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Tatyana F Svitova, Meng C Lin; Tear-lipid films: decoding enigma of “water-evaporation barrier” by studying model-lipid films in vitro. Invest. Ophthalmol. Vis. Sci. 2018;59(9):4955. doi: https://doi.org/.
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© ARVO (1962-2015); The Authors (2016-present)
To quantitatively evaluate the efficiency of model-tear-lipid (MTL) films of 30-100 nm as water-evaporation inhibitors and to elucidate the impact of different MTL components and their concentrations on evaporation retardation.
Sessile-drop technique with constant drop-volume control was used to volumetrically quantify the evaporation rates at constant 36°C temperature and humidity RH = 75 %. The 0.8 – 2.0 µl drops of model lipid-mixture solutions were deposited on the surface of 8-10 µl aqueous drops to create films of 40±100 nm; the drops were kept in sealed optical cell for 24-48 hours. The dynamic surface pressure measurements during expansion-contraction cycles were performed under the same conditions. Rame-Hart tensiometer was used for all the measurements. The set of MTL tested included model meibum (MM) composed of 1:1 mixture of Cholesteryl Oleate and (Behenyl Oleate:Oleyl Oleate = 2:1)+0.03 parts of C16-TAG. OAHFA (C16), Egg Sphingomyelins, DSPS, DOPC and SLPC were added in concentrations 0-10% wt. as representatives of polar tear lipids (PTL).
Water evaporation through MTL films decreased with film aging but in different degrees. Evaporative fluxes reached steady-state after 18-24 hours. Evaporation-retardation efficiency is strongly dependent on model mixture compositions. The flux through 80-nm MM films without any PTL reduced water-evaporative fluxes by 30±5% at most, while the addition of 5% OAHFA improved MM performance only marginally. However, MM mixtures with optimized PTL concentration decreased evaporative fluxes by 77±5%, comparable to evaporation reduction observed for healthy human tears in vivo. Evaporative fluxes through optimized MTL films decreased by a factor of 4.5-5.0 as film thickness was increased from 0 to 55-60 nm and remained practically constant for thicker MTL films.
Our methodology demonstrated that significant evaporation retardation by MTL films in vitro is possible and can be accurately and reproducibly quantified in contrast to findings of other in vitro studies showing that neither model lipids nor human meibum films of ocular thickness are able to retard evaporation significantly. It was found that polar lipids, especially phospholipids, can enhance the strength of lipid- films evaporative barrier to the level observed for human tear lipids in vivo.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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