Evaporation and Hydrocarbon Chain Conformation of Surface Lipid Films.

PURPOSE: The inhibition of the rate of evaporation (Revap) by surface lipids is relevant to reservoirs and dry eye. Our aim was to test the idea that lipid surface films inhibit Revap. METHODS: Revap were determined gravimetrically. Hydrocarbon chain conformation and structure were measured using a Raman microscope. Six 1-hydroxyl hydrocarbons (11-24 carbons in length) and human meibum were studied. Reflex tears were obtained from a 62-year-old male. RESULTS: The Raman scattering intensity of the lipid film deviated by about 7 % for hydroxyl lipids and varied by 21 % for meibum films across the entire film at a resolution of 5 µm2. All of the surface lipids were ordered. Revap of the shorter chain hydroxyl lipids were slightly (7%) but significantly lower compared with the longer chain hydroxyl lipids. Revap of both groups was essentially similar to that of buffer. A hydroxyl lipid film did not influence Revap over an estimated average thickness range of 0.69 to >6.9 µm. Revap of human tears and buffer with and without human meibum (34.4 µm thick) was not significantly different. Revap of human tears was not significantly different from buffer. CONCLUSIONS: Human meibum and hydroxyl lipids, regardless of their fluidity, chain length, or thickness did not inhibit Revap of buffer or tears even though they completely covered the surface. It is unlikely that hydroxyl lipids can be used to inhibit Revap of reservoirs. Our data do not support the widely accepted (yet unconfirmed) idea that the tear film lipid layer inhibits Revap of tears.

Sebum/Meibum Surface Film Interactions and Phase Transitional Differences.

PURPOSE: Sebum may contribute to the composition of the tear film lipid layer naturally or as a contaminant artifact from collection. The aims of this study were to determine: if sebum changes the rheology of meibum surface films; if the resonance near 5.2 ppm in the 1H-NMR spectra of sebum is due to squalene (SQ); and if sebum or SQ, a major component of sebum, interacts with human meibum. METHODS: Human meibum was collected from the lid margin with a platinum spatula. Human sebum was collected using lipid absorbent tape. Langmuir trough technology was used to measure the rheology of surface films. Infrared spectroscopy was used to measure lipid conformation and phase transitions. We used 1H-NMR to measure composition and confirm the primary structure of SQ. RESULTS: The NMR resonance near 5.2 ppm in the spectra of human sebum was from SQ which composed 28 mole percent of sebum. Both sebum and SQ lowered the lipid order of meibum. Sebum expanded meibum films at lower concentrations and condensed meibum films at higher concentrations. Sebum caused meibum to be more stable at higher pressures (greater maximum surface pressure). CONCLUSIONS: Physiological levels of sebum would be expected to expand or fluidize meibum making it spread better and be more surface active (qualities beneficial for tear film stability). Sebum would also be expected to stabilize the tear film lipid layer, which may allow it to withstand the high shear pressure of a blink.

Pilot study of the influence of eyeliner cosmetics on the molecular structure of human meibum.

BACKGROUND/AIMS: It has been suggested that eye makeup could interact with human meibum causing a decrease in the stability of the tear film. The aim of this pilot study was to measure makeup-human meibum interactions in vitro. METHODS: Human meibum-makeup interactions were quantified by measuring order-to-disorder lipid phase transitions using infrared spectroscopy. RESULTS: Makeup products exhibited lipid phase transition temperatures that were much higher than those for meibum. One product increased the lipid phase transition temperature by 4.2°C when combined with human meibum causing a large increase (from 30 to 49%) in the order of the meibum-lipid hydrocarbon chains and significantly decreased the minimum frequency, enthalpy and entropy of the phase transition of human meibum. Another eyeliner caused no significant (p < 0.05) change in the phase transition parameters of human meibum. CONCLUSION: Infrared spectroscopy may be used to measure interactions between human meibum and makeup. One makeup product increased the lipid order (viscosity) which could have adverse effects on tear film stability. Modern cosmetics are highly regulated and relatively safe to use; however, it could be beneficial to design makeup products that do not interact with meibum, especially since women have a higher prevalence of dry eye symptoms.