Christine Demore

Independent trapping and manipulation of microparticles using dexterous acoustic tweezers

An electronically controlled acoustic tweezer was used to demonstrate two acoustic manipulation phenomena: superposition of Bessel functions to allow independent manipulation of multiple particles and the use of higher-order Bessel functions to trap particles in larger regions than is possible with first-order traps. The acoustic tweezers consist of a circular 64-element ultrasonic array operating at 2.35 MHz which generates ultrasonic pressure fields in a millimeter-scale fluid-filled chamber. The manipulation capabilities were demonstrated experimentally with 45 and 90-μm-diameter polystyrene spheres. These capabilities bring the dexterity of acoustic tweezers substantially closer to that of optical tweezers.

Array-controlled ultrasonic manipulation of particles in planar acoustic resonator

Ultrasonic particle manipulation tools have many promising applications in life sciences, expanding on the capabilities of current manipulation technologies. In this paper, the ultrasonic manipulation of particles and cells along a microfluidic channel with a piezoelectric array is demonstrated. An array integrated into a planar multilayer resonator structure drives particles toward the pressure nodal plane along the centerline of the channel, then toward the acoustic velocity maximum centered above the subset of elements that are active. Switching the active elements along the array moves trapped particles along the microfluidic channel. A 12-element 1-D array coupled to a rectangular capillary has been modeled and fabricated for experimental testing. The device has a 300-μm-thick channel for a half-wavelength resonance near 2.5 MHz, with 500 μm element pitch. Simulation and experiment confirm the expected trapping of particles at the center of the channel and above the set of active elements. Experiments demonstrated the feasibility of controlling the position of particles along the length of the channel by switching the active array elements.

Real-time volume imaging using a crossed electrode array

This paper describes a unique crossed electrode array for real-time volume ultrasound imaging. By placing orthogonal linear array electrode patterns on the opposite sides of a hemispherically shaped composite transducer substrate, a 2D array can be fabricated using a small fraction of the elements required for a traditional 2D array. The performance of the array is investigated using a computer simulation of the radiation pattern. We show that by using a 288-element crossed electrode pattern it is possible to collect large field of view volume images (60deg times 60degsector) at real-time frame rates (>20 volume images/s), with image contrast and resolution comparable to what can be obtained using a conventional 128-element linear phased array.

Piezoelectric Micromachined Ultrasound Transducer (PMUT) Arrays for Integrated Sensing, Actuation and Imaging

Many applications of ultrasound for sensing, actuation and imaging require miniaturized and low power transducers and transducer arrays integrated with electronic systems. Piezoelectric micromachined ultrasound transducers (PMUTs), diaphragm-like thin film flexural transducers typically formed on silicon substrates, are a potential solution for integrated transducer arrays. This paper presents an overview of the current development status of PMUTs and a discussion of their suitability for miniaturized and integrated devices. The thin film piezoelectric materials required to functionalize these devices are discussed, followed by the microfabrication techniques used to create PMUT elements and the constraints the fabrication imposes on device design. Approaches for electrical interconnection and integration with on-chip electronics are discussed. Electrical and acoustic measurements from fabricated PMUT arrays with up to 320 diaphragm elements are presented. The PMUTs are shown to be broadband devices with an operating frequency which is tunable by tailoring the lateral dimensions of the flexural membrane or the thicknesses of the constituent layers. Finally, the outlook for future development of PMUT technology and the potential applications made feasible by integrated PMUT devices are discussed.

Characterization of piezocrystals for practical configurations with temperature- and pressure-dependent electrical impedance spectroscopy

Piezoelectric single crystal materials such as (x)Pb(Mg1/3Nb2/3)O3-(1-x)PbTiO3 (PMN-PT) have, by some measures, significantly better performance than established piezoelectric ceramics for ultrasound applications. However, they are also subject to phase transitions affecting their behavior at temperatures and pressures encountered in underwater sonar and actuator applications and in non-destructive testing at elevated temperatures. Materials with modified compositions to reduce these problems are now under development, but application-oriented characterization techniques need further attention. Characterization with temperature variation has been reported extensively, but the range of parameters measured is often limited and the effects of pressure variation have received almost no attention. Furthermore, variation in properties between samples is now rarely reported. The focus of this paper is an experimental system set up with commercially available equipment and software to carry out characterization of piezoelectric single crystals with variation in temperature, pressure, and electrical bias fields found in typical practical use. We illustrate its use with data from bulk thickness-mode PMN-29%PT samples, demonstrating variation among nominally identical samples and showing not only the commonly reported changes in permittivity with temperature for bulk material but also significant and complicated changes with pressure and bias field and additional ultrasonic modes which are attributed to material phase changes. The insight this provides may allow the transducer engineer to accelerate new material adoption in devices.

Acoustic devices for particle and cell manipulation and sensing.

