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P. Mudgil, T. J. Millar; The Non-Collapsible Nature of Human Meibomian Lipid Films at an Air-Liquid Interface. Invest. Ophthalmol. Vis. Sci. 2008;49(13):81.
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The lipid layer of the tear film helps in maintaining a stable tear film on the ocular surface by reducing its surface tension. It is subjected to enormous pressures during blinking. The purpose of this study was to investigate the collapsible/non-collapsible nature of human Meibomian lipids under high surface pressures and compare it with that of the individual lipids that are known to be present in the tear film and Meibomian lipids.
Human Meibomian lipids, collected from squeezing eyelids of a healthy volunteer, were applied to the surface of an artificial tear buffer in a specialised Langmuir ribbon trough. The surface is enclosed by a ribbon and by using two moveable barriers to reduce the surface area, high pressures can be applied to a film without causing breaching of the sides of the trough. Dynamic surface pressure changes were measured using a Wilhelmy plate. Temperature of the trough was maintained either at 37°C (~physiological temperature) or at 20°C (a temperature below melting point of Meibomian lipids). Pure films of lipids that are known components of Meibomian lipids or are found in the tear film were also tested.
Pressure-area profiles of human Meibomian lipids showed a gradual increase in pressure with no signs of collapse even at very high surface pressures (~47 mN/m) when large amounts of Meibomian lipids were spread on the surface. All the individual lipids tested (cholesterol, stearic acid, cholesterol palmitate and dipalmitoyl phosphatidylcholine) showed collapse under high surface pressures. These findings were independent of temperature.
A human Meibomian lipid film does not collapse at high surface pressures. It is highly compressible which could be due to reversible folding, or reversible formation of multiple layers. This is seen not only at physiological temperature but also at 20°C where the film is expected to be more rigid. On the other hand, the individual lipids do not possess this property by themselves. This might indicate that combination of different lipids and their interactions with each other contribute towards making lipid layer non-collapsible that is well adapted for blinking. In future experiments it would be of interest to determine if the Meibomian lipids from people with blepharitis also show flexibility in films from their Meibomian lipids, and to determine possible contribution of tear proteins to this property.
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