Lipid Layer Structure Research Articles

Tear Film Lipid Layer Structure: Self-Assembly of O-Acyl-ω-hydroxy Fatty Acids and Wax Esters into Evaporation-Resistant Monolayers.

In healthy eyes, the tear film lipid layer (TFLL) is considered to act as an evaporation resistant barrier, which prevents eyes from drying. Seeking to understand the mechanisms behind the evaporation resistance of the TFLL, we studied mixtures of lipid layer wax esters and O-acyl-ω-hydroxy fatty acids. Analyzing their self-assembly and biophysical properties led to new discoveries concerning the structure and function of the TFLL. We discovered how these lipids self-assemble at the air-water interface and form an efficient antievaporative barrier, demonstrating for the first time how the interaction of different tear film lipid species can improve the evaporation resistance compared with individual lipid classes on their own. These results provide a potential mechanism for the evaporation resistance of the lipid layer. In addition, the results serve as a base for the future development of improved dry eye treatments and other applications where the evaporation of water represents a significant challenge.

Bioinspired Nanoemulsions Stabilized by Phosphoethanolamine and Phosphoglycerol Lipids.

Water-in-oil (W/O) nanoemulsions stabilized by phospholipids (PLs) are increasingly exploited in a wide spectrum of applications, from pharmaceuticals to food and cosmetic formulations. In this work, we report the design and optimization of an innovative emulsion based on a mixture of phosphoethanolamine (PE) and phosphoglycerol (PG) PLs, inspired by the composition of the inner leaflet of a bacterial outer membrane. Using the natural oil squalene as the continuous organic phase, no additional emulsion stabilizer is needed. On the other hand, a small amount of Span 80 is required when dodecane is used. The obtained nanoemulsions are stable for at least two hours, thus allowing the droplet size and distribution to be characterized by Dynamic Light Scattering (DLS) and the lipid layer structure and dynamics to be analyzed by Electron Paramagnetic Resonance (EPR) spectroscopy. The results indicate that squalene shallowly intercalates among the lipid tail termini, being unable to deeply penetrate the adsorbed lipid monolayer. The altered lipid dynamics are proposed to be the reason for the enhanced emulsion stability, this paving the way to future implementations and possible applications.

Interactions of polar lipids with cholesteryl ester multilayers elucidate tear film lipid layer structure

PurposeThe tear film lipid layer (TFLL) covers the tear film, stabilizing it and providing a protective barrier against the environment. The TFLL is divided into polar and non-polar sublayers, but the interplay between lipid classes in these sublayers and the structure-function relationship of the TFLL remains poorly characterized. This study aims to provide insight into TFLL function by elucidating the interactions between polar and non-polar TFLL lipids at the molecular level. MethodsMixed films of polar O-acyl-ω-hydroxy fatty acids (OAHFA) or phospholipids and non-polar cholesteryl esters (CE) were used as a model of the TFLL. The organization of the films was studied by using a combination of Brewster angle and fluorescence microscopy in a Langmuir trough system. In addition, the evaporation resistance of the lipid films was evaluated. ResultsPhospholipids and OAHFAs induced the formation of a stable multilamellar CE film. The formation of this film was driven by the interdigitation of acyl chains between the monolayer of polar lipids and the CE multilayer lamellae. Surprisingly, the multilayer structure was destabilized by both low and high concentrations of polar lipids. In addition, the CE multilayer was no more effective in resisting the evaporation of water than a polar lipid monolayer. ConclusionsFormation of multilamellar films by major tear film lipids suggest that the TFLL may have a similar structure. Moreover, in contrast to the current understanding, polar TFLL lipids may not mainly act by stabilizing the non-polar TFLL sublayer, but through a direct evaporation resistant effect.

Concentration dependent cholesteryl-ester and wax-ester structural relationships and meibomian gland dysfunction

