Acoustic Micromixer Research Articles

Integration of acoustic micromixing with cyclic olefin copolymer microfluidics for enhanced lab-on-a-chip applications in nanoscale liposome synthesis

The integration of acoustic wave micromixing with microfluidic systems holds great potential for applications in biomedicine and lab-on-a-chip technologies. Polymers such as cyclic olefin copolymer (COC) are increasingly utilized in microfluidic applications due to its unique properties, low cost, and versatile fabrication methods, and incorporating them into acoustofluidics significantly expands their potential applications. In this work, for the first time, we demonstrated the integration of polymer microfluidics with acoustic micromixing utilizing oscillating sharp edge structures to homogenize flowing fluids. The sharp edge mixing platform was entirely composed of COC fabricated in a COC-hydrocarbon solvent swelling based microfabrication process. As an electrical signal is applied to a piezoelectric transducer bonded to the micromixer, the sharp edges start to oscillate generating vortices at its tip, mixing the fluids. A 2D numerical model was implemented to determine the optimum microchannel dimensions for experimental mixing assessment. The system was shown to successfully mix fluids at flow rates up to 150 µl h−1 and has a modest effect even at the highest tested flow rate of 600 µl h−1. The utility of the fabricated sharp edge micromixer was demonstrated by the synthesis of nanoscale liposomes.

Sharp-edge–driven spiral acoustic micromixers for functional nanoarray engineering

Acoustic microfluidics has a remarkable influence on mixing but still face significant challenges of low throughput. Here, a robust acoustic micromixer with a spiral microchannel and side-wall sharp-edge structures is first proposed for facilitating microscale fluid mixing at wide-range flow rates. A micromixer that integrates acoustics with hydrodynamics enables fast and homogeneous mixing, ranging from 4 μL/min to over 2500 μL/min. the mixing performance shows a transition from acoustics domination at low flow rates to hydrodynamics domination at high flow rates. The increases of transducer amplitude, curvature, and sharp-edge density are positively correlated with mixing performance, while the increase of a tilted angle shows the opposite. The acoustic micromixer is leveraged to synthesize a functional ZnO nanoarray inside glass capillaries as portable microanalytical systems. ZnO nanofiber and nanosheet arrays are developed and their performances are systematically investigated by photodegradation of organic dye and enrichment of a heavy metal ion. Generally, efficiencies in photodegradation and enrichment can be well regulated by residence time, and a ZnO nanofiber shows more outstanding activity and robust stability than a ZnO nanosheet. These findings not only shed new light on the rational design of advanced lab-on-a-chip devices but also provide important guidelines for the controllable synthesis and applications of functional nanomaterials.

A multi-vortex micromixer based on the synergy of acoustics and inertia for nanoparticle synthesis

Mixing is one of the most important steps in chemical reaction, sample preparation and emulsification. However, achieving complete mixing of fluids at high throughput is still a challenge for acoustic micromixers, which are limited by the intensity of the acoustic streaming. In this study, we proposed an acoustic-inertial micromixer based on multi-vortex synergy by introducing inertia into acoustic micromixer. The device contains side-wall sharp-edge structure and contraction-expansion array structure (SCEA) in the microchannel to enhance the acoustic streaming with inertial vortices. The mixing mechanism of SCEA was explored and the mixing process showed three modes: acoustic streaming dominant mode, acoustic-inertial synergy mode and inertial vortex dominant mode. On the basis of the “vortex seed” provided by the contraction-expansion structure, stronger chaotic advection was produced by the synergy of acoustic streaming and inertial vortices (including Dean vortex and horizontal vortex). Rapider mixing (0.20 m s) and wider operating ranges (0–3000 μL/min) were achieved in SCEA at lower driving voltages compared with conventional acoustic micromixers. Finally, more homogeneous and tunable chitosan nanoparticles and shellac nanoparticles were synthesized based on this device. The micromorphology, particle size distribution and drug loading properties of the products were measured and compared. This work provides a platform for control of mixing process in specific application environments with high operational flexibility, indicating potentially wider application of SCEA in multi-functional integration of lab-on-a-chip systems.

