Self-assembling microsphere-based dextran hydrogels for pharmaceutical applications

Abstract

In this thesis novel self-assembling hydrogels, based on physical interactions between dextran microgels, potentially suitable for controlled drug delivery and tissue engineering, are presented. Two different approaches to self-assemble the hydrogels were investigated: ionic interactions and hydrophobic interactions combined with stereocomplexation. In the first approach, dextran derivatized with hydroxyethyl methacrylate (dex-HEMA) was copolymerized with methacrylic acid or N,N-dimethylamino ethyl methacrylate resulting in negatively and positively charged microspheres, respectively, at physiological pH. Because 'opposites attract', hydrogel formation occurred through ionic interactions when aqueous dispersions of both types of microspheres were mixed. Importantly for application of these systems, it was demonstrated that the mainly elastic hydrogel has a reversible yield point, meaning that the material starts to flow above a certain applied stress, whereas gel reformation occurred when the stress is removed. The hydrogel strength was tailored by varying the water content of the system and the zeta-potential and size (distribution) of the microspheres. Injectability tests showed that hydrogels composed of equal amounts of oppositely charged microspheres (average particle size 7 micrometer) could be injected through 25G needles using a static load of 15 N, an ISO accepted value. Protein-loaded macroscopic gels were obtained by hydration of mixtures of oppositely charged microspheres with a protein solution. A continuous, diffusion-controlled release of model proteins (lysozyme, BSA and IgG) was observed for 25 to 60 days. Importantly, lysozyme was released quantitatively and with full preservation of its enzymatic activity in about 25 days, emphasizing the protein friendly gel-preparation technology. Swelling studies showed that dispersions containing cationic, neutral or anionic microspheres completely degraded within 30, 55 or 120 days, respectively. Combining the oppositely charged microspheres in different ratios makes it possible to tailor the network properties and the degradation behavior of these hydrogels, making these injectable matrices suitable for various applications in drug delivery and tissue engineering. In the second approach, hydrogels were obtained through hydrophobic self-assembly of and stereocomplexation between dex-HEMA microspheres grafted with oligolactates of opposite chirality. Both the number of oligolactate grafts (DS) and the degree of polymerization (DP) of the oligomers were varied. Rheological analysis of aqueous dispersions of oligolactate-grafted microspheres demonstrated that hydrophobic interactions between oligolactate chains on the surface of the microspheres resulted in the formation of an almost fully elastic gel. A mixture of microspheres substituted with L- or D-oligolactates of opposite chirality resulted in gels with the highest strength, likely due to stereocomplexation between the enantiomers. The network properties could be modulated by varying the solid content of the gel and the DS and the DP of the oligolactate grafts. The biocompatible nature of the material and the possibility to tailor the gel properties also makes this hydrogel system an attractive candidate for pharmaceutical and biomedical applications. In conclusion, these self-assembling microsphere-based hydrogels hold promise as injectable matrix for diffusion-controlled delivery of pharmaceutically active proteins or as cell-supporting scaffold in tissue engineering. Additionally they could offer the possibility to create an 'all-in-one' protein releasing scaffold: growth factors can be entrapped inside the dex-HEMA microspheres, while cells can be embedded in the macroscopic network.