Rheological behavior of commercial artificial tear solutions

PURPOSE: To measure the rheological behavior of artificial tears to gain insight into the potential role of rheology in predicting the efficacy of artificial tear solutions for the treatment of dry-eye disease (DED). SETTING: Research laboratories of I-MED Pharma, Canada, Rohn and Associates, Inc., New Jersey, and Hydan Technologies, New Jersey. DESIGN: Laboratory investigation. METHODS: Twenty commercially available artificial tear drops were purchased in Canada and the United Kingdom. Rheological measurements of viscosity and normal stress as a function of shear rate were performed at 25°C. RESULTS: For comparison of the rheological behavior, the various artificial tears were sorted into 3 groups: group A, which exhibit significant non-Newtonian shear-thinning behavior; group B, which exhibit moderate non-Newtonian shear-thinning behavior; and group C, which exhibit Newtonian behavior throughout the shear rate range. Results of normal stress difference, N1, as a function of shear rate were concordant with the rheological testing, indicated the viscoelastic nature of the samples in groups A and B, whereas members of group C did not exhibit any elasticity. CONCLUSIONS: The various artificial tear solutions were sorted into groups based on their Newtonian or non-Newtonian behaviors. The results suggest that non-Newtonian solutions should provide better comfort and longer-lasting symptomatic relief for DED. It remains to be confirmed clinically if there is a direct correlation between the rheological behavior of artificial tears and their ability to provide prolonged relief in DED, or if other factors are more important.