Tobias Lilliehorn

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.

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. 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 /spl mu/L/min.

Fabrication of multilayer 2D ultrasonic transducer microarrays by green machining

This paper presents a fabrication technique for multilayer 2D ultrasonic transducer arrays based on wet building and mechanical machining of piezoelectric PZT ceramics. The arrays are intended for microparticle trapping and manipulation using ultrasonic standing waves in microfluidic devices. The developed fabrication technique is relevant also for the miniaturization of other multilayer piezoceramic components. Array elements were fabricated monolithically on a common carrier with voids integrated behind each element, to work as acoustic reflectors. Their function was later proved by identification of thickness resonances of the supported multilayer transducers. The voids were integrated by using sacrificial material that was removed during firing of the component. Electrodes, vias, sacrificial material and finally the array itself were structured in the green state using a high precision dicing saw. The fabrication technique with integrated multilayer electrodes and electrical vias was shown to allow for miniaturized array elements working above 10 MHz with lateral dimensions down to 350 × 350 µm2. The resolution of electrode patterning was below 10 µm with insulating spaces down to 30 µm. The smallest dimension of via interconnects was 20 µm and integrated voids were fabricated with lateral dimensions reaching from 10 µm to 600 µm.

Multilayer telescopic piezoactuator fabricated by a prototyping process based on milling

Abstract A miniature piezoceramic actuator with telescoping multilayer d33 tubes is presented. The monolithic structure was realized using a rapid prototyping process, which is evaluated and discussed. The process based on wet building, green machining and lamination has proven to be flexible, fast and suitable for small-scale prototyping of multilayer piezoceramic actuators. Green machining in a milling machine is not only used to shape the actual device, but also to pattern the internal electrodes in the multilayer structure. Patterning of internal electrodes using milling demonstrated as good pattern definition as an ordinary screen-printing process would give, but with a superior flexibility due to the lack of mask changes. A comparison of the fabricated actuator has been made with state of the art piezoelectric actuators utilizing magnifying mechanisms. The d33-actuated multilayer telescopic actuator has proved to be a promising component with a displacement amplification of about 5 and an energy density comparable to other levered actuator designs. Several routes to improve the performance of the first design are identified.

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.

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.

Temperature and Trapping Characterization of an Acoustic Lateral Trap for μ TAS

An acoustic lateral trap for application in non-contact trapping of cells or particles in a microfluidic channel in a μTAS (micro total analysis system) is characterized by temperature and trapping efficiency measurements. Temperature is measured by fluorescent response of Rhodamine B in the microchannel. Trapping efficiency is measured as the projected area of a trapped particle cluster counter-balanced by hydrodynamic force. One of the main objectives is to obtain quantitative values of the temperature in the fluidic channel to ensure safe handling of cells and proteins. Other objectives are to evaluate optimal drive parameters. The optimal frequency, when temperature and trapping is considered, was found to be the parallel resonance frequency.