Rosemary J. Boltryk

Efficient finite element modeling of radiation forces on elastic particles of arbitrary size and geometry

A finite element based method is presented for calculating the acoustic radiation force on arbitrarily shaped elastic and fluid particles. Importantly for future applications, this development will permit the modeling of acoustic forces on complex structures such as biological cells, and the interactions between them and other bodies. The model is based on a non-viscous approximation, allowing the results from an efficient, numerical, linear scattering model to provide the basis for the second-order forces. Simulation times are of the order of a few seconds for an axi-symmetric structure. The model is verified against a range of existing analytical solutions (typical accuracy better than 0.1%), including those for cylinders, elastic spheres that are of significant size compared to the acoustic wavelength, and spheroidal particles.

Contrast agent-free sonoporation: The use of an ultrasonic standing wave microfluidic system for the delivery of pharmaceutical agents

Sonoporation is a useful biophysical mechanism for facilitating the transmembrane delivery of therapeutic agents from the extracellular to the intracellular milieu. Conventionally, sonoporation is carried out in the presence of ultrasound contrast agents, which are known to greatly enhance transient poration of biological cell membranes. However, in vivo contrast agents have been observed to induce capillary rupture and haemorrhage due to endothelial cell damage and to greatly increase the potential for cell lysis in vitro. Here, we demonstrate sonoporation of cardiac myoblasts in the absence of contrast agent (CA-free sonoporation) using a low-cost ultrasound-microfluidic device. Within this device an ultrasonic standing wave was generated, allowing control over the position of the cells and the strength of the acoustic radiation forces. Real-time single-cell analysis and retrospective post-sonication analysis of insonated cardiac myoblasts showed that CA-free sonoporation induced transmembrane transfer of fluorescent probes (CMFDA and FITC-dextran) and that different mechanisms potentially contribute to membrane poration in the presence of an ultrasonic wave. Additionally, to the best of our knowledge, we have shown for the first time that sonoporation induces increased cell cytotoxicity as a consequence of CA-free ultrasound-facilitated uptake of pharmaceutical agents (doxorubicin, luteolin, and apigenin). The US-microfluidic device designed here provides an in vitro alternative to expensive and controversial in vivo models used for early stage drug discovery, and drug delivery programs and toxicity measurements.

HeLa Cell Transfection Using a Novel Sonoporation System

Sonoporation has been shown to have an important role in biotechnology for gene therapy and drug delivery. This pa per presents a novel microfluidic sonoporation system that achieves high rates of cell transfection and cell viability by operating the sonoporation chamber at resonance. The paper presents a the oretical analysis of the resonant sonoporation chamber design, which achieves sonoporation by forming an ultrasonic standing wave across the chamber. A piezoelectric transducer (PZT 26) is used to generate the ultrasound and the different material thick nesses have been identified to give a chamber resonance at 980 kHz. The efficiency of the sonoporation system was determined exper imentally under a range of sonoporation conditions and different exposures time (5,10,15, and 20 s, respectively) using HeLa cells and plasmid (peGFP-Nl). The experimental results achieve a cell transfection efficiency of 68.9% (analysis of variance, ANOVA,p <; 0.05) at the resonant frequency of 980 kHz at 100 Vp-p (19.5 MPa) with a cell viability of 77% after 10 s of insonication.

Multi-modal particle manipulator to enhance bead-based bioassays

By sequentially pushing micro-beads towards and away from a sensing surface, we show that ultrasonic radiation forces can be used to enhance the interaction between a functionalised glass surface and polystyrene micro-beads, and identify those that bind to the surface by illuminating bound beads using an evanescent field generated by guided light. The movement towards and immobilisation of streptavidin coated beads onto a biotin functionalised waveguide surface is achieved by using a quarter-wavelength mode pushing beads onto the surface, while the removal of non-specifically bound beads uses a second quarter-wavelength mode which exhibits a kinetic energy maximum at the boundary between the carrier layer and fluid, drawing beads towards this surface. This has been achieved using a multi-modal acoustic device which exhibits both of these quarter-wavelength resonances. Both 1-D acoustic modelling and finite element analysis has been used to design this device and to investigate the spatial uniformity of the field. We demonstrate experimentally that 90% of specifically bound beads remain attached after applying ultrasound, with 80% of non-specifically bound control beads being successfully removed acoustically. This approach overcomes problems associated with lengthy sedimentation processes used for bead-based bioassays and surface (electrostatic) forces, which delay or prevent immobilisation. We explain the potential of this technique in the development of DNA and protein assays in terms of detection speed and multiplexing.

