On microdroplet chemistry and on-demand drug release systems

This thesis is divided into two parts. Part 1 relates to fundamentals of chemical reactions in microdroplets. Part 2 endeavors to develop an electrically controlled coating that can store and release drugs on demand for medical device applications. The connecting thread between the two parts is use of electric fields to manipulate ion gradients. Part 1 is motivated by experimental observations of unusual chemistry in aerosol microdroplets. It endeavors to establish a useful quantitative model of microdroplets and the chemistry happening within them. The main goal being to provide a strong foundation upon which to evaluate and consolidate the myriad of hypothesized mechanisms affecting chemical reactions in microdroplets. Charged surfactants are shown to give rise to an electric double layer (EDL) within microdroplets. This EDL is modeled using the Gouy-Chapman-Stern model in spherical coordinates and numerically solved. The Gouy-Chapman-Stern model, which is limited to strong electrolytes, is expanded upon to account for weak electrolytes in a geometry independent fashion. This allows for modeling of the EDL of microdroplets containing species in chemical equilibria. Several notable features of microdroplets emerge. First, significant concentration gradients exist within microdroplets - including a radial pH gradient. Depletion of ions to form the EDL can result in significant differences in concentration between the solution used to make the microdroplet and the microdroplet's center. Second, the strong electric field of the EDL is present in a significant portion of the microdroplet's volume. Mechanisms through which both the concentration gradient and electric field can affect the average reaction rate within a microdroplet are elaborated. Further consideration is given to chemical reactions which alter the EDL -- such as redox reactions of charged surfactants. A mechanism is proposed wherein chemical reactions can discharge the potential energy of the EDL; thereby gaining an additional driving force which acts in a similar manner to an applied voltage. This is demonstrated first for a one-dimensional case and then expanded to spherical microdroplets. Part 2 is motivated by a need in medical devices for triggerable release of drugs. The potential applications are wide ranging: an implantable epi-pen that can trigger on its own to counter act an allergic reaction; a glucose monitor that can give an emergency antidote to a diabetic who accidentally overdoses on insulin; implant coatings that can release a programable quantity of anti-inflammatory drugs, to prevent implant rejection while also minimizing the dose of anti-inflammatory drugs to prevent side effects. The goal of the project was to create coatings that when placed on an electrode could act as reservoirs that store a drug, and then release the drug on command by application of a voltage. Two different electrically triggerable mechanisms are developed, one of which is demonstrated in vivo. In both mechanisms, the drugs are stored as ions, confined to a location and the applied voltage is used to release the drugs on demand

Keywords: Chemistry.
Advisor: Zare Richard N.
Fayer Michael D.
Martinez Todd J. (Todd Joseph)