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Multifunctional Layered Magnetite Composites

 

Prof. Dr. Helmut Cölfen, Universität Konstanz, Fachbereich Chemie AG Physikalische Chemie , Universitätsstraße 10 78464 Konstanz

Dr. Damien Faivre, Max-Planck-Institut für Kolloid- und Grenzflächenforschung Wissenschaftspark Golm Abteilung Biomaterialien, Am Mühlenberg 1 14476 Potsdam

Dr. Vitaliy Pipich, Forschungszentrum Jülich GmbH; Jülich Centre for Neutron Science Außenstelle am FRM II c/o Technische Universität München, Lichtenbergstraße 1 85747 Garching

Professor Dr. Dirk Zahn, Friedrich-Alexander-Universität Erlangen-Nürnberg; Computer-Chemie-Centrum, Nägelsbachstraße 25 91052 Erlangen

Nature provides a variety of archetypes of highly ordered systems, of which many biomaterials are known for their remarkable mechanical properties. Nacre is one of these biominerals combining both stiffness and hardness. Its hierarchical structuring of highly organized crystal platelet layers separated by thin layers of organic material is responsible for the extraordinary fracture resistance. Another class of biominerals are magnetite incorporating chiton teeth, which are harder than any other known biomineral. Apart from that there are simple organisms, namely magnetotactic bacteria that can mineralize nano-sized magnetite particles and arrange them in chains which lead to coupling of the magnetic dipoles.

In the last project phase we realized a layered organic-inorganic hybrid material combining the structural features of these three biominerals. The chitin/peptide matrix of demineralized nacre was used as a scaffold and infiltrated with gelatin hydrogel (image A). The gel can be mineralized with magnetite, where repetition of the mineralization cycle allows for mineral contents between 15-65 wt-%. Thus, a hybrid material with a nacre-like structure containing a chiton tooth mimic could be synthesized (see image B). Variation of the synthesis allows for superparamagnetic (10 nm) and ferrimagnetic (50-90 nm) particles. The materials showed anisotropic mechanical properties, reaching hardness and elastic modulus comparable to human dentin and bone. Ferrogels ordered in a magnetic field showed further reinforcement with notably increased hardness and stiffness measured perpendicular to the field direction.

 

As the limited mineralization degree was seen as a limiting factor for further mechanical improvement, an additional mineral phase shall be applied to act as a “filler”. We also want to explore ways to stiffen the organic matrix. The mechanical properties shall be optimized by relating mechanical and structural features.

Small angle scattering techniques allow us a nondestructive view on the interaction between ions and the gelatin/chitin matrix and on the mineralization kinetics. The different components of the composite materials can be analyzed by variation of the scattering contrast. Polarization analysis to gain information on the magnetic phase structure and surface scattering methods completes the picture.

Simulation studies showed how both gelatin and chitin can act as nucleator for magnetite in this system. Previous simulations suggested the iron oxide mineralization as a two-step process: First, iron hydroxide-protein aggregates are formed. Then, after a few iron- and hydroxide layers are formed, the oxide is formed as a secondary nucleation event. We want to investigate influences on the peptide/ion interaction like the pH and different collagen/gelatin species. Furthermore the possible interfaces between the peptide, magnetite and the new filling minerals shall be investigated.