BackgroundWith dry eye, the ratio of cholesteryl ester (CE) to wax ester (WE) decreases substantially in meibum, but the functional and structural consequences of this change are speculative. The aim of this study is to confirm this finding and to bridge this gap in knowledge by investigating the effect of varying CE/WE ratios on lipid structure and thermodynamics. MethodsInfrared spectroscopy was use to quantify CE and WE in human meibum and to measure hydrocarbon chain conformation and thermodynamics in a cholesteryl behenate, stearyl stearate model system. ResultsThe CE/WE molar ratio was 36% lower for meibum from donors with dry eye due to meibomian gland dysfunction compared with meibum from donors without dry eye. CE (5 mol %) dramatically increased the phase transition temperature of pure WE from -0.12 °C to 63 °C in the mixture. Above 5 mol % CB, the phase transition temperature increased linearly, from 68.5 °C to 85 °C. In the ordered state, CE caused an increase in lipid order from about 72% trans rotamers to about 86% trans rotamers. Above 10% CE, the hydrocarbon chains were arranged in a monoclinic geometry. ConclusionsThe CE/WE is lower in meibum from donors with dry eye due to meibomian-gland dysfunction. Major conformational changes in the hydrocarbon chains of wax and cholesteryl ester mixtures begin to occur with just 5% CB and above. General significanceCE-WE interactions may be important for in understanding lipid layer structure and functional relationships on the surface of tears, skin and plants.

Role of Endogenous Ingredients in Meibum and Film Structures on Stability of the Tear Film Lipid Layer against Lateral Compression.

The property and structure of spread films of meibum extruded from rabbit eyelids and its fractions were investigated using the Langmuir film balance technique and Brewster angle microscopy in order to understand the influence of endogenous ingredients in meibum on the structure and stability of the tear film lipid layer against mechanical stimulus. Surface pressure (?)?film area ( A) isotherms for meibum were measured upon repetitive high-speed compression?expansion cycles and were found almost identical to each other with very small hysteresis, indicating the high stability of the meibum film. Brewster angle microcopy observation implied the spontaneous formation of condensed-phase network structures which consist primarily of wax esters and cholesteryl esters as nonpolar ingredients, coexisting with a monolayer phase of polar lipids two-dimensionally confined by the networks, which were spontaneously formed in the meibum film. The networks were gathered densely and deformed when the film was laterally compressed with barriers of Langmuir trough, but returned to the dispersed networks when expanded. The influence of temperature and salts dissolved in an aqueous subphase was also investigated. The results indicated that the temperature change (20 and 35 ?C) induced a difference of surface pressure at the same film areas in rather compressed films, and the presence of salts in the subphase expanded the films. However, the features of isotherm and surface morphology of the film, including their reversibility, were maintained. Phospholipid-removed meibum also formed a stable film, but slight changes were found in the hysteresis and film morphology compared to those in the meibum film. In contrast, in a film of phospholipid- and cholesterol-removed meibum, three-dimensional aggregates grew upon the first compression and not redispersed by the subsequent expansion, giving noticeable hysteresis between the isotherms. It is considered that high deformability upon compression and resilience upon expansion of the networks as well as reversible collapse and spreading property of the confined monolayer phase would hold the stability of the meibum film against repeated compressions.

Human Meibum Cholesteryl and Wax Ester Variability With Age, Sex, and Meibomian Gland Dysfunction.

PurposeRelationships between tear film lipid (TFL) layer composition, structure, and function could provide insight into the etiology of dry eye. The molar ratio of cholesteryl ester (CE)/wax ester (WE) was measured in meibum from normal donors (Mn) and compared with meibum from donors with meibomian gland dysfunction (MMGD).MethodsCE/WE was measured using nuclear magnetic resonance spectroscopy.ResultsCE/WE was distributed into two populations with 81% distributed near 0.55 and 19% near 0.3. CE/WE were higher in donors 13 to 19 years old compared with donors 1 to 12 years old and 20 to 88 years old. CE/WE for MMGD was 30% lower, 0.34 ± 0.04, compared with Mn, 0.49 ± 0.04. There were no sex differences in CE/WE. There were no significant racial differences between the CE/WE ratios for Asians and Caucasians. The CE/WE ratio was higher for blacks and lower for Hispanics compared to Caucasians. Due to the small number sampled, confirmation of the later racial results is needed. The packing of CE and WE in the TFL layer was proposed.ConclusionsAlthough MMGD contains much less CE than Mn, factors other than the CE content, such as the levels of saturation and/or proteins, may be responsible for the higher order of MMGD. In addition to saturation, CE could contribute to the increase in order of Mn between 0 and 20 years of age. Observed changes in the meibum content of CE alone is not likely to influence tear film stability.

