Evaluating Viscosity and Tear Breakup Time of Contemporary Commercial Ocular Lubricants on an In Vitro Eye Model

To evaluate the link between the viscosity of ophthalmic formulation and tear film stability using a novel in vitro eye model.The viscosities and noninvasive tear breakup time (NIKBUT) of 13 commercial ocular lubricants were measured to evaluate the correlation between viscosity and NIKBUT. The complex viscosity of each lubricant was measured three times for each angular frequency (ranging from 0.1 to 100 rad/s) using the Discovery HR-2 hybrid rheometer. The NIKBUT measurements were performed eight times for each lubricant using an advanced eye model mounted on the OCULUS Keratograph 5M. A contact lens (CL; ACUVUE OASYS [etafilcon A]) or a collagen shield (CS) was used as the simulated corneal surface. Phosphate-buffered saline was used as a simulated fluid.The results showed a positive correlation between viscosity and NIKBUT at high shear rates (at 10 rad/s, r = 0.67) but not at low shear. This correlation was even better for viscosities between 0 and 100 mPa*s (r = 0.85). Most of the lubricants tested in this study also had shear-thinning properties. OPTASE INTENSE, I-DROP PUR GEL, I DROP MGD, OASIS TEARS PLUS, and I-DROP PUR had higher viscosity in comparison to other lubricants (P < 0.05). All of the formulations had a higher NIKBUT than the control (2.7 ± 1.2 seconds for CS and 5.4 ± 0.9 seconds for CL) without any lubricant (P < 0.05). I-DROP PUR GEL, OASIS TEARS PLUS, I-DROP MGD, REFRESH OPTIVE ADVANCED, and OPTASE INTENSE had the highest NIKBUT using this eye model.The results show that the viscosity is correlated with NIKBUT, but further work is necessary to determine the underlying mechanisms.The viscosity of ocular lubricants can affect NIKBUT and tear film stability, so it is an important property to consider when formulating ocular lubricants.