Both protein adsorption and aggregation contribute to shear yielding and viscosity increase in protein solutions

The nonlinear viscoelasticity of hyaluronic acid and its role in joint lubrication

Hyaluronic acid solutions have been widely studied due to their relevance to the rheological behavior of synovial fluid and joint lubrication. Ambulatory joint motion is typically large oscillatory deflections; therefore, large amplitude oscillatory shear strain experiments are used to examine the relevant non-linear viscoelastic properties of these solutions. Using the sequence of physical processes method to analyze data provides time dependent viscoelastic moduli, which exhibit a clear physiologically relevant behavior to hyaluronic acids non-linear viscoelasticity. In particular, it is seen that during peak strain/acceleration, the time dependent elastic modulus peaks and the loss modulus is at a minimum. The hyaluronic acid can provide an immediate elastic response to sudden forces, acting like a shock absorber during sudden changes in direction of motion or maximum deflection. However, during peak rate, the elastic modulus is at a minimum and the loss modulus is at a maximum, which provides greater efficacy to hydrodynamic shear lubrication.

Rheology of reconstituted silk fibroin protein gels: the epitome of extreme mechanics

In nature, silk fibroin proteins assemble into hierarchical structures with dramatic mechanical properties. With the hope of creating new classes of on demand silk-based biomaterials, Bombyx mori silk is reconstituted back into stable aqueous solutions that can be reassembled into functionalized materials; one strategy for reassembly is electrogelation. Electrogels (e-gels) are particularly versatile and can be produced using electrolysis with small DC electric fields. We characterize the linear and nonlinear rheological behavior of e-gels to provide fundamental insights into these distinct protein-based materials. We observe that e-gels form robust biopolymer networks that exhibit distinctive strain hardening and are recoverable from strains as large as γ=27, i.e. 2700%. We propose a simple microscopic model that is consistent with local restructuring of single proteins within the e-gel network.

The role of protein content on the steady and oscillatory shear rheology of model synovial fluids

Recent studies have debated the role of protein content on the bulk rheology of synovial fluid; in particular, it has been questioned if proteins aggregate or interact with hyaluronic acid in synovial fluid to enhance bulk rheology, or if observed effects were due to systematic measurement error caused by interfacial rheology, stemming from protein adsorption to the interface. Utilizing several techniques to ensure results reflect only bulk rheology, an examination of the role of bovine serum albumin and γ-globulin on model synovial fluid rheology has been undertaken. When interfacial rheology caused by protein adsorption to the interface is abrogated, the bulk rheology of a model synovial fluid composed of bovine serum albumin, γ-globulin, and hyaluronic acid is found to be dominated solely by the hyaluronic acid over a wide range of shear rates, strains and frequencies. These results show that the previously reported enhanced rheological properties of model synovial fluids are solely due to interfacial rheology and not from any type of protein aggregation/interaction in bulk solution.

Effect of interfacial viscoelasticity on the bulk linear viscoelastic moduli of globular protein solutions

The role of interfacial rheology on the bulk linear viscoelastic moduli of low concentration bovine albumin solutions is probed. Previously reported soft gel properties of these systems were attributed to either protein aggregation or organization within the bulk. Instead, these behaviors are shown to be attributable to the measurement error caused by interfacial rheology due to adsorption of bovine serum albumin to the air and water interface. Even at low bulk concentrations, fast interfacial adsorption results in erroneous measurements. When these effects are removed, the solutions are viscous dominated with a dynamic viscosity slightly larger than water.