Monica Almqvist

Multilayer piezoelectric copolymer transducers

Process technology for fabricating multilayer transducers of piezoelectric polymers without the need of adhesive lamination has been developed. The technology is based on spin-coating of the piezoelectric copolymer poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)). A comparison between single- and multilayer transducers of the same dimensions fabricated using similar processing schemes has been performed with respect of generating ultrasound. Multilayer transducers become increasingly important in miniaturized systems, due to the low permittivity of the piezoelectric polymer material. Multilayer technology makes it possible to subject the polymer material to high electrical fields at low drive voltages, and also increases the capacitance of the transducers for impedance matching purposes. It has been shown that the multilayer transducers outperform similar single-layer transducers by 12 dB during pitch-catch measurements.

Characterization of air-coupled ultrasound transducers in the frequency range 40 kHz–2 MHz using light diffraction tomography

The aim of this work was to show the applicability of light diffraction tomography on airborne ultrasound in the frequency range 40 kHz-2 MHz. Seven different air-coupled transducers were measured to show the methods performance regarding linearity, absolute pressure measurements, phase measurements, frequency response, S/N ratio and spatial resolution. A calibrated microphone and the pulse-echo method were used to evaluate the results. The absolute measurements agreed within the calibrated microphones uncertainty range. Pulse waveforms and corresponding FFT diagrams show the methods higher bandwidth compared with the microphone. Further, the method offers non-perturbing measurements with high spatial resolution, which was especially advantageous for measurements close to the transducer surfaces. The S/N ratio was higher than or in the same range as that of the two comparison methods.

High resolution light diffraction tomography: nearfield measurements of 10 MHz continuous wave ultrasound

Light diffraction tomography is an ultrasound measurement method that offers possibilities to make high spatial resolution measurements. The aim of this study was to investigate the spatial resolution of light diffraction tomography by characterizing the complex pressure and phase patterns 0.5-1 mm from the surface of a 5 mm diameter 10 MHz ultrasound transducer. Three different transducers were measured with a detection width of 5 microm, sample distances of 20-40 microm and 50-100 projections/180 degrees. They were excited with a 10 Vpp CW-signal. To verify the results the optical measurements were compared with 75 microm diameter hydrophone measurements and with theoretical calculations. The light diffraction tomography results show very good agreement with the hydrophone measurements and pressure peaks separated 160 microm were resolved. Comparison with the theoretical calculation shows that small distortions, caused by defects in the matching layer or in the piezo-electric disc, disturb the symmetric ring-pattern characteristic of an ideal piston source.

Temperature and trapping characterization of an acoustic trap with miniaturized integrated transducers - towards in-trap temperature regulation

An acoustic trap with miniaturized integrated transducers (MITs) for applications in non-contact trapping of cells or particles in a microfluidic channel was characterized by measuring the temperature increase and trapping strength. The fluid temperature was measured by the fluorescent response of Rhodamine B in the microchannel. The trapping strength was measured by the area of a trapped particle cluster counter-balanced by the hydrodynamic force. One of the main objectives was to obtain quantitative values of the temperature in the fluidic channel to ensure safe handling of cells and proteins. Another objective was to evaluate the trapping-to-temperature efficiency for the trap as a function of drive frequency. Thirdly, trapping-to-temperature efficiency data enables identifying frequencies and voltage values to use for in-trap temperature regulation. It is envisioned that operation with only in-trap temperature regulation enables the realization of small, simple and fast temperature-controlled trap systems. The significance of potential gradients at the trap edges due to the finite size of the miniaturized transducers for the operation was emphasized and expressed analytically. The influence of the acoustic near field was evaluated in FEM-simulation and compared with a more ideal 1D standing wave. The working principle of the trap was examined by comparing measurements of impedance, temperature increase and trapping strength with impedance transfer calculations of fluid-reflector resonances and frequencies of high reflectance at the fluid-reflector boundary. The temperature increase was found to be moderate, 7°C for a high trapping strength, at a fluid flow of 0.5mms(-1) for the optimal driving frequency. A fast temperature response with a fall time of 8s and a rise time of 11s was observed. The results emphasize the importance of selecting the proper drive frequency for long term handling of cells, as opposed to the more pragmatic way of selecting the frequency of the highest acoustic output. Trapping was demonstrated in a large interval between 9 and 11.5MHz, while the main trapping peak displayed FWHM of 0.5MHz. A large bandwidth enables a more robust manufacturing and operation while allowing the trapping platform to be used in applications where the fluid wavelength varies due to external variations in fluid temperature, density and pressure.

