuated with a 633 MHz sinusoidal signal. This streaming field serves to focus fluid streamlines such that incoming particles interact with the acoustic field similarly regardless of their initial starting positions, and results in particle displacements that would not be possible with a travelling acoustic wave force alone. This streaming-induced manipulation of nanoscale particles is maximized with the formation of micro-vortices that extend the width of the microfluidic channel even with the imposition of a lateral flow, occurring when the streaming-induced flow velocities are an order of magnitude larger than the lateral one. We make use of this acoustic streaming to demonstrate the continuous and differential focusing of 100 nm, 300 nm and 500 nm particles.

Self-Aligned Interdigitated Transducers for Acoustofluidics

The surface acoustic wave (SAW) is effective for the manipulation of fluids and particles at microscale. The current approach of integrating interdigitated transducers (IDTs) for SAW generation into microfluidic channels involves complex and laborious microfabrication steps. These steps often require full access to clean room facilities and hours to align the transducers to the precise location. This work presents an affordable and innovative method for fabricating SAW-based microfluidic devices without the need for clean room facilities and alignment. The IDTs and microfluidic channels are fabricated using the same process and thus are precisely self-aligned in accordance with the device design. With the use of the developed fabrication approach, a few types of different SAW-based microfluidic devices have been fabricated and demonstrated for particle separation and active droplet generation.

Acoustofluidic Rotational Manipulation of Cells and Organisms Using Oscillating Solid Structures

A polydimethylsiloxane microchannel featuring sidewall sharp-edge structures and bare channels, and a piezoelement transducer is attached to a thin glass slide. When an external acoustic field is applied to the microchannel, the oscillation of the sharp-edge structures and the thin glass slide generate acoustic streaming flows which in turn rotate single cells and C. elegans in-plane and out-of-plane.

On-demand droplet splitting using surface acoustic waves

We demonstrated the operation of an acoustomicrofluidic device composed of a polydimethylsiloxane (PDMS) microchannel and a slanted-finger interdigitated transducer (SF-IDT), for the on-demand splitting of droplets in an active, accurate, rapid, and size-controllable manner. A narrow beam of surface acoustic waves (SAWs) that emanated from the SF-IDT exerted an acoustic radiation force (ARF) on the droplet's water-oil interface due to the acoustic contrast between the two fluids. The ARF split the mother droplet into two or more daughter droplets of various volumes in a split ratio that was readily controlled by varying the applied voltage or the flow rate. Theoretical estimates of the ARF acting on the droplet interface were used to investigate the mechanism underlying the droplet splitting properties and size control. The versatility of the acoustomicrofluidic device operation was demonstrated by selectively pushing/placing a suspended polystyrene particle into a specific/preferred split daughter droplet using the direct ARF acting on the particle.

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

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