Ultrasound contrast agents: bubbles drops and particles

Abstract

The research described in this thesis is part of the research program NanoNextNL, a micro and nanotechnology consortium of the Government of the Netherlands and 130 partners. This thesis was carried out at the Physics of Fluids group of the Faculty of Science and Technology of the University of Twente. Cover design:As a representation of a crucial goal of this work, the cover depicts a confocal microscopy image showing cells selectively porated by loaded microbubbles upon ultrasound exposure. The porated cells take up a blue fluorescent probe. By Ine Lentacker, Ine De Cock and Guillaume Lajoinie. Guide through the thesisIn this thesis, we investigate different types of innovative agents, designed for biomedical use in a context of molecular imaging. In Chapter 1, we review the agents that arouse the interest of the scientific community, most of them at the research stage of development, together with the experimental methods that can be used to study their interaction with cells. Such in vitro investigations are necessary to test on short scales the impact of the various agents on biological structures.We separate the agents in three main categories. The first group consists of stabilized microbubbles, usually by means of phospholipids that fill up the interfacial area between the gas core and the surrounding liquid (water). Such bubbles are clinically used for 40 years as contrast agents for ultrasound owing to their unique resonance behavior. When irradiated with an ultrasound pulse, a bubble experiences volumetric oscillations, that reemit an ultrasound wave back to the transducer. This process makes them unchallenged ultrasound scatterers. The role of these stable microbubbles in new biomedical research directions is discussed in Chapters 2 to 9.Phospholipid coatings were developed in the last decade to do much more than just prevent bubble dissolution. They are now a crucial tool to enrich the microbubbles with specific markers or drug payloads. In Chapter 2, we investigate the behavior of this coating when the microbubbles are subjected to an ultrasound field. We show that the coating is shed away from the microbubble, which is of major interest for molecular imaging combined with drug delivery using microbubbles. We pursue the idea in Chapter 3 where we investigate how a bubble payload, here liposomes labeled with a fluorescent dye, is released under ultrasound exposure. In Chapter 4, we link these direct observations of shedding of the previous Chapters to the subsequent dissolution of the microbubbles, no longer protected by a phospholipid shell. This dissolution, visible in the ultrasound scatter spectrum can be used to remotely quantify the effect of the microbubbles. In support of these observations, we dedicate the next Chapter (Chapter 5) to physically understand the mechanisms underlying the behavior of the phospholipid coating. The application of these findings in practice requires the production a sufficiently high production rate and with great control of such microbubbles. In Chapter 6, we th...