This project will quantify the structure-property relations at the dentin-enamel junction (DEJ) that are critical to tooth function and to determine if it embodies a fundamental design principle that can mimicked to produce fracture resistant interfaces of wide applicability. In addition it will also continue development of high resolution non- invasive imaging and measurement of nanolevel constituent properties that are critical to further understanding of calcified tissues. The DEJ unites two dissimilar calcified tissues. It plays a critical role in the biomechanical integrity of the tooth, is probably optimized for this function, and could serve as a biomimetic model for joining mechanically dissimilar biomaterials. Critical applications that could benefit from improved interfaces include orthopaedic and dental implants, as well as nearly all restorative dental procedures that relay on bonding or cementation. the central hypothesis is that the DEJ can be used as a biomimetic model for construction of new synthetic- biological substrate structures. Aim 1 will determine if the nanomechanical properties of the DEJ form a biomechanical interphase with graded mechanical properties across mammalian species (mouse, bovine, human) that leads to interfaces of enhanced fracture toughness. Novel methods of atomic force microscopy (AFM)-based nanoindentation allow measurement of modulus and nanohardness values. Fracture toughness will be determined using advanced methods that will validate new nanoscale fracture toughness methods for the DEJ. The structure- property relationships in transgenic mouse models with biomineralization defects of enamel or dentin will also be determined. Aim 2 will evaluate a 3-level (scallops, microscallops and nanostructure) DEJ model. X-ray tomographic microscopy, SEM and AFM will be used to define the architecture of the DEJ in human, bovine, mouse, and transgenic mouse teeth with mineralization defects. In Aim 3 current enamel bonding processes will be examined in light of the prior aims. The clinical success of enamel bonding procedures might stem from successful mimicry of the DEJ architecture as a byproduct of normal acid etching of enamel. The DEJ architecture may serve as an efficient means to bone and transmit forces between polymers and enamel, even in the absence of biological binding processes. Modifications that are more DEJ-like should improve bonding and fracture toughness. Aim 4 applies the most fracture resistant DEJ like architecture from the prior aims to a totally synthetic model to bond Si wafers to resin. The wafers will be patterned using lithography to determine the effects of each factor (microstructure, nanostructure, chemical bonding) on fracture toughness of the interface.