Your Supernatural Teeth
Aspects of human teeth appear over-designed for their function.
Apparently German scientists are less reticent about appealing to the supernatural in science. We’re saying this partly in jest, of course, because they did not really invoke spirits in the title of a press release from Charité – Universitätsmedizin Berlin, “Dentin nanostructures – a super-natural phenomenon.” They’re only explaining “Why it is superior to any synthetic filling material at making teeth last.” It’s super-human, perhaps.
Dentin is one of the most durable biological materials in the human body. Researchers from Charité – Universitätsmedizin Berlin were able to show that the reason for this can be traced to its nanostructures and specifically to the interactions between the organic and inorganic components. Measurements performed at BESSYII, the Helmholtz-Zentrum Berlin’s synchrotron radiation source, showed that it is the mechanical coupling between the collagen protein fibers and mineral nanoparticles which renders dentin capable of withstanding extreme forces. Results from this research have been recently published in the journal Chemistry of Materials.
How extreme are the forces dentin can withstand? They found that dentin is much stronger than actually needed for chewing.
As part of their experiments, the researchers increased the compressive stress inside the dentin samples. The samples were also dried by heating them to 125ºC. This resulted in the collagen fibers shrinking, leading to huge stress being exerted on the nanoparticles. The ability to withstand forces of up to 300 MPa is equivalent to the yield strength of construction grade steel, and is comparable to 15 times the pressure exerted during mastication of hard food, which usually remains well below 20 MPa. Heat treatment did not lead to the destruction of the protein fibers, suggesting that the mineral nanoparticles also have a protective effect on collagen.
Our teeth come into contact 5,000 times a day, the article says. That’s 1.8 million times a year, or almost 200 million times in a lifetime. “In spite of this, and although we often use large forces during mastication, it is surprisingly rare for healthy teeth to break,” they say. If evolution hit on this “super-natural” ability, the scientists didn’t say so. Instead, they appealed to design: “It is widely accepted that the design of teeth makes teeth tough, where an inner core, known as dentin, supports the outer hard enamel cap.”
Not only is dentin strong, it is organized in a way to resist stress. The material is a composite of collagen, a protein, and carbonated hydroxyapatite, or cHAP, a mineral (see diagram in the press release). The structure is “surprisingly more complex” than they expected.
Analysis of the data also showed a gradual reduction in the size of the cHAP crystal lattices as one moves deeper into the tooth. “Tissue found near the dental pulp, which is formed during the later stages of tooth development, contains mineral particles that are made up of smaller cell units,” explains Zaslansky. The nanoparticle length shows the same trend, with the mineral platelets situated near bone on the outer parts of the root measuring approximately 36 nm in length, while those found near the pulp are smaller, only 25 nm long.
The enamel at the tooth surface is also strong but more brittle. Having the cushioning layer of dentin below protects against breakage; “the organic fibers found in dentin appear to exert exactly the right pressure on the mineral nanoparticles that is required to increase the material’s repetitive, cyclic load-bearing capacity.” Dentin’s resistance to compressive stress is “why damage or cracks in enamel do not extend catastrophically into the dentin bulk,” they say.
Of course, bacteria can weaken teeth and form cavities; that’s why we have dentists. Bacteria can weaken the cHAP minerals and dissolve the collagen fibers. The new findings from Berlin can help. You might want to share this article with your dentist, or tell him or her to “keep teeth moist during dental procedures, such as when inserting dental fillings or installing crowns,” they say. “Avoiding dehydration may very well prevent build-up of internal stresses, the long-term effects of which remain to be studied.” Meanwhile, they hope what they learned can “be used as a model system for new materials development, for example when designing novel dental restoration materials.” If built the “super-natural” way, your next fillings could be far superior to the metals or composites currently in use.
This is another great example of design science at work, seeking to understand a design and apply it for mankind’s benefit. The material is but one aspect of the design. Think about how teeth are arranged in the jaw, two sets of matching pairs that fit against each other. You have a set of tools from cutters to grinders arranged front to back, kept moist by six salivary glands that keep them moist and in good operation. All these work in concert with the tongue and the palate. And we haven’t even begun to discuss the innate behaviors that guide our chewing. That requires additional design in nerves that instruct the muscles and provide feedback, so that we know not to bite our tongues or to bite off more than we can chew. Working with all this are the senses of taste and smell. The result: what could be more pleasurable than a hearty meal among friends? Your salivary glands start secreting in advance of imagining your favorite food.
That’s just human teeth. Dentition is widespread throughout the vertebrates, from fish to mammals. Even squid have a certain kind of teeth called sucker ring teeth not for digestion but for clinging (see pictures and description at Evolution News & Views). Each species has just the kind of teeth it needs for its diet: giant crushing molars for elephants, sharp teeth for piranha fish, huge sabers for dinosaurs. Some species can replace whole sets of teeth in a conveyor-belt fashion. We humans start with baby teeth that get replaced. It’s interesting to consider whether the original design provided for regeneration of teeth. There’s clearly enough design to make us stand in awe; and for the blessing of teeth we should be thankful to our great Designer.