Investigations of enamel’s mechanical behaviors at different length scales

Project manager: Prof. Dr.-rer. nat. Gerold A. Schneider
Projekt worker: M. Sc. Siang Fung Ang
Supported by: German Research Fondation (DFG)
(Grant number: SCHN 372/14)

Prof. Dr. med. dent. M.S. Arndt Klocke
Poliklinik für Kieferorthopädie
Universitätsklinikum Hamburg-Eppendorf, Germany

Division of Orthodontics, Department of Orofacial Sciences, University of California, San Francisco, USA.

Prof. Dr. M. V. Swain
Department of Biomaterials, Faculty of Dentistry,
University of Sydney, Australia

Start: 01.05.2008
End: 30.04.2012


Analysis of the mechanical properties of dental enamel is complex, due to its hierarchical structure (Fig. 1).   A proper interpretation of the mechanical behavior of enamel should be performed in correlation to its structural dimensions and compositional characteristics. For clinical practitioners, detailed knowledge of enamel’s mechanical characteristics is essential to improve dental restoration practices (better selection of dental materials etc). For materials scientists, the knowledge might provide guidance in developing bio-inspired materials.

Figure 1: Micro- and nanostructure of enamel. (a) Enamel is the outermost layer of teeth. The enlarged section shows an occlusal plane, where enamel rods are cross-sectioned and shows keyhole-shaped structures of ~5 mm (b) A schematic illustration of one enamel rod and the arrangement of the ~50 nm crystallites inside. [Ang et al. Biomaterials 31(7), 1955-1963 (2010)]


Experimental methods include the scanning probe microscopy (AFM, PFM) and the nanoindentation to probe the local mechanical properties in the micron and sub-micron range. In addition, uni-axial compression tests are conducted to determine the macroscopic elastic/inelastic and creep behaviors of enamel. One particular challenge is the mapping and analysis of the mechanical behavior of proteins between the enamel rods/crystallites.

Uni-axial compression tests and the spherical indentations with different indenter radii were used to probe enamel’s elastic/inelastic transitions over four hierarchical length scales, namely: ‘bulk enamel’ (mm), ‘multiple-rod’ (10’s μm), ‘intra-rod’ (100’s nm with multiple crystallites) and finally ‘single-crystallite’ (10’s nm with an area of approximately one hydroxyapatite crystallite). The transitions were observed at 0.4-17GPa (Fig. 2) and were compared with the values of synthetic hydroxyapatite. A map of elastic/inelastic regions of enamel from millimeter to nanometer length scale is presented (Fig. 3).

Figure 2: Enamel’s stress-strain curves. (a) Uni-axial compressive stress-strain curves for bovine enamel. Indentation stress-strain curves of human enamel with (b) R=3mm indenter [Staines et al. 1981] (c) R=8.3µm indenter and (d) R=63nm indenter. Depending on the contact areas, these length scales are called (a) ‘bulk enamel’ (b) ‘multiple-rod’ (c) ‘intra-rod’ with multiple crystallites and (d) ‘single-crystallite’. [Ang et al. Biomaterials 31(7), 1955-1963 (2010)]

Figure 3: A map of regions of the most probable deformation behaviors of enamel: elastic, plastic or micro-crack induced damage behavior. At the smallest investigated length scale, plastic behavior ensued after elastic deformation. However, for the biggest investigated length scale, micro-crack induced damage occurs beyond the elastic limit. [Ang et al. Biomaterials 31(7), 1955-1963 (2010)]

In another study, the crack tip toughness (KI0, KIII0), the crack closure stress and the cohesive zone size at the crack tip of enamel were quantified.



  1. Determination of the elastic/plastic transition of human enamel by nanoindentation
    Ang, S. F.; Scholz, T.; Klocke, A.; Schneider, G. A.
    Dental Materials, 25(11), 1403-1410 (2009).

  2. Size-dependent elastic/inelastic behavior of enamel over millimeter and nanometer length scales
    Ang, S. F.; Bortel, E. L.; Swain, M. V.; Klocke, A.; Schneider, G. A.
    Biomaterials, 31(7), 1955-1963 (2010).

  3. Sub-micron toughening and crack tip toughness of dental enamel
    Ang, S. F.; Schulz, A; Pacher Fernandes, R; Schneider, G. A.

    (Under review)



  1. S. F. Ang, T. Scholz, G. A. Schneider. Poster: „Nanoinentation study of human enamel“. Symposium Hochleistungskeramik 2008, Hamburg, Germany. 26-27 February 2008.

  2. S. F. Ang. Presentation: „Capturing the beauty of enamel’s organic design by advanced nanoindentation“. 5. Hamburger Studententagung zur Medizin- und Botechnologie, Hamburg, Germany. 5 May 2008.

  3. S. F. Ang, A. Klocke, M. Swain, G. A. Schneider. Presentation: „Nanoindentation study of human enamel.“ Hysitron User Meeting and Workshop, Ismaning, Germany. 29-30 October 2008.

  4. S. F. Ang, G. A. Schneider. Presentation: „Der mechanische Baukasten der Zähne.“ Workshop „Medizintechnik an der TUHH“, Hamburg, Germany. 11 December 2008.

  5. S. F. Ang, R. Pacher Fernandes, G. A. Schneider. Presentation: „AFM and PFM measurements of enamel in order to determine the crack tip toughness and cohesive zone.“ MRS Spring Meeting, San Francisco USA.13-17 April 2009.

  6. S. F. Ang, S. Habelitz, A. Klocke, M. Swain, G. A. Schneider, Poster: „Indentation and uni-axial compression study of enamel’s elastic/plastic behavior from milimeter to nanometer length scale“. MRS Spring Meeting, San Francisco USA.13-17 April 2009.