Acoustofluidics 9: Modelling and applications of planar resonant devices for acoustic particle manipulation

Separation of suspensions and emulsions via ultrasonic standing waves - a review

Ultrasonic standing waves (USW) separation is an established technology for micro scale applications due to the excellent control to manipulate particles acoustically achieved when combining high frequency ultrasound with laminar flow in microchannels, allowing the development of numerous applications. Larger scale systems (pilot to industrial) are emerging; however, scaling up such processes are technologically very challenging. This paper reviews the physical principles that govern acoustic particle/droplet separation and the mathematical modeling techniques developed to understand, predict, and design acoustic separation processes. A further focus in this review is on acoustic streaming, which represents one of the major challenges in scaling up USW separation processes. The manuscript concludes by providing a brief overview of the state of the art of the technology applied in large scale systems with potential applications in the dairy and oil industries.

Modeling and optimization of acoustofluidic micro-devices

We investigate how the combination of numerical simulation tools and optimization routines can be used to design micro-devices. Experimental devices that are designed in this way can only provide optimal performance if the simulation model, used in the optimization procedure, reflects the real device characteristics accurately. Owing to this fact, the modeling of acoustofluidic devices is summarized. The mathematical formulation of the optimization problem, the parameterization of the device design and the implementation of the optimization loop is addressed alongside with practical recommendations for the chosen genetic algorithm optimization. In order to validate the implementation, an optimized planar resonator is compared with the optimal geometry given in the literature. The optimization of a typical 3D micro-device shows that devices can be designed to generate any desired acoustic mode shape at maximum pressure amplitude. The presented automatic design approach is of great practical relevance for the development of highly optimized micro-devices and it can speed up and facilitate the design-process in the growing field of acoustofluidics.

A thin-reflector microfluidic resonator for continuous-flow concentration of microorganisms: a new approach to water quality analysis using acoustofluidics

An acoustofluidic device has been developed for concentrating vegetative bacteria in a continuous-flow format. We show that it is possible to overcome the disruptive effects of acoustic streaming which typically dominate for small target particles, and demonstrate flow rates compatible with the testing of drinking water. The device consists of a thin-reflector multi-layered resonator, in which bacteria in suspension are levitated towards a glass surface under the action of acoustic radiation forces. In order to achieve robust device performance over long-term operation, functional tests have been carried out to (i) maintain device integrity over time and stabilise its resonance frequency, (ii) optimise the operational acoustic parameters, and (iii) minimise bacterial adhesion on the inner surfaces. Using the developed device, a significant increase in bacterial concentration has been achieved, up to a maximum of ~60-fold. The concentration performance of thin-reflector resonators was found to be superior to comparable half-wave resonators.

Acoustic devices for particle and cell manipulation and sensing

An emerging demand for the precise manipulation of cells and particles for applications in cell biology and analytical chemistry has driven rapid development of ultrasonic manipulation technology. Compared to the other manipulation technologies, such as magnetic tweezing, dielectrophoresis and optical tweezing, ultrasonic manipulation has shown potential in a variety of applications, with its advantages of versatile, inexpensive and easy integration into microfluidic systems, maintenance of cell viability, and generation of sufficient forces to handle particles, cells and their agglomerates. This article briefly reviews current practice and reports our development of various ultrasonic standing wave manipulation devices, including simple devices integrated with high frequency (>20 MHz) ultrasonic transducers for the investigation of biological cells and complex ultrasonic transducer array systems to explore the feasibility of electronically controlled 2-D and 3-D manipulation. Piezoelectric and passive materials, fabrication techniques, characterization methods and possible applications are discussed. The behavior and performance of the devices have been investigated and predicted with computer simulations, and verified experimentally. Issues met during development are highlighted and discussed. To assist long term practical adoption, approaches to low-cost, wafer level batch-production and commercialization potential are also addressed.