Tryptophan to Arginine Substitution in Puroindoline-b Alters Binding to Model Eukaryotic Membrane

We have studied how puroindoline-b (PINB) mutants bind to model eukaryotic membranes dependent on binary composition of anionic:zwitterionic phospholipids and the presence of cholesterol and sphingomyelin in the model membrane. We have found that the trends in lipid binding behavior are different for wild-type PINB compared to its naturally occurring PINB(Trp44Arg) mutant form and have seen evidence of protein-induced domain formation within the lipid layer structure. Results show that selective binding of antimicrobial peptides to different membrane types is as a result of differences in lipid composition and the arrangement of lipids within the membrane surface. However, membrane-binding behavior is not easily predicted; it is determined by net charge, hydrophobicity, and the amphiphilicity of the protein/peptide lipid-binding domain.

Homogeneous PLGA-lipid nanoparticle as a promising oral vaccine delivery system for ovalbumin

In this study, a polymeric lipid nanoparticle (NP) (simplified as Lipid NP) was reported as a promising oral vaccine delivery system. The Lipid NPs composed of a hydrophobic polymeric poly(d,l-lactide-co-glycolide) (PLGA) core and a surface coating of lipid monolayer. Membrane emulsification technique was used to obtain uniform-sized Lipid NPs. Ovalbumin (OVA) was used as a model vaccine. Compared with the pure PLGA NPs, the Lipid NPs achieved higher loading capacity (LC) and entrapment efficiency (EE) for the encapsulated OVA. An in vitro oral release profile showed that the OVA-Lipid NPs were with lower initial burst and could protect the loaded OVA from the harsh gastrointestinal (GI) environment for a long time. In addition, a human microfold cell (M-cell) transcytotic assay demonstrated that due to a lipid layer structure on the particle surface, the Lipid NPs showed higher affinity to the M-cells. Since the M-cell in the intestinal epithelium played an important role in particle transportation as well as intimately associated with the underlying immune cells, the OVA-Lipid NPs effectively induced mucosal and humoral immune responses.

Molecular-Level Organization of the Tear Film Lipid Layer: A Molecular Dynamics Simulation Study

Tear film is essential to the health and optics of the human eye. It refreshes with every blink and ruptures if blinking is suppressed. Its instabilities lead to the dry eye syndrome. The outermost layer of the tear film consists of lipids which provide an optically smooth surface over the cornea and retard water evaporation. The tear film lipid layer formation and structure are still debated. We employ coarse grain molecular dynamics simulations to study molecular-level structure and dynamics of the tear film lipid layer. A novel, realistic tear film model under conditions mimicking those experienced by the tear film under physiological conditions was built and employed.Simulations show that polar phospholipids separate their non-polar counterparts from the water phase, constituting a monomolecular uniform platform at which a thick non-polar lipid layer is formed. This lipid arrangement is stable upon lateral compression. A significant restructuring of the film occurs upon non-equilibrium lateral compression which mimics eye blinks. The water/lipid interface undulates, and lipids are sorted based on their head group size. Undulations of the water/lipid boundary are followed by transfer of some of polar lipids and water toward the non-polar phase, resulting in formation of inverse micelles in the non-polar lipid layer with water encapsulated by polar lipids. Moreover, some of non-polar lipids are transferred into the water phase in vesicles formed by polar lipids.We predict that the tear film lipid layer at the molecular level is a dynamic and non-uniform assembly with lipids and water forming numerous three-dimensional structures in the vicinity of the lipid/water interface. These structures form and reorganize due to the action of eye lids during blinks. They may serve as lipid reservoirs and thus effectively increase stability of the tear film.

Lipid-Containing Lubricants for Dry Eye

Dry eye is a common eye condition with significant impact on its sufferers' quality of life. We conducted a systematic review of randomized controlled treatment trials to assess the effectiveness of lipid-containing lubricants (LCLs) on dry eye treatment. An online database was searched without any language or date restrictions. Clinical trials on LCL with non-lipid-containing control eyedrops were included. The outcome measures of each study (including symptoms and clinical signs), their risk of bias (assessed using the Cochrane tool), and strength of evidence were discussed. Three studies on liposomal lid sprays and four on lipid-containing eyedrops were included. All studies found significant improvements in most of the symptoms/signs of dry eye in patients who used LCL during the study period. Subjects in all trials reported greater symptom relief with LCLs, although this was shown to be short lived in two studies. Lipid layer structure improvement after LCLs application was demonstrated in two studies, although only up to 90 min in one study. Improvement in Schirmer test results was shown in two out of four studies, tear breakup time in four out of five studies, meibomian gland status in three out of three studies, conjunctival folds and tear evaporation rates in two out of two studies each. None of the studies were completely free of risks of bias; only three studies were double masked. Three of the studies were assigned high level of evidence, three were assigned as moderate evidence, and the final one low level of evidence. Most studies have demonstrated the effectiveness of LCLs on improving select signs of dry eye. Based on the overall substantial level of evidence, this type of eyedrop can be recommended for use in clinical practice where the aim is to reduce the signs and symptoms of dry eye.