Characterisation and comparison of a cMUT versus a piezoelectric transducer for air applications

The main objectives with this investigation are firstly to compare important parameters of a capacitive micromachined ultrasonic transducer (cMUT) for air applications with a corresponding piezoelectric ultrasonic transducer. Secondly, to characterise the cMUTs radiation field using light diffraction tomography. The cMUTs consist of a silicon backplate with small pyramid-shaped cavities and a metallised polymeric membrane. The piezoelectric transducers are nonfocused with a resonance frequency of 450 kHz. Time waveforms, FFT-spectra, S/N-ratio, linearity plots and distance working range are presented for different kinds of combinations of cavity size, membrane thickness and bias voltage. Pressure maps from light diffraction tomography measurements are also presented. The principal conclusions are that the cMUTs have comparable linearity and sensitivity but exhibit superior bandwidth to the compared piezoelectric transducers.

ULTRASOUND DOPPLER VECTOR TOMOGRAPHY MEASUREMENTS OF DIRECTIONAL BLOOD FLOW

An experimental system has been developed to verify the possibility of detecting flow activity using a technique called ultrasound Doppler vector tomography. A tomography algorithm is used to reconstruct blood flow vector fields using data from computer-controlled ultrasound continuous-wave Doppler scanning equipment. The result is a picture in which the brightness variations represent the reconstructed values of the curl of the velocity field (inverted delta x v). Continuous ultrasound is transmitted into a region with flow activity and the Doppler-shifted signals are received. To obtain measurement data suited for fan beam tomography, the scanning is performed in a plane from points encircling the region. Reconstructions have been achieved using measurement data from two different flow phantoms. A comparison between the experimental results and simulations shows good conformity.

47-channel burst-mode recording hydrophone system enabling measurements of the dynamic echolocation behavior of free-swimming dolphins

Detailed echolocation behavior studies on free-swimming dolphins require a measurement system that incorporates multiple hydrophones (often >16). However, the high data flow rate of previous systems has limited their usefulness since only minute long recordings have been manageable. To address this problem, this report describes a 47-channel burst-mode recording hydrophone system that enables highly resolved full beamwidth measurements on multiple free-swimming dolphins during prolonged recording periods. The system facilitates a wide range of biosonar studies since it eliminates the need to restrict the movement of animals in order to study the fine details of their sonar beams.

Outreach initiatives operated by universities for increasing interest in science and technology

ABSTRACT Since the 1990s, the low number of students choosing to study science and technology in higher education has been on the societal agenda and many initiatives have been launched to promote awareness regarding career options. The initiatives particularly focus on increasing enrolment in the engineering programmes. This article describes and compares eight European initiatives that have been established and operated by universities (and in some cases through collaboration with other actors in society). Each initiative is summarised in a short essay that discusses motivation, organisation, pedagogical approach, and activities. The initiatives are characterised by comparing the driving forces behind their creation, how the initiative activities relate to the activities at the university, size based on the number of participants and cost per participant and pedagogical framework. There seem to be two main tracks for building outreach activities, one where outreach activities are based on the universitys normal activities, and one where outreach activities are designed specifically for the visiting students.

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 arid 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.

An evaluation of the temperature increase from pzt micro-transducers for acoustic trapping

This paper reports a comparison of soft and hard piezoceramic transducer materials used for ultrasonic standing wave particle trapping in a microfluidal bioanalytical platform. The investigation is made with the objective to obtain high acoustic forces with a minimum of temperature increase. Temperature is a critical parameter for bioassays and most often need to be kept below a certain level to allow handling of cells and proteins. The main conclusions in this paper are that it is possible to get efficient trapping with a temperature increase of only a few degrees and that a hard piezoceramic material has advantages in an application such as this.