(725b) Engineering Stress Relaxation in Protein Hydrogels Containing Both Chemical and Physical Crosslinking

Hydrogels developed for biomedical engineering applications routinely experience stresses such as those generated by cellular traction, during tissue loading under tension, compression, or shear, and during injection or implantation. Engineering the appropriate response to these stresses is critical for material performance. At the molecular level, this response is governed by the network structure and dynamics. In hydrogels prepared from recombinant artificial proteins, these properties can be encoded within the protein sequence. Using this approach, we have developed an artificial protein capable of forming simultaneous chemical crosslinks through terminal cysteine residues and physical crosslinks through the formation of coiled-coils between helical domains on nearby chains. In hydrogels prepared from this protein, the chemical network junctions are permanent and store stress elastically while the physical junctions are transient and result in stress relaxation and energy dissipation as the coiled coils dissociate and reform. In this work, we extend this approach to engineer two important properties of viscoelastic hydrogels: the amount of stress relaxation and the relaxation rate. Both properties can be tuned by changes to the protein sequence that are readily generated by recombinant methods. The amount of stress relaxation, which is determined by the ratio of transient physical crosslinking to permanent chemical crosslinking, can be increased by increasing the number of coiled-coil domains per protein. The relaxation rate, which is related to the lifetime of a physical crosslink, can be tuned through sequence variation of the helical coiled-coil domain. The materials described here demonstrate how desired viscoelastic behavior can be engineered using recombinant proteins and may have applications as replacements for viscoelastic tissues.