High resolution images of the tear film lipid layer: effect of the blink cycle

AbstractPurpose Deficiency of the lipid layer causes evaporative dry eye. We have therefore developed a high‐resolution microscope to investigate lipid layer structure and how it may relate to evaporation and the blink cycle.Methods The microscope images a 200 um diameter spot at the center of the cornea, with a resolution of about 1 um. Time after a blink is also recorded. Over 10,000 images of at least “fair” quality were recorded from 375 subjects including dry eyes and normals.Results 1. Within about 0.1 seconds after a blink, lipid layer reflectance was less than at later times, implying that lipid thickness was lower. In this early interval, cloud‐like patches of lipid were more often seen than at later times; they were surrounded by darker (thin) lipid. 2. Some structures in the lipid layer were very fluid, showing pronounced shape changes in a fraction of a second, whereas other structures were stable, with no evident change over several seconds.Conclusion 1. Immediately after a blink, the lipid layer tends to be thin and often has an unusual appearance. 2. At later times, some structures vary rapidly in shape, indicating that the lipid layer is fluid with a low viscosity. Other structures are stable, suggesting that the lipid layer can have a gel‐like structure.

The relation of lipid layer structure to meibum ‐ Results from a new high resolution microscope

AbstractPurpose Evaporative dry eye is a common disorder, in which the lipid layer of the tear film is a poor barrier to evaporation. We therefore developed a high resolution microscope as an aid in understanding the properties of the lipid layer.Methods The microscope has a resolution of about 1 um, and uses a stroboscopic light source to prevent blurring from eye movements. Over 10,000 images of a least "fair" quality have been obtained from 375 subjects, including dry eyes and normals.Results Two characteristics of the images are described here. 1. In some dry eye patients, irregular "islands" and circular "lenses" of lipid are seen within an apparently bare surrounding area. 2. In most images, "droplets" of lipid are observed which may be circular or irregular, and are surrounded by lipid of irregular thickness. Droplets do not appear to change in the inter‐blink interval.Conclusion 1. Rapid evaporation is to be expected through the bare surface between islands and lenses. This pattern may be due to insufficient surfactant (e.g., polar lipid) to help spread lipid over the aqueous layer. 2. "Droplets" do not seem to be generated in the inter‐blink interval and it is not obvious how they could be generated by blinks, so their origin may be the "fat droplets" which are observed histologically in meibomian glands (Sirigu et al., 1992, IOVS 33, 2284). It is suggested that meibum is an emulsion of these droplets.

Lessons from the biophysics of interfaces: Lung surfactant and tear fluid

The purpose of this review is to provide insight into the biophysical properties and functions of tear fluid and lung surfactant--two similar fluids covering the epithelium of two distinctive organs. Both fluids form a layer-like structure that essentially comprise of an aqueous layer next to the epithelium and an anterior lipid layer at the air-water interface. The aqueous layers contain soluble proteins and metabolites, and they are responsible for the host defence system and nutrition of the organ. However, many proteins also interact with the lipid layer and are important for the surface-active function of the fluid film. The lipid layer of lung surfactant comprises mainly of phospholipids, especially phosphatidylcholines, and only small amounts of non-polar lipids, mainly cholesterol. In contrast, tear fluid lipid layer comprises of a mixture of polar and non-polar lipids. However, the relative proportion and the spectrum of different polar and non-polar lipids seem to be more extensive in tear fluid than in lung surfactant. The differing lipid compositions generate distinctive lipid layer structures. Despite the structural differences, these lipid layers decrease the surface tension of the air-water interface. The structure of the tear film lipid layer also minimises the evaporation of the tear fluid. In lung surfactant surface activity is crucial for the function of the organ, as the lipid layer prevents the collapse of the lung alveoli during the compression-expansion cycle of breathing. Similarly the tear film experiences a compression-expansion cycle during blinking. The dynamics of this cycle have been studied to a lesser extent and are not as clear as those of lung surfactant. The common structure and properties suggest a similar behaviour under rapid compression-expansion for both fluids.

