Shear-Stress-Dependent Viscous Properties of Hyaluronic-Based Lubricants

Background/Objectives: The physical properties of topical eyedrop formulations used for the treatment of dry-eye disease play an important role in the lubrication of the ocular surface. In the present study, we investigate the shear-stress-dependent viscous properties of seven different commercially available lubricants using a novel optical rheometer for accurate analysis of the viscosity of liquid samples. In addition, the viscosity of natural tears was studied. Methods: Viscosity measurements were performed using Fluidicam RHEO technology (FORMULACTION, Toulouse, France), an automated optical rheometer combining microfluidic and imaging technologies. Measurements were conducted at a temperature of 34 °C and at shear rates ranging from 3000 s^-1 to 30,000 s^-1 to mimic conditions during eye blinking. Results: Natural tears showed minimal change in viscosity in response to changes in shear stress, with viscosity values of 0.91 mPa·s at 3000 s^-1 and 0.80 mPa·s at 30,000 s^-1. Among the artificial tear formulations, Thealoz^® Duo had the lowest viscosity (2.62 ± 0.01 mPa·s at 3000 s^-1), followed by Ivizia^® (2.42 ± 0.02 mPa·s), Hylo Comod^® (3.69 ± 0.01 mPa·s), Hylo Parin^® (3.87 ± 0.01 mPa·s), Xailin HA^® (4.73 ± 0.02 mPa·s), Vismed^® (5.42 ± 0.02 mPa·s), and Systane Hydration^® (7.76 ± 0.1 mPa·s). Conclusions: This study demonstrates that commercially available ocular lubricants exhibit varying degrees of shear-thinning behavior, a finding that is clinically relevant for their performance on the ocular surface. Formulations containing low-molecular-weight hyaluronic acid, such as Thealoz^® Duo, exhibited viscosity values closest to those of natural tears at the measured shear rates.