Anne Bernassau

Manipulation of particles in two dimensions using phase controllable ultrasonic standing waves

The ability to manipulate dense micrometre-scale objects in fluids is of interest to biosciences with a view to improving analysis techniques and enabling tissue engineering. A method of trapping micrometre-scale particles and manipulating them on a two-dimensional plane is proposed and demonstrated. Phase-controlled counter-propagating waves are used to generate ultrasonic standing waves with arbitrary nodal positions. The acoustic radiation force drives dense particles to pressure nodes. It is shown analytically that a series of point-like traps can be produced in a two-dimensional plane using two orthogonal pairs of counter-propagating waves. These traps can be manipulated by appropriate adjustment of the relative phases. Four 5 MHz transducers (designed to minimize reflection) are used as sources of counter-propagating waves in a water-filled cavity. Polystyrene beads of 10 μm diameter are trapped and manipulated. The relationship between trapped particle positions and the relative phases of the four transducers is measured and shown to agree with analytically derived expressions. The force available is measured by determining the response to a sudden change in field and found to be 30 pN, for a 30 Vpp input, which is in agreement with the predictions of models of the system. A scalable fabrication approach to producing devices is demonstrated.

Controlling acoustic streaming in an ultrasonic heptagonal tweezers with application to cell manipulation

Acoustic radiation force has been demonstrated as a method for manipulating micron-scale particles, but is frequently affected by unwanted streaming. In this paper the streaming in a multi-transducer quasi-standing wave acoustic particle manipulation device is assessed, and found to be dominated by a form of Eckart streaming. The experimentally observed streaming takes the form of two main vortices that have their highest velocity in the region where the standing wave is established. A finite element model is developed that agrees well with experimental results, and shows that the Reynolds stresses that give rise to the fluid motion are strongest in the high velocity region. A technical solution to reduce the streaming is explored that entails the introduction of a biocompatible agar gel layer at the bottom of the chamber so as to reduce the fluid depth and volume. By this means, we reduce the region of fluid that experiences the Reynolds stresses; the viscous drag per unit volume of fluid is also increased. Particle Image Velocimetry data is used to observe the streaming as a function of agar-modified cavity depth. It was found that, in an optimised structure, Eckart streaming could be reduced to negligible levels so that we could make a sonotweezers device with a large working area of up to 13 mm × 13 mm.

Interactive manipulation of microparticles in an octagonal sonotweezer

An ultrasonic device for micro-patterning and precision manipulation of micrometre-scale particles is demonstrated. The device is formed using eight piezoelectric transducers shaped into an octagonal cavity. By exciting combinations of transducers simultaneously, with a controlled phase delay between them, different acoustic landscapes can be created, patterning micro-particles into lines, squares, and more complex shapes. When operated with all eight transducers the device can, with appropriate phase control, manipulate the two dimensional acoustic pressure gradient; it thus has the ability to position and translate a single tweezing zone to different locations on a surface in a precise and programmable manner.

Direct patterning of mammalian cells in an ultrasonic heptagon stencil

We describe the construction of a ultrasonic device suitable for micro patterning particles and cells for tissue engineering applications. The device is formed by seven transducers shaped into a heptagon cavity. By exciting two and three transducers simultaneously, lines or hexagonal shapes can be formed with beads and cells. Furthermore, phase control of the transducers allows shifting the standing waves and thus patterning at different positions on a surface in a controlled manner. The paper discusses direct patterning of mammalian cells by ultrasound “stencil”.

Patterning of microspheres and microbubbles in an acoustic tweezers

We describe the construction of an ultrasonic device capable of micro-patterning a range of microscopic particles for bioengineering applications such as targeted drug delivery. The device is formed from seven ultrasonic transducers positioned around a heptagonal cavity. By exciting two or three transducers simultaneously, lines or hexagonal shapes can be formed with microspheres, emulsions and microbubbles. Furthermore, phase control of the transducers allows patterning at any desired position in a controlled manner. The paper discusses in detail direct positioning of functionalised microspheres, emulsions and microbubbles. With the advantages of miniaturization, rapid and simple fabrication, ultrasonic tweezers is a potentially useful tool in many biomedical applications.

