Tooth enamel is the hardest biological tissue in the human body, providing strong wear resistance and efficiency to meet the needs for life-long oral food processing. Professor Yeau-Ren Jeng, a renowned scholar in tribology, has explored the underlying mechanisms of why tooth enamel could also bear significant resistance to fracture and crack compared to engineering materials. Together with Professor Dar-Bin Shieh of the Department of Dentistry at NCKU, students and international collaborators, they recently completed a fantastic voyage of research and exploration on this amazing biomaterial.

Unlock insight into unique mechanical properties of mineralized tissue
Previous research using the nano tribological tools developed by Professor Jeng's laboratory was able to measure the tribological properties of fluoride-treated teeth and interpret how fluoride contributes to caries prevention. Surprisingly, his team also discovered that the hardness of the mammalian enamel presents a gradient characteristic that gradually declined from the tooth's surface to the dentino-enamel junction. It turns out that this gradient is the key to both the hardness and toughness of the enamel layer. To better understand how the gradient feature resolves the conflict between hardness and toughness and establish guidelines for the development of cutting-edge materials, the teams of Prof. Jeng and Prof. Shieh explored the enamel layers of various mammals.

Diet drives the enamel composition and adaptive hardness gradient profiles
The results of the present study indicate that feeding habits are the main evolutionary factor affecting the hardness of mammalian tooth enamel. Herbivorous and omnivorous mammals need to chew much resistant food when eating, so they evolved harder tooth enamel. Mammals with longer lifespans also have a thicker layer of enamel, and species with harder and thicker enamel layers also developed more significant hardness gradients to prevent brittle cracking of the enamel layer.

Interdisciplinary cross-country collaboration inspires cutting-edge material development
The Fourier Transform Infrared Spectroscopy (FTIR) performed by the Taiwan Light Source synchrotron radiation provided an explanatory model for this gradient. It showed that nature uses trace elements and protein matrix to regulate enamel hardness. These results indicate that nature delicately configured the mineral and protein components to develop the hardness and gradient properties of the tooth enamel layer required to maximize dental efficiency and survival.

Many people contributed to the research, including Professor and Academician Robert Reisz of the University of Toronto in Canada, who kindly provided valuable advice and guidance. We also acknowledge the generous donation of mammalian teeth from many sources. This exploration provides critical insight for developing revolutionary materials—a case in point is a multiple nano-layers thin film for the coating of the advanced machining tool.