Acoustic trapping as a generic non-contact incubation site for multiplex bead-based assays

In this study, we show a significantly reduced assay time and a greatly increased bead recovery for a commercial Luminex-based multiplex diagnostic immunoassay by performing all liquid handling steps of the assay protocol in a non-contact acoustic trapping platform. The Luminex assay is designed for detecting antibodies in poultry serum for infectious bursal disease virus, infectious bronchitis virus, Newcastle disease virus and avian reovirus. Here, we show proof-of-concept of a microfluidic system capable of being fully automated and handling samples in a parallel format with a miniature physical footprint where the affinity beads are retained in a non-contact levitated mode in a glass capillary throughout the assay protocol. The different steps are: incubation with the serum sample, secondary antibodies and fluorescent reporters and finally washing to remove any non-specifically bound species. A Luminex 200 instrument was used for the readout. The flow rates applied to the capillary during the initial trapping event and the wash steps were optimised for maximum bead recovery, resulting in a bead recovery of 75% for the complete assay. This can be compared to a bead recovery of approximately 30% when an automatic wash station was used when the assay was performed in the conventional manual format. The time for the incubation steps for a single assay was reduced by more than 50%, without affecting assay performance, since intermediate wash steps became redundant in the continuously perfused bead trapping capillary. We analyzed seven samples, in triplicates, and we can show that the readout of the assay performed in the acoustic trap compared 100% to the control ELISAs (positive or negative readout) and resulted in comparable S/P values as the conventional manual protocol. As the acoustic trapping does not require the particles to have magnetic properties, a greater degree of freedom in selecting microparticles can be provided. In extension, this can provide an opportunity to develop cheaper and more effective microparticles.

Deformation of red blood cells using acoustic radiation forces

Acoustic radiation forces have been used to manipulate cells and bacteria in a number of recent microfluidic applications. The net force on a cell has been subject to careful investigation over a number of decades. We demonstrate that the radiation forces also act to deformcells. An ultrasonic standing wave field is created in a 0.1 mm glass capillary at a frequency of 7.9 MHz. Using osmotically swollen red-blood cells, we show observable deformations up to an aspect ratio of 1.35, comparable to deformations created by optical tweezing. In contrast to optical technologies, ultrasonic devices are potentially capable of deforming thousands of cells simultaneously. We create a finite element model that includes both the acoustic environment of the cell, and a model of the cell membrane subject to forces resulting from the non-linear aspects of the acoustic field. The model is found to give reasonable agreement with the experimental results, and shows that the deformation is the result of variation in an acoustic force that is directed outwards at all points on the cell membrane. We foresee applications in diagnostic devices, and in the possibility of mechanically stimulating cells to promote differentiation and physiological effects.

Impedance matched channel walls in acoustofluidic systems

Acoustophoresis in bulk acoustic wave (BAW) devices typically operates with an ultrasonic standing wave in a microfluidic channel between two opposing silicon walls, which act as both the acoustic and the fluidic boundary. In this paper, we describe BAW devices with an additional material layer of polydimethylsiloxane (PDMS). This PDMS wall is introduced to decouple the acoustic boundary (silicon wall) from the fluidic boundary (PDMS wall) by acoustic impedance matching. The acoustic field and the resulting particle manipulation are thereby less restricted than in conventional BAW devices. In the presented devices, particle accumulation lines can be placed arbitrarily within the fluidic domain, which strongly increases the possibility of acoustophoresis. The paper covers experimental results, an analytical model in good agreement and microfabrication techniques for PDMS enclosed in a microchannel. An application example for microparticle concentration is demonstrated. The presented approach offers further potential for biotechnological applications such as particle separation, enhanced particle sensors and cell handling.

Ultrasound assisted particle and cell manipulation on-chip

Ultrasonic fields are able to exert forces on cells and other micron-scale particles, including microbubbles. The technology is compatible with existing lab-on-chip techniques and is complementary to many alternative manipulation approaches due to its ability to handle many cells simultaneously over extended length scales. This paper provides an overview of the physical principles underlying ultrasonic manipulation, discusses the biological effects relevant to its use with cells, and describes emerging applications that are of interest in the field of drug development and delivery on-chip.

