Institute Advanced Ceramics

Nature has inspired scientists and engineers for thousands of years. With high-resolution microscopes and nano-testing equipment being developed, biological materials came into focus of materials research. Being fascinated by their structural functionality – but also by their beauty and elegancy – much effort is done at the moment by material scientists to detect structures, mechanical and functional properties of this class of materials as a source of inspiration for new materials design.

Many biological materials are hierarchically structured which means that they are designed from the nano- to the macroscale with characteristic structural elements on several length scales. Although lots of data for many biological materials as bone, shells, teeth and wood is available for properties on the smallest length scales (determined via nanoindentation) and largest length scale (bulk material testing) it is still unclear how hierarchical structuring influences bulk mechanical properties.

Within this project we chose a model-material, namely dental enamel, with the goal to determine mechanical properties as strength and toughness of all hierarchical levels and further evaluate this data theoretically by using and expanding a mechanics model developed by Huajian Gao (currently Brown University, Providence, USA) and co-workers. With this approach we want to describe material properties on higher structural levels as functions of the material properties on subordinate hierarchical levels.

Dental enamel consists of three hierarchical levels, see Figure 1. On the smallest level, long and thin nano-fibers are embedded within a protein matrix. On the next level, these nano-fibers are bundled together to micro-fibers. This is basically the most prominent structural level of enamel. The micro-fibers are called enamel rods and are roughly five microns in diameter. In some parts of the tooth the rods are interwoven and form a decussation pattern which varies between species. In human and many mammal teeth the decussating rods are organized in groups of rods of same orientation forming the so-called Hunter-Schreger bands.

At current state of research we determined fracture properties of enamel on the largest hierarchical level by bending experiments conducted by using our self-made bending device (see “Experiments with stable crack growth in structural and functional ceramics”). We further showed by using the determined and additional literature data the applicability of the theoretical model developed by Gao to enamel and other biological materials. Further research will focus on determination of enamel’s material properties on smaller size scales with subsequent data analysis as described above.

Figure 1: Hierarchical structure of enamel. Enamel is built by hydroxyapatite (HAP) nano-fibers which are glued together by proteins to form micro-fibers (the so-called enamel ‘rods’) which on the largest size scale form decussation patterns. Hunter-Schreger Bands contain groups of rods of same orientation and are visible as dark and bright reflexion bands in light microscope images (level 3).

Publications

Bechtle S, Ang SF, Schneider GA. On the mechanical properties of hierarchically structured biological materials. Leading Opinion Paper in Biomaterials, 2010.

Bechtle S, Habelitz S, Klocke A, Fett T, Schneider GA. The fracture behaviour of dental enamel. Biomaterials 31: 375-384, 2010. (PDF)

Bechtle S, Fett T, Rizzi G, Habelitz S, Schneider GA. Mixed-mode stress intensity factors for kink cracks with finite kink length – Application to enamel and dentin. J Mech Behav Biomed Mater 3: 303-312, 2010. (PDF)

Bechtle S, Fett T, Rizzi G, Habelitz S, Klocke A, Schneider GA. Crack arrest at the DEJ caused by elastic modulus mismatch. Biomaterials 31: 4238-4247, 2010. (PDF)