Exceptional contact elasticity of human enamel in nanoindentation test

Objective: Tooth enamel has unsurpassed hardness and stiffness among mammalian tissue structures. Such stiff materials are usually brittle, yet mature enamel can survive for a lifetime. Understanding the nanoscale origin of enamel durability is important for developing advanced load-bearing biomaterials. Here, nanoscale exceptional contact elasticity of the human tooth enamel, based on nanoindentation tests, is reported. Methods: Spherical indenter tips with radii of 243 and 1041 nm were used to determine stress-strain curves of enamel. Force-displacement curves were recorded using quasi-static loading strain rates of 0.031, 0.041, and 0.061 s −1. The storage moduli from a superimposed signal amplitude (dynamic strain at 220 Hz) embedded during primary quasi-static loading and from quasi-static elastic theory were simultaneously measured. Modulus mapping was considered to be an extremely low quasi-static loading strain rate indentation test. Results: The elastic limits were 7-9 GPa and 5-6 GPa for the small and large indenters, respectively. The elastic-plastic transition point and elastic modulus value increased with substantially increased quasi-static loading strain rate. The results suggested that the increase of the elastic limit during high-loading strain was associated with exceptional contact elasticity at the nanoscale of the enamel structure and the consequent extension of the contact area (i.e., a temporary pile-up response, dependent on the enamel nanocrystals and protein glue). Significance: Structural modification at this scale effectively prevents the initiation of cracking from localized strain, thus reinforcing the bulk structure. These results may provide valuable insight for conceptualizing bio-inspired nanocomposites.