High throughput acoustic microfluidic mixer controls self-assembly of protein nanoparticles with tuneable sizes

HypothesisProtein nanoparticles have attracted increased interest due to their broad applications ranging from drug delivery and vaccines to biocatalysts and biosensors. The morphology and the size of the nanoparticles play a crucial role in determining their suitability for different applications. Yet, effectively controlling the size of the nanoparticles is still a significant challenge in their manufacture. The hypothesis of this paper is that the assembly conditions and size of protein particles can be tuned via a mechanical route by simply modifying the mixing time and strength, while keeping the chemical parameters constant. ExperimentalWe use an acoustically actuated, high throughput, ultrafast, microfluidic mixer for the assembly of protein particles with tuneable sizes. The performance of the acoustic micro-mixer is characterized via Laser Doppler Vibrometry and image processing. The assembly of protein nanoparticles is monitored by dynamic light scattering (DLS) and transmission electron microscopy (TEM). FindingsBy changing actuation parameters, the turbulence and mixing in the microchannel can be precisely varied to control the initiation of protein particle assembly while the solution conditions of assembly (pH and ionic strength) are kept constant. Importantly, mixing times as low as 6 ms can be achieved for triggering protein assembly in the microfluidic channel. In comparison to the conventional batch process of assembly, the acoustic microfluidic mixer approach produces smaller particles with a more uniform size distribution, promising a new way to manufacture protein particles with controllable quality.

Versatile hybrid acoustic micromixer with demonstration of circulating cell-free DNA extraction from sub-ml plasma samples.

Acoustic micromixers have attracted considerable attention in the last years since they can deliver high mixing efficiencies without the need for movable components. However, their adoption in the academic and industrial microfluidics community has been limited, possibly due to the reduced flexibility and accessibility of previous designs since most of them are application-specific and fabricated with techniques that are expensive, not widely available and difficult to integrate with other manufacturing technologies. In this work, we describe a simple, yet highly versatile, bubble-based micromixer module fabricated with a combination of low-cost rapid prototyping techniques. The hybrid approach enables the integration of the module into practically any substrate and the individual control of multiple micromixers embedded within the same monolithic chip. The module can operate under static and continuous flow conditions showing enhanced mixing capabilities compared to similar devices. We show that the system is capable of performing cell-free DNA extractions from small volumes of blood plasma (≤500 μl) with up to a ten-fold increase in capture efficiency when compared to control methods.

Ultrafast star-shaped acoustic micromixer for high throughput nanoparticle synthesis.

We present an acoustically actuated microfluidic mixer, which can operate at flowrates reaching 8 ml min-1, providing a 50-fold improvement in throughput compared to previously demonstrated acoustofluidic approaches. The device consists of a robust silicon based micro-mechanical oscillator, sandwiched between two polymeric channels which guide the fluids in and out of the system. The chip is actuated by application of an oscillatory electrical signal onto a piezoelectric disk coupled to the substrate by adhesive. At the optimal frequency, this acoustofluidic system can homogenise two fluids with a relative mixing efficiency of 91%, within 4.1 ms from first contact. The micromixer has been used to synthesize two different systems: Budesonide nanodrugs with an average diameter of 80 ± 22 nm, and DNA nanoparticles with an average diameter of 63.3 ± 24.7 nm.

An Acoustic Micromixer Using Low-Powered Voice Coil Actuation

A low-powered acoustic micromixer has been presented in this paper. This newly invented system consists of an open-top mixing chamber and a permanent magnet mixer base. A voice coil is attached to the bottom of the mixing chamber with a polydimethylsiloxane membrane. The optimal driving frequency of each mixing test can be co-determined during (or prior to) the mixing via an electrical impedance ( $Z_{e}$ ) measurement owing to inherent electro-acoustic coupling of the system. When the mixer is driving at its resonance frequency found from the $Z_{e}$ , mixing efficiency is maximized characterizing millie-watt efficiency to achieve a mixing time scale comparable to other acoustic mixers. In addition, the present mixer demonstrates feasibility of a portable microfluidic system, wherein the degree of mixing may be tracked by $Z_{e}$ monitoring from a smart-phone, which may allow each experiment to be controlled remotely over existing network environment. We believe that our study demonstrates a feasible application of microelectromechanical systems technology for the Internet of Things. [2017-0119]