Human LDL core cholesterol ester packing: three-dimensional image reconstruction and SAXS simulation studies

Human LDL undergoes a reversible thermal order-disorder phase transition associated with the cholesterol ester packing in the lipid core. Structural changes associated with this phase transition have been shown to affect the resistance of LDL to oxidation in vitro studies. Previous electron cryo-microscopy studies have provided image evidence that the cholesterol ester is packed in three flat layers in the core at temperatures below the phase transition. To study changes in lipid packing, overall structure and particle morphology in three dimensions (3D) subsequent to the phase transition, we cryo-preserved human LDL at a temperature above phase transition (53°C) and examined the sample by electron microscopy and image reconstruction. The LDL frozen from 53°C adopted a different morphology. The central density layer was disrupted and the outer two layers formed a "disrupted shell"-shaped density, located concentrically underneath the surface density of the LDL particle. Simulation of the small angle X-ray scattering curves and comparison with published data suggested that this disrupted shell organization represents an intermediate state in the transition from isotropic to layered packing of the lipid. Thus, the results revealed, with 3D images, the lipid packing in the dynamic process of the LDL lipid-core phase transition.

Update on pathogenic factors of Malassezia species

As a part of normal cutaneous microflora, Malassezia species may become pathogenic under certain conditions. The pathogenicity of Malassezia species results from a synthetic action of external and internal factors. The internal factors of Malassezia species involve invasive enzymes, metabolites, interaction with epidermic cells, phase switching between yeast and hyphal states, and special structure of lipid layer of the cell wall; host risk factors include microenvironmental abnormality, androgen levels, hypoimmunity, and so on. This article adresses the factors affecting the pathogenicity of Malassezia species. Key words: Malassezia; Risk factors; Pathogenesis

A preliminary investigation into the relationship between ocular surface temperature and lipid layer thickness

The aim of this study was to establish the relationship between OST, tear film stability as assessed by NIBUT and subjective evaluation of the lipid layer thickness in a young, asymptomatic, sample group (N=29). Non-invasive tear break-up time (NIBUT) and tear lipid layer structure were evaluated through a slit-lamp mounted Tearscope Plus. A self-calibrating infrared thermography camera was used to record two OST values (one immediately post-blink and one immediately pre the subsequent blink). The most common lipid layer pattern observed was the amorphous pattern (48.3%). Differences between post- and pre-blink OST values were observed (paired t-test; p<0.001). Significant differences of pre-blink OST values were observed between the closed marmoreal group with that from the amorphous and flow groups (Tukey post hoc test, p<0.05). There were no differences of NIBUT values between each lipid layer thickness (Kruskal-Wallis test; p=0.152). A no significant tendency for higher OST in eyes with increased NIBUT was observed. This study suggests that higher OST values could be associated with thicker tear lipid layer in normal subjects. The lack of significant results in relation with tear film stability may be due to only normal subjects were included.

Hydrophobicity, topography in membranes and photosensitization of silicon phthalocyanines with axial ligands of varying lengths

Six amphiphilic silicon phthalocyanines (SiPc's) axially linked to a dimethylated amino alkyl group of varying length have been examined for their potential suitability as photosensitizers for photodynamic therapy (PDT). This group of molecules was chosen because the length of the axial ligand might place the chromophoric part of the molecule at different vertical depths in the membrane and possibly affect the extent of membrane localized damage caused by singlet oxygen. We tested the relative penetration depth of the SiPc groups in the membrane by the extent to which their fluorescence was quenched by external iodide ions. We also measured singlet oxygen quantum yields of the SiPc's in a liposome membrane, using the fluorescent target for singlet oxygen, 9,10-dimethylanthracene. The hydrophobicity parameters, LogP, were calculated and were also measured. Some correlation was found between them and Kb's, the binding constants for liposomes. The effect of the axial ligand's length is less striking than in similar cases with hematoporphyrins and protoporphyrins. We link this smaller effect with a bending of the linker chain that enables, sterically, a better positioning of the sensitizer molecules within the ordered lipid layer structure.

