RESEARCH ARTICLE Copyright © 2014 American Scientific Publishers All rights reserved Printed in the United States of America Journal of Colloid Science and Biotechnology Vol. 3, 1–6, 2014 Investigating the Release Mechanism of Calcein from eLiposomes at Higher Temperatures Ghaleb A. Husseini 1 ∗ , William G. Pitt 2 , Jacob B. Williams 2 , and Marjan Javadi 2 1 American University of Sharjah, Chemical Engineering Department EB1-230, AUS, Sharjah, United Arab Emirates 2 Brigham Young University, Department of Chemical Engineering 350 CB, BYU, Provo, UT, 84604 Numerous nanocarriers are currently being investigated as drug delivery vehicles for transporting chemotherapeutics to cancer cells. Our research group has recently synthesized a new generation of echogenic liposomes based on the concept of encapsulating one or more nanoemulsion droplets inside a liposome (called an “eLiposome”). The concept is to use a nanoemulsion droplet with a low boiling point near body temperature, thus requiring only a small acoustic nudge to vaporize the droplet from liquid to gas and break open the eLiposome, thus releasing its contents. The purpose of this note is twofold. First, we wanted to show experimentally that eLiposomes remained stable at body temperature and retained their potential to deliver drugs through the ultrasonically-activated expansion of the emulsion nanodroplet. Additionally, we examined the physical mechanism poten- tially involved in the release of calcein at higher temperatures. Experimental results using calcein as a model drug confirmed the eLiposome stability at physiological temperatures and suggested that heterogeneous nucleation theory was capable of capturing the general release characteristics observed in this study. Heterogeneous nucleation of gas is possibly the main mechanism at play in passive release from eLiposomes at temperatures above body temperature. More research is needed to confirm the definitive physics of the model drug’s release from these novel nanovehicles. Keywords: eLiposomes Stability, Calcein Release, Heterogeneous Nucleation, Fluorometry, Acoustic Droplet Vaporization (ADV). 1. INTRODUCTION The first decade and a half of the 21st Century has seen a plethora of nanotechnology devices and con- cepts. This includes the field of medicine in which nanodevices have been employed in diagnostic and ther- apeutic applications. 1–3 Selective identification, precise delivery and successful repair are the hallmarks of use- able nanotechnology in medicine. In the realm of drug delivery, nanodevices employ passive triggering and active triggering to deliver a therapeutic only at a designated site or tissue. 4 Passively triggered release is actuated by variances in the local environment, such as pH, tem- perature or redox state, which produce a change in the nanocarriers to release the sequestered therapeutic. Active release is triggered by some external action that can be focused on a particular site in the body, such as light, other electromagnetic fields, pressure waves (including ultrasound), or thermal heating. This report describes an ultrasonically-activated nanodevice that tightly sequesters drugs or nucleic acids and releases them only upon expo- sure to ultrasound (insonation). ∗ Author to whom correspondence should be addressed. Our lab has developed a nanodevice called an eLipo- some (see Fig. 1) which consists of a nanosized liposome (a bilayer lipid shell enveloping an aqueous interior) con- taining the therapeutic and at least 1 nanodroplet of a per- fluorocarbon liquid with a low boiling point. 5–7 The name “eLiposome” comes from emulsion droplet in a liposome. Release from an eLiposome is triggered by ultrasound via the phenomenon of acoustic droplet vaporization, in which ultrasonic pressure waves cause a liquid droplet to change phase to a gas bubble of much larger volume. 8 The expansion of the liquid emulsion droplet to a gas bubble causes the rupture of the liposomal membrane sur- rounding it, and thus releases the therapeutic payload upon insonation. We have demonstrated the controlled release of calcein (a model drug), doxorubicin (a chemotherapeu- tic) and DNA plasmids using eLiposomes and ultrasound of low power and low frequency. 9 We have used per- fluoropentane (PFC5) and perfluorohexane (PFC6) as the emulsion nanodroplets in these eLiposomes. The former has a normal boiling point of 29  C and the latter of 56  C. They are both non-toxic and have very low solubility in water. J. Colloid Sci. Biotechnol. 2014, Vol. 3, No. 3 2164-9634/2014/3/001/006 doi:10.1166/jcsb.2014.1100 1