Microbubble oscillation induced acoustic micromixing in microfluidic device

Microfluidic is of great significance for biomedical research and chemical engineering. The mixing of liquids is an essential and necessary procedure for the sample preparation. Due to the low Reynolds number, laminar flow is dominant in a microfluidic channel and it is difficult to mix the fluids in the microchannel quickly and effectively. To improve the mixing efficiency of the liquids in microfluidic channels, we develop an acoustic mixer based on single microbubble oscillation. By designing the cylinder structure on the bottom surface, when the fluid flows through cylinder structure with a diameter of 40 m, the microbubble can be generated by the surface tension of the liquid. The device is fabricated by using standard soft lithography and the replica moulding technique, ensuring the stability and repeatability of the mixing. A piezoelectric transducer (PZT) with a resonant frequency of 165 kHz is attached to the polydimethylsiloxane microfluidic device on the glass substrate by ultrasound coupling gel. When the microbubble is excited by the PZT at a resonant frequency of 165 kHz, microbubble oscillates immediately. To verify whether ultrasound can induce microbubble cavitation, a passive cavitation detection system is established. The results show that the higher harmonics can be detected, indicating that the stable cavitation occurs. The microstreaming induced by the oscillating microbubble disturbs the fluid dramatically, achieving the mixture of liquids. Particle image velocimetry method is utilized to characterize the microstreaming, and a pair of counter-rotating vortices in the microchannel is detected. Furthermore, to test the performance of the device, the deionized water and rhodamine B are injected into the Y-shape microchannel. Relative mixing index is used to quantitatively analyze the mixing performance by measuring the grayscale values of the optical images. The results indicate that with the increase of the input power, mixing time can be shortened correspondingly. When the input power is 14.76 W, the mixing process is ultrafast, within 37.5 ms the high mixing uniformity can be achieved to be 92.7%. With the advantages of simple design, high efficient and ultrafast mixing, and low power consumption, this oscillating microbubble-based acoustic micromixer may provide a powerful tool for various biochemical studies and applications.

Multiplexed detection of cancer biomarkers using a microfluidic platform integrating single bead trapping and acoustic mixing techniques.

It is critical to reliably and rapidly detect multiple disease biomarkers in tiny liquid samples with high sensitivity to meet the growing demand for point-of-care diagnostics. This paper reports a microfluidic platform integrating magnetic-based single bead trapping in conjunction with acoustic micromixing for simultaneous detection of multiple cancer biomarkers within minutes. Individual beads retained by permalloy (NiFe81/19) microarray were used to capture biomarkers and facilitate the fluorescence identification. A numerical study indicates that the magnetic force keeping a bead in the trap is proportional to the thickness of the permalloy array and the external magnetic field strength, while inversely proportional to the size of the trap. The acoustic microstreaming activated by a piezo transducer was applied to generate fast-switching flow patterns to minimize the diffusion length scales. The flow at various driving frequencies was experimentally tested to achieve the optimal mixing effect. The flow field of the microstreaming was subsequently described by a mathematical model to understand the flow further. Finally, the prostate-specific antigen (PSA) and carcinoembryonic antigen (CEA) were employed as model analytes to demonstrate the capability of the platform for rapid biomarker detection. With the aid of acoustic micromixing, the detection can be finished in 20 minutes. The respective limit of detection of PSA and CEA is 0.028 ng mL-1 (0.8 pM) and 3.1 ng mL-1 (17 pM), which is respectively 1/142 and 1/3 of the cutoff value of PSA and CEA. Our results indicate this platform has great potential for the rapid detection of multiple biomarkers in future point-of-care diagnostics.

Acoustic micromixing increases antibody-antigen binding in immunoassays.

Sound wave-assisted acoustic micromixing has been shown to increase the binding of molecules in small volumes (10-100 μL) where effective mixing is difficult to achieve through conventional techniques. The aim of this work is to study whether acoustic micromixing can increase the binding efficiency of antibodies to their antigens, a reaction that forms the basis of immunoassays, including enzyme-linked immunosorbent assay (ELISA). Using a procedure from a general ELISA and immobilizing an antigen on wells of 96-well plates, it was found that acoustic micromixing at 125-150 Hz increased the initial rate of antibody-antigen binding by over 80 % and the total binding at the end point (i.e., 45 min) by over 50 %. As a result, acoustic micromixing achieved a binding level in 9 min that would otherwise take 45 min on a standard platform rocking mixer. Therefore acoustic micromixing has the potential to increase the detection sensitivity of ELISA as well as shorten the antigen-antibody binding times from typically 45-60 min to 15 min.

