Structure and Optical Anisotropy of Spider Scales and Silk: The Use of Chromaticity and Azimuth Colors to Optically Characterize Complex Biological Structures

Abstract

Herein, we give an overview of several less explored structural and optical characterization techniques useful for biomaterials. New insights into the structure of natural fibers such as spider silk can be gained with minimal sample preparation. Electromagnetic radiation (EMR) over a broad range of wavelengths (from X-ray to THz) provides information of the structure of the material at correspondingly different length scales (nm-to-mm). When the sample features, such as the alignment of certain fibers, cannot be characterized optically, polarization analysis of the optical images can provide further information on feature alignment. The 3D complexity of biological samples necessitates that there be feature measurements and characterization over a large range of length scales. We discuss the issue of characterizing complex shapes by analysis of the link between the color and structure of spider scales and silk. For example, it is shown that the green-blue color of a spider scale is dominated by the chitin slab’s Fabry–Pérot-type reflectivity rather than the surface nanostructure. The use of a chromaticity plot simplifies complex spectra and enables quantification of the apparent colors. All the experimental data presented herein are used to support the discussion on the structure–color link in the characterization of materials.

Determination of Stokes Vector from a Single Image Acquisition

“…Another conventional method to solve this issue of fast versus slow axis is to use a color-shifting plate (usually a 530 nm wavelplate) in cross-polarized imaging, which determines positive and negative contributions to the refractive index with blue-red perpendicular directions corresponding to the n e (slow)-n o (fast) orientations of silk. [28] Finally, with 𝛿, 𝜃 determined via fit or from analytical expression using single a image from a 4-pol. camera, all four Stokes parameters can be defined: S 0 (intensity) and S 1,2,3 .…”

Four-Polarisation Camera for Anisotropy Mapping at Three Orientations: Micro-Grain of Olivine

“…In the case of absorption-dominated transmission changes, it is shown that a cos-fit can be used for each pixel of the camera and obtained using only three points (three from four polarisation images). This technique is demonstrated for transmission and reflection modes using natural olivine micro-crystals, but it is fully transferable to other polarisationresolved imaging techniques; e.g., bio-medical techniques [14,15], 3D optical coherent tomography [16,17], analysis of structural colours in natural materials [18], and industrial applications, such as edge detection of fast moving materials and workpieces [19].…”