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PA Simmons, T Zhou; Comparative Rheology of High-viscosity Artificial Tear Preparations . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3113.
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Purpose: Primary treatment for various forms of dry eye disease consists of administration of artificial tear supplements. These products vary greatly in form from simple saline drops to heavy ointments. Higher viscosity supplements generally have longer ocular retention, with potential for greater clinical benefit. However, depending on composition, viscosity on-eye will be affected by a number of factors including temperature, pH, ionic balance, and the shearing force of the lids. This study investigates the effects of electrolyte concentration and shear force on the viscosity of four current tear products with moderate to high initial viscosities. Methods: Viscosity measurements were made on an AR1000 rheometer (TA Instruments) at 35 deg C, with shear rates ranging from 1 to 1000 sec-1. Two carbomer (carbopol 980)-based products and two carboxymethylcellulose (CMC)-based products were tested, each under three conditions: as delivered from the container, diluted 1:1 with distilled water, and diluted 1:1 with an electrolyte solution similar to natural tears, in order to simulate ocular conditions. Results: As delivered, the carbomer-based products showed marked shear-thinning, with apparent viscosities ranging from ≷10,000 cps at low shear rates to 150-250 cps at high shear rates. CMC-based products had much less shear thinning, ranging from ≷300 cps at low shear rates to 27 cps at high shear rates. Dilution with distilled water lowered apparent viscosity approximately 50% for all products. When mixed with tear electrolytes, the carbomer-based gel products completely converted to the sol state, yielding viscosities <5 cps (90% reduction from the values with water dilution alone) at moderate to high shear rates. CMC-based products were not as strongly affected, with minimum viscosities of 8 to 15 cps, depending on formula. Conclusion: The carbomer-based gel products exhibit large amounts of shear thinning and have very low residual ocular viscosity with addition of electrolytes simulating mixture with the tear fluid. CMC-based products are more Newtonian, and maintain higher viscosity under simulated ocular conditions. This suggests that, on eye, the potential for lubrication and protection is greater for CMC-based products, and may explain in part their reported clinical efficacy for dry eye treatment.
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