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L3.6.1/BB3.6.1

Quantitatively Studying Nano-mechanical Properties within the Prism and Organic Sheath of Enamel Fuzhai Cui, Jun Ge, and Xiumei Wang Biomaterials Laboratory, Department of Materials Science & Engineering, Tsinghua University, Beijing 100084, China. ABSTRACT Enamel is made up of enamel prisms separated by thin layer of organic sheaths. The mechanical properties of the prisms and the organic sheaths are obviously different from each other due to different compositions and microstructures. However, quantitative measurements of such differences have been a challenge in the past. The objective of this study is to accurately study the mechanical properties in the isolated domains within single enamel prism. The technique of nanoindentation combined with Atomic Force Microscopy (AFM) was employed to test the enamel specimens from mature human maxillary third molar. It was revealed that the nanohardness and elastic modulus of the sheaths were about 73.6% and 52.7% lower than those of the prisms. AFM topographies of the residual indent impressions also visually confirmed the differences. In addition to nanoindentation tests, the microstructures of enamel were carefully investigated in terms of hierarchical levels of organization to understand the structural reasons of the mechanical differences. We found a close relation between the variations of mechanical properties of enamel and its hierarchical structure. The analysis of the mechanical properties within enamel upon hierarchy is not only helpful to understand its unique property, but may also inspire ideas for the design of novel synthetic materials. INTRODUCTION As the most highly mineralized tissue found in the human body, dental enamel consists of 96% mineral and 4% organic material and water in weight [1]. Previous studies of dental enamel have revealed some of its unique microstructure in terms of hierarchy [2, 3]. The prevailing concept of enamel structure is that the fundamental elements of enamel are the nanosized fibrillike hexagonal hydroxyapatite crystals [4], which are further clustered into groups forming higher hierarchical levels. Then the most readily apparent structure of enamel termed prisms and interprisms are imposed. As far as we know, the only substantial difference between them is the orientation of the crystals [5, 6]. The crystals in the prisms, particularly along the center of the prism tend to align lengthways and lie parallel to the prism axis, but deviate more and more as their distance from the centre increases. In the latter location, the crystals tend to orientate perpendicular to the incremental lines [7]. Where the crystals meet, the structure is discontinuous, leaving a gap, the prism sheath, which is filled with organic matter either from residual matrix or salivary material. In human enamel, this gap is keyhole shaped in cross section [8]. Recently, as the development of a powerful technique to measure mechanical properties at nanoscale, nanoindentation, knowledge of the nanomechanical properties of dental enamel in det