Acoustofluidics 17: theory and applications of surface acoustic wave devices for particle manipulation

In this paper, number 17 of the thematic tutorial series “Acoustofluidics – exploiting ultrasonic standing waves, forces and acoustic streaming in microfluidic systems for cell and particle manipulation” – we present the theory of surface acoustic waves (SAWs) and some related microfluidic applications. The equations describing SAWs are derived for a solid-vacuum interface before generalisations are made about solid-solid and solid-fluid interfaces. Techniques for SAW generation are discussed before an overview of applications is presented

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

This article introduces the design, construction and applications of planar resonant devices for particle and cell manipulation. These systems rely on the pistonic action of a piezoelectric layer to generate a one dimensional axial variation in acoustic pressure through a system of acoustically tuned layers. The resulting acoustic standing wave is dominated by planar variations in pressure causing particles to migrate to planar pressure nodes (or antinodes depending on particle and fluid properties). The consequences of lateral variations in the fields are discussed, and rules for designing resonators with high energy density within the appropriate layer for a given drive voltage presente

Mode-switching: a new technique for electronically varying the agglomeration position in an acoustic particle manipulator

Acoustic radiation forces offer a means of manipulating particles within a fluid. Much interest in recent years has focussed on the use of radiation forces in microfluidic (or “lab on a chip”) devices. Such devices are well matched to the use of ultrasonic standing waves in which the resonant dimensions of the chamber are smaller than the ultrasonic wavelength in use. However, such devices have typically been limited to moving particles to one or two predetermined planes, whose positions are determined by acoustic pressure nodes/anti-nodes set up in the ultrasonic standing wave. In most cases devices have been designed to move particles to either the centre or (more recently) the side of a flow channel using ultrasonic frequencies that produce a half or quarter wavelength over the channel, respectively.It is demonstrated here that by rapidly switching back and forth between half and quarter wavelength frequencies – mode-switching – a new agglomeration position is established that permits beads to be brought to any arbitrary point between the half and quarter-wave nodes. This new agglomeration position is effectively a position of stable equilibrium. This has many potential applications, particularly in cell sorting and manipulation. It should also enable precise control of agglomeration position to be maintained regardless of manufacturing tolerances, temperature variations, fluid medium characteristics and particle concentration

Effects of surface profile on a boundary-driven acoustic streaming field

Acoustic streaming fields in two-dimensional rectangular enclosures that have structured boundaries are simulated and the effects of surface profile amplitude on a boundary-driven acoustic streaming field are numerically investigated. The standing wave fields in the enclosures are generated by excitation of a boundary and a sine-wave shaped profile on a boundary parallel to the particle oscillations is considered. This surface profile is found to have a large influence on the magnitude of both outer and inner streaming velocities. In terms of streaming pattern, it is found that the number of inner streaming vortices is dependent on the wavelength of profile while this profile has a less significant effect on the outer vortex pattern

Numerical simulation of 3D acoustophoretic motion of microparticles in an acoustofluidic device

Acoustic streaming is typically found in addition to acoustic radiation forces in acoustofluidic devices. Simulation of acoustic streaming is a crucial step for the understanding of its origins, which can provide efficient guidance on creating designs to limit or control this phenomenon. However, most existing methods can only simulate the streaming field in a local area, typically a cross-section of fluid channel. In this work, the three-dimensional (3D) Rayleigh streaming pattern in an acoustofluidic device is simulated and its effects on the movement of microparticles with various sizes are demonstrated. The viability of the simulation of 3D Rayleigh streaming presented here not only can provide better understanding and more comprehensive prediction of experiments in full acoustofluidic devices, but also can offer instructions on the simulation of unusual acoustic streaming patterns, e.g. transducer-plane streamin

