Flying by the Skin of Our Teeth

TAU says that teeth give us clues for building lighter airplanes and space vehicles

It's been a mystery: how can our teeth withstand such an enormous amount of pressure, over many years, when tooth enamel is only about as strong as glass? A new study by Prof. Herzl Chai of Tel Aviv University's School of Mechanical Engineering and his colleagues at the National Institute of Standards and Technology and George Washington University gives the answer.

The researchers applied varying degrees of mechanical pressure to hundreds of extracted teeth, and studied what occurred on the surface and deep inside them. The study, published in the May 5, 2009, issue of the Proceedings of the National Academy of Science, shows that it is the highly-sophisticated structure of our teeth that keeps them in one piece — and that structure holds promising clues for aerospace engineers as they build the aircraft and space vehicles of the future.

"Teeth are made from an extremely sophisticated composite material which reacts in an extraordinary way under pressure," says Prof. Chai. "Teeth exhibit graded mechanical properties and a cathedral-like geometry, and over time they develop a network of micro-cracks which help diffuse stress. This, and the tooth's built-in ability to heal the micro-cracks over time, prevents it from fracturing into large pieces when we eat hard food, like nuts."

News the aviation industry can bite into

The automotive and aviation industries already use sophisticated materials to prevent break-up on impact. For example, airplane bodies are made from composite materials — layers of glass or carbon fibers — held together by a brittle matrix.

In teeth, though, fibers aren't arranged in a grid, but are "wavy" in structure. There are hierarchies of fibers and matrices arranged in several layers, unlike the single-thickness layers used in aircrafts. Under mechanical pressure, this architecture presents no clear path for the release of stress. Therefore, "tufts" — built-in micro cracks — absorb pressure in unison to prevent splits and major fractures. As Prof. Chai puts it, tooth fractures "have a hard time deciding which way to go," making the tooth more resistant to cracking apart. Harnessing this property could lead to a new generation of much stronger composites for planes.

Prof. Chai, himself an aerospace engineer, suggests that if engineers can incorporate tooth enamel's wavy hierarchy, micro-cracking mechanism, and capacity to heal, lighter and stronger aircraft and space vehicles can be developed. And while creating a self-healing airplane is far in the future, this significant research on the composite structure of teeth can already begin to inspire aerospace engineers — and, of course, dentists.

Creating a super-smile

Dental specialists looking for new ways to engineer that picture-perfect Hollywood smile can use Dr. Chai's basic research to help invent stronger crowns, better able to withstand oral wear-and-tear. "They can create smart materials that mimic the properties found in real teeth," he says.

In natural teeth, there may not be any way to speed up the self-healing ability of tooth enamel, which the Tel Aviv University research found is accomplished by a glue-like substance that fills in micro-cracks over time. But fluoride treatments and healthy brushing habits can help to fill in the tiny cracks and keep teeth strong.

Clinically assess condition of tooth enamel using lasers

A group of researchers in Australia and Taiwan has developed a new way to analyze the health of human teeth using lasers. As described in the latest issue of Optics Express, the Optical Society's (OSA) open-access journal, by measuring how the surface of a tooth responds to laser-generated ultrasound, they can evaluate the mineral content of tooth enamel -- the semi-translucent outer layer of a tooth that protects the underlying dentin.

This is the first time anyone has been able to non-destructively measure the elasticity of human teeth, creating a method that can be used to assess oral health and predict emerging dental problems, such as tooth decay and cavities.

"The ultimate goal is to come up with a quick, efficient, cost-effective, and non-destructive way to evaluate the mineralization of human dental enamel," says David Hsiao-Chuan Wang, a graduate student at the University of Sydney in Australia and first author on the paper in Optics Express. Wang and his advisor Simon Fleming, a physics professor at the University of Sydney's Institute of Photonics and Optical Science, collaborated on the study with dental researchers at the University of Sydney and ultrasonic evaluation researchers at National Cheng Kung University in Tainan City, Taiwan.

Stronger than bone, enamel is the hardest and the most mineralized substance of the human body -- one of the reasons why human teeth can survive for centuries after a person has died. It envelops teeth in a protective layer that shields the underlying dentin from decay.

Throughout a person's lifetime, enamel constantly undergoes a cycle of mineral loss and restoration, in which healthy teeth maintain a high mineral content. If the balance between mineral loss and gain is lost, however, teeth can develop areas of softened enamel -- known as carious lesions -- which are precursors to cavities and permanently damaged teeth.

Enamel demineralization is caused by bad oral hygiene. Not brushing, for instance, can lead to the build-up of dental plaques, and bacteria in these plaques will absorb sugars and other carbohydrates a person chews and produce acids that will dissolve the minerals in tooth enamel.

Quantifying the mineral content of tooth enamel can help dentists determine the location and the severity of developing dental lesions. Existing methods for evaluating enamel are limited, however. Dentists can visually assess the teeth, but dental lesions can be hard to spot in certain parts of the mouth because they are obscured by dental plaque, saliva, or the structure of a tooth itself. Dentists can use sharp instruments to probe the enamel, but this can be destructive to the teeth and gums. X-ray scans can reveal dental lesions, but they give no information on the level of mineralization.

For research purposes, "nano-indentation" is commonly used for gaining information on the elasticity of tooth enamel -- a measure of its mineral content -- but nano-indentation destroys the measured regions of the enamel in the process and is only used to look at extracted teeth.

What Wang, Fleming, and their colleagues wanted to do was to develop a clinical method that would give as much information as nano-indentation and could be used to assess tooth enamel in actual patients while being completely non-destructive. So they developed a way to measure the elasticity of tooth enamel by adapting laser ultrasonic surface wave velocity dispersion, a method similar to what industrial engineers use to evaluate the integrity of thin films and metals.

The method uses short duration laser pulses to excite ultrasonic waves that propagate along the surface and penetrate only a small distance into a tooth. The velocity of these waves is influenced by the elastic properties of the enamel on a tooth, and by detecting the ultrasonic waves with fiber optics at various points, they can determine the enamel's elasticity, which is directly related to its mineralization.

In their Optics Express article, Wang, Fleming, and their colleagues showed that they could use this technique on extracted human teeth. They have not yet tested the technique on a living person's teeth, and it will likely take several years before any eventual device is ready for use in the dentist's office.