Preparation and Characterization of Macroporous Cryostructured Materials

Sammanfattning: Macroporous hydrogels are regarded as interesting materials both within biotechnology and biomedicine due to their properties. These materials can be prepared from a wide range of synthetic or naturally occurring compounds using a number of different techniques for their production. For this thesis, cryostructuring was used to prepare macroporous hydrogels, often named cryogels. The work utilized the fact that freezing an aqueous solution or suspension results in the formation of ice crystals. As these crystals grow any solutes or particles are expelled and accumulate in a liquid non-frozen phase around the ice crystals. Gel formation takes place in this non-frozen phase resulting in a crosslinked gel-network. The properties of this non-frozen phase are determined by the freezing conditions and the composition of the sample that was frozen. The ice crystals that form act as pore-forming agents and when the sample melts after gelation a macroporous material is formed with the pores being a replica of the ice crystals. Nuclear magnetic resonance (NMR) was utilized in this thesis to study the formation of cryogels produced by free radical polymerization of aqueous solutions of monomers. This technique allowed in situ studies of both the freezing process and the polymerization reaction. It could be seen from these studies that the starting concentration of monomers influenced the size of the non-frozen phase and that the properties of this phase influenced the reaction conditions. Furthermore, studying these reactions at -10 °C made it possible to investigate the differences between polymerization in a semi-frozen state and at supercooled conditions. Polymerization of a supercooled sample generated a non-porous material similar to materials formed above the freezing point, whereas in the semifrozen sample a macroporous structure was produced. It was shown that the structure of cryogels produced from monomeric precursors could be modulated by adding different inert solutes to the monomeric mixture. Adding salts resulted in materials with thicker pore walls and smaller pores sizes since the added solutes created a larger non-frozen phase. The addition of solvents which were poor solvents for the forming polymer resulted in cryogels with a bimodal pore size distribution. Macropores were formed due to the cryogelation process while a secondary porosity within the pore walls formed due to a polymerization-induced phase separation caused by the presence of the solvent. Using the principal that growing ice crystals expel compounds, a method for producing cryostructured materials from suspensions was described. Suspensions of synthetic particles or microorganisms were frozen and the material became closely packed between the ice crystals. In this state inter-particle covalent bonds were formed which prevented the structure from disintegrating into individual particles when the sample was thawed. The covalent bonds could be formed either through the addition of a crosslinker or through the reactions of functional groups on the surfaces of the particles. Structuring particles using this approach made it possible to incorporate activated carbon particles into the structure without blocking the internal porosity of the carbon. When a composite cryogel based on monomers was used to immobilize the carbon, blockage of the internal porosity of the carbon was observed. An evaluation of these new structures for biotechnological and biomedical applications would be interesting.