Preparation and Characterization of Macroporous Cryostructured Materials

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

Cryostructuration as a tool for preparing highly porous polymer materials

Cryostructuration is a technique which can be used to produce highly porous polymer materials from either monomeric or polymeric starting material. The technique utilizes freezing of dilute solutions or suspensions for the formation of porous materials. The process has been used in both aqueous and organic media for the preparation of porous materials. This mini-review highlights some recent trends for cryostructuration where it is based on a freeze/thawing approach

Building Macroporous Materials from Microgels and Microbes via One-Step Cryogelation (dagger).

Macroporous materials are prepared from microgels or microbes by one-step chemical cross-linking under semifrozen conditions. This avoids the use of freeze drying of the sample because a chemically stable structure is prepared under semifrozen conditions. Cryostructuration results in a material with pore walls composed of closely packed particles

Reversible in situ precipitation: a flow-through approach for coating macroporous supports with metal hydroxides

In this study we report on the production of metal-hydroxide-coated macroporous polymers (MHCMPs), which mainly involves a polyacrylamide backbone coated with iron-aluminium double hydroxides. The coating process is fast, occurs using relatively mild reagents at room temperature, and can be repeated multiple times, thus making it a very simple and flexible process. Electron microscopy and energy dispersive X-ray spectroscopy studies showed that metal hydroxide coating occurred throughout the polymer backbone. It was shown that the mass of metal hydroxides incorporated in the MHCMPs could be adjusted by varying the initial salt solution concentration or the number of cycles in the process. Under the studied conditions, on a polymer backbone of mass 25 mg, we observed a maximum metal hydroxide mass incorporation of 18 mg for the MHCMPs produced at 6 cycles by using 0.4 M iron and aluminium salt solution. Nitrogen adsorption isotherms indicated that the surface area of the MHCMPs increased linearly with the increase in the mass of metal hydroxides incorporated. The polymer backbone with no mass incorporated showed a BET surface area of 18 m(2) g(-1) and the MHCMPs with maximum mass incorporation under the studied conditions showed a BET surface area of 63 m(2) g(-1). MHCMPs with varying mass incorporations were applied for arsenic (As(III)) adsorption and showed a high As(III) removal, indicating that they can serve as potential adsorbents. In addition, MHCMPs incorporated with other metal hydroxides were also produced and characterized to show that this method is applicable for coating these as well

Arsenite adsorption on cryogels embedded with iron-aluminium double hydrous oxides: Possible polishing step for smelting wastewater?

Arsenic is among the most toxic elements and it commonly exists in water as arsenite (As(III)) and arsenate (As(V)) ions. As(III) removal often requires a pre-oxidation or pH adjustment step and it is a challenge to adsorb As(III) at circumneutral pH. In this study, iron-aluminium double hydrous oxides were synthesized and incorporated into cryogels. The resulting composite cryogels were evaluated for As(III) adsorption. Initial experiments indicated that the adsorbent showed similar adsorption kinetics for both As(V) and As(III) ions. The adsorption of As(III) best fit the Langmuir isotherm and the maximum adsorption capacity was 24.6mg/g. Kinetic modeling indicated that the mechanism of adsorption was chemisorption, making the adsorbent-adsorbate interactions independent of charge and hence allowing the adsorbent to function equally efficient across pH 4-11. A Swedish smelting wastewater was used to evaluate the adsorption performance in continuous mode. The studies showed that the adsorbent was successful in reducing the arsenic concentrations below the European Union emission limit (0.15mg/l) in a smelting wastewater collected after two precipitation processes. The arsenic removal was obtained without requiring a pH adjustment or a pre-oxidation step, making it a potential choice as an adsorbent for As(III) removal from industrial wastewaters

Ultrafast Responsive Poly(N-isopropylacrylamide) Gel Produced by Cryostructuring of Self-crosslinkable Polymer Microgels

A macroporous material composed of closely aggregated particles was prepared by cryostructuration of N-isopropylacrylamide-co-N-hydroxymethylacrylamide (NIPA-co-HMAm) particle suspensions. The formed structure was maintained by the formation of covalent bonds through self-crosslinking between the particles while the system was in a semi-frozen state thus avoiding the need to freeze-dry the sample. This resulted in macroporous structure composed of closely aggregated thermoresponsive particles which exhibit an ultrafast temperature response. The response rate can be attributed both to the macroporous structure as well as the fast responsive properties of the individual particles

Oxidized Dextran as Crosslinker for Chitosan Cryogel Scaffolds and Formation of Polyelectrolyte Complexes between Chitosan and Gelatin.

Macroporous scaffolds composed of chitosan and using oxidized dextran as a crosslinker are produced through cryogelation. Introducing gelatin as a third component into the structure results in the formation of mesopores in the pore walls, which are not seen if gelatin is excluded. The mesoporous structure is explained by the formation of polyelectrolyte complexes between chitosan and gelatin before crosslinking takes place. The scaffolds exhibit highly elastic properties withstanding compressions up to 60%. The in vitro biocompatibility of the cryogels is evaluated using fibroblasts from a mouse cell line (L929) and it is seen that the cells adhere and proliferate on the scaffolds. The mesoporous structure seems to have a positive effect on proliferation

Stimuli-Responsive Polymers in the 21st Century: Elaborated Architecture to Achieve High Sensitivity, Fast Response, and Robust Behavior

Life is polymeric in its essence. The living cell contains a range of biopolymers such as proteins, carbohydrates, and nucleic acids. The cells are often compartmentalized via membranes that are composed of lipids. These are small molecules, but they spontaneously aggregate into supermolecular structures. The building blocks of these lipids are among others fatty acids, structures built from methylene oligomers. Biopolymers are sensitive to external stimuli. There are examples where the molecules show a highly non-linear response to external stimuli. This is seen as moderate changes in structural properties in response to changes in an external parameter until a critical point is reached where a dramatic change in molecular properties takes place upon an incremental change in the external conditions. After the transition, the system responds poorly to further changes. Such non-linear responses contribute to dramatic cooperative conformational changes leading to strong effects in the biological system. The strong response is an integrated effect of many weak interactions, and it is the cooperativity between all these interactions that are the driving forces for processes occurring in such systems. (c) 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 49: 173-178, 201