P2 “Enamel”: Natural enamel and water-glass based composites – decoding structure-function relationship regarding fatigue resistance of two complementary systems

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Enamel is the hardest material in the human body and forms the outermost protection of teeth. It has a complex hierarchical structure based on 95% hydroxyapatite (HA) nanofiber crystallites bundled to rods that are oriented into different directions. A small amount of organic matrix accumulates at sites of low mineral density, e.g. between crystals and prisms. The micro- and nanoarchitecture of natural tooth enamel and the arrangement of inorganic nanocrystals in a low amount of organic matrix contribute significantly to the outstanding fatigue resistance that enables lifelong function. Nevertheless, under certain conditions, crack formation and propagation can affect and damage the natural tooth enamel. Little is known, however, about the long-time fatigue behavior of enamel. Sodium-silicate solution (“water-glass”, WG), is a versatile, inorganic material with tuneable (visco-)elastic properties. Its chemistry and the adjustable water content allow manufacturing of WG based composites by a variety of processing routes. In combination with (bio-) polymers WG can be used to process organic-inorganic model structures with varying mineral content. It is therefore an ideal precursor for nanocomposites inspired by the complex fiber-based composite structure of enamel. The aim of this project is to transfer natural enamel architecture to WG-based composite structures, from the nano- to the macro-scale, while exploiting the versatility of WG processing. We will combine 3D-printing, electrospinning and electro-writing techniques to reach versatile novel processing routes. Tailoring the interface between the polymer and inorganic phases will further be inspired by the experience from dental applications and inorganic materials. This will be an asset contributing also to the other FOR projects. By investigating fatigue behavior and resistance of natural enamel and WG-based composites we will identify structural strategies and features that contribute to fatigue resistance in a similar or different way and how this affects longevity. To understand the relative contribution of the hierarchical nano- and microstructure on the long-time fatigue resistance of enamel and of 2D and 3D WG-based composites, we will perform fatigue tests on macro-specimens in a chewing simulator, and on the micro- and nanoscale with cyclic nanoindentation in the central fatigue lab. Important facets throughout the project are water content and temperature, influencing processing and properties of the WG-based composites, as well as water content, composition of the environmental medium and temperature during specimen preparation and fatigue testing of specimens of both material groups. Thus, together with the other FOR members, we will contribute to develope reliable specimen preparation and fatigue testing procedures for biological and bio-inspired nanocomposites.