Imprinted Polymer-Based Ultrasound Transducers for Medical Imaging

• Autor(a) principal: Gheorghica, Dan
• Data de Publicação: 2019
• Tipo de documento: Dissertação
• Idioma: eng
• Título da fonte: Repositório Científico de Acesso Aberto de Portugal (Repositórios Cientìficos)
• Texto Completo: http://hdl.handle.net/10362/90933
• topic: ultrasound technology; piezoelectricity; P(VDF-TrFE); pillars; imprinting; Domínio/Área Científica::Engenharia e Tecnologia::Nanotecnologia

This project focuses on the development of a new generation of imprinted polymer-based ultrasound transducers that can be directly processed on top of a backplane. It is intended to use thin-film technologies that were previously developed at Holst Centre for future application in large area flexible displays, to produce these transducers. This technology can promise affordable fabrication costs to develop large (> 10 x 10 cm) and flexible ultrasound arrays. The proposed device consists of a structured active layer of P(VDF-TrFE), in the form of 100 μm pillars, comprised between conductive electrodes. Upon application of an electric field, the vertical vibration of the piezoelectric material generates ultrasonic waves. Different approaches (e.g. lamination and imprint lithography) were investigated to structure P(VDF-TrFE) into pillars on a substrate. Experiments regarding film thickness, annealing parameters, pillars height and device poling conditions were conducted. Through a combination of pressure and temperature inside a differential pressure laminator, a PDMS mould was used to structure a commercial 50 μm sheet of P(VDF-TrFE) into pillars. Sputtering was used to deposit conductive MoCr electrodes onto the structured layer of P(VDF-TrFE). After poling the resulting device up to 6 kV, additional measurements were performed to obtain the subscript piezoelectric coefficient, the remanent polarization and the voltage correspondent to the coercive field. The resulting sample shows a remanent polarization value of 6.8 μC/cm2, a coercive voltage of 4100 V and a subscript d33 coefficient of 18 pC/N. In many samples, high voltage breakdown issues were encountered like burned spots and permanent electrode damage. Acoustic characterization and modelling tests were performed in collaboration with the TNO Acoustics and Sonar Group, where the device successfully transmitted and received ultrasonic waves with an 8 MHz frequency.