Microstructure and mechanical properties of the enameloid of pacu and piranha fishes: The role of different diets

An understanding of the relationship between the structure, mechanical properties and functions of teeth is required regarding the development of effective and durable bio-inspired synthetic dental materials. Many studies have investigated this relationship in many species. However, few studies have been conducted on the teeth of Serrasalmidae fishes. The family of Serrasalmidae, though, offers an excellent opportunity to study this relationship because of the diversity of their diet.The tooth consists of three distinct layers which are called (going from the center of the tooth to the outside): the pulp, the dentin and the enamel/enameloid. In some fishes, including those of the family Serrasalmidae, an additional superficial layer called the cuticle is also present.The topic of the present thesis is to investigate structural and mechanical adaptation of the enameloid in two Serrasalmidae fishes having different diets: the carnivorous Pygocentrus nattereri, preferentially feeding on soft prey, and the herbivorous pacu Piaractus brachypomus, preferentially eating hard shells.Enameloid microstructure is first characterized. Microscopic analysis of fractured teeth as well as surface etching performed on teeth sections allow identifying precisely the structure of the enameloid in the two species. Comparison between the structures found in the two species highlights that despite their different diets, no structural differences are observed between species with different feeding strategies (slicing vs. crushing). The enameloid of both fishes possesses a two-part organization. The inner enameloid is characterized by hydroxyapatite fiber bundles oriented and curved in a random manner forming a very sophisticated interlocking structure. The outer enameloid is organized with hydroxyapatite bundles aligned with each outer and oriented either parallel or perpendicular to the tooth surface, depending on the region analyzed.Second, the potential correlation between microstructure and mechanical properties is investigated through the assessment of local fracture behavior. Indeed, fracture resistance is an essential feature allowing the enameloid and, in general, the tooth to avoid catastrophic failure when cracks nucleate on the other surface due to repeated cycles of chewing. High load indentation tests in combination with scanning microscopy are used to explore fracture properties of the different teeth. Although the difference is not significant, a quantitative evaluation of the fracture toughness as well as a qualitative observation of the cracks morphology demonstrates that inner enameloid possesses a higher resistance to crack initiation and propagation than the outer enameloid. Furthermore, indentation-based cracks invariably propagate along the internal interfaces, especially at the interface between the hydroxyapatite bundles, and that several extrinsic toughening mechanisms, such as crack deflection/curvature and un-cracked hydroxyapatite bundles are used by the enameloid to increase fracture resistance.Novel addictive manufacturing routes such as freeze casting or magnetically assisted manufacturing may allow the fabrication of ceramic scaffolds replicating the structure seeing in the enameloid to improve fracture resistance of synthetic teeth.