Negative Ions in Space.

Until a decade ago, the only anion observed to play a prominent role in astrophysics was H-. The bound-free transitions in H- dominate the visible opacity in stars with photospheric temperatures less than 7000 K, including the Sun. The H- anion is also believed to have been critical to the formation of molecular hydrogen in the very early evolution of the Universe. Once H2 formed, about 500 000 years after the Big Bang, the expanding gas was able to lose internal gravitational energy and collapse to form stellar objects and "protogalaxies", allowing the creation of heavier elements such as C, N, and O through nucleosynthesis. Although astronomers had considered some processes through which anions might form in interstellar clouds and circumstellar envelopes, including the important role that polycyclic aromatic hydrocarbons might play in this, it was the detection in 2006 of rotational line emission from C6H- that galvanized a systematic study of the abundance, distribution, and chemistry of anions in the interstellar medium. In 2007, the Cassini mission reported the unexpected detection of anions with mass-to-charge ratios of up to ∼10 000 in the upper atmosphere of Titan; this observation likewise instigated the study of fundamental chemical processes involving negative ions among planetary scientists. In this article, we review the observations of anions in interstellar clouds, circumstellar envelopes, Titan, and cometary comae. We then discuss a number of processes by which anions can be created and destroyed in these environments. The derivation of accurate rate coefficients for these processes is an essential input for the chemical kinetic modeling that is necessary to fully extract physics from the observational data. We discuss such models, along with their successes and failings, and finish with an outlook on the future.

The real reason for having a meibomian lipid layer covering the outer surface of the tear film - A review.

This review critically evaluates a broad range of literature in order to show the relationship between meibum, tear lipids and the tear film lipid layer (TFLL). The relationship of meibum composition to dry eye syndrome is briefly discussed. The review also explores the interactions between aqueous and the TFLL by examining the correlations between meibomian lipids and lipids extracted from whole tears, and by considering protein adsorption to the TFLL from the aqueous. Although it is clear to the authors that a normal tear film resists evaporation, an emerging idea from the literature is that the main purpose of the TFLL is to allow the spread of the tear film and to prevent its collapse onto the ocular surface, rather than to be an evaporative blanket. Current models on the possible structure of the TFLL are also examined.

An investigation of the likely role of (O-acyl) ω-hydroxy fatty acids in meibomian lipid films using (O-oleyl) ω-hydroxy palmitic acid as a model.

(O-acyl) ω-hydroxy fatty acids (OAHFAs) are a recently found group of polar lipids in meibum. Since these lipids can potentially serve as a surfactant in the tear film lipid layer, the surface properties of a molecule of this lipid class was investigated and compared with a structurally related wax ester and a fatty acid. (O-oleyl) ω-hydroxy palmitic acid was synthesized and used as the model OAHFA. It was spread either alone or mixed with human meibum on an artificial tear buffer in a Langmuir trough, and pressure-area isocycle profiles were recorded at different temperatures and compared with those of palmityl oleate and oleic acid. These measurements were accompanied by fluorescence microscopy of meibum mixed films during pressure-area isocycles. The pressure area curves indicated that pure films of the model OAHFA are as surface active as oleic acid films, cover a much larger surface area than either palmityl oleate or oleic acid and show a distinct biphasic pressure-area isocycle profile. The OAHFAs appeared to remain on the aqueous surface and show only a minor re-arrangement into multi-layered structures during repetitive pressure area isocycles. All these properties can be explained by OAHFAs binding weakly to the aqueous surface via an ester group and strongly via a carboxyl group. By contrast, the pressure area profiles of palmityl oleate films indicate that they form multi-layers and oleic acid presumably forms micelles and desorbs into the subphase. When mixed with meibum, similar features as for pure films were observed. In addition, meibum-OAHFA films appeared very homogeneous; a feature not seen with other mixtures. In conclusion these data support the notion that the tested OAHFA is a very potent surfactant which is important in spreading and stabilising meibomian lipid films.

Tear film stability: a review.

Tear film stability can be assessed via a number of tools designed for clinical as well as research purposes. These techniques can give us insights into the tear film, and allow assessment of conditions that can lead to dry eye symptoms, and in severe cases, to significant ocular surface damage and deterioration of vision. Understanding what drives tear film instability and its assessment is also crucial for evaluating existing and new therapies. This review examines various techniques that are used to assess tear film instability: evaluation of tear break-up time and non-invasive break-time; topographic and interferometric techniques; confocal microscopic methods; aberrometry; and visual function tests. It also describes possible contributions of different tear film components; namely meibomian lipids, ocular mucins and proteins, and factors such as age, contact lens wear, ocular surgery and environmental stimuli, that may influence tear film instability.