Ultrasonic manipulation of particles in an open fluid film

Ultrasonic manipulation is a noncontact method of trapping and holding particles in suspension, and has found many applications in microfluidic systems. Typically, ultrasonic standing waves are used; this approach is well established in fully enclosed microfluidic systems consisting of channels or chambers with an attached piezoelectric actuator. In this work, we examine the use of ultrasonic manipulation in open fluid films, which offer a high degree of accessibility. A piezoelectric actuator is presented which can be lowered into a separate fluid tray. This two-part system offers a high degree of flexibility; indeed the actuator can be removed with little disturbance to the particle patterns, so manipulation could potentially be periodically applied as required. Particle manipulation is shown to be possible over a distance many times the size of the actuator. Furthermore, particle manipulation can also be achieved in a tilted fluid film, so alignment between the two parts of the system is not critical to its operation.

Varying the agglomeration position of particles in a micro-channel using Acoustic Radiation Force beyond the resonance condition

It is well-known that particles can be focused at mid-height of a micro-channel using Acoustic Radiation Force (ARF) tuned at the resonance frequency (h=λ/2). The resonance condition is a strong limitation to the use of acoustophoresis (particles manipulation using acoustic force) in many applications. In this study we show that it is possible to focus the particles anywhere along the height of a micro-channel just by varying the acoustic frequency, in contradiction with the resonance condition. This result has been thoroughly checked experimentally. The different physical properties as well as wall materials have been changed. The wall materials is finally the only critical parameters. One of the specificity of the micro-channel is the thickness of the carrier and reflector layer. A preliminary analysis of the experimental results suggests that the acoustic focusing beyond the classic resonance condition can be explained in the framework of the multilayered resonator proposed by Hill [1]. Nevertheless, further numerical studies are needed in order to confirm and fully understand how the acoustic pressure node can be moved over the entire height of the micro channel by varying the acoustic frequency. Despite some uncertainties about the origin of the phenomenon, it is robust and can be used for improved acoustic sorting or manipulation of particles or biological cells in confined set-ups.

Acoustic streaming in the transducer plane in ultrasonic particle manipulation devices

In acoustofluidic manipulation and sorting devices, Rayleigh streaming flows are typically found in addition to the acoustic radiation forces. However, experimental work from various groups has described acoustic streaming that occurs in planar devices in a plane parallel to the transducer face. This is typically a four-quadrant streaming pattern with the circulation parallel to the transducer. Understanding its origins is essential for creating designs that limit or control this phenomenon. The cause of this kind of streaming pattern has not been previously explained as it is different from the well-known classical streaming patterns such as Rayleigh streaming and Eckart streaming, whose circulation planes are generally perpendicular to the face of the acoustic transducer. In order to gain insight into these patterns we present a numerical method based on Nyborg's limiting velocity boundary condition that includes terms ignored in the Rayleigh analysis, and verify its predictions against experimental PIV results in a simple device. The results show that the modelled particle trajectories match those found experimentally. Analysis of the dominant terms in the driving equations shows that the origin of this kind of streaming pattern is related to the circulation of the acoustic intensity.

Acoustofluidics 23: acoustic manipulation combined with other force fields

In this, the final paper of the Acoustofluidics series of tutorial articles, we discuss applications in which acoustic radiation forces are used in conjunction with competing or complementary force-fields. This may be in order to enable manipulation operations that would not be easily performed by either force-field alone, or may be used to effect separation based on the different physical principals underlying competing fields. Examples are given of a number of different applications in which acoustic forces are combined with gravitational fields, hydrodynamic forces, electric fields (including dielectrophoresis), magnetic forces and optical forces.

Acoustofluidics 22: multi-wavelength resonators, applications and considerations

One important niche for multi-wavelength resonators is the filtration of suspensions containing very high particle concentration. For some applications, multi-wavelength ultrasound enhanced sedimentation filters are second only to the centrifuge in efficiency but, unlike the centrifuge they are easily adapted for continuous flow. Multi-wavelength resonators are also an obvious consideration when half-wavelength chambers are too small for a specific application. Unfortunately the formula, bigger = higher-throughput, does not scale linearly. Here we describe the relationships between chamber size and throughput for acoustic, electrical, flow and thermal convection actions, allowing the user to define initial parameters for their specific applications with some confidence. We start with a review of some of the many forms of multi-wavelength particle manipulation systems.