Effects of surface profile on a boundary-driven acoustic streaming field

Control of boundary-driven streaming in acoustofluidic systems is vital for various microfluidic applications either to generate it as a positive mechanism (e.g. microfluidic mixing, heat/mass transfer and fluid pumping) or suppressing it as an undesired disturbance (e.g. particle/cell focusing). It has been shown that two-dimensional (2D) and three-dimensional (3D) boundary-driven streaming can be solved from the limiting velocity method as long as the curvature of the surface is small compared to the viscous penetration depth. In this work, acoustic streaming fields in 2D rectangular enclosures that have structured textures, which do not satisfy this condition are numerically studied by full modelling of Reynolds stresses and the effects of surface profile amplitude on a boundary-driven acoustic streaming field are investigated. Specifically, a sine-wave shaped profile on a boundary parallel to the particle oscillations is considered, which is found to have large influences on both the magnitude of acoustic streaming velocities and streaming patterns

Modelling and control of acoustic streaming in standing wave fields

In acoustofluidic particle manipulation and sorting devices streaming flows are typically found in addition to the acoustic radiation forces. Understanding their origins is essential for creating designs to limit or control this phenomenon.In addition to the classical Rayleigh streaming, experimental work from various groups has described ‘unusual’ acoustic streaming, transducer-plane streaming, typically a four-quadrant streaming pattern with the circulation parallel to the transducer face. 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[1] and Eckart streaming[2].In this work, both 3D Rayleigh streaming and transducer-plane streaming are investigated using both experimental and numerical methods. Furthermore, acoustic streaming field due to two orthogonal standing wave fields in a microfluidic device is simulated and analysed

Mithramycin encapsulated in polymeric micelles by microfluidic technology as novel therapeutic protocol for beta-thalassemia

This report shows that the DNA-binding drug, mithramycin, can be efficiently encapsulated in polymeric micelles (PM-MTH), based on Pluronic® block copolymers, by a new microfluidic approach. The effect of different production parameters has been investigated for their effect on PM-MTH characteristics. The compared analysis of PM-MTH produced by microfluidic and conventional bulk mixing procedures revealed that microfluidics provides a useful platform for the production of PM-MTH with improved controllability, reproducibility, smaller size, and polydispersity. Finally, an investigation of the effects of PM-MTH, produced by microfluidic and conventional bulk mixing procedures, on the erythroid differentiation of both human erythroleukemia and human erythroid precursor cells is reported. It is demonstrated that PM-MTH exhibited a slightly lower toxicity and more pronounced differentiative activity when compared to the free drug. In addition, PM-MTH were able to upregulate preferentially ?-globin messenger ribonucleic acid production and to increase fetal hemoglobin (HbF) accumulation, the percentage of HbF-containing cells, and their HbF content without stimulating ?-globin gene expression, which is responsible for the clinical symptoms of ß-thalassemia. These results represent an important first step toward a potential clinical application, since an increase in HbF could alleviate the symptoms underlying ß-thalassemia and sickle cell anemia. In conclusion, this report suggests that PM-MTH produced by microfluidic approach warrants further evaluation as a potential therapeutic protocol for ß-thalassemia.<br/

Acoustofluidic particle steering

Steering micro-objects using acoustic radiation forces is challenging for several reasons: Resonators tend to create fixed force distributions that depend primarily on device geometry, and even when using switching schemes, the forces are hard to predict a priori. In this paper an active approach is developed that measures forces from a range of acoustic resonances during manipulation using a computer controlled feedback loop based in matlab, with a microscope camera for particle imaging. The arrangement uses a planar resonator where the axial radiation force is used to hold particles within a levitation plane. Manipulation is achieved by summing the levitation frequency with an algorithmically chosen second resonance frequency, which creates lateral forces derived from gradients in the kinetic energy density of the acoustic field. Apart from identifying likely resonances, the system does not require a priori knowledge of the structure of the acoustic force field created by each resonance. Manipulation of 10 μm microbeads is demonstrated over 100 s μm. Manipulation times are of order 10 s for paths of 200 μm length. The microfluidic device used in this work is a rectangular glass capillary with a 6 mm wide and 300 μm high fluid chamber.</p