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. 2004 Mar 9;101(10):3358-63.
doi: 10.1073/pnas.0307843101. Epub 2004 Mar 1.

Biological Glass Fibers: Correlation Between Optical and Structural Properties

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Free PMC article

Biological Glass Fibers: Correlation Between Optical and Structural Properties

Joanna Aizenberg et al. Proc Natl Acad Sci U S A. .
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Abstract

Biological systems have, through the course of time, evolved unique solutions for complex optical problems. These solutions are often achieved through a sophisticated control of fine structural features. Here we present a detailed study of the optical properties of basalia spicules from the glass sponge Euplectella aspergillum and reconcile them with structural characteristics. We show these biosilica fibers to have a distinctive layered design with specific compositional variations in the glass/organic composite and a corresponding nonuniform refractive index profile with a high-index core and a low-index cladding. The spicules can function as single-mode, few-mode, or multimode fibers, with spines serving as illumination points along the spicule shaft. The presence of a lens-like structure at the end of the fiber increases its light-collecting efficiency. Although free-space coupling experiments emphasize the similarity of these spicules to commercial optical fibers, the absence of any birefringence, the presence of technologically inaccessible dopants in the fibers, and their improved mechanical properties highlight the advantages of the low-temperature synthesis used by biology to construct these remarkable structures.

Figures

Fig. 1.
Fig. 1.
(a) Photograph of a typical specimen of E. aspergillum, showing the lattice-like skeleton of fused siliceous spicules and a crown-like organization of basalia at the base. (bd) SEM micrographs of the spined proximal region (b) and the smooth distal region of a basalia spicule (c) and its apical process (d). The SEM micrographs are related to a schematic representation of an individual spicule from the anchoring region (Center).
Fig. 2.
Fig. 2.
Structural and compositional characterization of the basalia spicules. (a) SEM image of a mechanically cleaved freestanding spicule, revealing an axial organic filament (OF), a smooth central cylinder (CC), and a striated shell region (SS). (Inset) SEM image of a similar spicule that was gently polished after embedding in an epoxide. A smooth homogeneous cross section is now seen. (b) Etch figures produced after HF treatment. The exposed CC region with the hollow core (indicated by an arrow) and surrounded by a receding series of SS layers is seen. (Inset) High-magnification scanning electron micrograph of the etched core region (≈2 μm in diameter), exposing the OF. (c) Etch figures produced after bleach (NaOCl) treatment reveal that the spicule's building blocks are silica spheres. Arrows indicate the interfaces between the shell layers, from which organic material is selectively removed. (Inset) Magnified view of the interface, revealing remnants of the organic bridges. (d Left) EDX carbon map of Microtome-cut spicule cross section highlights the presence of an OF. Note that the carbon content in the CC region is higher than that in the SS region. (Right) EDX sodium map indicates an enhanced concentration of sodium ions within the spicule cross section, with the highest density of Na occurring in the 2-μm core. (Insets) SEM images of the corresponding spicules.
Fig. 3.
Fig. 3.
Interferometric refractive index analysis of spicules. (a) Typical inter-ferogram of an individual spicule. (b) Quantitative refractive index values extracted from interferograms of spicules similar to those in a.(c) Polarization-specific refractive index measurements reveal negligible differences in the refractive index of fiber for either transversely or axially polarized light. (d) Schematic refractive index profile of the spicule in seawater. The structure can be considered as a single-mode fiber with a low refractive index cladding, which comprises both the CC and the SS regions, and a high refractive index core. Alternatively, because of the large refractive index difference between silica and seawater, the entire spicule can be considered as a high refractive index core region surrounded by a lower refractive index seawater cladding, thus forming a multimode waveguide.
Fig. 4.
Fig. 4.
Light-coupling experiments. (a) Free-space coupling of light to a free fractured spicule results in the entire fiber being illuminated. Right and Left are images of the experimental setup in the presence and absence of room light. Referenced with arrows (a and b) are the input and output ends of the spicule. (Inset) Output of the spicule imaged on a screen. (b) Transmission optical image of a spicule embedded in epoxide. The 2-μm core region is brighter than the cladding, showing that the fiber acts as a single-mode (few-mode) waveguide. (c) Transmission image of a freestanding spicule. Entire fiber is lit up, showing that the fiber acts as a highly multimode waveguide. (d) Light coupling into the spined regions of the fiber. (Upper) Optical micrograph of the original fiber with the spine positions labeled with arrows. (Lower) Optical image of the same fiber upon free-space coupling of white light into the fiber. Although the bulk of the fiber is substantially dark, select illumination points corresponding to the spine positions are observed. (Inset) Interferometric image showing the extension of the striated shell into the spined region of the spicule.

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