Acoustic levitation of a large solid sphere

We demonstrate that acoustic levitation can levitate spherical objects much larger than the acoustic wavelength in air. The acoustic levitation of an expanded polystyrene sphere of 50 mm in diameter, corresponding to 3.6 times the wavelength, is achieved by using three 25 kHz ultrasonic transducers arranged in a tripod fashion. In this configuration, a standing wave is created between the transducers and the sphere. The axial acoustic radiation force generated by each transducer on the sphere was modeled numerically as a function of the distance between the sphere and the transducer. The theoretical acoustic radiation force was verified experimentally in a setup consisting of an electronic scale and an ultrasonic transducer mounted on a motorized linear stage. The comparison between the numerical and experimental acoustic radiation forces presents a good agreement.

Characterisation of an epoxy filler for piezocomposite material compatible with microfabrication processes

High frequency ultrasound transducer arrays that can operate at frequencies above 30 MHz are needed for high resolution medical imaging. A key issue in the development of these miniature imaging arrays is the need for photolithographic patterning of array electrodes. To achieve this directly on a 1-3 piezocomposite requires not only planar, parallel and smooth surfaces, but also an epoxy composite filler that is resistant to chemicals, heat and vacuum. This paper reports the full characterisation of an epoxy filler suitable for fine-scale piezocomposite fabrication as well as photolithographic processes.

2F-5 Surface Preparation of 1-3 Piezocomposite Material for Microfabrication of High Frequency Transducer Arrays

A key issue in the development of ultrasound imaging arrays to operate at frequencies above 30 MHz is the need for photolithographic patterning of array electrodes. To achieve this directly on a 1-3 piezocomposite requires planar, parallel and smooth surfaces. This paper reports an investigation of the surface finishing of 1-3 piezocomposite material by mechanical lapping and/polishing that has demonstrated that excellent surface flatness can be obtained. Subsequently, high frequency array elements have been fabricated on these surfaces using a low temperature lift-off photolithography process. A 50 MHz linear array with 30 mum element pitch has been patterned on the lapped and polished surface of a low frequency 1-3 piezocomposite. Good electrode edge definition and electrical contact to the composite were obtained. Additionally, patterning has been demonstrated on a fine-scale composite, itself suitable for operation above 30 MHz.

Concepts and issues in piezo‐on‐3D silicon structures

Purpose – The purpose of this paper is to explore concepts and manufacturing issues for the emerging piezo on silicon technology being used in ultra‐sound devices. Development of 3D silicon‐on‐silicon structures is now under way. Additional functionality can be achieved using piezoelectric‐on‐silicon structures and work in this area has started. A commercialisation road map is required, specifying development of the design and fabrication techniques from research to high volume and lower volume high‐value manufacture of niche products.Design/methodology/approach – This conceptual paper outlines processes needed, along with their possible sources with illustrations of present capabilities. Included are surface finishing techniques such as grinding, bonding technology for dissimilar materials, and through‐wafer‐via fabrication. Control of acoustic propagation, thermal expansion and electric field fringing effects will be considered.Findings – Areas that require research and development are identified with p...

Progress towards wafer‐scale fabrication of ultrasound arrays for real‐time high‐resolution biomedical imaging

Purpose – High‐frequency transducer arrays that can operate at frequencies above 30 MHz are needed for high‐resolution medical ultrasound imaging. The fabrication of such devices is challenging not only because of the fine‐scale piezocomposite fabrication typically required but also because of the small size of arrays and their interconnects. The purpose of this paper is to present an overview of research to develop solutions for several of the major problems in high‐frequency ultrasound array fabrication.Design/methodology/approach – Net‐shape 1‐3 piezocomposites operating above 40 MHz are developed. High‐quality surface finishing makes photolithographic patterning of the array electrodes on these fine scale piezocomposites possible, thus establishing a fabrication methodology for high‐frequency kerfless ultrasound arrays.Findings – Structured processes are developed and prototype components are made with them, demonstrating the viability of the selected fabrication approach. A 20‐element array operating...