Multifunctional nanogels for biomedical applications

Hildebrandt, Haika; Möller, Martin (Thesis advisor); Groll, Jürgen (Thesis advisor)

Aachen (2018)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2018

Abstract

This thesis focuses on the synthesis, functionalisation and characterisation of nanogels for biomedical applications. Two different synthetic routes for nanogel preparation were explored. The first approach dealt with the investigation of reactive self-assembly in water, whereas the second one made use of the synthesis in inverse miniemulsion. The nanogels were prepared in both cases from a six-arm, star-shaped pre-polymer with isocyanate end groups (NCO-star-(PEO-co-PPO)). The backbone of the polymeric building block consisted of a statistical copolymer of ethylene oxide and propylene oxide in a ratio of 4:1 with a molecular weight of 12 kDa. Isocyanate groups were introduced through reaction of the terminal hydroxy group with isophorone diisocyanate (IPDI). The amphiphilic polymer backbone in combination with the isocyanate end groups enabled the synthesis of densely crosslinked particles by reactive self-assembly in water. The particles were stable upon dilution and external influences like salt, buffer and temperature changes. The size of the nanogels could be controlled by the initial concentration of the pre-polymer. The isocyanate end groups allowed not only covalent cross-linking of the pre-polymer, but also further modification with targeting peptides. Time and concentration dependent in vitro studies with HeLa cells demonstrated that the nanogels were cytocompatible. Moreover, specific cellular uptake was observed for the peptide functionalised nanogels. With regard to drug delivery applications, the incorporation of a hydrophobic model compound was successfully accomplished. Hybrid particles were prepared by encapsulation of iron oxide nanoparticles. Monodisperse magnetite particles were synthesized by thermal decomposition of iron(III)acetylacetonate (Fe(acac)3). The hydrophobic, oleic acid coated particles were encapsulated by the NCO-star-(PEO-co-PPO) pre-polymer in toluene and transferred to water where nanogel formation took place. The aqueous dispersions were stable for more than one year. TEM studies revealed that the magnetite particles were embedded as isolated particles. MRI analysis that were performed on a nanogel coated mesh revealed the capability of the nanogels to be applied as contrast agent. Targeting peptides and labelling tags could be immobilised via a maleimide linker. Cell tests showed no influence on the cell viability of Huvec cells. Selective uptake of the hybrid particles in hCMEC, hDEMC and Huvec cells could be induced by immobilized targeting peptides. Nanogels could also be prepared by polyaddition reaction of the isocyanate functionalised pre-polymer with a bifunctional cross-linker in inverse miniemulsion. Since the hydrolysis of isocyanates is very slow, different diamine cross-linkers were explored. This resulted in monodisperse nanogels that constituted a replicate of the emulsion droplet. Biodegradable nanogels were obtained by using cystamine as cross-linker. The addition of a reducing agent led to cleavage of the disulfide bond. The size of the colloidal particles could be controlled by the amount of the emulsifiers Span 80 and Tween 80. Easy immobilization of tagging ligands was possible in situ as well as after nanogel synthesis. Moreover, the incorporation of inorganic magnetite particles could be accomplished. Furthermore, poly(t-butyl glycidyl ether)-b-poly(ethylene glycol)-b-poly(t-butyl glycidyl ether) triblock copolymers were synthesized and their use as emulsifier for the preparation of nanogels in inverse miniemulsion investigated. The copolymers were prepared by living anionic polymerisation of tert-butyl glycidyl ether using polyethylene glycol as macroinitiator. The use of these copolymers as emulsifiers led to stable emulsions with a small droplet size. As a result, it was also in this case possible to prepare nanogels as a replicate of the emulsion droplets.