Lactic Acid-Induced Colloidal Microrheology of Synovial Fluids.

The presence of colloidal scaffolds composed of proteins and hyaluronic acid engenders unique viscous and elastic properties to the synovial fluid (SF). While the elastic resistance of SF due to the presence of such nanoscale structures provides the load-bearing capacity, the viscous nature enables fluidity of the joints during the movements to minimize the wear and tear of the adjacent muscle, cartilage, or bone tissues. It is well-known that the hypoxic conditions at the bone joints often increase the lactic acid (LA) concentration due to the occurrence of excess anaerobic respiration during either hyperactivity or arthritic conditions. The present study uncovers that in such a scenario, beyond a critical loading of LA, the colloidal nanoscaffolds of SF break down to precipitate higher molecular weight (MW) proteins and hyaluronic acid (HA). Subsequently, the viscosity and elasticity of SF reduce drastically to manifest a fluid that has reduced load bearing and wear and tear resistance capacity. Interestingly, the study also suggests that a heathy SF is a viscoelastic fluid with a mild Hookean elasticity and non-Newtonian fluidity, which eventually transforms into a viscous watery liquid in the presence of a higher loading of LA. We employ this knowledge to biosynthesize an artificial SF that emulates the characteristics of the real one. Remarkably, the spatiotemporal microscopic images uncover that even for the artificial SF, a dynamic cross-linking of the high MW proteins and HA takes place before precipitating out of the same from the artificial SF matrix, emulating the real one. Control experiments suggest that this phenomenon is absent in the case when LA is mixed with either pure HA or proteins. The experiments unfold the specific role of LA in the destruction of colloidal nanoscaffolds of synovia, which is an extremely important requirement for the biosynthesis and translation of artificial synovial fluid.

Microrheology of Mucin-Albumin Assembly Using Diffusing Wave Spectroscopy.

Mucus plays an important role in the protection of the epithelial cells from various pathogens and low pH environments besides helping in the absorption of nutrients. Alteration of the rheology of the mucus layer leads to various disease conditions such as cystic fibrosis, Crohn's disease, and gastric ulcers, among others. Importantly, mucus consists of various mucins along with proteins such as immunoglobulin, lysozyme, and albumin. In the present study, we explore the effect of pH on the interactions between bovine serum albumin (BSA) and porcine gastric mucins using diffusing wave spectroscopy (DWS). The study unveils that BSA actively binds with mucin to form mucin-BSA complexes, which is largely driven by electrostatic interactions. Interestingly, such physical interactions significantly alter the microrheology of these biomaterials, which is indicated by a reduction in the diffusivity of tracer particles in DWS. An array of DWS experiments suggests that the interaction between mucin and BSA is the highest at pH 7.4 and the least at pH 3. Further analyses using atomic force microscopy showed the formation of a compact cross-linked colloidal network of mucin-BSA complexes at pH 7.4, which is the main reason for the reduction in the diffusivity of the tracer particles in DWS. Furthermore, the circular dichroism analysis revealed that the secondary structures of mucin-BSA complexes are markedly different from those of only mucin at pH 7.4. Importantly, such a difference has not been observed at pH 3, which confirms that largely electrostatic interactions drive the formation of mucin-BSA complexes at neutral pH. In such a scenario, the presence of Ca2+ ions is also found to facilitate bridging between BSA molecules, which is also reflected in the microrheology of the suspension of BSA-mucin complexes.