Silk Polymer Models for Structure-Function Relationships

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Intellectual Merit - Silk proteins are a useful model system for the study of novel functional properties derived from hydrophobic polymers in aqueous systems. The impressive mechanical properties of spun silk fibers rest with the unusual control of structure development in glands in silkworms and spiders, all achieved through control of water content in combination with appropriate sequence chemistry. The objective is to understand the mechanistic basis for silk protein assembly, through control of water content in silk solutions, as a route to new and improved processing options for hydrophobic polymers in general. This insight has strong implications for: (a) new materials engineering from silks, and (b) for new polymer chemistries/sequences that mimic silk in that the designs (chemical sequences) must consider processing environments as well as functional outcomes. A systematic investigation will be undertaken into the formation and structural features of silk gels containing high concentrations of protein achieved via osmotic stress, and subsequent transitions and structural and morphological features induced through chemical and mechanical factors. Specifically, the following aims will be pursued: (a) to compare structure development in aqueous systems by systematic control of water content, including comparisons between native gels from silkworm glands and reconstituted silkworm silk; and, (b) to study the role of specific environmental factors (e.g., pH, divalent cations, temperature) on rates and nature of structure and morphology development at different water contents. To address these questions, model materials will be characterized using spectroscopic techniques (Fourier Transform infrared, FTIR; Raman), scattering techniques (wide and small angle X-ray scattering, WAXS/SAXS; small angle light scattering, SALS), and imaging techniques (atomic force microscopy, AFM; scanning electron microscopy, SEM; optical ellipsometry; differential interference contrast microscopy, DIC) to assess morphologies and structures at a variety of length scales. These assessments will permit the formulation of phase diagrams for quiescent silk gels. Then, to approximate the step of fiber formation, the effects of stress (using tensile deformation and shear) on structure evolution and the corresponding mechanical properties of reconstituted silks will be studied. The planned studies build off the recent discoveries from the lab on the solution behavior of silk proteins and control of this behavior. %%%Broader Impact - The proposed experiments represent an interdisciplinary approach to the study of this unique polymer, including a biochemist (David Kaplan) and a physicist (Peggy Cebe). The graduate and undergraduate students involved will gain direct insight from both perspectives during their research, while also contributing to new directions in polymer science and engineering. The research approaches and outcomes will be exported into classroom settings in a number of ways for a broader audience of students including lecture modules in both undergraduate and graduate courses, windows on research laboratory experiences for undergraduate students, and specific programs for deaf and hearing impaired students.