Shedding light on the viscoelastic behavior of artificial and human tears: A microrheological approach

This study conducts research on the viscoelastic properties of biological fluids, including both hyaluronic acid-containing lubricating eye drops and human tears, by means of passive microrheology. By tracking the Brownian motion of tracer particles of various sizes in the fluids, we were able to probe their viscoelastic properties. The results showed that the viscoelastic properties of artificial tears, such as Newtonian viscosity and relaxation time, scale to the concentration and macromolecular size of hyaluronic acid, resembling unentangled semidiluted solutions. Moreover, human tears were found to have a viscosity that is 50% greater than that of pure water, comparable to artificial tears containing 0.1% hyaluronic acid, but with a relaxation time one order of magnitude longer than ophthalmic solutions. This behavior was attributed to their intricate composition. The distinctive aspect of this study lies in demonstrating the feasibility of measuring the rheological properties of human tears, a biofluid of great interest, using extremely small sample volumes and microrheology, providing comprehensive information across a wide range of frequencies approaching those corresponding to blinking upon the application of the Cox-Merz rule. These findings are of significant value as they pave the way for future research on small volumes of tears from ophthalmic patients.