A therapeutic delivery platform has been investigated with the ultimate goal of designing a sustained protein release matrix utilizing an in-situ gelling, acidic polysaccharide derived from the Aloe vera plant. The Aloe vera polysaccharide (AvP) has been examined in order to determine how chemical composition, structure, molecular weight and solution behavior affect gelation and protein/peptide delivery. Correlations are drawn between structural characteristics and solution behavior in order to determine the impact of polymer conformation and solvation on gel formation under conditions designed to simulate nasal applications. Steady state and dynamic rheology, classic and dynamic light scattering, zeta potential, pulse field gradient nuclear magnetic resonance and fluorescence spectroscopy have been employed to gain insight into the effects of galacturonic acid content, degree of methylation, entanglement and ionic strength on both solution behavior and the hydrogel state which ultimately governs protein/peptide release.
This dissertation is divided into two sections. In the first section, a series of Aloe vera polysaccharides (AvP), from the pectin family have been structurally characterized indicating high galacturonic acid (GalA) content, low degree of methylester substitution (DM), low numbers of rhamnose residues and high molecular weight with respect to pectins extracted from traditional sources. The behavior of AvP was examined utilizing dilute solution, low-shear rheological techniques for specific molecular weight samples at selected conditions of ionic strength. From these dilute aqueous solution studies, the Mark-Houwink-Sakurada (MHS) constants (K and a), persistence length (Lp) and inherent chain stiffness (B parameter) were determined, indicating an expanded random coil in aqueous salt solutions. The critical concentration for transition from dilute to concentrated solution, Ce, was determined by measuring both the zero shear viscosity and fluorescence emission of the probe molecule 1,8-anilino-l -naphthalene sulphonic acid (1,8-ANS) as a function of polymer concentration. Correlations are drawn between viscosity experiments and measurement of zeta potential. Increased degrees of intermolecular interactions are responsible for a shift of Ce to lower polymer concentrations with increasing ionic strength. Additionally, dynamic rheology data are presented highlighting the ability of AvP to form gels at low polymer and calcium ion concentrations, exemplifying the technological potential of this polysaccharide for in-situ drug delivery. In the second section, properties of Aloe vera galacturonate hydrogels formed via Ca + crosslinking have been studied in regard to key parameters influencing gel formation including molecular weight, ionic strength and molar ratio of Ca2+ to COO functionality. Dynamic oscillatory rheology and pulsed field gradient NMR (PFG-NMR) studies have been conducted on hydrogels formed at specified Ca concentrations in the presence and absence of Na+ and K+ ions, in order to assess the feasibility of in situ gelation for controlled delivery of therapeutics. Aqueous Ca concentrations similar to those present in nasal and subcutaneous fluids induce the formation of elastic Aloe vera polysaccharide (AvP) hydrogel networks. By altering the ratio of Ca to COO functionality, networks may be tailored to provide elastic modulus (G\u27) values between 20 and 20,000 Pa. The Aloe vera polysaccharide exhibits time dependent phase separation in the presence of monovalent electrolytes. Thus the relative rates of calcium induced gelation and phase separation become major considerations when designing a system for in situ delivery applications where both monovalent (Na+, K+) and divalent (Ca2+) ions are present. PFG-NMR and fluorescence microscopy confirm that distinctly different morphologies are present in gels formed in the presence and absence 0.15 M NaCl. Curve fitting of theoretical models to experimental release profiles of fluorescein labeled dextrans indicate diffusion rates are related to hydrogel morphology. These studies suggest that for efficient in situ release of therapeutic agents, polymer concentrations should be maintained above the critical entanglement concentration (Ce, 0.60 wt%) when [Ca ]/[COO ] ratios are less than 1. Additionally, the monovalent electrolyte concentration in AvP solutions should not exceed 0.10 M prior to Ca2+ crosslinking
