I celebrate my silk, and shear my silk: assembly and rheology of reconstituted silk protein
Tabatabai, Alan Pasha
Reconstituting silk cocoons into aqueous protein solutions allows for the mechanical properties of silk to be utilized in non-fibrous forms. However, the exact impact of reconstitution on the protein was previously unknown. I quantify the molecular weight distribution of the protein after reconstitution as a function of extraction time and suggest a possible mechanism for the degradation of the protein. Additionally, the structure and mechanical properties of films made from silk with different molecular weight distributions is explored.Once a reconstituted silk solution is made, it can then be transformed into a gel by running an electric current through the sample. The electric current creates an acidic domain near the positive electrode, leading to the assembly of an electrogel. I discuss the mechanical properties of these electrogels through rheological measurements.The structure of the protein and protein-aggregates is measured by the controlled addition of acid instead of using an electric current. Aggregate sizes are measured with dynamic light scattering, while protein structure is observed through small angle neutron scattering. A possible explanation of the assembly of protein is generated by interpreting both protein structural and ionic concentration changes.Additionally, I move towards the covalent enzymatic gelling of reconstituted silk, where bonds have a particular fluorescent signal and are monitored during gelation. A comparison between bond growth and modulus growth, as measured through bulk rheology, indicates that percolation drives the initial stages of gelation. However, depending on the molecular weight of the protein, either gelation ends after percolation, or another mode of growth continues gelation. This secondary mode of growth is not related to the covalent bonds signifying that the molecular weight of the protein governs whether or not a secondary mechanism contributes to network formation.
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