Study of Biomechanical Properties of Protein-Based Hydrogels Using Force-Clamp Rheometry

Protein-based hydrogels are unique materials that combine the durability and wetting properties of polymeric hydrogels with the biocompatibility and elasticity of proteins. Additionally, protein hydrogels present a promising system to probe the mechanical unfolding and refolding of proteins in an ensemble approach. Current rheometry methods apply tensile deformation under constant elongation while tethered materials experience changes in length and tension due to viscoelastic effects. These approaches limit the pulling speed in stress-relaxation experiments and the ability to extract information on the molecular behavior of proteins within hydrogels. Here we introduce a force-clamp (FC) hydrogel rheometer that can measure the extension of protein hydrogels at controlled setpoint forces. We demonstrate this system using protein hydrogels made of bovine serum albumin (BSA), polymerized via a photoactivated reaction. We measure the mechanical response of these hydrogels by maintaining a setforce using an analog proportional-integral-differential (PID) system. We investigate how protein concentration and solution conditions affect the mechanical properties of protein-based hydrogels in two modes: constant force mode where the hydrogel is exposed to constant pulling and relaxation forces and force ramp mode where the applied stress is linearly increased and decreased. Our measurements suggest that BSA molecules under force inside hydrogels behave similar to a Hoberman sphere. This exciting new method enables new experiments to study the effect of mechanical unfolding through a bulk approach and will pioneer the discovery and characterization of new superelastic biomaterials.