Energy & Fuels 2006, 20, 2189-2191
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Biomimetics: From Teeth to Photonic Crystal Solar Light Collectors Andrei P. Sommer*,† and Michael Gente‡ Materials DiVision, UniVersity of Ulm, 89081 Ulm, Germany, and DiVision of Prosthetic Dentistry, School of Dental Medicine, Philipps UniVersity, 35039 Marburg, Germany ReceiVed May 22, 2006. ReVised Manuscript ReceiVed July 3, 2006
Increasing insecurity about the fossil energy supply and environmental changes force us to intensify the search for clean alternative energy sources (e.g., the sun). Here, we show that the photonic crystal structure of dentin, already exploited to treat tooth inflammations, inspires a practical method to harvest sunlight. Arrays of toothlike structures mounted on silicon permit collection of sunlight virtually independent of the angle of incidence of the sun, which could be vital, for instance, in Antarctica.
Why do we like to show our teeth when we smile? It is because of their photonic crystal structure. Photonic crystals1 are characterized by a periodic dielectric structure, which determines the light-flow pattern (i.e., allows radiation of certain wavelengths to enter its structure but blocks others). In human dentin, the periodic structure consists of a regular array of tubes with diameters reaching from a few hundred nanometers at the dentin-enamel interface and a number on the order of 15 000 per mm2 to 2 µm at the pulp and a number on the order of 40 000 per mm2 (Figure 1a). Their length is 2-5 mm. As in opals2 (Figure 1b) and on butterfly wings,3 the only known natural photonic crystals, the interplay between light and the tooth is controlled by the variation in the index of refraction, n, of the material: ∼1.3 (aqueous liquid) in the tubes and ∼1.6 (hydroxyapatite) between them. Figure 2a and b provides a representative view of the architecture of a human tooth with the location of the pulp-dentin-enamel compartments, as well as the tubular structure and orientation. As long as there is a small index contrast between the tubular content and the surrounding mineral, light incident from any direction will be directed toward the pulp (Figure 2c). Previously, we filled the dentin tubes with liquids with different indexes of refraction including water (1.3), alcohol (1.4), and cinnamon oil (1.6).4 With equalization of the intratubular index with that of the surrounding hard tissue there was a spontaneous disappearance of the anisotropy effect exposed in Figure 2c. Index equalization was accompanied by an instant increase in the translucency of the dentin sample,4 indicating that as long as an index-contrast persisted, the light was transmitted exclusively via the hard tissue and not, as one might expect, through the holes. The finding that in periodic structures holes act as antiguides, channelling the transmission through the sample, was confirmed later for * To whom correspondence should be addressed. E-mail:
[email protected]. † University of Ulm. ‡ Philipps University. (1) Yablonovitch, E. Nat. Mater. 2003, 2, 648-649. (2) Blanco, A.; Chornski, E.; Grabtchak, S.; Ibisate, M.; John, S.; Leonard, S. W.; Lopez, C.; Meseguer, F.; Miguez, H.; Mondia, J. P., Ozin, G. A.; Toader, O.; van Driel, H. M. Nature 2000, 405, 437-440. (3) Vukusic, P.; Hooper, I. Science 2005, 310, 1151. (4) Hoff, N. Light Distribution in Human Dentin. Thesis, Philipps University, Marburg, 1997.
2D square arrays with 100 nm diameter holes spaced 500 nm apart and also square arrays with 1 µm holes spaced several microns apart by illumination via the tip of a near-field scanning optical microscope.5 The light-collector mechanism in dentin could serve as model for the design of solar concentrator arrays consisting of similarly operating hemispherical 3D photonic crystals mounted on silicon chips. The spacing between individual units could be adjusted to the prevalent position of the sun, thereby exploiting its irradiation, even at very low angles of incidence, and minimizing shadow effects. An important practical advantage of the curved collector surfaces is a self-cleaning effect: as on the back of beetles, rain drops would successively roll down, leaving the curved surface relatively dry. Wind is a critical factor and puts size limits to conventional collector panels. Photonic crystal-based solar concentrator arrays can have any size, and they promise to be instrumental in harvesting solar energy under extreme climate conditions and to work self-sufficiently in wind-intensive zones on Earth (research stations in Antarctica) and in planetary missions. Solar concentrator arrays could be used, in principle, on any stationary or mobile system with an extended surface exposed to the sun at variable angles and, thereby, provide energy to power-hungry systems. They might work alone or as modules in remote area power supply (RAPS) systems. It is instructive to calculate the ratio of effective light-flow area to blocked area for dentin and compare the total irradiated area in a hemispherical photonic crystal (cf. Figure 1b) with that of its projection onto a flat surface. For a hemispherical body of the size of a ladybird (convex surface area, Ac ≈ 5.6 × 10-5 m2; area at base, Ap ≈ 2.8 × 10-5 m2), a number of 15 000 tubes per mm2 and a mean tube diameter of 1 µm, we get for the blocked surface area Ab ≈ 6.7 × 10-7 m2, and for the total light collector area (Ac - Ab ) 5.5 × 10-5 m2), we obtain a value exceeding Ap. In analogy to the position and the extension of the pulp chamber in the tooth, the hemispherical photonic crystal should have its focal spot at its base and tolerate the passage of the same number of tubules which cross its (5) Campillo, A. L.; Hsu, J. W. P.; Bryant, G. W. Proceedings of the 6th International Conference on Near Field Optics and Related Techniques, 27-31 August, 2000, Twente, Netherlands.
