(715a) Invited Talk: Dynamics of Artificially Engineered Protein Gels Based on Coiled-Coil Associating Groups

Artificially engineered protein hydrogels provide an attractive platform for biomedical materials due to their similarity to components of the native extracellular matrix; however, a better understanding of the polymer science underlying their mechanical response can help to extend the range of mechanical properties achievable with such systems.  One particular area of interest is improving the toughness of protein gels.  Herein, we demonstrate an approach to biomaterial toughening through engineering both topological entanglements and physical associations into the gel to produce two distinct length and timescales of network interaction.  Oxidation or coupling of cysteine residues in proteins enables chain extension through protein end coupling, producing polymer gelators of high molecular weight that are not easily achievable directly by protein expression.  When the molecules are processed to induce a chain extended state, the plateau modulus of the network remains largely unchanged, but significant increases in low frequency modulus and elastic recovery after loading are observed.  In addition, the resulting gels can be extended to over 3,000% strain before break.  These materials serve as models for comparison with theories of sticky reptation.  Using responsive coupling chemistries allows entanglements to be produced dynamically within the material, enabling responsive enhancement of the network. 

When the materials are prepared with a molar mass below the entanglement molar mass, they can be compared to sticky Rouse theories of polymer dynamics.  Clear deviations from single mode relaxation behavior are observed in shear rheology, and superdiffusive behavior can be identified in the materials.  Comparisons between multi-sticker and simple telechelic proteins provide insight into the effects of molecular structure on the observed behavior.  Finally, we report the development of a simple associative network model capable of calculating chain end-to-end distance distributions under flow as a first step to understanding the specific behaviors observed in these systems.  Interconversion between dangling, looped, and bridging chains is modeled with a Smoluchowski equation using the dumbbell model and capturing chain end association and flow terms.  This model shows for the first time the effect of looped chains on stress in gels and the presence of flow instabilities.  Furthermore, by tracking the entire chain end distribution, this model is able to quantify the effects of chain tumbling on stress in the polymer networks.