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P.A. Simmons; Modeling of Ocular Viscosity for Mid–viscosity Artificial Tear Preparations . Invest. Ophthalmol. Vis. Sci. 2004;45(13):3893.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:Viscosity of artificial tear preparations is an important property related to ocular retention time and potential for clinical benefit. The ocular viscosity on the eye will be affected by temperature, dilution and mixture with natural tears, pH, and the shear forces due to blinking. Ocular viscosity is likely to be substantially different from manufacturers’ specifications. This study modeled ocular conditions to compare viscosity of several different tear preparations. Methods: A programmable rheometer was used to measure viscosity at ocular temperature and with varying shear forces to simulate interaction with the lids during blinking. Two levels of dilution with a model tear fluid containing salts and proteins (20% and 50%) were used to simulate mixing with natural tears. pH was also adjusted to ocular levels. Three artificial tear formulas were tested, with comparison between packaged viscosity performance and ocular values. Results: Initial packaged viscosity measured at 25 deg at low shear conditions was very high for a carbomer–based gel (>5000 cps), and lower for a 1% carboxymethylcellulose (CMC)–based product (60 cps) and a .18% guar gum–based product (18 cps). All 3 materials exhibited some degree of shear thinning under these conditions. Increasing measurement temperature to 35 deg C reduced viscosity of all formulas by approximately 10%. Mixture with model tear solution caused the carbomer gel to solubilize, and reduced viscosity due to dilution for the other formulas. At 50% (1:1 mixture) dilution with model tear, the carbomer, CMC, and guar formulas had mean viscosities (respectively) of 11.5, 11.0, and 4.3 cps at low shear and 4.4, 6.8, and 2.3 cps at high shear. Adjustment of pH to ocular levels (pH 7.4 +/– 0.1) did not substantially change the viscosity of any of the products. Outside of the physiological range (>pH 7.6), the guar formula increased viscosity and exhibited shear–thickening (dilation and instability) behavior. Conclusions: Under ocular conditions of temperature, dilution, and pH, viscosity performance of artificial tears may be substantially different from that of the packaged product. Overall, the 1% CMC formula maintained more stable viscosity performance, and generally higher viscosity, than either the carbomer or guar gum formulas.
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