Surface Acoustic Wave Microfluidic Device Research Articles

Aerosol jet printing of surface acoustic wave microfluidic devices

The addition of surface acoustic wave (SAW) technologies to microfluidics has greatly advanced lab-on-a-chip applications due to their unique and powerful attributes, including high-precision manipulation, versatility, integrability, biocompatibility, contactless nature, and rapid actuation. However, the development of SAW microfluidic devices is limited by complex and time-consuming micro/nanofabrication techniques and access to cleanroom facilities for multistep photolithography and vacuum-based processing. To simplify the fabrication of SAW microfluidic devices with customizable dimensions and functions, we utilized the additive manufacturing technique of aerosol jet printing. We successfully fabricated customized SAW microfluidic devices of varying materials, including silver nanowires, graphene, and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS). To characterize and compare the acoustic actuation performance of these aerosol jet printed SAW microfluidic devices with their cleanroom-fabricated counterparts, the wave displacements and resonant frequencies of the different fabricated devices were directly measured through scanning laser Doppler vibrometry. Finally, to exhibit the capability of the aerosol jet printed devices for lab-on-a-chip applications, we successfully conducted acoustic streaming and particle concentration experiments. Overall, we demonstrated a novel solution-based, direct-write, single-step, cleanroom-free additive manufacturing technique to rapidly develop SAW microfluidic devices that shows viability for applications in the fields of biology, chemistry, engineering, and medicine.

Glycine polymorphs screening by antisolvent crystallization in a surface acoustic wave microfluidic device

Polymorph control is crucial in the pharmaceutical field. In this work, we utilized a surface acoustic wave (SAW)-assisted micromixer with thermal control for glycine polymorphs screening via antisolvent crystallization. Different crystal forms were screened out by altering the input voltages and temperature. • A SAW-assisted micromixer with thermal control was utilized to screen polymorphs. • Increasing the input voltage, the ratio of the β-glycine gradually grows to dominant. • Increasing the temperature of mixing solutions, the γ-glycine increases sharply to predominant. Polymorph control is crucial in the pharmaceutical field. In this work, we utilized a surface acoustic wave (SAW)-assisted micromixer with thermal control for glycine polymorphs screening via antisolvent crystallization. Different crystal forms were screened out by altering the input voltage and temperature.

Numerical investigation of particle deflection in tilted-angle standing surface acoustic wave microfluidic devices

• Numerical model developed for particle trajectories in acoustofluidic separation. • Effect of related parameters of acoustic and flow fields on trajectories is analyzed. • Two-stage separation demonstrated for exosome-sized particles from whole blood. • The model has potential for optimal design of acoustofluidic separation systems. The tilted-angle standing surface acoustic wave (taSSAW) microfluidic device has become a powerful tool for biosample separation due to its biocompatibility and non-contact, label-free, and high-efficiency nature. Studying and modeling particle deflection in a microfluid environment containing a taSSAW field is essential in the design of robust taSSAW-based microfluidic devices. Here, we present a numerical model taking into consideration fluid viscous drag force and the acoustic radiation force induced by scattering of acoustic waves for the study of particle deflection. The reliability of the model is validated by comparing our predictions with data from existing literature. In order to support our prediction experimentally, we fabricated a taSSAW microfluidic chip using 128° YX LiNbO 3 , and the deflection results for 3- and 7-μm polystyrene microspheres concur with the numerical estimation. The effects on particle deflection by parameters such as pre-focusing, pre-focusing width, average flow velocity, acoustic pressure amplitude, tilted angle, and surface acoustic wave frequency on particle deflection are then analyzed. This model could be used to optimize the design and better understand the mechanism of taSSAW microfluidic devices.

