P3 - Fishbone: Bones without cells as blueprints for durable, cyclically loaded nanocomposites: how does anosteocytic fishbone cope with fatigue?

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For mammalian bone, it is widely accepted that osteocyte cells sense strains and initiate remodelling and repair of fatigue-damage. But bone of advanced teleost fish lacks osteocytes. How do they sustain millions of loading cycles? This tough and fatigue resistant material presents an ideal role model for bioinspired composites.In this project we elucidate which structural entities on which length scales dominate fatigue resistance of anosteocytic fishbone and how they interact, in comparison with cellular fishbone. Composition, fibre orientation and porosity will be evaluated quantitatively by histology/materialography, with an emphasis on 3D high resolution investigations. A focus will be on identfying pre-existing micro-cracks, nano-/ microscale porosity, and nm-sized residual stresses. Finite element modelling, based on whole bone microcomputed tomography data, will help us, in synergy with strain data based on digital image correlation to better understand connections between geometry, macro-porosity, stress/strain distributions and in vivo loading conditions. We will transfer structural features identified as important for the fatigue resistance to organic-inorganic composites, while modifying their relative amount, across length scales and by combining different processing routes. This will involve understanding the impact of composite processing on the organic structure and the organic/inorganic interfaces. Composite fibers with nano- or microscale diameters (to investigate hierarchical structures) and different stiffness will be made by melt electrowriting (MEW), electrospinning, and their combination from natural or synthetic organic precursors. MEW scaffolds with different macro-geometries will be produced with/without hydroxyapatite (HA) nanoparticles to enhance fatigue properties, considering effects of anisotropy by using different crystal shapes. Sandwich structures will be obtained by supporting these macrostructures with nanofibers with/without incorporated HA nanoparticles. All structures will be investigated in terms of their physical and chemical properties. All tests are performed in simulated body fluid, considering viscoelastic deformation by holding times and frequency variations. Fatigue-induced structural changes will be investigated in 2D/3D, as for the unloaded state. From the viewpoint of a biological material with intermediate mineralization degree, we will contribute to the FOR by providing knowledge on how composition affects fatigue resistance, and how the nanocomposites can be tested with minimum of artefacts.