Effects on the Human Tear Film of Applying Skin Lipids to the Ocular Surface.

PURPOSE: The effect of skin lipids on the formation and stability of the human tear film was investigated. METHODS: Skin swab substances (SSSs) were applied to the eyes of volunteers and studied using fluorescein or with TearView, which records infrared emissivity showing tear film integrity in real time. Results were compared with similar experiments using castor oil, freshly collected meibum, or acetic acid, which simulated the low pH of the skin. RESULTS: Fluorescein and TearView results were comparable. TearView showed the natural unaltered tear film over the whole eye, instant changes to the tear film, and meibomian gland activity. Minimal amounts of SSS destroyed the integrity of the film and caused pain. Corneal epithelial damage could be detected. TearView showed that SSS stimulated meibomian gland secretion if applied directly to the posterior eyelid margin. Excess meibum had no effect on the tear film spread or integrity. Castor oil formed floating lenses on the tear film which were spread by a blink but then condensed back toward themselves. There was no pain or surface damage with these oils. CONCLUSIONS: SSS contamination of the ocular surface disrupts the tear film, causes stinging, and fluorescein staining of the corneal epithelial cells after a blink. SSS stimulates meibomian gland activity. It is possible that various ocular conditions associated with dry eye, such as blepharitis and ocular rosacea, may compromise a meibomian lipid barrier of the eye lid margin. Skin lipids would then have access to the ocular surface and cause dry eye symptoms.

Dry eye disease in astronauts: a narrative review.

Long-duration spaceflight can have adverse effects on human health. One of the most common ocular conditions experienced by astronauts is dry eye disease (DED). Symptoms of DED include feelings of eye irritation, eye strain, foreign body sensation and blurred vision. Over 30% of International Space Station expedition crew members reported irritation and foreign body sensation. We reviewed the current literature on the prevalence and mechanisms of DED in astronauts and its potential implications for long-duration spaceflight, including the influence of environmental factors, such as microgravity and fluid shift on tear film physiology in space. DED has negative effects on astronaut performance, which is why there is a need for further research into the pathophysiology and countermeasures. As an in-flight countermeasure, neurostimulation seems to be among the most promising options.

The interfacial viscoelastic properties and structures of human and animal Meibomian lipids.

As the interface between the aqueous layer of the tear film and air, the lipid layer plays a large role in maintaining tear film stability. Meibomian lipids are the primary component of the lipid layer; therefore the physical properties of these materials may be particularly crucial to the functionality of the tear film. Surface pressure versus area isotherms, interfacial shear and extensional rheology, and Brewster angle microscopy (BAM) were used to characterize the Meibomian lipids from different species known to have different lipid compositions. The isotherms of humans, bovinae, wallabies, rabbits and kultarrs (a small desert marsupial) were qualitatively similar with little hysteresis between compression and expansion cycles. In contrast, several isocycles were necessary to achieve equilibrium behavior in the koala lipids. With the exception of kultarr lipids, the interfacial complex viscosity of all samples increased by one or two orders of magnitude between surface pressures of 5 mN/m and 20 mN/m and exhibited classic gel behavior at higher surface pressures. In contrast, the kultarr lipids were very fluid up to 22 mN/m; the behavior did not depend on surface pressure. Human lipids were very deformable in extensional flow and the BAM images revealed that the film became more homogeneous with compression as the elasticity of the film increased. The morphology of the kultarr lipids did not change with compression indicating a strong correlation between film structure and behavior. These results suggest that the lipid layer of the tear film forms a gel in vivo, which may aid in mechanically stabilization of the tear film.

Nanoscale phase dynamics of the normal tear film.

