Introduction to macroporous cryogels.

Cryogels are highly elastic three-dimensional materials consisting of a network of interconnected macropores. This unique morphology combined with high mechanical and chemical stability provides excellent mass flow properties. The matrices are synthesized at subzero temperatures from almost any gel-forming precursor. The main fields of application are in biotechnology as 3D-scaffold for cell cultivation, and tissue engineering, or bioseparation as chromatographic media for the separation and purification of biomolecules. This chapter briefly highlights the preparation, properties, and application of these materials.

Electron beam-induced immobilization of laccase on porous supports for waste water treatment applications.

The versatile oxidase enzyme laccase was immobilized on porous supports such as polymer membranes and cryogels with a view of using such biocatalysts in bioreactors aiming at the degradation of environmental pollutants in wastewater. Besides a large surface area for supporting the biocatalyst, the aforementioned porous systems also offer the possibility for simultaneous filtration applications in wastewater treatment. Herein a "green" water-based, initiator-free, and straightforward route to highly reactive membrane and cryogel-based bioreactors is presented, where laccase was immobilized onto the porous polymer supports using a water-based electron beam-initiated grafting reaction. In a second approach, the laccase redox mediators 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) and syringaldehyde were cross-linked instead of the enzyme via electron irradiation in a frozen aqueous poly(acrylate) mixture in a one pot set-up, yielding a mechanical stable macroporous cryogel with interconnected pores ranging from 10 to 50 µm in size. The membranes as well as the cryogels were characterized regarding their morphology, chemical composition, and catalytic activity. The reactivity towards waste- water pollutants was demonstrated by the degradation of the model compound bisphenol A (BPA). Both membrane- and cryogel-immobilized laccase remained highly active after electron beam irradiation. Apparent specific BPA removal rates were higher for cryogel- than for membrane-immobilized and free laccase, whereas membrane-immobilized laccase was more stable with respect to maintenance of enzymatic activity and prevention of enzyme leakage from the carrier than cryogel-immobilized laccase. Cryogel-immobilized redox mediators remained functional in accelerating the laccase-catalyzed BPA degradation, and especially ABTS was found to act more efficiently in immobilized than in freely dissolved state.

Electron-beam generated porous dextran gels: experimental and quantum chemical studies.

PURPOSE: The aim of this work was to investigate the reaction mechanism of electron-beam generated macroporous dextran cryogels by quantum chemical calculation and electron paramagnetic resonance measurements. METHODS: Electron-beam radiation was used to initiate the cross-linking reaction of methacrylated dextran in semifrozen aqueous solutions. The pore morphology of the resulting cryogels was visualized by scanning electron microscopy. Quantum chemical calculations and electron paramagnetic resonance studies provided information on the most probable reaction pathway and the chain growth radicals. RESULTS: The most probable reaction pathway was a ring opening reaction and the addition of a C-atom to the double-bond of the methacrylated dextran molecule. CONCLUSIONS: First detailed quantum chemical calculation on the reaction mechanism of electron-beam initiated cross-linking reaction of methacrylated dextran are presented.

Biocompatible polysaccharide-based cryogels.

This study focuses on the development of novel biocompatible macroporous cryogels by electron-beam assisted free-radical crosslinking reaction of polymerizable dextran and hyaluronan derivatives. As a main advantage this straightforward approach provides highly pure materials of high porosity without using additional crosslinkers or initiators. The cryogels were characterized with regard to their morphology and their basic properties including thermal and mechanical characteristics, and swellability. It was found that the applied irradiation dose and the chemical composition strongly influence the material properties of the resulting cryogels. Preliminary cytotoxicity tests illustrate the excellent in vitro-cytocompatibility of the fabricated cryogels making them especially attractive as matrices in tissue regeneration procedures.

Amino-functionalized monolithic spin-type columns for high-throughput lectin affinity chromatography of glycoproteins.

