CHARGE-SHIFTING POLYCATIONS FOR BIOMATERIAL APPLICATIONS

Abstract

Polycations are used extensively in a wide range of applications from enhanced oil recovery, wastewater treatment, and to biomaterials for delivery of therapeutics. This thesis focuses on the design, synthesis, and study of polycations towards improving the cyto- and immuno- compatibility in polyelectrolyte-based biomaterial applications. Alginate-based polyelectrolyte encapsulation and DNA delivery were chosen as applications to demonstrate the properties of the polycations developed, as a proof of concept. The work in this thesis represents significant contributions to the field, in particular, in the study of polymers based on N,N-(dimethylamino)ethyl acrylate (DMAEA) as charge-shifting polycations where it provides an in-depth study of the hydrolysis of PDMAEA that probes the mechanism of its unique reactivity. The hydrolysis of polymers of DMAEA was found to be highly pH dependent and sensitive to various neighboring groups on its pendent side chain, polymer backbone, and of comonomers. The effects were hypothesized to be due to a combination of interactions including hydrogen-bonding, steric hinderance, as well as ionic, hydrophilic, and hydrophobic interactions. These studies led to the synthesis of polycations with various rates of hydrolysis, with half-lives ranging from years to minutes. As these esters hydrolyze, the polymers have the potential to undergo transitions in net charge from cationic to zwitterionic, followed by anionic, due to the formation of carboxylate groups. Hence, the polymers are known as “charge-shifting” polycations in the field. As a proof of concept, charge-shifting polycations based on DMAEA were studied in alginate-based polyelectrolyte materials for encapsulation. Copolymers of DMAEA and 3-aminopropyl methacrylamide (APM), called PAD copolymers, were used as coating materials with polyanionic calcium alginate hydrogel capsules. The charge-shifting ability was confirmed as the coatings were shown to degrade over time due to disruption of electrostatic interactions. It was further demonstrated that covalent cross-linking could preserve the structural integrity of the membrane coating. Electrophoretic mobility measurements of PAD coated microspheres confirmed the shift in net surface charge from cationic to anionic with progressive hydrolysis. The charge-shifting ability of PAD copolymers was then tested for improving cytocompatibility as DNA delivery agents. PAD copolymers were shown to condense 60bp DNA to form polyplexes with very high cellular uptake efficiency in comparison to the gold standard polycation in the field, branched polyethylenimine (PEI). Cells exposed to PAD copolymers with the greatest charge-shifting ability were shown to have improved viabilities relative to non-charge-shifting polycations. This was attributed to the reduction of cytotoxic cationic charge of the polymer with progressive DMAEA hydrolysis. The results described in this thesis provide fundamental structure–function information to help design and develop new polymers and materials with tunable properties towards biomaterial applications.