I. Introduction
Microbubbles are ultrasound contrast agents with diameters typically less than that are comprised of a polymer, albumin, or lipid shell that surrounds a gas core [1]. Microbubbles have demonstrated utility for molecular-targeted imaging [2]–[5] and therapeutic applications [6]–[10]. Molecular targeting is achieved by conjugating ligands to the microbubble shell, which are targeted to specific cell surface proteins, such as disease markers [1], [2], [11]. Thus, disease can be detected based on a molecular signature before anatomical or functional manifestation, providing the opportunity for preventative action such as drug therapy or lifestyle change [1]. In addition to early disease detection, enhanced local delivery of therapeutic agents (i.e., genes or drugs) has been demonstrated using microbubbles through a process known as sonoporation [12], [13]. A variety of methods for drug/gene loading of microbubbles have been adapted to incorporate different therapeutic agents (e.g., plasmid DNA, lipophilic drug, hydrophilic drug) according to solubility [9], [14]–[21]. Incorporation of a therapeutic agent into a molecular-targeted microbubble produces a microbubble capable of molecular targeting and therapy [17], [22]. The application of ultrasound in the presence of microbubbles and therapeutic agent simultaneously increases cell permeability and releases therapeutic agent at the precise location of disease, achieving enhanced therapeutic agent delivery relative to treatment with ultrasound and therapeutic agent without microbubbles [23]. Increasing acoustic pressure promotes unstable microbubble cavitation which enhances cell permeabilization but decreases cell viability f241, f251·