Hydrophilic poly(ethylene glycol)-based monoliths were synthesized in the spin-tip format for high-throughput applications via pulsed electron beam irradiation. Monoliths with a homogeneous porous structure and a total porosity of 69% were obtained. The cross-linked polymeric structure was further mechanically stabilized via the incorporation of silica nanoparticles. Amino-functionalization of the monoliths was accomplished by a straightforward, water-based, one-step approach that entailed the electron-beam irradiation-induced grafting of poly(allylamine). The amine functionalized spin columns showed very low unspecific protein adsorption and were successfully applied as adsorbents in lectin affinity chromatography for the purification of ovalbumin. The novel columns also outperformed a commercially available system.

Functionalization of solid surfaces with hyperbranched polyesters to control protein adsorption.

Thin films of hyperbranched polyesters were studied in dry state and in aqueous buffer solution regarding their swelling behaviour and protein adsorption potential. The influence of the degree of branching, the backbone structure, flexibility as well as the polarity was varied. By changing the backbone structure from aromatic, aromatic-aliphatic to aliphatic the surface properties can be controlled from protein active to protein repelling. The higher adsorption potential observed in comparison to linear polyesters is the result of the large amount of end groups allowing the formation of hydrogen bonds, and the larger swellability of the more flexible linear polymers. The protein adsorption process was studied intensively by in-situ spectroscopic ellipsometry. Different approaches towards a proper optical model for the vis-ellipsometry data evaluation for the determination of the correct layer thickness and refractive index are discussed. IR-ellipsometric measurements using a new in-situ cell gave the full chemical evidence for the formation of thin protein adsorption layer on the polymer films in the aqueous buffer environment.

Immobilization of a hyperbranched polyester via grafting-to and electron beam irradiation.

Stable thin films of an aromatic-aliphatic hyperbranched polyester with hydroxyl groups were fabricated on silicon substrates using electron beam irradiation and a grafting-to approach. We present a detailed study on the influence of the dose, dose rate, and temperature on the film properties and degradation behavior of the polyester immobilized by electron beam irradiation. A patterned polyester film was prepared on the substrate using a masking technique. In the second part of this work, we report on a method for the strong binding of the hyperbranched polyester onto the surface of an "activated" silicon substrate without using any coupling agent. The results are compared with the grafting-to of the hydroxyl-terminated polyester using thin PGMA anchoring layers ( Reichelt et al. Macromol. Symp. 2007, 254, 240- 247 ). The optimal conditions and mechanism of the anchoring procedures were investigated. The surface and film properties of all immobilized polymer films were characterized by atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), electrokinetic measurements, contact-angle measurements by drop-shape analysis, spectroscopic and imaging ellipsometry, and infrared spectroscopy. It is shown that all immobilization methods can be optimized in such a way that the polymer surface properties remain unchanged compared to those of nongrafted polyesters.

ADSA-TRIS: a new method to study interfacial phenomena at polymer-aqueous solution interfaces.

Axisymmetric drop shape analysis-profile (ADSA-P) was combined with total reflectometric interference spectroscopy (TRIS) in one experimental setup to study the interfacial phenomena at solid-liquid and liquid-vapor interfaces caused by adsorption/desorption (dissolution) of surface-active substances. Using sessile liquid droplets on polymer film/chromium-coated glass substrates that were optically matched with an immersion oil to a TRIS reflection prism, the optical thickness (product of physical thickness d and refractive index n) of the polymer film can be estimated by evaluating the wavelength-dependent intensity of reflected light. The sessile droplet is analyzed simultaneously by an ADSA setup arranged in a transverse direction to the path of the white-light beam of TRIS. From this analysis, the solid-vapor interfacial tension gamma (lv)(t), contact angle theta(t), contact radius r(t), drop volume V(t), and solid-liquid interfacial tension gamma (sl)(t) can be monitored as a function of time. The new method was applied to study polystyrene and poly(4-hydroxystyrene) surfaces in contact with aqueous buffer solutions and with protein solutions. The time-dependent changes in the optical film thickness caused by the adsorption of human serum albumin (HSA) and lysozyme (LSZ) were accompanied by changes in the solid-liquid interfacial tension. From the detailed study of both parameters, conclusions can be drawn with regard to the adsorption kinetics of the proteins on the hydrophobic polystyrene surfaces and to conformational changes occurring within the adsorbed protein layers.