Multiscale Engineered Artificial Tooth Enamel

This review appeared in the February 2022 edition of the Nanomechanical Testing Journal Club — a monthly email brief highlighting leading-edge research and the latest discoveries supported by Bruker nanomechanical testing technology.

Tooth enamel, the shiny white layer that covers our teeth, is the hardest material in our body. It contains 96% mineral components—the highest percentage of minerals in any tissue in our body—making it durable and damage resistant.

In this study, the authors conducted a detailed microstructural investigation of natural tooth enamel with transmission electron microscopy, and intelligently manufactured a very similar microstructural material using assembled hydroxyapatite (HA) nanowires with amorphous intergranular phase of ZrO₂. HA nanowires were synthesized by a solvothermal synthesis process. Enamel-like aligned lamellar composite materials were prepared by bidirectional freeze casting technique. This artificial tooth enamel (ATE) was designed to closely mimic the composition of the natural enamel.

To understand mechanical performance of individual components, amorphous-ZrO₂ coated HA nanowires and non-coated HA nanowires were transferred to a tensile device, PTP (Bruker, Minneapolis, MN). The fracture strength and strain of individual nanowires were determined from load-displacement data obtained by Hysitron PI 85L SEM PicoIndenter (Bruker, Minneapolis, MN). The results from PTP experiments showed the fracture strength and strain of A-ZrO₂ coated HA nanowires of ~1.6 GPa and ~6.2%, which are 2.5 times and 1.6 times higher than non-coated HA nanowires (~0.65 GPa and ~4%, respectively). Overall, the fracture properties of nanowires are found to be much higher than the properties of bulk HA. The stiffness of different samples were measured by Hysitron TI 950 TriboIndenter (Bruker, Minneapolis, MN) with a Berkovich diamond tip using a continuous depth-sensing indentation technique. Nanoindentation experiments confirmed that elastic modulus and hardness obtained from ATE are much higher than that of natural enamel and ATE exhibits high viscoelasticity without sacrificing its stiffness and hardness. The article claims such extraordinary engineered structure is machinable and can be modified to any macroscopic shape.

The experimental work was conducted using a Hysitron SEM PicoIndenter combined with our patented tensile device Push-to-Pull (PTP) device inside an environmental scanning electron microscope (ESEM) and Hysitron TriboIndenter under ambient conditions (25°C, 15% RH).

      _{KEY TERMS:}

• Artificial Tooth Enamel; Biomimetic Materials; Engineered Structural Materials; Microarchitectures