Generation of anisotropic hydrogel membranes, mimicking plant cell wall structures, and exploration of new bio-inspired mechanical devices based on gel swelling.
Final Report Abstract
In this project on the generation of anisotropic hydrogel membranes, mimicking plant cell wall structures, two specific goals were followed. On the one hand it was intended to build hydrogel membranes in which preferentially oriented nanofibres are embedded. Concept generator for these hydrogel-nanocomposites was the primary cell wall of plants in which cellulose fibrils and a hydrogel-like matrix form a fibre composite structure. The anisotropy and mechanical performance of the cell wall is mainly controlled by the distribution and the orientation of the cellulose. On the other hand it was aimed to develop testing techniques that allow for characterizing the nanostructure and mechanical performance of these membranes when exposed to lateral stress. Here it was intended to monitor both phases of the nanocomposite; i) the spatial re-orientation of the polymer molecules which form the hydrogels and ii) the orientation/re-orientation of the nanofibres embedded in the hydrogel. Control of the orientation of reinforcing elements in a hydrogel could be achieved by loading nanofibres with magnetic nanoparticles. As a first approach a simple and versatile routine was developed to directly load hydrophobic inorganic nanoparticles into hydrogel microparticles bya solvent exchange reaction in the hydrogel. This process does not require a chemical modification of the surfaces of the nanoparticles and the hydrogel networks. Unfortunately it was not possible to achieve a loading of multilayered nanofibers with magnetic nanoparticles via electrochemical deposition in porous membranes. As an alternative we moved to a model system consisting of cellulose nanowhiskers embedded in an agarose matrix. Anisotropy of these hydrogel-nanocomposites was constituted by tilting the whiskers from a primarily random distribution to a preferential orientation by applying an external load. To tilt the whiskers during tensile straining an initial drying process of the hydrogel was required to establish a sufficient interaction between the cellulose whiskers and the hydrogel. For the development of experimental techniques to monitor changes in the orientation of polymers forming the hydrogel during mechanical loading, polyelectrolyte multilayers were used as a model system. An in-situ testing setup was built that allowed for applying lateral mechanical stress and simultaneously measuring changes at the molecular level by elipsometry, IR-ATR and UV-Vis. With this approach we were able to correlate the mechanical performance of polyelectrolyte multilayers with the molecular coiling of the polyelectrolyte molecules. For studying the orientation/re-orientation of the nanofibres embedded in the hydrogel a microtensile setup was developed which combines mechanical straining under varying moisture conditions and monitoring of structural information by X-ray diffraction. With this setup hydrogels filled with cellulose nanowhiskers were investigated in an in-situ experiment at a synchrotron beamline (BESSY). This setup can not only be used for characterizing hydrogels but also for creating anisotropic hydrogels since a preferential orientation of the nanowhiskers can be achieved during tensile straining. Finally a testing setup was developed which allows for characterizing the degree of anisotropy of hydrogel membranes, but has so far only been used for membranes without nanofibres. This setup combines bi-axial mechanical loading with electronic speckle pattern interferometry (ESPI) to study the deformation of the sample with high-resolution. Although no specific attention has been paid to a commercial exploitability of this study, we believe that anisotropic hydrogels filled with oriented reinforcing whiskers may well serve as moisture sensors and actuators in nanotechnology and biomedical applications.
Publications
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Fabrication of colloidal stable, thermosensitive, and biocompatible magnetite nanoparticles and study of their reversible agglomeration in aqueous millieu. Chem. Mater. 21(2009), 1906-1914
Chanana M, Jahn S, Georgieva R, Lutz JF, Bäumler H, Wang D
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Hydrogen bonding selective phase transfer of nanoparticles across liquid/gel Interfaces. Angew. Chem. Int. Ed. 48(2009), 4953-4956
Mao Z, Gao J, Bai S, Nguyen TL, Xia H, Huang Y, Mulvaney P, Wang D
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Using polymers to make up magnetic nanoparticles for biomedicine. J. Biomed. Nanotechnol. 5(2009), 652-668
Chanana M, Mao Z, Wang D
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Changes of the molecular structure in polyelectrolyte multilayers under stress. Langmuir 25(2010),15516-15552
Früh J, Köhler K, Möhwald H, Krastev R
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Moisture changes in the plant cell wall force cellulose crystallites to defonn. J Structural Biol. 171(2010), 133-141
Zabler SA, Paris O, Burgert I, Fratzl P
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Synthesis of monodisperse quasi-spherical gold nanoparticles in water via silver (I) - assisted citrate reduction. Langmuir 26(2010), 3585-3589
Xia H, Bai S, Hartmann J, Wang D
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Using hydrogel microparticles to transfer hydrophilic nanoparticles and enzymes to organic media via stepwise solvent exchange. Langmuir 26 (2010), 12980-12987
Bai S, Wu C, Gawlitza K, v. Klitzing R, Ansorge-Schumacher M, Wang D
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Using hydrogels to accommodate hydrophobic nanoparticles in aqueous media via solvent exchange. Adv. Mater 22(2010), 3247-3250
Bai S, Nguyen TL, Mulvaney P, Wang D