An emerging demand for the precise manipulation of cells and particles for applications in cell biology and analytical chemistry has driven rapid development of ultrasonic manipulation technology. Compared to the other manipulation technologies, such as magnetic tweezing, dielectrophoresis and optical tweezing, ultrasonic manipulation has shown potential in a variety of applications, with its advantages of versatile, inexpensive and easy integration into microfluidic systems, maintenance of cell viability, and generation of sufficient forces to handle particles, cells and their agglomerates. This article briefly reviews current practice and reports our development of various ultrasonic standing wave manipulation devices, including simple devices integrated with high frequency (>20 MHz) ultrasonic transducers for the investigation of biological cells and complex ultrasonic transducer array systems to explore the feasibility of electronically controlled 2-D and 3-D manipulation. Piezoelectric and passive materials, fabrication techniques, characterization methods and possible applications are discussed. The behavior and performance of the devices have been investigated and predicted with computer simulations, and verified experimentally. Issues met during development are highlighted and discussed. To assist long term practical adoption, approaches to low-cost, wafer level batch-production and commercialization potential are also addressed.

Intraoperative Ultrasound-Guided Resection of Gliomas: A Meta-Analysis and Review of the Literature

BACKGROUND Image-guided surgery has become standard practice during surgical resection, using preoperative magnetic resonance imaging. Intraoperative ultrasound (IoUS) has attracted interest because of its perceived safety, portability, and real-time imaging. This report is a meta-analysis of intraoperative ultrasound in gliomas. METHODS Critical literature review and meta-analyses, using the MEDLINE/PubMed service. The list of references in each article was double-checked for any missing references. We included all studies that reported the use of ultrasound to guide glioma-surgery. The meta-analyses were conducted according to statistical heterogeneity between the studies using Open MetaAnalyst Software. If there was no heterogeneity, fixed effects model was used for meta-analysis; otherwise, a random effect model was used. Statistical heterogeneity was explored by χ(2) and inconsistency (I(2)) statistics; an I(2) value of 50% or more represented substantial heterogeneity. RESULTS A wide search yielded 19,109 studies that might be relevant, of which 4819 were ultrasound in neurosurgery; 756 studies used ultrasound in cranial surgery, of which 24 studies used intraoperative ultrasound to guide surgical resection and 74 studies used it to guide biopsy. Fifteen studies fulfilled our stringent inclusion criteria, giving a total of 739 patients. The estimated average gross total resection rate was 77%. Furthermore, the relationship between extent of surgical resection and study population was not linear. Gross total resection was more likely under IoUS when the lesion was solitary and subcortical, with no history of surgery or radiotherapy. IoUS image quality, sensitivity, specificity, and positive and negative predictive values deteriorated as surgical resection proceeded. CONCLUSION IoUS-guided surgical resection of gliomas is a useful tool for guiding the resection and for improving the extent of resection. IoUS can be used in conjunction with other complementary technologies that can improve anatomic orientation during surgery. Real-time imaging, improved image quality, small probe sizes, repeatability, portability, and relatively low cost make IoUS a realistic, cost-effective tool that complements any existing tools in any neurosurgical operating environment.

Transducer arrays for ultrasonic particle manipulation

Ultrasonic particle manipulation tools have many promising applications in life sciences research, expanding on the capabilities of current manipulation technologies. In this paper the feasibility of ultrasonic manipulation of particles and cells along a microfluidic channel with an array is investigated. An array integrated into a multilayer resonator structure drives particles towards the pressure nodal plane along the axis of the channel, then towards the acoustic velocity maximum centered above the driven elements. Switching the active elements along the array moves trapped particles along the microfluidic channel. A 1-D array coupled to a rectangular capillary has been simulated and fabricated for experimental testing. The device has a 300 µm thick channel for a half wavelength resonance near 2.5 MHz, and 500 µm element pitch. Simulation and experiment confirm the expected trapping of particles at the centre of the channel and above the set of driven elements. Experiments demonstrated the feasibility of controlling the position of particles along the length of the channel by switching the driven array elements.

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-6 Properties and Application-Oriented Performance of High Frequency Piezocomposite Ultrasonic Transducers

Routine high frequency piezocomposite fabrication is now becoming possible. However, specialized characterisation techniques and reference results are absent from the literature and there is an urgent demand for characterisation to support the use of this material in transducers for biomedical diagnosis and non-destructive evaluation. In this paper, we report on a novel procedure for high frequency piezocomposite fabrication and results of intensive characterisation based on electrical impedance spectroscopy and other techniques. 1-3 composite structures have been made with circular PZT pillars with diameter and kerf down to 12 mum and 6 mum respectively, aspect ratio > 3, volume fractions around 40% and active area of around 4 mm using a process which is capable of scalable commercial production. Experimental electrical impedance measurements have proved to be closely matched to one-dimensional models and piezocomposite material performance greatly exceeds that of other materials such as PVDF and LiNbO3. Comprehensive characterization of practical performance has been carried out including both through-transmission and pulse-echo measurements, generating single point results and high resolution 1D and 2D scans to produce 2D and 3D data sets, including tungsten wires test objects to determine axial and lateral resolution and transducer sensitivity. A typical prototype focused transducer is described here, with f- number of 4 and centre frequency of 37 MHz. Performance investigation yielded a through-transmission -3 dB bandwidth of 24 MHz, pulse-echo -6 dB bandwidth of 22 MHz, and lateral and axial resolutions of 250 mum and 150 mum respectively.