10.1021/ef0602325 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/12/2006
2190 Energy & Fuels, Vol. 20, No. 5, 2006
Sommer and Gente
Figure 1. (a) Scanning electron microscopy image of human dentin. Of the natural photonic crystals (opals and butterfly wings), dentin comes closest to the perfect periodic man made structures (bar 10 µm). (b) Photograph of a natural opal on silicon wafer.
Figure 2. (a) Light microscopy photograph of human tooth sample cut parallel to its axis. Enamel, dentin, and pulp chamber are marked 1, 2, 3, respectively. (b) Dentin-enamel interface at higher magnification. Clearly visible is the hair-like tubular structure of dentin. (c) Human tooth sample: light incident from the right is directed by the dentin’s photonic crystal structure toward the pulp, the live center of the tooth.
convex surface. In a prototype, it could be reasonable to start with the antiguide/guide ratios designed by nature: around 1:65 at the dentin-enamel interface and 1:21 at the pulp-dentin interface. The related light-flow in the tooth, always directed toward the pulp, has two practical aspects: it restricts the curing efficiency for the external irradiation of light-curing composites in large dental cavities6 and facilitates the use of low-level lasers7 for noninvasive treatment of acute inflammations of the pulp (A. P. Sommer, personal communication). In a realistic scenario the size of the solar cell may be adjusted to the cross-section illuminated by the concentrator array. From their construction mimicking teeth, the solar concentrators could work even with miniaturized photovoltaic devices. Mounted, for instance, on PbSe-nanocrystal quantum dots, which showed a dramatic enhancement of the quantum efficiency (carrier multiplication via multiple excitons),8 they may be useful to harvest solar light in the visible spectrum of the sun. A closer inspection of (6) Sommer, A. P.; Gente, M. Biomed. Tech. 1999, 44, 290-293. (7) Sommer, A. P.; Pinheiro, A. L. B.; Mester, A. R.; Franke, R. P.; Whelan, H. T. J. Clin. Laser Med. Surg. 2001, 19, 29-33. (8) Qi, D.; Fischbein, M.; Selmic, S. Appl. Phys. Lett. 2005, 86, 093103.
Figure 1a reveals imperfections in the natural light concentrator (irregularities in hole diameters and intertubular distances). In addition, Figure 2b shows clearly that the dentin tubules are not straight, but slightly curved. These aspects, together with the opacity of dentin, are the reasons for the apparent loss in light intensity shown in Figure 2c. In an artificial solar concentrator, the orientation of the waveguide structures would be straight, the material less opaque, and the focusing geometry (parameters analogue to the progressive change in tubular diameter and intertubular space in dentin) would be optimized for a certain range of wavelengths. In this way, the efficiency of man-made solar concentrators would surpass that of the tooth by a considerable factor. Photovoltaic cells-based power plants cannot compete with oil: one problem is cost. Actually, the cells are the most cost intensive parts in photovoltaic systems. Dielectric waveguides serving as solar concentrators could be produced at low costs from cheap materials. The harvesting of light via smaller photovoltaic units could help cut the cost of large solar panels. Because they work at low solar incidence angles and at panel positions compatible with extreme wind speeds (wind is a major
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solar panel problem),9 they seem suitable for implementation in autonomously operating RAPS, which could be necessary for a variety of monitoring systems in polar regions. Advanced and clean energy-harvesting solutions are of special importance
in polar marine environments, which are presumably more sensitive to toxic effects of contaminants than other geographical zones.10
(9) How to protect solar energy collectors from the wind. New Scientist, 23 June, 2006, p 29.
(10) Chapman, P. M.; Riddle, M. J. EnViron. Sci. Technol. 2005, 39, 200A-206A.
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