Flexible and bendable acoustofluidics for particle and cell patterning

Surface Acoustic Wave (SAW) based microfluidic devices provide active techniques to manipulate fluid and particles, which can be used for precise and controllable patterning of microparticles and biological cells, with a high efficiency in a non-invasive and contact-free manner. This paper investigates flexible and bendable SAW microfluidic devices and explores the effects of bending and twisting of SAW devices on microparticle and cell patterning, using both experimental and numerical modelling. We showed that bending flexible SAW devices changes the distribution of particle pattern lines significantly. In devices with concave bending the particle pattern lines converge towards the centre of the curvature, whereas for devices with convex bending, they diverge away from it. Comparing the particle patterning using Lamb and Rayleigh wave devices with concave bending, we found that particle alignment is more efficient in the flexural mode Lamb wave device, whereas for the devices with convex bending, the particle patterning is more clear and regular when Rayleigh waves are used. We further investigated the effects of twisting the flexible SAW devices and observed that the particles are patterned into lines parallel to the deformed interdigital transducers (IDTs). We finally patterned yeast cells using our flexible SAW devices, and demonstrated the possibility of using our flexible acoustofluidic device for biomechanical systems such as body conforming technologies, wearable bio-sensors, and flexible point-of-care devices for personalized health monitoring.

On-chip surface acoustic wave and micropipette aspiration techniques to assess cell elastic properties.

The cytoskeletal mechanics and cell mechanical properties play an important role in cellular behaviors. In this study, in order to provide comprehensive insights into the relationship between different cytoskeletal components and cellular elastic moduli, we built a phase-modulated surface acoustic wave microfluidic device to measure cellular compressibility and a microfluidic micropipette-aspiration device to measure cellular Young's modulus. The microfluidic devices were validated based on experimental data and computational simulations. The contributions of structural cytoskeletal actin filament and microtubule to cellular compressibility and Young's modulus were examined in MCF-7 cells. The compressibility of MCF-7 cells was increased after microtubule disruption, whereas actin disruption had no effect. In contrast, Young's modulus of MCF-7 cells was reduced after actin disruption but unaffected by microtubule disruption. The actin filaments and microtubules were stained to confirm the structural alteration in cytoskeleton. Our findings suggest the dissimilarity in the structural roles of actin filaments and microtubules in terms of cellular compressibility and Young's modulus. Based on the differences in location and structure, actin filaments mainly contribute to tensile Young's modulus and microtubules mainly contribute to compressibility. In addition, different responses to cytoskeletal alterations between acoustophoresis and micropipette aspiration demonstrated that micropipette aspiration was better at detecting the change from actin cortex, while the response to acoustophoresis was governed by microtubule networks.

Particle separation in surface acoustic wave microfluidic devices using reprogrammable, pseudo-standing waves

We report size and density/compressibility-based particle sorting using on-off quasi-standing waves based on the frequency difference between two ultrasonic transducers. The 13.3 MHz fundamental operating frequency of the surface acoustic wave microfluidic device allows the manipulation of particles on the micrometer scale. Experiments, validated by computational fluid dynamics, were carried out to demonstrate size-based sorting of 5–14.5 μm diameter polystyrene (PS) particles and density/compressibility-based sorting of 10 μm PS, iron-oxide, and poly(methyl methacrylate) particles, with densities ranging from 1.05 to 1.5 g/cm3. The method shows a sorting efficiency of >90% and a purity of >80% for particle separation of 10 μm and 14.5 μm, demonstrating better performance than similar sorting methods recently published (72%–83% efficiency). The sorting technique demonstrates high selectivity separation of particles, with the smallest particle ratio being 1.33, compared to 2.5 in previous work. Density/compressibility-based sorting of polystyrene and iron-oxide particles showed an efficiency of 97 ± 4% and a purity of 91 ± 5%. By varying the sign of the acoustic excitation signal, continuous batch acoustic sorting of target particles to a desired outlet was demonstrated with good sorting stability against variations of the inflow rates.

Size-amplified acoustofluidic separation of circulating tumor cells with removable microbeads

Isolation and analysis of rare circulating tumor cells (CTCs) is of great interest in cancer diagnosis, prognosis, and treatment efficacy evaluation. Acoustofluidic cell separation becomes an attractive method due to its contactless, noninvasive, simple, and versatile features. However, the indistinctive physical difference between CTCs and normal blood cells limits the purity of CTCs using current acoustic methods. Herein, we demonstrate a size-amplified acoustic separation and release of CTCs with removable microbeads. CTCs selectively bound to size-amplifiers (40 μm-diameter anti-EpCAM/gelatin-coated SiO2 microbeads) have significant physical differences (size and mechanics) compared to normal blood cells, resulting in an amplification of acoustic radiation force approximately a hundredfold over that of bare CTCs or normal blood cells. Therefore, CTCs can be efficiently sorted out with size-amplifiers in a traveling surface acoustic wave microfluidic device and released from size-amplifiers by enzymatic degradation for further purification or downstream analysis. We demonstrate a cell separation from blood samples with a total efficiency (Etotal) of ∼ 77%, purity (P) of ∼ 96%, and viability (V) of ∼83% after releasing cells from size-amplifiers. Our method substantially improves the emerging application of rare cell purification for translational medicine.

