Ivo Leibacher

Acoustofluidics 6: Experimental characterization of ultrasonic particle manipulation devices

Because of uncertainties in material and geometrical parameters in ultrasonic devices, experimental characterization is an indispensable part in their successful application for the manipulation of particles or cells. Its miniaturized size precludes the use of many of the usual tools used for macroscopic systems. Also, a further challenge is the fact that the resulting motion due to the electromechanical actuation has both high frequency and small amplitudes. Contactless methods like laser interferometry are therefore promising methods. In addition, as long as there is strong electromechanical coupling between the transducer and the device also electrical measurements like admittance curves give insight into the frequencies at which the devices might work best. This is the case for example for piezoelectric transducers working at one of their resonance frequencies. Because the devices usually are used in resonant modes, narrow frequency detection methods like lock in amplifiers help to improve the signal to noise ratio. Also many analysis tools have been established in the context of modal analysis, which is based on frequency domain methods. Special emphasis is placed here on the determination of the quality factor Q of the resonator, as Q determines the efficiency of a device.

Enhanced single-cell printing by acoustophoretic cell focusing

Recent years have witnessed a strong trend towards analysis of single-cells. To access and handle single-cells, many new tools are needed and have partly been developed. Here, we present an improved version of a single-cell printer which is able to deliver individual single cells and beads encapsulated in free-flying picoliter droplets at a single-bead efficiency of 96% and with a throughput of more than 10 beads per minute. By integration of acoustophoretic focusing, the cells could be focused in x and y direction. This way, the cells were lined-up in front of a 40 μm nozzle, where they were analyzed individually by an optical system prior to printing. In agreement with acoustic simulations, the focusing of 10 μm beads and Raji cells has been achieved with an efficiency of 99% (beads) and 86% (Raji cells) to a 40 μm wide center region in the 1 mm wide microfluidic channel. This enabled improved optical analysis and reduced bead losses. The loss of beads that ended up in the waste (because printing them as single beads arrangements could not be ensured) was reduced from 52% ± 6% to 28% ± 1%. The piezoelectric transducer employed for cell focusing could be positioned on an outer part of the device, which proves the acoustophoretic focusing to be versatile and adaptable.

Ultrasonic robotics in microfluidic cavities

Ultrasonic standing waves are often used in biomedical applications. It has become quite common to move beads, cells, droplets, and other particles for sorting or biomedical analysis in microfluidic cavities by bulk acoustic waves or by vibrations excited by piezoelectric transducers. The motion of particles is determined by streaming and radiation forces. For the calculation of the radiation forces acting on single particles Gorkov’s potential is considered to be the modeling tool of choice, once the acoustic field and the properties of constituents (fluid and particle density and compressibility, respectively) are known. For the acoustic streaming, predictions can be made numerically. For both aspects large uncertainties exist, due to the complexity of the system and fluid structure interaction at multiple levels. In this paper, first various characterization tools for the acoustic field in the cavity are described. They consist of the interplay between numerical modeling of the device, impedance analys...

Experimental and numerical acoustofluidics in bulk acoustic wave devices at ETH Zurich

Ultrasonic fluid cavity resonances in acoustofluidic micro-devices can be exploited to miniaturize important operations for the handling of beads, cells, droplets, and other particles. With a growing number of experimentally tested unit operations, acoustofluidics holds increasing promise for emerging applications in bio- and microtechnology on lab-on-a-chip systems. We provide an overview of our research activities during the last years with a focus on the latest experimental setups and advances in the numerical simulation. Specifically, we present micro-devices with impedance matched cavity walls that allow a more flexible device design. Further, we show devices for the handling of fluid droplets and report on a method for the direct measurement of the acoustic radiation force on micro-particles. Due to the rapidly growing computational capabilities, numerical simulation has become a valuable tool in acoustofluidics research. We present a numerical model that accurately mimics the boundary layer damping...

Chapter 20:Experimental Characterization of Ultrasonic Particle Manipulation Devices

Because of uncertainties in the material and geometrical parameters of ultrasonic devices, experimental characterization is an indispensable part of their successful application for the manipulation of particles or cells. Their miniaturized size precludes the use of many of the usual tools used for macroscopic systems. Also, a further challenge is the fact that the resulting motion due to the electromechanical actuation has both high frequency and small amplitudes. Contactless methods such as laser interferometry and schlieren imaging are therefore promising methods. In addition, as long as there is strong electromechanical coupling between the transducer and the device, electrical measurements such as admittance curves give insight into the frequencies at which the devices might work best. This is the case, for example, for piezoelectric transducers working at one of their resonance frequencies. Because the devices are usually used in resonant modes, narrow frequency detection methods such as lock in amplifiers help to improve the signal to noise ratio. Also many analysis tools have been established in the context of modal analysis, which is based on frequency domain methods. Special emphasis is placed here on the determination of the quality factor Q of the resonator, as Q determines the efficiency of a device.