(696a) Structure and Nonlinear Mechanics of Injectable Telechelic Protein Hydrogels

Injectable biomaterials show great promise in tissue engineering, where they can be used to facilitate the repair or regeneration of tissues through minimally invasive surgical techniques. Shear-thinning physical hydrogels offer an attractive route to injectable materials because they can be prepared and stored in a stable gel state outside of the body, then they thin into a liquid-like state for injection and reset inside the body. We have developed a system of shear-thinning artificial protein hydrogels that afford sufficient design flexibility to incorporate biofunctionality and tailor the physical properties of the gel state, and we have investigated the relationships between molecular engineering, structure, and nonlinear mechanics.

Using protein expression in E. coli allows a high degree of control over the molecular engineering of these systems. The polymers are prepared as triblock proteins containing coiled-coil endblocks linked together by a random coil polyelectrolyte midblock. Association of the coiled-coils into multimeric aggregates with four or five coils per aggregate results in gelation of the materials, and the gelation is reversible with temperature, pH, and ionic strength by disrupting the association of the coiled-coils. Biosynthesis allows the production of exactly monodisperse polymers with the network topology controlled through the specificity and multiplicity of the coiled-coil aggregates and the length of the polyelectrolyte midblock.

The temperature of the sol-gel transition can be controlled by engineering the sequence of the coiled-coiled block. Heating the materials through the reversible sol-gel transition demonstrates that there are two separate gel regimes, one where the coiled-coils are folded and one where they are unfolded, suggesting that both specific and nonspecific aggregation of the endblocks may result in gelation in these systems. The microstructure of these physical gels depends strongly on the midblock molecular weight as well as the coiled-coil multiplicity. The gel molecules assemble in a hierarchical fashion, with monodisperse aggregates on the 15-100 nm length scale assembling into larger micron-scale structures that are responsible for gelation. The length scale of the primary aggregates is tuned through molecular design.

Several formulations of these gels present a combination of strong shear-thinning and extremely rapid recovery that makes them ideal for injectable tissue engineering applications. The gels demonstrate approximately a two order of magnitude drop in elastic modulus with less than ten seconds of applied oscillatory shear, and even after 15 minutes of oscillatory shear at 500% strain they can recover to greater than 95% of their initial strength in less than 30 seconds. This allows the materials to be precisely placed and sculpted during injection. Analysis of the nonlinear rheology of the materials suggests that this behavior is due to yielding or shear banding within the materials. Ongoing research is focused on understanding this mechanism and optimizing the nonlinear mechanics of the gels through molecular design.