(503c) Uniaxial Extension of Associative Proteins Reveals Chain Alignment Mechanism in Highly Extensible and Tough Protein Hydrogels

Engineering artificial protein materials for medical applications requires precise control over their mechanical properties, including stiffness, toughness, extensibility, and stability in the physiological environment. Previously, topological entanglement was employed as an effective strategy to improve the tensile toughness and extensibility of an associative protein hydrogel network. High molecular weight associative protein polymers were synthesized by oxidative chain coupling of N- and C-terminal cysteine residues in recombinant coiled-coil proteins. The resulting entangled associative gels exhibit remarkable toughness (65 000 J m^–3) and extensibility (approximately 3000% engineering strain) with minimal changes in material stiffness (only a 1.5-fold increase in the plateau modulus) compared to unentangled associative gels.

Here, we apply uniaxial extension to show that the high extensibility and toughness of entangled associative protein hydrogels are related to molecular alignment within the gel under deformation, which is not observed in unentangled associative protein gels. Uniaxial strain-induced structural changes are investigated in protein hydrogels up to ~600% engineering strain using in situ small-angle X-ray scattering, laser light scattering, and polarized optical microscopy. These methods reveal that the hydrogel develops an anisotropic optical response to uniaxial strain at the nano-, micro-, and macro-scales, respectively. Macroscale anisotropy suggests bulk chain alignment occurs along the straining axis, which is confirmed with depolarized light scattering. This behavior does not emerge in hydrogels with molecular weight below the entanglement cutoff. The findings suggest that both entanglements and freedom of individual chains to align at the nanoscale due to junction relaxation are critical to achieving high extension in physical gels.