PhD Thesis
Dual-Responsive Polymer and Hybrid Systems: Applications for Gene Delivery and Hydrogels
Alexander Majewski (09/2013)
Support: Axel H. E. Müller
Summary
This thesis focused on the synthesis of functional materials based on water-soluble and responsive polymers, in particular poly((2-dimethylamino)ethyl methacrylate) (PDMAEMA). The dual-responsive behavior and polycationic character at physiological pH of PDMAEMA lead to outstanding properties and thus, to a versatile component for water-based applications. The main concept of the thesis was to combine the ability for gene delivery of PDMAEMA with the magnetic properties of iron oxide nanoparticles to enable an activity of the gene vector in an applied external magnetic field. Another point was to apply the dual-responsive behavior of PDMAEMA (temperature and pH) for physically cross-linked hydrogels. Initial studies on magnetic dual-responsive gene vectors revealed a facile synthesis of PDMAEMA-grafted iron oxide nanoparticles utilizing dopamine as physically binding anchor group for the polymer chains. Here, a dopamine-based ATRP initiator was applied for the surface modification of the nanoparticles, which enabled a controlled polymerization technique via the “grafting-from” approach. Gene transfection experiments with CHO-K1 cells show that the transfection efficiency is significantly higher than for poly(ethyleneimine) (PEI), which is regarded as the “gold standard” among the polycationic gene vectors. Although the hybrid particles show a considerably high molecular weight (4.3 MDa), which should lead to a significant increase of the cytotoxicity as observed for linear PDMAEMA their cytotoxicity is remarkably low, lower than that of PEI. Thus, the excellent performance in gene delivery experiments can be attributed to the star-like architecture of the PDMAEMA. Moreover, the uptake of our superparamagnetic gene vector into the cells enables a magnetic cell separation by applying an external magnetic field. However, due to the non-covalent bonds of dopamine to the iron oxide nanoparticles, the PDMAEMA chains undergo a detachment with time from the nanoparticle surface. This led to the synthesis of PDMAEMA-based magnetic core-shell-corona nanoparticles. Here, the iron oxide nanoparticles were covered with a thin silica shell in order to link the PDMAEMA chains covalently to the inorganic core via silane chemistry. This approach revealed stable dual-responsive hybrid nanoparticles with irreversible binding of the polymer chains and a high long-term stability in aqueous media. These hybrid star-like particles also show excellent gene delivery. The inter-polyelectrolyte complex formation between the PDMAEMA corona of the core-shell-corona particles and pDNA showed that the pDNA molecules are individually complexed with single nanoparticles at N/P ratios (polymer nitrogen / pDNA phosphorous) where the best transfection results are obtained. The magnetic cell separation was further improved by using a Magnetic Activated Cell Sorting system (MACSTM). The magnetically separated cells maintain a high transfection efficiency as well as viability and could even be further cultivated. Another aspect of this thesis was to include PDMAEMA as stimuli-responsive block in a double switchable block copolymer-based hydrogel. For this purpose, we chose a physically cross-linked ABCBA pentablock terpolymer system, which was polymerized via sequential ATRP and consist of a water-soluble PEO middle block, two dual-responsive (temperature/pH) PDMAEMA B-blocks as well as two thermo-responsive poly(di(ethylene glycol) methyl ether methacrylate) (PDEGMA) A-blocks (PDEGMA-b-PDMAEMA-b-PEO-b-PDMAEMA-b-PDEGMA). The aggregation behavior in dilute solution was investigated via temperature-dependent Dynamic Light Scattering (DLS) revealing that both stimuli-responsive blocks can be triggered separately and the coil-to-globule transition temperatures of the stimuli-responsive blocks were found to be strongly dependent on the block lengths for low molecular weights. In concentrated solutions, however, rheology studies did not show a further change in the mechanical properties after gelation for the investigated ABCBA pentablock terpolymer compositions. As a result, the principle of our complex system points towards a successful synthesis of a dual-responsive ABCBA pentablock terpolymer hydrogel system, which may show two distinct phase transition even for the gel state, if longer block lengths of the outer A- and B-blocks would be applied.