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Antti H. Rantamaki, Juha M. Holopainen; Evaporation Rate of Artificial Tear Fluid. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4252.
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The function of the tear film lipid layer remains partly unknown. The main functions are believed to be the reduction of surface tension and the evaporation control of water from the ocular surface. Little evidence, however, has been shown in support for the latter. Here we have studied the influence of differing lipid compositions on the evaporation rate in near-physiological temperatures. The lipid compositions were based on the present knowledge of tear film lipid layer.
The evaporation rates were measured in a thermostated Langmuir trough. The temperature of the subphase, depending on the lipid composition, was between 34-37 °C. The Langmuir device was placed into a cabinet having a constant flow of filtered and dried air diminishing the effect of fluctuations in relative humidity. The airflow also caused a slight vibrant movement of the air-water interface replicating the natural environment of the tear film. The surface pressure of the air-water interface was measured to assess the packing density of the lipid layer. The lipid compositions used in the measurements contained from one to four lipid species including polar egg-yolk PC (eggPC), non-polar lipids cholesteryl ester (CE), triglyceride mixture (TG), and wax ester (WE). The evaporation rate was based on the evaporated mass of the subphase.
The evaporation rate of phosphate buffered saline (pH 7.4) was 296 ±4 µL/min. A mixture of eggPC and WE having a molar ratio of 1:9 reduced the evaporation rate to 233 ±3 µL/min at surface pressure of ~21 mN/m. The same amount of pure WE at the air-water interface produced the slowest evaporation rate of 230 ±9 µL/min. Lipid mixtures additionally containing CE and TGs did not have an evident effect on evaporation rate.
The effect of the lipid layer on evaporation rate proved out to be lower than expected. As the precise proportions of the lipid species in tear film lipid layer are unknown, experiments were performed using differing proportions. Based on the results, only WEs seem to have a beneficial effect on preventing tear fluid evaporation. Due to the hydrophobic nature of wax esters, however, they need to be accompanied by a certain proportion of polar lipids in order to form a uniform layer at the air-water interface.
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