Biophysicists have known for a long time that polar-bear fur and other biological materials with an ordered microstructure can guide light, but this has always been assumed to be because of fibre-optic effects. Now, however, physicists in Germany have shown that the light-guiding effect in a component of teeth called dentin is due to light scattering. The researchers say that the finding could also be relevant to other biomaterials and call for all light-guiding effects in nature to be reviewed (Phys. Rev. Lett. 97 018104).
Alwin Kienle and Raimund Hibst at the University of Ulm came to the surprising conclusion that light can be guided by scattering by looking at how laser light scatters off the faces of cubes of dentin varying in thickness from 20 microns to 1 mm. Using an optical microscope, the researchers observed that almost all of the light was transmitted from one of the faces perpendicular to the first face but that very little light was transmitted from the other faces (figure 1).
According to the team, this anisotropic light propagation is due to multiple scattering from the microstructure of dentin, which is made up of “tubules” — cylindrical channels that run from the pulp to the enamel-dentin junction in a tooth (figure 2). The team confirmed this result by studying exactly how the light was transmitted using a CCD camera and comparing these measurements to computer simulations of light propagation through dentin.
“This light-guiding effect could be important for therapeutic and diagnostic applications of light in medicine because many tissues exhibit a similar elongated, cylindrical microstructure as dentin — for example, muscle, skin, tendon, bone, enamel and ligaments,” explains Kienle. He also thinks that this effect could be seen in biomaterials other than human tissue. For example, nature could be using it as an ‘inexpensive’ way to harvest light in seeds, leaves, or plants. Kienle believes the effect could even be used to focus sunlilght and generate solar power.