Three-dimensional numerical simulation and experimental investigation of boundary-driven streaming in surface acoustic wave microfluidics.

Acoustic streaming has been widely used in microfluidics to manipulate various micro-/nano-objects. In this work, acoustic streaming activated by interdigital transducers (IDT) immersed in highly viscous oil is studied numerically and experimentally. In particular, we developed a modeling strategy termed the "slip velocity method" that enables a 3D simulation of surface acoustic wave microfluidics in a large domain (4 × 4 × 2 mm3) and at a high frequency (23.9 MHz). The experimental and numerical results both show that on top of the oil, all the acoustic streamlines converge at two horizontal stagnation points above the two symmetric sides of the IDT. At these two stagnation points, water droplets floating on the oil can be trapped. Based on these characteristics of the acoustic streaming field, we designed a surface acoustic wave microfluidic device with an integrated IDT array fabricated on a 128°YX LiNbO3 substrate to perform programmable, contactless droplet manipulation. By activating IDTs accordingly, the water droplets on the oil can be moved to the corresponding traps. With its excellent capability for manipulating droplets in a highly programmable, controllable manner, our surface acoustic wave microfluidic devices are valuable for on-chip contactless sample handling and chemical reactions.

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.

Analysis improvement of standing surface acoustic wave microfluidic devices for bio-particles separation

In this paper, the study and analysis to improve the Standing Surface Acoustic Wave (SSAW) microfluidic devices for particle separation are described. This study provides a theoretical analysis towards highly accurate particles separation compared to other published papers. To decrease the power consumption, the calculation of minimum AC signal power required to produce SSAW force for the separation process is briefly described and analysed. The horizontal component ratio of SSAW force and its attenuation in the liquid medium are analysed and represented. To estimate the effective width of collecting microchannels, the relations between the difference in volumes or densities of particles and the difference in the displacement of particles from the centre of the microchannel of SSAW microfluidic at the end of the SSAW force working area are represented. The separation of three different volumes of polystyrene particles and three different densities of particles in distilled water are simulated.

Analysis improvement of standing surface acoustic wave microfluidic devices for bio-particles separation

In this paper, the study and analysis to improve the Standing Surface Acoustic Wave (SSAW) microfluidic devices for particle separation are described. This study provides a theoretical analysis towards highly accurate particles separation compared to other published papers. To decrease the power consumption, the calculation of minimum AC signal power required to produce SSAW force for the separation process is briefly described and analysed. The horizontal component ratio of SSAW force and its attenuation in the liquid medium are analysed and represented. To estimate the effective width of collecting microchannels, the relations between the difference in volumes or densities of particles and the difference in the displacement of particles from the centre of the microchannel of SSAW microfluidic at the end of the SSAW force working area are represented. The separation of three different volumes of polystyrene particles and three different densities of particles in distilled water are simulated.

Continuous flow actuation between external reservoirs in small-scale devices driven by surface acoustic waves

We have designed and characterized a surface acoustic wave (SAW) fluid actuation platform that significantly improves the transmission of sound energy from the SAW device into the fluid in order to obtain enhanced performance. This is in distinct contrast to previous SAW microfluidic devices where the SAW substrate is simply interfaced with a microchannel without due consideration given to the direction in which the sound energy is transmitted into the fluid, thus resulting in considerable reflective and dissipative losses due to reflection and absorption at the channel walls. For the first time, we therefore demonstrate the ability for continuous fluid transfer between independent reservoirs driven by the SAW in a miniature device and report the associated pressure-flow rate relationship, in which a maximum flow rate of 100 μl min(-1) and pressure of 15 Pa were obtained. The pumping efficiency is observed to increase with input power and, at peak performance, offers an order-of-magnitude improvement over that of existing SAW micropumps that have been reported to date.