In-chip direct laser writing of a centimeter-scale acoustic micromixer

A centimeter-scale micromixer was fabricated by two-photon polymerization inside a closed microchannel using direct laser writing. The structure consists of a repeating pattern of 20 μm×20 μm×155 μm acrylate pillars and extends over 1.2 cm. Using external ultrasonic actuation, the micropillars locally induce streaming with flow speeds of 30 μm s −1 . The fabrication method allows for large flexibility and more complex designs

Design and Fabrication of an Acoustic Micromixer for Biological Media Activation

Abstract In this work, we present the design, fabrication and characterization of a silicon-based device dedicated for activation of biological solution under dynamic and static flow regimes by fluid mixing using acoustic low-frequency vibrations. The acoustic vibrations are generated by a bulk piezoelectric lead-zirconate-titanate (PZT). In the dynamic flow regime, biological compounds (as antibodies) were used in order to build immunochips taking advantages from the remarkable properties of such acoustic device to optimize the efficiency of Abs grafting onto chemically functionalized chip. The evaluation of the performance of the system to mix and activate fluids is presented. The experiments show the mixing capacity of the device in static and dynamic flow regimes using colored water and a biological model respectively.

An acoustofluidic micromixer based on oscillating sidewall sharp-edges

Rapid and homogeneous mixing inside a microfluidic channel is demonstrated via the acoustic streaming phenomenon induced by the oscillation of sidewall sharp-edges. By optimizing the design of the sharp-edges, excellent mixing performance and fast mixing speed can be achieved in a simple device, making our sharp-edge-based acoustic micromixer a promising candidate for a wide variety of applications.

Novel microfluidic technologies for portable diagnostics systems

Novel microfluidic technologies being developed for portable diagnostic systems have potential for use in TB serodiagnosis or as platforms for cell and biomolecular assays. Using microfluidics, with its miniaturized channels and reservoirs for portable devices, offers many advantages, including: high surface-to-volume ratio; a low Reynolds number; increased speed of reaction; reduced cost of reagents; decreased cost of power consumption; precise mixing/dosage and heating. Its integration capability offers advantages as well, such as low manufacturing cost and multiplex capability, i.e. an increased number of parameters can be monitored per sample. Two microfluidic platforms may be helpful in protein microarray analysis. Microfluidic electrical sensing platforms are capable of detecting antigen/antibody binding in real time. The development of this platform will allow for faster detection of certain diseases with a patient’s sample compared to standard techniques such as ELISA. This platform requires microchannel-sealed microarrays with electrical detection capability. By measuring electrical charges, the scientists can determine protein-binding activity. Additionally, microfluidic colorimetric platforms are being developed using acoustic micromixing instead of fluorescence scanning. The process uses a scanner that detects an enzyme–based change of color that allows measurement of the protein-binding event. Our team is developing these platforms using components that have very small energy requirements, which means the device may not require batteries to operate. A ‘lab-in-a-droplet’ technique using digital microfluidics rather than using microarrays, which produce a static substrate, diffuses samples into as many as millions of droplets, presenting a three-dimensional picoliter reactor array. This process has several advantages including: rapid mixing, homogeneous reactions, precise control of volumes and concentrations, and an ability to mimic cellular reactions. Our team is using this technique in the development of lipoplexes and to encapsulate a cell in a single droplet. Lastly, a platform is under development for cell sorting using electrodes to track cells in order to determine what function the cell will take on. In addition to its value in the diagnosis of TB, microfluidics technology has several other applications, including: broader distribution of new assays to biologic labs, rapid analyses in the field for epidemiologic studies, and the ability to study hybridization of molecules on microarrays. For example, Lab-on-a-chip technologies are an excellent vehicle for integrating diagnostic and therapeutic functions. Digital (droplet) microfluidics enables high throughput cellular and molecular assays and cell sorting technologies enable more practical cellular analyses and therapies.

Chaotic Micromixing in Open Wells Using Audio-Frequency Acoustic Microstreaming

Mixing fluids for biochemical assays is problematic when volumes are very small (on the order of the 10 microL typical of single drops), which has inspired the development of many micromixing devices. In this paper, we show that micromixing is possible in the simple open wells of standard laboratory consumables using appropriate acoustic frequencies that can be applied using cheap, conventional audio components. Earlier work has shown that the phenomenon of acoustic microstreaming can mix fluids, provided that bubbles are introduced into a specially designed microchamber or that high-frequency surface acoustic wave devices are constructed. We demonstrate a key simplification: acoustic micromixing at audio frequencies by ensuring the system has a liquid-air interface with a small radius of curvature. The meniscus of a drop in a small well provided an appropriately small radius, and so an introduced bubble was not necessary. Microstreaming showed improvement over diffusion-based mixing by 1-2 orders of magnitude. Furthermore, significant improvements are attainable through the utilization of chaotic mixing principles, whereby alternating fluid flow patterns are created by applying, in sequence, two different acoustic frequencies to a drop of liquid in an open well.

