Name of the Student: Mr. Kaustav Roy
Advisors: Prof. Rudra Pratap, (CeNSE)
Date: 22nd April 2021 (Thursday), Time: 11 AM
Venu: Online on Microsoft Teams https://tinyurl.com/3sazhshc
Abstract:
Ultrasound is a scientific miracle with its magical contributions in so many branches of modern-day technology. Its connection with piezoelectrics started with the discovery of the ‘piezoelectric effect’ by the Curie brothers, who demonstrated electromechanical coupling in cane sugar, as a result of which, it was possible to generate electricity from a typical sugar crystal by straining it. This led to the dawn of piezoelectric ultrasound transducers, which revolutionized the way sound was produced till then. Comprehensive research followed these initial developments, which transcended the worlds of non-destructive techniques, medical diagnosis and prognosis, medical therapeutics, ranging and security, etc. The next stage of reformation in this research progression was written by the hands of the VLSI micromachining technique, which ushered the development of low-cost, low-powered, compact, and above all, ‘little’ transducers which can actually fit inside the smallest veins found inside the human body. These transducers are the present and the future of ultrasound and bear the fate of the next big technological change that the world is likely to witness in the upcoming years.
In this doctoral dissertation, we construct piezoelectric ultrasound transducers and put them to various novel applications. We divide the thesis into three parts, with the first part describing the design and fabrication of piezoelectric micromachined ultrasound transducer (PMUT) and applying them to real-time fluid density sensing in the macro-scale. We have described the various effects observed in the special kind of experimental arrangement, which was created to sense fluid density by using the transmission of ultrasound through the fluid. We name this arrangement as PMUT-Fluid-PMUT (PFP) which was observed to successfully sense fluid density over a broad density range.
In the second part of the thesis, we make a special kind of a PMUT, which is capable of self-sensing, while getting actuated. We apply this technique to fluid density sensing in a single platform, thereby eliminating the need for a through-transmission arrangement as described in the first part. We subsequently progress and integrate such self-actuation self-sensing PMUTs into microfluidic channels, thereby creating an independent device to handle and analyze fluid samples in small volumes. We call this system the PMUT-Microfluidic-Integration (PMI) which is suitable for sensing changes in the blood density at the microscale.
In the final part of the work, we explore a different branch of medical ultrasound, known as photoacoustics. We start off by fabricating flexible bulk ultrasound transducers, which conform to objects of any geometry and thereby image any targets present inside them. This is of significant importance in designing wearables for continuous human health monitoring applications by photoacoustically imaging the tissues underneath. Next, we roll on to making an optofluidic integration by using a pulsed laser and microfluidic channel, and we demonstrate the use of such a system to sense the concentration of a solute in a solvent. Finally, we fabricated a typical variant of a 32 channel PMUT array and demonstrate its application in photoacoustic imaging, thereby proving that PMUTs can function successfully as photoacoustic detectors. This development can lead to the creation of portable bedside health monitoring systems based on photoacoustics.
In summary, the work describes in a comprehensive manner the creation of both bulk and thin-film piezoelectric material-based ultrasound transducers and demonstrates some of their novel applications in both industry and biomedical diagnosis and imaging.