Investigating the transverse optical structure of spider silk micro-fibers using quantitative optical microscopy

Abstract

Abstract:The transverse optical structure of two orbweaver (family Araneidae) spider dragline silks was investigated using a variant of the inverse-scattering technique. Immersing the silks in a closely refractive index-matched liquid, the minimum achievable image contrast was greater than expected for an optically homogeneous silk, given what is currently known about the optical absorption of these silks. This "excess contrast" indicated the presence of transverse optical structure within the spider silk. Applying electromagnetic scattering theory to a transparent double cylinder, the minimum achievable irradiance contrast for the Plebs eburnus and Argiope keyserlingi dragline silks was determined to be consistent with step index refractive index contrasts of 1−4 × 10 −4 and 6-7 × 10 −4 , respectively, supposing outer-layer thicknesses consistent with previous TEM studies (50 nm and 100 nm, respectively). The possibility of graded index refractive index contrasts within the spider silks is also discussed. This is the strongest evidence, to date, that there is a refractive index contrast associated with the layered morphology of spider silks and/or variation of proportion of nanocrystalline components within the spider silk structure. The method is more generally applicable to optical micro-fibers, including those with refractive index variations on a sub-wavelength scale.

Photoreflectance/scattering measurements of spider silks informed by standard optics

“…For spiders that have evolved to occupy aerial locations, especially locations of high brightness during the day, it is underappreciated that the silks of ecribellate orb webs (i.e. sticky prey capturing wheel-shaped webs) mostly comprise transparent materials of high optical quality [11][12][13][14][15][16][17]. The species Argiope keyserlingi and Plebs eburnus have been studied and shown to have high optical quality dragline/radial (i.e.…”

“…The species Argiope keyserlingi and Plebs eburnus have been studied and shown to have high optical quality dragline/radial (i.e. major ampullate) silks [11][12][13][14][15][16][17]. The orb webs of adult ecribellate spiders have radial and capture silks with widths from one to a few micrometres.…”

“…A study of webs under UV light [27] did note that the measurements in [19] may have been affected by the form of the silks in ways that had not been taken into account. The silk of A. keyserlingi has a measured refractive index of approximately 1.55 [14][15][16] and has low absorption in the visible [17]. Treating it as a homogeneous optical material, i.e.…”

Spider silk colour covaries with thermal properties but not protein structure

“…Second, pigments and other compounds, such as phenols, porphyrins, quinones and carotenoids, may reside within the skin layer and are thought to be responsible for some of the species-specific silk coloration variation [6,30,37,38]. Third, the β-sheet secondary structures and crystalline tertiary structures of MA silk are anisotropic, thus highly birefringent [39,40]. Given that the size, density and alignment of β-sheets can vary across spider species [41], this may contribute to variation in MA silk optical properties across species.…”