Lighting the World with a Smile

Hank Hogan

According to researchers in Germany, teeth may provide more than just a smile that helps gain friends and influence enemies. Instead, the structure of these dental fixtures could lead to the development of solar collectors that function under low-angle light conditions and strong winds or to better restorative dentistry techniques. Such are the beliefs of a team with members from the University of Ulm and Philipps University School of Dental Medicine in Marburg.

A cross section of a human tooth shows layers of enamel, dentin and pulp. Differences in refractive index among these layers — combined with the characteristic waveguide structure of the dentin — bring light from the outer tooth surface to the pulp. The upper inset shows a high-resolution microscopic image of the dentin's tubes. The lower inset is a scanning electron microscopic image of the photonic crystal structure of tubular dentin.

Andrei P. Sommer, a senior materials scientist at the University of Ulm, noted that such applications arise because teeth do not solely reflect light. “Teeth are, indeed, unique natural solar collectors. The biological purpose of the light-harvesting function is presently not totally clear,” he said.

Teeth consist of enamel on the outside and living pulp on the inside. Between these substances is dentin, which, as with the enamel, is composed of hydroxyapatite, a relatively transparent mineral with a refractive index of roughly 1.6. Unlike the partly crystalline solid enamel, however, dentin is full of hollow tubes that are up to 5 mm long and full of a liquid that has a refractive index of about 1.3. At the dentin-enamel interface, the tubes are a few hundred nanometers in diameter; at the dentin-pulp interface, the tubes are ~2 μm in diameter.

The combination of structure and refractive indices makes teeth act as near-perfect photonic crystals, with the tubes functioning as antiguides that force light down the tube walls. Using a 40-mW, 670-nm laser from Lasotronic International Ltd. of Budapest, Hungary, the investigators found that light striking teeth from almost any angle is directed inward toward the pulp. By filling the tubes with cinnamon oil, which has a refractive index matching that of hydroxyapatite, they extinguished the effect.

Light funneling enables the treatment of inflamed pulp with a low-level therapeutic laser, which could affect restorative dentistry, because large composite fillings are photopolymerized from the top and side. Side illumination, according to Sommer, tends to be ineffective because the dentin’s structure prevents it from effectively reaching the filling.

Beyond the implications for dentistry, the research offers a blueprint for a solar collector, particularly in challenging environments. A solar collector built with a toothlike structure could collect light from any angle, and the vulnerable solar cell would be protected by a tough photonic crystal. Also, a rounded shape would shed obscuring snow, water and dust.

“Panels with arrays of photonic-crystal-based solar collectors represent a reasonable modality to generate energy in Antarctica,” Sommer said.

Energy & Fuels, September 2006, pp. 2189-2191.