Scaleable production of microbubbles using an ultrasound-modulated microfluidic device.

Surfactant-coated gas microbubbles are widely used as contrast agents in ultrasound imaging and increasingly in therapeutic applications. The response of microbubbles to ultrasound can be strongly influenced by their size and coating properties, and hence the production method. Ultrasonic emulsification (sonication) is the most commonly employed method and can generate high concentrations of microbubbles rapidly, but with a broad size distribution, and there is a risk of contamination and/or degradation of sensitive components. Microfluidic devices provide excellent control over microbubble size, but are often challenging or costly to manufacture, offer low production rates (<106s-1), and are prone to clogging. In this study, a hybrid sonication-microfluidic or "sonofluidic" device was developed. Bubbles of ∼180 µm diameter were produced rapidly in a T-junction and subsequently exposed to ultrasound (71-73 kHz) within a microchannel, generating microbubbles (mean diameter: 1-2 µm) at a rate of >108s-1 using a single device. Microbubbles were prepared using either the sonofluidic device or conventional sonication, and their size, concentration, and stability were comparable. The mean diameter, concentration, and stability were found to be comparable between techniques, but the microbubbles produced by the sonofluidic device were all <5 µm in diameter and thus did not require any post-production fractionation.

Acoustic micro-vortexing of fluids, particles and cells in disposable microfluidic chips.

We demonstrate an acoustic platform for micro-vortexing in disposable polymer microfluidic chips with small-volume (20 µl) reaction chambers. The described method is demonstrated for a variety of standard vortexing functions, including mixing of fluids, re-suspension of a pellet of magnetic beads collected by a magnet placed on the chip, and lysis of cells for DNA extraction. The device is based on a modified Langevin-type ultrasonic transducer with an exponential horn for efficient coupling into the microfluidic chip, which is actuated by a low-cost fixed-frequency electronic driver board. The transducer is optimized by numerical modelling, and different demonstrated vortexing functions are realized by actuating the transducer for varying times; from fractions of a second for fluid mixing, to half a minute for cell lysis and DNA extraction. The platform can be operated during 1 min below physiological temperatures with the help of a PC fan, a Peltier element and an aluminum heat sink acting as the chip holder. As a proof of principle for sample preparation applications, we demonstrate on-chip cell lysis and DNA extraction within 25 s. The method is of interest for automating and chip-integrating sample preparation procedures in various biological assays.

Temperature-controlled MPa-pressure ultrasonic cell manipulation in a microfluidic chip.

We study the temperature-independent impact on cell viability of relevant physical parameters during long-term, high-acoustic-pressure ultrasonic exposure in a microfluidic chip designed for ultrasonic-standing-wave trapping and aggregation of cells. We use a light-intensity method and 5 µm polymer beads for accurate acoustic pressure calibration before injecting cells into the device, and we monitor the viability of A549 lung cancer cells trapped during one hour in an ultrasonic standing wave with 1 MPa pressure amplitude. The microfluidic chip is actuated by a novel temperature-controlled ultrasonic transducer capable of keeping the temperature stable around 37 °C with an accuracy better than ±0.2 °C, independently on the ultrasonic power and heat produced by the system, thereby decoupling any temperature effect from other relevant effects on cells caused by the high-pressure acoustic field. We demonstrate that frequency-modulated ultrasonic actuation can produce acoustic pressures of equally high magnitudes as with single-frequency actuation, and we show that A549 lung cancer cells can be exposed to 1 MPa standing-wave acoustic pressure amplitudes for one hour without compromising cell viability. At this pressure level, we also measure the acoustic streaming induced around the trapped cell aggregate, and conclude that cell viability is not affected by streaming velocities of the order of 100 µm s(-1). Our results are important when implementing acoustophoresis methods in various clinical and biomedical applications.

Measuring acoustic energy density in microchannel acoustophoresis using a simple and rapid light-intensity method.

We present a simple and rapid method for measuring the acoustic energy density in microchannel acoustophoresis based on light-intensity measurements of a suspension of particles. The method relies on the assumption that each particle in the suspension undergoes single-particle acoustophoresis. It is validated by the single-particle tracking method, and we show by proper re-scaling that the re-scaled light intensity plotted versus re-scaled time falls on a universal curve. The method allows for analysis of moderate-resolution images in the concentration range encountered in typical experiments, and it is an attractive alternative to particle tracking and particle image velocimetry for quantifying acoustophoretic performance in microchannels.