Nanomechanical and Microstructural Properties of Bombyx mori Silk Films
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Nanomechanical and Microstructural Properties of Bombyx mori Silk Films Donna M. Ebenstein1, Jaehyung Park2, David L. Kaplan2, and Kathryn J. Wahl1 1 Chemistry Division, U.S. Naval Research Laboratory, Washington, DC 20375, USA 2 Department of Biomedical Engineering, Tufts University, Medford, MA 02155, USA ABSTRACT Instrumented indentation and micro-Raman spectroscopy were used to investigate the mechanical and structural properties of Bombyx mori silk films. Twelve different films were prepared from B. mori silk fibroin protein using a variety of post-deposition processing treatments (e.g. soaking in methanol, soaking in water, stretching, and/or enzymatic etching). The results show that different treatments lead to changes in both the conformation of the silk fibroin protein and the mechanical properties of the films. INTRODUCTION Native silkworm and spider silk fibers have remarkably high strength and elasticity [1]. These desirable mechanical properties have inspired researchers to develop synthetic materials from silk proteins. The goal is to develop biomimetic materials that have comparable or even improved mechanical properties over the native silk fibers. Mechanical properties of silks have been shown to be related to spinning conditions, which in turn influence silk fibroin protein microstructures [2-5]. These microstructures include a non-crystalline (random coil) phase, a silk II crystalline phase composed of highly structured beta-sheets, and a less structurally organized silk I crystalline phase [6]. Silk fibers spun by silkworms or spiders have a large percentage of the silk II structure, which is thought to impart much of their high strength. In this study, silk films were prepared from B. mori silkworm fibroin protein by a variety of different processing techniques. Post-deposition processing treatments (soaking in methanol, soaking in water, and stretching) were investigated to correlate the processing approach to the silk II component of the films, and to the mechanical properties. Samples were etched (via a digestive enzyme) to selectively remove random coil and silk I protein conformations and reveal silk II structures. Mechanical properties of the films were examined by instrumented indentation, and near-surface protein conformations were characterized by micro-Raman spectroscopy. EXPERIMENTAL DETAILS Sample preparation Silk films were cast from an 8 wt% B. mori silk fibroin protein solution. Preparation of the silk fibroin solution has been explained in detail elsewhere [7-8]. In short, the sericin proteins coating the silk fibers were extracted through Na2CO3 treatment and the purified silk fibroin protein was then suspended in LiBr solution and dialyzed to produce an 8% wt/vol aqueous silk fibroin solution. Twelve different specimens were produced (as illustrated in figure 1): Five samples that underwent combinations of methanol, water, and stretching treatments; a control specimen that underwent none of those three treatments; and these six samples following an enzymatic etch
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