AFM-Based Nanomechanical Properties and Storage of Dentin and Enamel
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AFM-Based Nanomechanical Properties and Storage of Dentin and Enamel Stefan Habelitz, Mehdi Balooch*, Grayson W. Marshall, Thomas M. Breunig, Sally J. Marshall, Dept of Preventive and Restorative Dental Sciences, University of California, 707 Parnassus Ave. San Francisco, CA 94143, USA; *Dept Chemistry and Materials Science, Lawrence Livermore National Laboratory, 7000 East Avenue, L-357, Livermore, CA 94551
ABSTRACT This study evaluated the affects of 2 different solutions, e.g. deionized water and Hanks’ Balanced Salt Solution (HBSS), on nanohardness and elastic modulus of dentin and enamel from human third molars at storage times of 0, 1, 7, 14 and 28 days using a modified AFM (Triboscope). The pH values of the solutions were monitored throughout the test periods. Storing the specimens in deionized water resulted in a large decrease of mechanical properties, e.g. the reduced elastic modulus of dentin decreased from 24.0 ±1.5 GPa to 21.0 ±1.6, 8.6 ±1.1 and 5.2 ±1.1 GPa for storage times of 1, 7 and 14 days, respectively. Mechanical properties of dentin and enamel dropped by more than 12% after one day and more than 50% after a week when stored in deionized water. The observed changes in mechanical response were attributed to a superficial demineralization process during storage. In contrast, storing teeth in HBSS did not significantly alter the mechanical properties of dentin or enamel over the time studied.
INTRODUCTION Nanoindention has become a common technique for the determination of local mechanical properties of structural features in biological hard tissues[1-4]. Nanoindentations usually do not exceed indentation depths of 500 nm. Although only a thin surface layer is examined, the mechanical properties obtained are assumed to be representative of the bulk material. Chemical changes in the surfaces of mineralized tissues resulting from storage solutions are, thus, important considerations for the accurate determination of mechanical properties. Tooth structure is composed of three calcified tissues: the outer enamel; the bulk of the tooth, dentin; and cementum. Enamel is the most highly calcified tissue (85 vol.% mineral) and therefore the hardest tissue in the human body [4]. Its unique microstructure consists of keyhole-like rods, which are aligned parallel and run from the dentino-enamel junction towards the surface of the tooth (Figure 2). Each rod consists of carbonated apatite fibers of 30 to 80 nm diameter and up to several millimeters length. These fibers, which are covered with a thin protein layer, are aligned parallel in the center of the rod and flare laterally towards the outer rod. The oriented microstructure results in anisotropy of the mechanical properties [3]. Dentin is a complex hydrated biological composite structure containing approximately 45 vol.% mineral in the form of carbonated apatite, 35 vol.% organic components (mostly type I collagen) and 20 vol.% fluids. Its distinct microstructure is characterized by tubules of 1 to PGLDPHWHU (Figure 1) that were the paths of the odon
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