SOFT MATERIALS BASED ON VESICLES AND BIOPOLYMERS

Abstract

Vesicles are hollow spherical structures formed by the self-assembly of amphiphilic molecules in aqueous solution. They are of great interest for applications ranging from drug delivery and controlled release to separations and sensing. However, the limited stability of vesicles to external conditions such as pH, temperature or ionic strength has hampered their applicability. In this dissertation, we explore the integration of vesicles with biopolymers as a route to creating vesicle-bearing soft materials with increased stability. Two specific types of such materials are studied: (a) vesicle gels, where the vesicles are linked into a network by biopolymer chains; and (b) vesicle-loaded capsules, where the vesicles are embedded in biopolymer-based capsules. The materials we have developed could be useful for the controlled and targeted release of drugs, cosmetics, and other chemicals.

The first part of this dissertation focuses on vesicle gels, obtained by adding a hydrophobically-modified polysaccharide, chitosan (denoted as hm-chitosan) to a solution of surfactant vesicles. The resulting gel shows an elastic rheological response, and is able to hold its own weight upon tube inversion. Small-angle neutron scattering (SANS) and cryo-transmission electron microscopy (cryo-TEM) are used to confirm the existence of vesicles within the gel. Based on these results, the likely structure in these gels is a network of vesicles connected by hm-chitosan chains, with the hydrophobes on the polymer embedded in vesicle bilayers. The SANS and cryo-TEM data also reveal interesting differences in the morphology of the vesicles at low and high polymer concentrations. In particular, adding a high concentration of polymer to unilamellar vesicles is shown to transform some of these into bilamellar (double-bilayered) structures. A similar co-existence of unilamellar and bilamellar vesicles is observed in all eukaryotic cells, but this is the first systematic demonstration of the phenomenon in an in vitro formulation.

The final part of this dissertation focuses on vesicle-loaded capsules. Capsules are created spontaneously when a solution of a cationic biopolymer is added dropwise into a solution of an anionic biopolymer. The driving force for capsule formation is the electrostatic interaction between the biopolymers at the interface of the drop. We modify the above procedure to create capsules with embedded vesicles. Additionally, to demonstrate the potential use of these capsules in targeted drug delivery, we load them with magnetic nanoparticles, and attach antibodies to the capsule surface. Controlled release experiments are conducted with both the vesicle-bearing capsules and with the vesicle gels. In each case, a model dye encapsulated in the vesicles is shown to release slowly over an extended period of time due to the combination of transport resistances from the vesicle bilayer and the capsule/gel. The results indicate the potential utility of these materials for drug delivery applications.