A millisecond micromixer via single-bubble-based acoustic streaming

We present ultra-fast homogeneous mixing inside a microfluidic channel via single-bubble-based acoustic streaming. The device operates by trapping an air bubble within a "horse-shoe" structure located between two laminar flows inside a microchannel. Acoustic waves excite the trapped air bubble at its resonance frequency, resulting in acoustic streaming, which disrupts the laminar flows and triggers the two fluids to mix. Due to this technique's simple design, excellent mixing performance, and fast mixing speed (a few milliseconds), our single-bubble-based acoustic micromixer may prove useful for many biochemical studies and applications.

Microfluidic Mixer and Transporter Based on PZT Self-Focusing Acoustic Transducers

This paper describes noninvasive acoustic micromixer and transporter as well as a microfluidic subsystem (on a single silicon chip) based on the two microfluidic actuators. The actuators use a piezoelectric PZT sheet to generate about 10-12.5MHz acoustic waves (corresponding to the PZT thickness), an array of sectored Fresnel annular rings [for the electrodes sandwiching the piezoelectric transducer (PZT)] to self-focus the acoustic waves, and acoustic streaming effect to produce a body force for fluidic motion. The self-focused waves from the PZT array propagate through 200-/spl mu/m-thick silicon without any significant energy loss, and move the liquid contained in the channels and chambers (without a cover) on a micromachined silicon chip, up to 123 mm/s in-plane speed. The PZT array (consisting of a PZT sheet with an array of electrode patterns for sectored Fresnel rings) is bonded to the micromachined silicon chip to produce a fast in-plane liquid motion in the open channels and chambers. When the liquid motion is produced in a channel, we have a liquid transporter; while when the motion is produced in a large area chamber, we have a micromixer. Various channel and chamber structures have been fabricated to verify the efficacies of the liquid transport and mixing. A microfluidic subsystem integrating a micromixer, transporters and reservoirs has been shown to transport two types of liquids from their reservoirs to a mixer and to mix the liquids together within 2 s. The experimental results agree well with theoretical studies and simulation results.

Acoustic micromixer for enhancement of DNA biochip systems

Microfluidics is emerging as one of the fastest growing segments of micro-electro-mechanical system (MEMS) technologies due to its potential applications in biotechnology, chemical microreactors, and drug discovery. Micromixing is one of the most challenging problems in microfluidic systems, since it is a diffusion-limited process and can be very inefficient. A micromixing device based on an acoustic microstreaming principle is developed to enhance micromixing. The micromixer uses air bubbles as actuators that can be set into vibration by a sound field. The vibration of the air bubbles generates steady circulatory flows, resulting in global convection flows and thus rapid mixing. The time to fully mix dyed solutions in a 50-μL shallow chamber using acoustic microstreaming is significantly reduced from hours (a pure diffusion-based mixing) to 6 s. We demonstrate the use of this micromixer to enhance the performance of conventional DNA microarray biochips that often suffer from lengthy hybridization and poor signal uniformity due to a diffusion-limited hybridization process. Experiments showed that the acoustic micromixer results in five-fold hybridization signal enhancement with significantly improved signal uniformity, as compared to conventional diffusion-based biochips. Acoustic microstreaming has many advantages over most existing micromixing techniques, including a simple apparatus, ease of implementation, low power consumption (∼ 2 mW), and low cost.

Bubble-induced acoustic micromixing.

A mixing technique based on the principle of bubble-induced acoustic microstreaming was developed. The mixer consists of a piezoelectric disk that is attached to a reaction chamber, which is designed in such a way that a set of air bubbles with desirable size is trapped in the solution. Fluidic experiments showed that air bubbles resting on a solid surface and set into vibration by the sound field generated steady circulatory flows, resulting in global convection flows and thus rapid mixing. The time to fully mix a 22 microL chamber is significantly reduced from hours (for a pure diffusion-based mixing) to tens of seconds. Numerical simulations showed that the induced flowfield and thus degree of mixing strongly depend on bubble positions. Optimal simulated mixing results were obtained for staggered bubble distribution that minimizes the number of internal flow stagnation regions. Immunomagnetic cell capture experiments showed that acoustic microstreaming provided efficient mixing of bacterial cell (Esherichia coli K12) matrix suspended in blood with magnetic capture beads, resulting in highly effective immunomagnetic cell capture. Bacterial viability assay experiments showed that acoustic microstreaming has a relatively low shear strain field since the blood cells and bacteria remained intact after mixing. Acoustic microstreaming has many advantages over most existing chamber micromixing techniques, including simple apparatus, ease of implementation, low power consumption (2 mW), and low cost.