Polarized light microscopy, variability in spider silk diameters, and the mechanical characterization of spider silk

Spider silks possess a remarkable combination of high tensile strength and extensibility that makes them among the toughest materials known. Despite the potential exploitation of these properties in biotechnology, very few silks have ever been characterized mechanically. This is due in part to the difficulty of measuring the thin diameters of silk fibers. The largest silk fibers are only 5-10 mu m in diameter and some can be as fine as 50 nm in diameter. Such narrow diameters, coupled with the refraction of light due to the anisotropic nature of crystalline regions within silk fibers, make it difficult to determine the size of silk fibers. Here, we report upon a technique that uses polarized light microscopy (PLM) to accurately and precisely characterize the diameters of spider silk fibers. We found that polarized light microscopy is as precise as scanning electron microscopy (SEM) across repeated measurements of individual samples of silk and resulted in mean diameters that were similar to 0.10 mu m larger than those from SEM. Furthermore, we demonstrate that thread diameters within webs of individual spiders can vary by as much as 600%. Therefore, the ability of PLM to non-invasively characterize the diameters of each individual silk fiber used in mechanical tests can provide a crucial control for natural variation in silk diameters, both within webs and among spiders.