Efficient finite element modeling of acoustic radiation forces on inhomogeneous elastic particles

An efficient and flexible numerical model is developed to investigate acoustic radiation forces on a particle of arbitrary size, shape, elasticity, composition and material characteristics. The model compares well with analytical solutions and has suggested useful results on shape dependency, and the effects of shear waves in ultrasonic particle manipulation. We present initial results for the forces on inhomogeneous particles using the example of a cell with a nucleus inside.

Chapter 7. Modelling and Applications of Planar Resonant Devices for Acoustic Particle Manipulation

This chapter 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 presented.

Controlling non-inertial cavitation microstreaming for applications in biomedical research

Within this paper non-inertial cavitation microstreaming is investigated as a method for applying a mechanical stress to biological cells in vitro. A microfluidic device is designed in which microstreaming generated around microbubbles adhered to the floor of the acoustic chamber can be targeted at cells. The repeatability and controllability of microstreaming are investigated by computing μPIV vector fields of microstreaming flows. The uniformity of the acoustic pressure field in the x-y plane of the acoustic chamber is characterized by measuring the prevalence of microstreaming throughout the device.

A new thin-reflector mode for ultrasonic particle manipulation in layered resonators

Previous literature has described two major classes of sub-wavelength planar acoustic particle manipulation devices: (a) those where the dominant resonance is in the fluid layer, leading to agglomeration at one or more pressure nodes within the fluid layer; and (b) those where a resonant reflector layer provides a pressure release boundary condition, causing the agglomeration position to occur at a pressure node close to the fluid/ reflector interface (Quarter-wave devices). We describe here a new arrangement which operates at the first thickness resonance of a layered structure. This leads to pressure nodes at the air boundaries of a device. By designing with only a thin reflector layer (significantly less than λ/4) particles at all positions within the channel are forced to the reflector/fluid layer boundary. We model and experimentally characterize a device, and show that it can produces forces of order 55pN on a 10µm diameter polystyrene bead with transducer excitation of 25Vpp. We also explore the parameter space to find optimum designs, and present a particle concentration device using this mode. We demonstrate that this configuration will work efficiently with lossy polymer reflector layers, making possible cheap, disposable devices.

Micromanipulation of cells and particles using ultrasonic fields

Ultrasonic standing waves allow concentration, washing, fractionation, or trapping against a flow of cells in microfluidic environments and can potentially enhance biosensor performance.

Development of an ultrasonic particle trap incorporating 3-dimensional geometric features

This paper describes the development of a device designed to collect a concentrate of cells for environmental monitoring and bio-hazard detection using ultrasonic radiation forces. As an alternative to a continuous flow-through design it is proposed to use batch trapping, periodically diverting the trapped cells through a high concentration outlet. Trapping against the flow is possible if acoustic radiation forces oppose fluid drag forces, but the challenge is to generate these strong lateral forces. This device uses a series of machined pegs which promote lateral variations within the acoustic field, thus generating lateral forces to oppose fluid drag forces. It has been shown that 20 and 1 µm diameter polystyrene beads can be trapped against a flow and then eluted in order to collect a 10 times concentrate of the particles. The pegs promote lateral trapping forces generated by a combination of enclosure and structural modes, especially of the pegs themselves. The fabrication method allows a large number of geometries to be explored and therefore there is much scope to optimize peg design and position. In summary, the concept of this device (geometric design and operation) offers the potential for practical advances for sample processing in sensor instruments.