Abstract
Purpose:
Meibum hydrocarbon chain conformation changes with dry eye and may be important for function. The conformation of meibum on the surface of tears is unknown. Others have shown that surface films composed of meibum do not inhibit the rate of evaporation in vitro. We tested the idea that components in human tears such as proteins interact with human meibum placed on the surface of human reflex tears and together they inhibit the rate of evaporation. Much of what we know about the evaporation through lipid films comes from studies focused on retarding the evaporation from large reservoirs. We repeated conflicting earlier studies relevant to reservoirs involving the inhibition of evaporation by lipids.
Methods:
Raman spectroscopy (of 5 mm2 areas) was used to measure the hydrocarbon chain conformation of human meibum and lipid alcohols containing 12 to 26 carbons on the surface of human reflex tears in vitro and to confirm the formation of a continuous film. A range of lipid thicknesses up to a very thick layer of lipids relative to physiological thicknesses were studied. Evaporation rates were measured gravimetrically.
Results:
Both lipids naturally present in tears and added meibum formed 5 mm2 “islands” of lipid aggregates on the surface of reflex tears. Raman spectra characteristic of lipid and water were uniform at all regions of the surface, even regions without islands, indicating that a lipid film completely covered the surface. When layered on the aqueous surface, the hydrocarbon chain conformation of meibum and 1-undecanol changed to become dramatically more ordered, containing more trans rotamers. A surface film of meibum and a series of lipid alcohols did not inhibit the rate of evaporation significantly.
Conclusions:
Meibum is fluid enough to be expressed from the meibomian glands and is likely to become more ordered (viscous) on the surface of the eye. Our previous studies showed that a stiff ordered molecular arrangement results in a more elastic lipid layer in which molecules are able to rearrange during compression and expansion. Our results concur with studies suggesting that it is questionable whether lipid layers significantly inhibit the rate of evaporation on the surface of the eye or on the surface of reservoirs.