Characterization of micromachined ultrasonic transducers using light diffraction tomography

This paper demonstrates that light diffraction tomography can be used to measure the acoustic field of micromachined ultrasonic transducers (MUT) in cases in which standard methods like hydrophone and microphone measurements fail. Two types of MUTs have been characterized with the method, one air-coupled capacitive MUT (cMUT) and one waterloaded continuous wave (CW) miniature multilayer lead zirconate titanate (PZT) transducer. Light diffraction tomography is an ultrasound measurement method with some special characteristics. Based on the interaction of light and ultrasound, it combines light intensity measurements with tomography algorithms to produce a measurement system. The method offers nonperturbing pressure measurements with high spatial resolution. It has been shown that, under certain circumstances, light diffraction tomography can be used as an absolute pressure measurement method with accuracy in the order of 10% in water and 13% in air. The results show that air-coupled cMUTs in the frequency range of about 1 MHz as well as the extreme near field of a miniaturized CW 10 MHz waterloaded transducer were successfully characterized with light diffraction tomography

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 functionalized glass surface and polystyrene micro-beads, and distinguish those that bind to the surface, ultimately by using an integrated optical waveguide implanted in the reflector to facilitate optical detection. The movement towards and immobilization of streptavidin coated beads onto a biotin functionalized 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 maxima 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 these quarter-wavelength resonances. Both 1-D acoustic modelling and finite element analysis has been used to design this device and 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

Miniaturized flowthrough microdispenser with piezoceramic tripod actuation

In this paper, the further development of a silicon flowthrough microdispenser is described. Previously reported designs of the dispenser used bimorph, and later multilayered, piezoelectric actuator elements for the generation of droplets. The introduction of a multilayered actuator significantly reduced the voltage amplitude needed to dispense droplets. Dispenser properties relevant for chemical analysis systems, e.g., reduced sample volume, internal surface area, and dispersion, were improved by miniaturization of the device. In this paper, a new actuator design, the tripod, is presented to enable further dispenser miniaturization and to facilitate device assembly. Tripod actuators were manufactured using a prototyping process, based on micromilling, for multilayer piezoceramic components. A building technique for miniaturized electrical interconnects, based on microstructured flexible printed circuits, is also suggested in line with the prospect of future miniaturization. The microfluidic properties of the tripod- actuated dispenser were evaluated. Stable droplet generation in the frequency range from 0 to 3 kHz was demonstrated, providing a maximum dispensed flow rate of 7.8 muL/min

Dynamic arraying of microbeads for bioassays in microfluidic channels

This paper proposes a new dynamic mode of generating bioanalytical arrays in microfluidic systems, based on ultrasonic trapping of microbeads using acoustic forces in standing waves. Trapping of microbead clusters in an array format within a flow-through device is demonstrated for the first time using a device with three integrated ultrasonic microtransducers. The lateral extension of each trapping site was essentially determined by the corresponding microtransducer dimensions, 0.8 mm x 0.8 mm. The flow-through volume was approximately 1 μ l and the trapping site volumes about 100 nl each. The strength of trapping was investigated, showing that 50% of the initially trapped beads were still trapped at a perfusion rate of 10 μ l/min. A fluorescence based avidin bioassay was successfully performed on biotin-coated microbeads trapped in the flow-through device, providing a first proof of principle of the proposed dynamic arraying concept. The dynamic arraying is believed to be expandable to two dimensions, thus, with a prospect of performing targeted and highly parallel protein analysis in microfluidic devices

Versatile microchip utilising ultrasonic manipulation of microparticles

This paper presents the concept and initial work on a microfluidic platform for bead-based analysis of biological sample. The core technology in this project is ultrasonic manipulation and trapping of particle in array configurations by means of acoustic forces. The platform is ultimately aimed for parallel multistep bioassays performed on biochemically activated microbeads (or particles) using submicrolitre sample volumes. A first prototype with three individually controlled particle trapping sites has been developed and evaluated. Standing ultrasonic waves were generated across a microfluidic channel by integrated PZT ultrasonic microtransducers. Particles in a fluid passing a transducer were drawn to pressure minima in the acoustic field, thereby being trapped and confined laterally over the transducer. It is anticipated that acoustic trapping using integrated transducers can be exploited in miniaturised total chemical analysis systems (μTAS), where e.g. microbeads with immobilised antibodies can be trapped in arrays and subjected to minute amounts of sample followed by a reaction, detected using fluorescence. Preliminary results indicate that the platform is capable of handling live cells as well as microbeads. A first model bioassay with detection of fluorescein marked avidin binding to trapped biotin beads has been evaluate

Acoustic resonances in straight micro channels: Beyond the 1D-approximation

Acoustic actuation can be used to perform several tasks in microfluidic systems. In this paper, we investigate an acoustic separator through micro-PIV analysis in stop-flow mode and numerical simulations, and a good agreement between the two is found. Moreover, we demonstrate that it is not sufficient only to characterize devices in flow-through mode, since in these systems much different resonant patterns can result in similarly looking band formations. Furthermore, we conclude that extended 1D approximations of the acoustic radiation force are inadvisable, and instead, a 2D model is preferred. The results presented here provide valuable insight into the nature and functionality of acoustic microdevices, and should be useful in the interpretation and understanding of the same