The tear film is a dynamic multilayered structure. The interactions and the interfacial dynamics between the layers that occur during a blink cycle must be such that they allow for maintenance of a stable tear film. Attempts to understand these dynamics have been limited by the techniques and biomarkers used. Quantum dots (qdots) offer a new potential to monitor the dynamics of the tear film layers in vivo without the drawbacks of previously used methodologies. Indium phosphide-gallium qdots were used to differentially assess the dynamics of the lipid and aqueous layers of the tear film in real time. In the aqueous, qdots dispersed to form a stable local region that was swept away into the upper and lower menisci during a blink. They did not redisperse onto the ocular surface but were progressively removed from the menisci through the puncta. Some of these qdots adhered to the mucin layer on the ocular surface in a meshlike pattern and remained there for five to six blinks before they were removed. The organic qdots dispersed quickly but patchily over the whole outer surface of the tear film. They also strongly marked both eyelid margins and slowly dispersed onto the skin and eyelashes and not through the puncta. Some were trapped in the menisci as blobs that rolled along the meniscus. These data support the view of a distinct three-layered tear film: an inner mucin layer attached to the epithelial cells, a fluid aqueous layer, and an outer viscoelastic lipid layer.

An Experimental Model to Study the Impact of Lipid Oxidation on Contact Lens Deposition In Vitro.

PURPOSE: This study was to establish a controlled in vitro test system to study the effect of lipid oxidation on lipid deposition on contact lenses. METHODS: Fatty acids with varying degree of unsaturation were oxidized using the Fenton reaction. The degree of lipid oxidation and the lipid moieties formed during the oxidation were identified and estimated by various lipid staining techniques following separation with thin-layer chromatography, and by measuring thiobarbituric acid reactive substances or peroxides in solution. Two different silicone hydrogel-based contact lenses (Balafilcon A and Senofilcon A) were incubated with fatty acids laced with radioactive tracer oxidized to varying degrees, and the amount of lipid deposition was measured using unoxidized lipid samples as controls. RESULTS: The Fenton reaction together with the analytical methods to analyze the lipid oxidation can be used to control oxidation of lipids to a desired amount. In general, saturated fatty acids are not oxidized, the monounsaturated oleic acid produced peroxides while poly-unsaturated lipids initially produced peroxides and then fragmented into reactive aldehydes. Incubation with mildly oxidized lipids (most likely lipid peroxides) resulted in increased lipid deposition on Balafilcon A lenses compared to unoxidized lipids, but this was not observed for Senofilcon A lenses. Further oxidation of the lipids (carbon chain breakup) on the other hand resulted in diminished lipid deposition for both contact lens types. CONCLUSIONS: This study provides a method for inducing and controlling lipid oxidation so that the effect of lipid oxidation on contact lens binding can be compared. It could be shown that the degree of lipid oxidation has different effects on the lipid deposition on different contact lens types.

The surface activity of purified ocular mucin at the air-liquid interface and interactions with meibomian lipids.

PURPOSE: Ocular mucins are thought to contribute to the stability of the tear film by reducing surface tension. The purpose of this study was to compare the effect of different mucins and hyaluronic acid (HA) alone and mixed with meibomian lipids on the surface pressure at an air-liquid interface. METHODS: A Langmuir trough and Wilhelmy balance were used to measure and compare the surface activity of bovine submaxillary gland mucin (BSM), purified BSM, purified bovine ocular mucin and HA, and mixtures of these with meibomian lipids, phosphatidylcholine, and phosphatidylglycerol. Their appearance at the surface of an air-buffer interface was examined using epifluorescence microscopy. RESULTS: Purified ocular mucin had no surface activity even at concentrations that were 100 times more than normally occur in tears. By contrast, commercial BSM caused changes to surface pressure that were concentration dependent. The surface pressure-area profiles showed surface activity with maximum surface pressures of 12.3-22.5 mN/m depending on the concentration. Purified BSM showed no surface activity at low concentrations, whereas higher concentrations reached a maximum surface pressure of 25 mN/m. HA showed no surface activity, at low or high concentrations. Epifluorescence showed that the mucins were located at the air-buffer interface and changed the appearance of lipid films. CONCLUSION: Purified bovine ocular mucin and HA have no surface activity. However, despite having no surface activity in their own right, ocular mucins are likely to be present at the surface of the tear film, where they cause an increase in surface pressure by causing a compression of the lipids (a reorganization of the lipids) and alter the viscoelastic properties at the surface.

Adsorption of lysozyme to phospholipid and meibomian lipid monolayer films.

