Evaporation retardation by model tear-lipid films: The roles of film aging, compositions and interfacial rheological properties.

A novel methodology for assessing evaporation (up to 48 h) through lipid-nanofilms in vitro was developed. The influence of lipid-mixture compositions on evaporation rates was studied. The evaporative fluxes and rheology of lipid-nanofilms were compared with those of human tear-lipid nanofilms in vitro.A sessile-drop technique with precise drop-volume control was adapted to measure evaporation rates at constant temperature of 36 °C and humidity of 75 %. Model lipid solutions were deposited on the surface of aqueous drops to create nanofilms of 10-100 nm. The measurements of dynamic surface pressure vs. nanofilm-thickness were performed under the same conditions. The lipid-mixtures compositions were chosen to mimic that of human tear lipids. Evaporation through lipid nanofilms decreased with film thickness and aging. Evaporation through 70-nm films was 2.5-3 time slower than through 10-nm-thick films. Nonpolar-lipid mixtures reduced evaporation by approximately 35 %. The optimized model-lipid mixtures containing polar phospholipids reduced evaporation by 70-75 %, matching the evaporation-reduction by human-lipid nanofilms in vitro. These model mixtures exhibited interfacial rheology similar to human tear lipids in vitro.This methodology substantiated that aged lipid-nanofilms significantly reduced evaporation in vitro. These findings contradict to the previous reports suggesting that model lipid and meibum films do not retard evaporation in vitro. Polar phospholipids enhance evaporative resistance close to the level observed for human tear-lipid films in vivo. We hypothesize that unique rheological properties of tear-lipid nanofilms are germane to the specific mono- and bi-layered structures formed by phospholipids at the lipid-air and lipid-aqueous interfaces.

Accounting for Ethnicity-Related Differences in Ocular Surface Integrity as a Step Toward Understanding Contact Lens Discomfort.

Contact lens discomfort is a common problem that can lead to unsuccessful or limited contact lens wear. Although many factors may contribute to contact lens discomfort, limited research has explored the influence of ethnicity-related differences in the anatomy and physiology of the ocular surface. Therefore, we performed a search of the literature in PubMed using key words related to "ocular surface" paired with the terms "race" and "ethnicity." The goal of this review was to determine potential areas of research regarding ethnicity differences, particularly between Asian and non-Asian eyes, in ocular surface integrity to advance our understanding of contact lens discomfort.

Dynamic interfacial properties of human tear-lipid films and their interactions with model-tear proteins in vitro.

This review summarizes the current state of knowledge regarding interfacial properties of very complex biological colloids, specifically, human meibum and tear lipids, and their interactions with proteins similar to the proteins found in aqueous part of human tears. Tear lipids spread as thin films over the surface of tear-film aqueous and play crucial roles in tear-film stability and overall ocular-surface health. The vast majority of papers published to date report interfacial properties of meibum-lipid monolayers spread on various aqueous sub-phases, often containing model proteins, in Langmuir trough. However, it is well established that natural human ocular tear lipids exist as multilayered films with a thickness between 30 and 100nm, that is very much disparate from 1 to 2nm thick meibum monolayers. We employed sessile-bubble tensiometry to study the dynamic interfacial and rheological properties of reconstituted multilayered human tear-lipid films. Small amounts (0.5-1µg) of human tear lipids were deposited on an air-bubble surface to produce tear-lipid films in thickness range 30-100nm corresponding to ocular lipid films. Thus, we were able to overcome major Langmuir-trough method limitations because ocular tear lipids can be safely harvested only in minute, sub-milligram quantities, insufficient for Langmuir through studies. Sessile-bubble method is demonstrated to be a versatile tool for assessing conventional synthetic surfactants adsorption/desorption dynamics at an air-aqueous solution interface. (Svitova T., Weatherbee M., Radke C.J. Dynamics of surfactant sorption at the air/water interface: continuous-flow tensiometry. J. Colloid Interf. Sci. 2003;261:1170-179). The augmented flow-sessile-bubble setup, with step-strain relaxation module for dynamic interfacial rheological properties and high-precision syringe pump to generate larger and slow interfacial area expansions-contractions, was developed and employed in our studies. We established that this method is uniquely suitable for examination of multilayered lipid-film interfacial properties. Recently it was compellingly proven that chemical composition of human tear lipids extracted from whole tears is substantially different from that of meibum lipids. To be exact, healthy human tear lipids contain 8-16% of polar lipids, similar to lung lipids, and they are mostly double-tailed phospholipids, with C16 and longer alkyl chains. Rationally, one would assume that the results obtained for meibum lipids, devoid of surface-active components such as phospholipids, and, above all, in a form of monolayers, are not pertinent or useful for elucidating behavior and stability of an averaged 60-nm thick ocular tear-lipid films in vivo. The advantage of sessile-bubble technique, specifically, using a small amount of lipids required to attain multilayered films, unlocks the prospect of evaluating and comparing the interfacial properties of human tear lipids collected from a single individual, typically 100-150µg. This is in sharp contrast with several milligrams of lipids that would be required to build equally thick films for Langmuir-trough experiments. The results of our studies provided in-depth understanding of the mechanisms responsible for properties and stability of human tear-lipid films in vivo. Here we summarize recent publications and our latest findings regarding human tear-lipid interfacial properties, their chemical composition, and their interaction with model proteins mimicking the proteins found in human tear-aqueous phase.