Acoustofluidics 21: ultrasound-enhanced immunoassays and particle sensors

In part 21 of the tutorial series "Acoustofluidics--exploiting ultrasonic standing wave forces and acoustic streaming in microfluidic systems for cell and particle manipulation", we review applications of ultrasonic standing waves used for enhancing immunoassays and particle sensors. The paper covers ultrasonic enhancement of bead-based immuno-agglutination assays, bead-based immuno-fluorescence assays, vibrational spectroscopy sensors and cell deposition on a sensor surface.

Acoustofluidics 20: applications in acoustic trapping

This part of the Acoustofluidics tutorial series reviews applications in acoustic trapping of micron-sized particles and cells in microfluidic systems. Acoustic trapping enables non-invasive and non-contact immobilisation of cells and particles in microfluidic systems. Acoustic trapping has been used for reducing the time needed to create 3D cell clusters, enhance particle-based bioassays and facilitated interaction studies of both cells and particles. An area that is increasingly interesting is the use of acoustic trapping for enriching low concentration samples and the washing or fractioning of cell populations prior to sensitive detection methods (MALDI-MS, PCR etc.) The main focus of the review is systems where particles can be retained against a flow while applications in which particles are positioned in a stationary fluid will be addressed in part 21 of the Acoustofluidics tutorial series (M. Wiklund, S. Radel and J. J. Hawkes, Lab Chip, 2012, 12, ).

Standing surface acoustic wave (SSAW) based multichannel cell sorting

We introduce a novel microfluidic device for cell sorting in continuous flow using tunable standing surface acoustic waves. This method allows individual cells to be precisely directed into five different outlet channels in a single step. It is versatile, simple, label-free, non-invasive, and highly controllable.

On-chip manipulation of single microparticles, cells, and organisms using surface acoustic waves

Techniques that can dexterously manipulate single particles, cells, and organisms are invaluable for many applications in biology, chemistry, engineering, and physics. Here, we demonstrate standing surface acoustic wave based "acoustic tweezers" that can trap and manipulate single microparticles, cells, and entire organisms (i.e., Caenorhabditis elegans) in a single-layer microfluidic chip. Our acoustic tweezers utilize the wide resonance band of chirped interdigital transducers to achieve real-time control of a standing surface acoustic wave field, which enables flexible manipulation of most known microparticles. The power density required by our acoustic device is significantly lower than its optical counterparts (10,000,000 times less than optical tweezers and 100 times less than optoelectronic tweezers), which renders the technique more biocompatible and amenable to miniaturization. Cell-viability tests were conducted to verify the tweezers' compatibility with biological objects. With its advantages in biocompatibility, miniaturization, and versatility, the acoustic tweezers presented here will become a powerful tool for many disciplines of science and engineering.

Array-controlled ultrasonic manipulation of particles in planar acoustic resonator

Ultrasonic particle manipulation tools have many promising applications in life sciences, expanding on the capabilities of current manipulation technologies. In this paper, the ultrasonic manipulation of particles and cells along a microfluidic channel with a piezoelectric array is demonstrated. An array integrated into a planar multilayer resonator structure drives particles toward the pressure nodal plane along the centerline of the channel, then toward the acoustic velocity maximum centered above the subset of elements that are active. Switching the active elements along the array moves trapped particles along the microfluidic channel. A 12-element 1-D array coupled to a rectangular capillary has been modeled and fabricated for experimental testing. The device has a 300-μm-thick channel for a half-wavelength resonance near 2.5 MHz, with 500 μm element pitch. Simulation and experiment confirm the expected trapping of particles at the center of the channel and above the set of active elements. Experiments demonstrated the feasibility of controlling the position of particles along the length of the channel by switching the active array elements.