It is believed that a lipid layer forms the outer layer of the pre-ocular tear film and this layer helps maintain tear film stability by lowering its surface tension. Proteins of the aqueous layer of the tear film (beneath the lipid layer) may also contribute to reducing surface tension by adsorbing to, or penetrating the lipid layer. The purpose of this study was to compare the penetration of lysozyme, a tear protein, into films of meibomian lipids and phospholipids held at different surface pressures to determine if lysozyme were part of the surface layer of the tear film. Films of meibomian lipids or phospholipids were spread onto the surface of a buffered aqueous subphase. Films were compressed to particular pressures and lysozyme was injected into the subphase. Changes in surface pressure were monitored to determine adsorption or penetration of lysozyme into the surface film. Lysozyme penetrated a meibomian lipid film at all pressures tested (max=20 mN/m). It also penetrated phosphatidylglycerol, phosphatidylserine or phosphatidylethanolamine lipid films up to a pressure of 20 mN/m. It was not able to penetrate a phosphatidylcholine film at pressures >or=10 mN/m irrespective of the temperature being at 20 or 37 degrees C. However, it was able to penetrate it at very low pressures (<10 mN/m). Epifluorescence microscopy showed that the protein either adsorbs to or penetrates the lipid layer and the pattern of mixing depended upon the lipid at the surface. These results indicate that lysozyme is present at the surface of the tear film where it contributes to decreasing the surface tension by adsorbing and penetrating the meibomian lipids. Thus it helps to stabilize the tear film.

Surface pressure measurements of human tears and individual tear film components indicate that proteins are major contributors to the surface pressure.

PURPOSE: Tear film stability has been associated with a low surface tension (high surface pressure), which has been attributed to a variety of tear film components. In this study, we examined the contribution of various tear proteins, mucin, and meibomian lipids to the surface pressure of human tears. METHODS: A Langmuir trough was used to measure and compare the surface activities of albumin, lipocalin, beta-lactoglobulin, lactoferrin, lysozyme, secretory IgA, mucin, meibomian lipid, and tears. RESULTS: All proteins exhibited surface activity. The surface pressure-area (Pi-A) profiles of most protein films at equilibrium surface pressure (Pieq) were sigmoidal and showed hysteresis between the expansion and compression phases of the cycle. Pieq of most proteins took 4-9 hours to occur. By contrast, the Pi-A profiles for meibomian lipid films were hyperbolic rather than sigmoidal and had little hysteresis, and Pieq was attained within 1 hour. The Pi-A profiles of mucin films showed mostly hyperbolic characteristics with small hysteresis. The Pi-A profiles of films of tears were sigmoidal, showed strong hysteresis, and reached Pieq at about 5 hours. Partitioning of the proteins and whole tears into the subphase also occurred. CONCLUSION: Comparison between the dynamic Pi-A profiles of tears and those of individual tear film components shows that tear film proteins not only are capable of surface activity but also are major contributors to the surface activity of the tear film.

The effects of novel amphipathic block copolymers on stabilization of the rat tear film.

PURPOSE: To determine whether various novel amphipathic polymers could be used to stabilize the tear film of the rat. The rheologic properties of these polymers were examined to investigate whether particular structural or physical characteristics improve the stability of the tear film. METHODS: Amphipathic polymers or particular phospholipids were mixed with a test solution of tears and saline and applied to the clean, dry corneal surface of a rat. The specular reflection of the tear film was observed at high magnification and recorded. For each of the polymers or lipids, the effects on surface regularity and tear break-up time were compared. After the experiments, histologic sections of the tested eyes were prepared and examined for acute cytotoxic effects on the cornea and ocular conjunctiva. RESULTS: Tear film break-up time was markedly affected by differences in polymer structure. Copolymers consisting of separate hydrophobic and hydrophilic regions appeared to be the best stabilizers. No acute cytotoxic effects were observed in histologic sections of corneas to which the polymers had been applied. CONCLUSIONS: Amphipathic polymers can be designed to increase tear film stability. Increased tear film stability occurred more readily with copolymers, possibly through their interaction with both lipid and aqueous tear components.

Clinical appearance and microscopic analysis of mucin balls associated with contact lens wear.

PURPOSE: The structure of mucin balls collected from silicone hydrogel contact lens wearers was examined to determine their nature. METHODS: Tears containing mucin balls were collected using a capillary tube. These were processed for light microscopic histochemistry, scanning electron microscopy, and electron microscopic elemental analysis. Mucin balls were also observed in vivo using confocal microscopy. RESULTS: Histology showed that the mucin balls were PAS positive, indicating that glycoproteins form a major component. Lipids and bacteria were not detected. Scanning electron microscopy did not show the surface to be smooth but revealed a variation in density across the surface. Elemental analysis was inconclusive. CONCLUSIONS: Mucin balls are likely to be made from collapsed mucin and are unlikely to have been formed as a result of pearling around a silicon, lipid, or bacterial kernel.

