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C. Cerretani, C. J. Radke; Tear Evaporation Reduction by Model Thin Oily Films. Invest. Ophthalmol. Vis. Sci. 2010;51(13):6285.
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Reduction of tear evaporation through the human tear-film lipid layer (TFLL) is crucial to dry-eye syndrome. Recent studies, however, claim that films of oily substances, including human and bovine meibum, play no measureable role in controlling water evaporation rates1,2. We hypothesize, however, that oily films of thickness near that of human TFLL do reduce water evaporation rates by forcing water to dissolve in and diffuse through the oil film.
Following Holly3, a model lipid film is studied consisting of oxidized mineral oil and bovine submaxillary mucin. Resulting oily films are mechanically spread in a blinking motion on an aqueous substrate in a miniature Langmuir trough. Evaporation rates are measured gravimetrically under controlled temperature, humidity, and air flow for films of thickness ranging from 100 nm (thickness of TFLL) to 100 um. To verify uniform coverage, films are visualized using white-light interferometry.
As film thickness increases from 100 nm to 100 um (100, 250, 500, 750, 1000, 10,000, 100,000 nm), evaporation rates reduce from 24% to 97% (24±6, 40±6, 42±6, 56±5, 61±5, 89±5, 97±2; n=7 at each corresponding thickness). The newly measured water evaporation rates are in agreement with water dissolution/diffusion theory. Theory fitting gives a water permeability in the model lipid films of 5x10-5 cm2/s.
In contrast to current literature1,2, in-vitro thin oily films of thickness similar to that of the human TFLL reduce water evaporation rates. The mechanism of evaporation-rate reduction is the resistance afforded by water dissolving in and diffusing through the oil film. Additionally, the oil film must cover the entire aqueous substrate uniformly and remain stable against rupture. Our findings suggest that to provide an effective barrier against tear evaporation, the human TFLL should have low water solubility and low water diffusivity.1. Borchman D, Foulks GN, Eye & Contact Lens 1, 32 (2009)2. Herok GH, Mudgil P, et al, Curr. Eye. Res. 34, 589 (2009)3. Holly FJ, J. Coll. Interf. Sci. 49, 221 (1974)
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