DEVELOPMENT OF PIEZOELECTRIC MEMS DEVICES

Ph.DDOCTOR OF PHILOSOPH

Additive manufacturing (3D print) of air-coupled diaphragm ultrasonic transdrucers

Air-coupled ultrasound is a non-contact technology that has become increasingly common in Non Destructive Evaluation (NDE) and material evaluation. Normally, the bandwidth of a conventional transducer can be enhanced, but with a cost to its sensitivity. However, low sensitivity is very disadvantageous in air-coupled devices. This thesis proposes a methodology for improving the bandwidth of an air-coupled micro-machined ultrasonic transducer (MUT) without sensitivity loss by connecting a number of resonating pipes of various length to a cavity in the backplate. This design is inspired by the pipe organ musical instrument, where the resonant frequency (pitch) of each pipe is mainly determined by its length. The −6 dB bandwidth of the "pipe organ" inspired air-coupled transducer is 55.7% and 58.5% in transmitting and receiving modes, respectively, which is ∼5 times wider than a custom-built standard device. After validating the concept via a series of single element low-frequency prototypes, two improved designs: the multiple element and the high-frequency single element pipe organ transducers were simulated in order to tailor the pipe organ design to NDE applications.Although the simulated and experimental performance of the pipe organ inspired transducers are proved to be significantly better than the conventional designs, conventional micro-machined technologies are not able to satisfy their required 3D manufacturing resolution. In recent years, there has been increasing interest in using additive manufacturing (3D printing) technology to fabricate sensors and actuators due to rapid prototyping, low-cost manufacturing processes, customized features and the ability to create complex 3D geometries at micrometre scale. This work combines the ultrasonic diaphragm transducer design with a novel stereolithographic additive manufacturing technique. This includes developing a multi-material fabrication process using a commercial digital light processing printer and optimizing the formula of custom-built functional (conductive and piezoelectric) materials. A set of capacitive acoustic and ultrasonic transducers was fabricated using the additive manufacturing technology. The additive manufactured capacitive transducers have a receiving sensitivity of up to 0.4 mV/Pa at their resonant frequency.Air-coupled ultrasound is a non-contact technology that has become increasingly common in Non Destructive Evaluation (NDE) and material evaluation. Normally, the bandwidth of a conventional transducer can be enhanced, but with a cost to its sensitivity. However, low sensitivity is very disadvantageous in air-coupled devices. This thesis proposes a methodology for improving the bandwidth of an air-coupled micro-machined ultrasonic transducer (MUT) without sensitivity loss by connecting a number of resonating pipes of various length to a cavity in the backplate. This design is inspired by the pipe organ musical instrument, where the resonant frequency (pitch) of each pipe is mainly determined by its length. The −6 dB bandwidth of the "pipe organ" inspired air-coupled transducer is 55.7% and 58.5% in transmitting and receiving modes, respectively, which is ∼5 times wider than a custom-built standard device. After validating the concept via a series of single element low-frequency prototypes, two improved designs: the multiple element and the high-frequency single element pipe organ transducers were simulated in order to tailor the pipe organ design to NDE applications.Although the simulated and experimental performance of the pipe organ inspired transducers are proved to be significantly better than the conventional designs, conventional micro-machined technologies are not able to satisfy their required 3D manufacturing resolution. In recent years, there has been increasing interest in using additive manufacturing (3D printing) technology to fabricate sensors and actuators due to rapid prototyping, low-cost manufacturing processes, customized features and the ability to create complex 3D geometries at micrometre scale. This work combines the ultrasonic diaphragm transducer design with a novel stereolithographic additive manufacturing technique. This includes developing a multi-material fabrication process using a commercial digital light processing printer and optimizing the formula of custom-built functional (conductive and piezoelectric) materials. A set of capacitive acoustic and ultrasonic transducers was fabricated using the additive manufacturing technology. The additive manufactured capacitive transducers have a receiving sensitivity of up to 0.4 mV/Pa at their resonant frequency

A comprehensive review on photoacoustic-based devices for biomedical applications

The photoacoustic effect is an emerging technology that has sparked significant interest in the research field since an acoustic wave can be produced simply by the incidence of light on a material or tissue. This phenomenon has been extensively investigated, not only to perform photoacoustic imaging but also to develop highly miniaturized ultrasound probes that can provide biologically meaningful information. Therefore, this review aims to outline the materials and their fabrication process that can be employed as photoacoustic targets, both biological and non-biological, and report the main components’ features to achieve a certain performance. When designing a device, it is of utmost importance to model it at an early stage for a deeper understanding and to ease the optimization process. As such, throughout this article, the different methods already implemented to model the photoacoustic effect are introduced, as well as the advantages and drawbacks inherent in each approach. However, some remaining challenges are still faced when developing such a system regarding its fabrication, modeling, and characterization, which are also discussed.This work was supported by Fundação para a Ciência e Tecnologia national funds, under the national support to R&D units grant, through the reference project UIDB/04436/2020 and UIDP/04436/2020

Photoacoustics for Cardiovascular Applications

In the thesis entitled Photoacoustic imaging for Cardiovascular Applications, two cardiovascular diseases were tackled, namely atrial fibrillation and coronary atherosclerosis. An imaging algorithm was also devised to enhance imaging target super-localization. Photoacoustic imaging is an imaging modality which provides molecular information, based on optical absorption and subsequent thermoelastic expansion resulting in detectable pressure waves with common ultrasonic detectors. Capability of imaging tissue molecular changes was shown relevant to enable real-time monitoring of lesion formation in catheter-based ablation for atrial fibrillation as well as to assess lipid content of atherosclerotic plaques in an anima