The tachykinin peptide neurokinin B binds copper(I) and silver(I) and undergoes quasi-reversible electrochemistry: towards a new function for the peptide in the brain.

The tachykinin neuropeptide family, which includes substance P and neurokinin B, is involved in a wide array of biological functions. Among these is the ability to protect against the neurotoxic processes in Alzheimer's Disease, but the mechanisms driving neuroprotection remain unclear. Dysregulation of metal ions, particularly copper, iron and zinc is a common feature of Alzheimer's Disease, and other amyloidogenic disorders. Copper is known to be released from neurons and recent work has shown that some tachykinins can bind Cu(II) ions, and that neurokinin B can inhibit copper uptake into astrocytes. We have now examined whether neurokinin B is capable of binding Cu(I), which is predicted to be available in the synapse. Using a combination of spectroscopic techniques including cyclic voltammetry and magnetic resonance we show that neurokinin B can bind Cu(I) either directly from added CuCl or by reduction of Cu(II)-bound neurokinin B. The results showed that the Cu(I) binding site differs greatly to that of Cu(II) and involves thioether coordination via Met2 and Met10 and an imidazole nitrogen ligand from His3. The Cu(I) coordination is also different to the site adopted by Ag(I). During changes in oxidation state, copper remains bound to neurokinin B despite large changes to the inner coordination sphere. We predict that neurokinin B may be involved in synaptic copper homeostasis.

Effects of keratin and lung surfactant proteins on the surface activity of meibomian lipids.

PURPOSE: In vitro studies indicate that surface tension and surface viscosity of the tear film lipid layer (TFLL) are governed by interactions between meibomian lipids and proteins from the aqueous layer. The role of minor tear proteins with strong lipophilic properties or those correlated with pathological states is still unknown. The discovery of lung surfactant proteins (SPs) in tears and keratin in normal and abnormal meibomian gland excretions warrants investigation into their effects on the surface activity of meibomian lipid films. METHODS: Commercial keratin and bovine lung SPs were used in vitro to assess the surface pressure of meibomian lipid films using a Langmuir trough. RESULTS: The pressure-area profiles of meibomian lipid films seeded with SPs (2.5 µL; 0.1 µg) demonstrated hybrid characteristics between meibomian lipid films alone and SPs alone but reached much higher maximum surface pressures (approximately 30 vs. 24 mN/m). Microscopically, the appearance of meibomian lipid films was not altered by SPs. Maximum surface pressure of meibomian films premixed with keratin was much higher than meibum alone. The pressure-area isocycles appeared more like those of meibomian lipids with a low concentration of protein and more like pure keratin films at a high concentration. CONCLUSIONS: The data strongly indicate that SPs and keratin likely interact with the TFLL. SPs are likely to act as strong surfactants and to reduce the surface tension of the lipid layer. Excess concentrations of keratin as identified in patients with meibomian gland dysfunction could disrupt the normal structure of the meibomian lipid film.

Interfacial dilatational viscoelasticity of human meibomian lipid films.

PURPOSE/AIM: The meibomian lipid layer is able to withstand the enormous stresses and deformations that occur during blinking due to the combination of its elastic and viscous properties. The purpose of this study was to measure the dilatational viscoelasticity of in vitro meibomian lipid films and compare how these properties differ between room temperature and physiological temperatures. Viscoelasticity was also compared with meibomian lipid films seeded with cholesterol or ß-carotene (the levels of these lipid species change in disease states). MATERIALS AND METHODS: Dilatational viscoelasticity (E) was measured using an oscillating pendant drop method. Measurements were carried out on spread films at the air-water interface as a function of frequency (0.1256-12.56 rad/s) at various temperatures between 18-43 °C. RESULTS: Generally, E gradually decreased as the overall temperature was increased. At both 37 and 20 °C, films demonstrated that the elastic modulus (E') was more dominant than the viscous modulus (E″), indicating films were more solid-like than fluid-like, regardless of temperature. E' and E″ were also dependant on frequency, indicating some molecular rearrangements of the lipid molecules as films were compressed and expanded. Films seeded with cholesterol or ß-carotene showed a modest increase in the moduli. CONCLUSIONS: These results are consistent with previous findings which have predicted and indicated that the meibomian lipid layer is a viscoelastic film at the air-liquid interface. These properties are integral to how the tear film lipid layer is able to maintain its structure, and hence integrity of the ocular surface.

