Manipulation of in vitro toxicant sensors in an ultrasonic standing wave

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 presented.

Particle separation in microfluidics using a switching ultrasonic field

We present a new method for separation of micro-sized constituents with positive acoustic contrast factors in a microfluidic channel using ultrasound. The ultrasound field is switched between the first and third resonant modes of the fluid channel, and the suspended constituents are separated onto the side and center pressure nodal lines according to their sizes or acoustic contrast factors. Initial hydrodynamic focusing of the constituents within a region of the channel near to the side nodal line is a crucial step in this separation method. This new method is shown to provide a novel "parallel-stream" separation of two species of particles with good robustness. Prior numerical simulations provide essential information on this operating region and also the voltage cycle to be applied to the ultrasonic actuators for optimal separation. Experiments were conducted using a prototype of the design with polystyrene microspheres of different sizes to demonstrate the efficiency and robustness of the separation process.

Cell Adhesion Dynamics and Actin Cytoskeleton Reorganization in HepG2 Cell Aggregates

The temporal dependence of cytoskeletal remodelling on cell-cell contact in HepG2 cells has been established here. Cell-cell contact occurred in an ultrasound standing wave trap designed to form and levitate a 2-D cell aggregate, allowing intercellular adhesive interactions to proceed, free from the influences of solid substrata. Membrane spreading at the point of contact and change in cell circularity reached 50% of their final values within 2.2 min of contact. Junctional F-actin increased at the interface but lagged behind membrane spreading, reaching 50% of its final value in 4.4 min. Aggregates had good mechanical stability after 15 min in the trap. The implication of this temporal dependence on the sequential progress of adhesion processes is discussed. These results provide insight into how biomimetic cell aggregates with some liver cell functions might be assembled in a systematic, controlled manner in a 3-D ultrasound trap.

Acoustically levitated droplets: a contactless sampling method for fluorescence studies

Acoustic levitation is used as a new tool to study concentration-dependent processes in fluorescence spectroscopy. With this technique, small amounts of liquid and solid samples can be measured without the need for sample supports or containers, which often limits signal acquisition and can even alter sample properties due to interactions with the support material. We demonstrate that, because of the small sample volume, fluorescence measurements at high concentrations of an organic dye are possible without the limitation of inner-filter effects, which hamper such experiments in conventional, cuvette-based measurements. Furthermore, we show that acoustic levitation of liquid samples provides an experimentally simple way to study distance-dependent fluorescence modulations in semiconductor nanocrystals. The evaporation of the solvent during levitation leads to a continuous increase of solute concentration and can easily be monitored by laser-induced fluorescence.