Doctor of Philosophy (Ph.D.)
Hannes C. Schniepp
Christopher A. Del Negro
Silk has enormous potential as a next-generation material: it is a biopolymer spun from protein at ambient temperature and pressure, and the best spider silks are as strong as steel and tougher than Kevlar. Because of its green production, mechanical robustness, and biocompatibility, silk has been researched for use in a range of engineering and biomedical applications. However, despite exciting recent advances in artificial silk fabrication via recombinant techniques, reserachers remain unable to fully replicate the complex assembly and hierarchical structure of native silks. Herein, we describe our morphological and mechanical characterization of a uniquely simple model silk system that revealed novel aspects of assembly and morphology: the thin ribbon silk spun by the recluse genus of spiders (Loxosceles). We characterized the Loxoscles silk ribbon, which is 7–10 µm wide and only 50–80 nm thick, at molecular-scale resolution using atomic force microscopy (AFM), then designed a custom AFM-based test to probe the silk’s mechanical properties. Our findings revealed a nanofibrillar substructure and mechanical performance typical of other spider silks, as well as hitherto undescribed protrusions (“nanopapillae”) on the surface. To complement these results, we investigated the 5 nm–thin cribellate fibrils spun by the southern house spider, a close relative of Loxosceles, and observed both nanofibrils and nanopapillae. We also assembled silkworm silk using a simple spin-coating routine, revealing nanofibrils in AFM scans that we quantified using extensive imaging analysis. The similarities and differences between these thin silk systems give a blueprint of silk’s core structural constituents and show the effects of thin assembly. Beyond these studies of molecular-scale structure, we discovered that the recluse uses an intricate spinning mechanism to form its ribbons into loops—a previously unknown web organization. By performing mechanical tests of looped silk and designing a mechanical model of the system, we found that introducing sacrificial loops into any fiber can significantly enhance its toughness, and we identified which looping and fiber parameters optimize the effect. We then fabricated a proof of concept—a looped strand of tape—and showed it to be far tougher than non-looped tape of equivalent length. Thus, our research of thin silk systems and our derivation of a bioinspired model have the potential to significantly impact the design of novel artificial silks and ultra-tough fibers.
© The Author
Koebley, Sean Robert, "The Thin Ribbon Silk of the Brown Recluse Spider: Structure, Mechanical Behavior, and Biomimicry" (2016). Dissertations, Theses, and Masters Projects. Paper 1516639558.
Available for download on Saturday, October 06, 2018