Antimicrobial Peptide-Lipid Binding Interactions and Binding Selectivity

Surface pressure measurements, external reflection-Fourier transform infrared spectroscopy, and neutron reflectivity have been used to investigate the lipid-binding behavior of three antimicrobial peptides: melittin, magainin II, and cecropin P1. As expected, all three cationic peptides were shown to interact more strongly with the anionic lipid, 1,2 dihexadecanoyl- sn-glycerol-3-(phosphor-rac-(1-glycerol)) (DPPG), compared to the zwitterionic lipid, 1,2 dihexadecanoyl- sn-glycerol-3-phosphocholine (DPPC). All three peptides have been shown to penetrate DPPC lipid layers by surface pressure, and this was confirmed for the melittin-DPPC interaction by neutron reflectivity measurements. Adsorption of peptide was, however, minimal, with a maximum of 0.4 mg m −2 seen for melittin adsorption compared to 2.1 mg m −2 for adsorption to DPPG (from 0.7 μM solution). The mode of binding to DPPG was shown to depend on the distribution of basic residues within the peptide α-helix, although in all cases adsorption below the lipid layer was shown to dominate over insertion within the layer. Melittin adsorption to DPPG altered the lipid layer structure observed through changes in the external reflection-Fourier transform infrared lipid spectra and neutron reflectivity. This lipid disruption was not observed for magainin or cecropin. In addition, melittin binding to both lipids was shown to be 50% greater than for either magainin or cecropin. Adsorption to the bare air-water interface was also investigated and surface activity followed the trend melittin>magainin>cecropin. External reflection-Fourier transform infrared amide spectra revealed that melittin adopted a helical structure only in the presence of lipid, whereas magainin and cecropin adopted helical structure also at an air-water interface. This behavior has been related to the different charge distributions on the peptide amino acid sequences.

Chemical specificity in short-chain fatty acids and their analogues in increasing osmotic fragility in rat erythrocytes in vitro

We examined the role of the chemical specificity of short-chain fatty acids (SCFAs) and their derivatives in increasing osmotic fragility (OF) in rat red blood cells (RBCs). Except for formic acid, normal SCFAs with 2 to 8 carbons increased the OF in rat RBCs with increasing number of hydrocarbons in a dose-dependent manner. Replacement of the carboxylic group with sulfonic group inhibited, but did not abolish, the SCFA-mediated increase in OF. Introduction of another carboxylic group (dicarboxylic acids) completely abolished the SCFA-mediated increase in OF. Transformation of the hydrocarbon chains in SCFAs from straight to branched or cyclic chains affected the degree of the OF-increasing effect. Introduction of double- or triple-carbon bonds to the hydrocarbon chain in parent SCFAs did not affect the increase in OF. Both hydrophilic (carboxylic group) and hydrophobic elements (hydrocarbons) at opposite sides of a molecule were required to affect the RBC membrane, and the size and form of hydrophobic element were important factors in determining the SCFA-mediated increase in OF. The hydrocarbon chains probably enter the plasma membrane, with the hydrophilic carboxylic base remaining outside of the membrane, and interact with phospholipid in cell membrane and disturb the structure of lipid layer resulting in the increase in OF in the rat RBCs.

Effect of an Oil-in-Water Emulsion on the Tear Physiology of Patients With Mild to Moderate Dry Eye

To determine the effect of an oil-in-water emulsion eye drop compared with a conventional dry eye supplement (hypromellose) on tear physiology in dry eye. A randomized parallel, longitudinal, and investigator-masked study of the efficacy of 1.25% castor oil emulsion and 0.32% hypromellose solution was carried out. A total of 53 patients with mild to moderate dry eye (27 in emulsion group and 26 in hypromellose group) were recruited for the study. Patients were enrolled if they reported at least 2 symptoms on a McMonnies Dry Eye Questionnaire together with 1 of the following screening tests: noninvasive tear breakup time (5-10 seconds) and Schirmer test without anesthesia (2-5 mm in 5 minutes). Patients were instructed to use the test solutions 3 times a day for 30 days. Tear production, evaporation, lipid layer structure, and osmolality were measured before and 30 days after use of the drops. A statistically significant decrease was seen after 1 month in tear evaporation rates with both emulsion (7.25 +/- 5.43 g/m2/h) and hypromellose (2.02 +/- 4.75 g/m2/h). However, the decrease with emulsion was significantly greater than with hypromellose (P < 0.001). Lipid layer structure improved from day 1 to day 30 of the study with the emulsion but not with the hypermellose. No significant changes were seen in tear production and osmolality with either of the drops. The oil-water emulsion was more effective in reducing tear evaporation than hypromellose after repeated application over a 1-month period. This finding signifies the potential of the emulsion in the management of evaporative dry eye.