Barriers to clinical uptake of tear osmolarity measurements.

AIM: The aim of the study was to examine the possibilities of measuring tear osmolarity in a general clinical setting, and to identify the barriers preventing the uptake of new methodologies for its measurement. METHODS: Five non-contact-lens wearers were recruited to evaluate the diagnostic capability of the TearLab. Three osmolarity measurements were taken at 1 min intervals in the morning at 09:00, midday between 12:00 and 13:00 and afternoon at 16:00 for two consecutive days. Forty more osmolarity measurements were carried out at different times on one subject with low and one subject with high tear osmolarity over 4 months. The osmolarity of a standard solution, 290 mOsm/l, was measured 19 times alternatively with the TearLab by two examiners. RESULTS: Consecutive tear osmolarity readings in an individual varied up to 35 mOsm/l, but an average over three readings was found to be a reliable indicator of tear osmolarity at 95% confidence level. For population studies, a power analysis based on the variability of the data showed that three repeat measurements would be required to obtain reliable data for a study with <50 subjects, whereas one measurement would suffice for 490 or more subjects. There were no interobserver or interinstrumental differences, but readings obtained for the standard solution varied up to 89 mOsm/l. CONCLUSION: Three consecutive readings are required with the TearLab to obtain a reliable measure of tear osmolarity. The variation in recorded tear osmolarity makes it difficult to use the technique for the diagnosis of mild dry eye.

Analysis of Comparison of Human Meibomian Lipid Films and Mixtures with Cholesteryl Esters In Vitro Films using High Resolution Color Microscopy.

PURPOSE: The lipid layer of the tears has been studied in vivo using high resolution color microscopy (HRCM). The purpose of these experiments was to gain further insight into the structure of the lipid layer by applying HRCM to in vitro meibomian lipid films. METHODS: Films of human meibomian lipids, cholesteryl nervonate, cholesteryl palmitate, or their mixtures, were spread on a Langmuir trough. Changes to the films were monitored using HRCM as the films were compressed to different surface pressures. The penetration of albumin into a meibomian lipid film also was studied. RESULTS: Small amounts of meibomian lipids at low pressures formed very thin films estimated to be 5.2 nm thick. Compression caused spots to appear in the films. At higher concentrations, micro lenses were a feature of the film. Cholesteryl nervonate formed a multilayered oil slick that did not change with surface pressure. Cholesteryl palmitate formed a stiff film that collapsed at high compression. Mixtures of cholesteryl nervonate and meibomian lipids showed that they mixed to increase surface pressures above that of the individual components. HRCM also allowed albumin to be seen penetrating the meibomian lipid film. CONCLUSIONS: HRCM combined with in vitro surface pressure measurements using a Langmuir trough is useful for modeling meibomian lipid films. The films often resemble the appearance of the lipid layer of in vivo films. The data indicate that the lipid layer might be modeled best as a duplex film containing an array of liquid crystals.

Lipid component contributions to the surface activity of meibomian lipids.

PURPOSE: Meibomian lipid films have very complex physical properties that enable them to be compressed and expanded without collapsing. These properties can be attributed to the self assembly of the individual components, mainly wax and cholesteryl esters (WE and CE). Here, the surface pressure properties of WEs and CEs films have been compared to evaluate their contributions to meibomian lipid films. METHODS: Films of different WEs and CEs were spread on a Langmuir trough and their surface pressure area profiles were compared with a particular emphasis on the effects caused by the degree of saturation of the alkyl/alkene chains. RESULTS: Fully saturated WEs and CEs formed unstable films that collapsed upon compression. Very unsaturated waxes and CEs tended to have two distinct phases, one that reflects interaction with the aqueous subphase, while the second appeared to be with the multilayered bulk of the lipid film. With aging of the films, the WEs tended to move off the surface into the bulk. When meibomian lipid films were seeded with large amounts of WEs, only minor changes could be seen unless the WE was very unsaturated. CONCLUSIONS: These data are consistent with meibomian lipid films having a surfactant layer with a complex bulk layer external to this. It is speculated that the bulk layer contains thermotropic smectic chiral liquid crystals of CEs that are interacting with the WEs. This structure would tend to prevent collapse of the meibomian lipids onto the ocular surface and be very tolerant of lipophilic contaminants.