Prior research has demonstrated synthesis and vaporization of hydrocarbon (HC)-in-fluorocarbon (H/F/W) endoskeletal droplets in aqueous suspension through the use of a fluorosurfactant and their potential utility as photoacoustic contrast agents. Here, we describe the synthesis of novel fluorocarbon-in-hydrocarbon (F/H/W) exoskeletal droplets through the choice of phospholipid surfactant. Exoskeletal drops tended to form nonspherical structures due to the elasticity of the solid HC shell. Vaporization of exoskeletal droplets occurred over a longer time scale than endoskeletal droplets due to constricted vapor nucleation and growth, and the resulting bubbles were significantly smaller. The size distribution of the exoskeletal droplets and the resulting bubbles could be further reduced through filtration. In contrast to endoskeletal droplets, which rapidly vaporized from a single nucleation site, exoskeletal droplets formed multiple nucleation sites during vaporization. Heat treatment with quenching increased the number of nucleation sites on each vaporizing droplet. The improved biocompatibility and ability to control the process of vaporization for exoskeletal droplets may be advantageous for medical imaging and other applications.
Frantz et al. (Wed,) studied this question.