Image Courtesy of Kayla Yup.
If you find it difficult to brush and floss regularly, you might be in luck. Researchers from China and the US engineered an artificial alternative to enamel that was designed to be even stronger. But could this man-made enamel trick a tooth fairy?
Enamel is the tooth’s outer shell responsible for shielding teeth from damage. This tissue usually serves the body for over sixty years and cannot be regenerated. In addition to enamel’s incredible durability, it possesses outstanding viscoelasticity—the ability to endure vibration and deformational damage for long periods, such as when chewing. While viscoelasticity is key to enamel’s longevity, its hardness allows teeth to bite through tough material. However, these mechanical properties are traditionally considered trade-offs and are difficult to reproduce in man-made materials.
The challenge was to figure out how to copy what nature had already designed. The key to enamel’s seemingly paradoxical combination of properties turned out to be its hierarchical structure of elements, represented by the dense packing of nanowires interlaced with soft organic matter. The organic material is known as the amorphous intergranular phase (AIP), which effectively forms a connection between adjacent nanowires through strong chemical bonds. According to Lin Guo, corresponding author and Beihang University Professor of Chemistry, the abundance of unsaturated chemical bonds in amorphous materials allows for this tight binding.
“The notion of functional complexity, which requires some amount of order and some amount of disorder, is the great representation for this amorphous, disordered layer on the surface of the nanorods forming enamel,” said Nicholas Kotov, corresponding author and University of Michigan Professor of Chemical Sciences and Engineering. “The inside of [the enamel’s nanorod structure] is ordered for the stiffness, and the outside is disordered for the adaptability of the interfaces.”
The interface is the area between the inorganic material—the surface of the nanowires—and the polymer around it. The AIP acts as a buffer layer that not only facilitates the transfer of force, but strengthens the interface. A strong interface is essential to protecting the nanowires from environmental attacks by acids, alkaline substances, and sharp temperature changes, while enhancing enamel’s mechanical performance. This dynamic layer effectively restricts the propagation of cracks that tend to occur along the interfaces.
In order to successfully arrange the structural elements of their synthetic enamel, Kotov drew inspiration from previous studies of chemical engineering processes based on self-assembly. The self-organization of nanorods was achieved through a double freezing technology approach. By applying a freezing gradient, the nanorods were forced to align along one axis, resulting in their proper parallel alignment.
“Everything in living matter is based on self-organization,” Kotov said. “This is ubiquitous. Manufacturing based on self-assembly is very attractive for its low-temperature requirements, high energy efficiency, and applicability to a multiplicity of structures—from nanoparticles to nanorods to microscale particles to microscale and nanoscale dental works.”
To test this structure’s strength, the team applied an external load to a sample of their synthetic enamel. The nanowires initially slid in response, but the confinement of the organic matter in the gaps between nanowires restricted motion. This hierarchical structure ultimately improved crack deflection, allowing the synthetic enamel to withstand more force than natural enamel.
“Does it mean that we have created material better than what has been created by living organisms after billions of years of evolution? The answer is yes, that’s exactly the case,” Kotov said.
Kotov proposed using this synthetic enamel to engineer ‘smart teeth.’ These sophisticated implants would detect disease inside of the mouth through sensors for anything from bacterial composition to inflammation. This enamel would afford these implants the same type of protection as normal teeth.
Beyond teeth, the simplification of enamel down to layers enables this structure to be built at multiple scales. The successful reproduction of tooth enamel opens the door to engineering other high mechanical performance materials.
“The combination of high hardness and stiffness plus viscoelasticity, strangely enough, is very much needed for buildings because of earthquakes, especially for sensitive structures such as nuclear plants,” Kotov said. “So, scaling up the preparation of materials like enamel and implementing enamel-like materials in other areas of technology are squarely in our plans.”
For the inexperienced tooth fairy, this synthetic enamel could pose an evolutionary feat thanks to its strength. But to trick a seasoned tooth fairy, more research on mimicking the 3-D structure of teeth will be required.