Lens-care-solution-induced alterations in dynamic interfacial properties of human tear-lipid films.

PURPOSE: To evaluate the influence of lens care solutions (LCS) on interfacial dynamics and rheological properties of human tear-lipid films. METHODS: Tear lipids were extracted from Schirmer strips collected from 6 healthy subjects. Sessile bubble tensiometry was used to study interfacial properties at 22°C. Lipids were deposited on an air bubble immersed into electrolytes solution to form 90±20nm films. Lipid films were subjected to expansion-compression cycles for dynamic interfacial properties and to step-strain relaxations for assessments of rheological properties. LCS (BioTrue [BT], PureMoist [PM], Revitalens [RL], ClearCare [CC]) were injected into optical chamber and equilibrated for 2h without or with lipid films. Dynamic interfacial properties of films were measured. Then electrolyte solution was pumped through chamber and properties of films were re-evaluated. RESULTS: Equilibrium surface tension (EST), elasticity modulus (E), and relaxation times (τ) of tear lipids were 22±2.1mN/m, 10.7-14.8mN/m, and 80-150s, respectively. EST for LCS was 43.6±0.2 for CC, 38.3±0.2 for BT, 36.5±1.2 for RL, and 32.8±0.2mN/m for PM. [corrected]. E for LCS varied within 0.5-6.7mN/m, and τ varied from 49 to 68±5s. For mixed lipids+LCS films, EST remained unchanged whereas E and τ were reduced for all LCS types. Exposure to PM and RL noticeably altered the shape of lipid-film iso-cycles. These changes persisted after LCS washout. CONCLUSIONS: Some components of LCS bind irreversibly to lipid films and make them less viscous and less elastic. These findings suggest the possibility of tear-film destabilization upon LCS exposure.

Racial variations in interfacial behavior of lipids extracted from worn soft contact lenses.

PURPOSE: To explore interfacial behaviors and effects of temperature and dilatation on dynamic properties of multilayered human tear lipids extracted from silicone hydrogel (SiH) lenses worn by asymptomatic Asian and white subjects. METHODS: Interfacial properties of lipids extracted from Focus N&D lenses worn by 14 subjects continuously for 1 month were studied. The lipids were deposited on an air bubble immersed in a model tear electrolyte (MTE) solution to form 100 ± 20-nm-thick films. Surface pressure was recorded during slow expansion/contraction cycles to evaluate compressibility and hysteresis of lipid films. Films were also subjected to fast step-strain dilatations at temperatures of 22 to 45°C for their viscoelastic property assessment. RESULTS: Isocycles for Asian and white lipids were similar at low surface pressures but had distinctly different compressibility and hysteresis at dynamic pressures exceeding 30 mN/m. Rheological parameters of reconstituted lipids were also dissimilar between Asian and white. The elastic modulus E∞ for white lipids was 1.5 times higher than that for Asian lipids, whereas relaxation time (t) was on average 1.3 times higher for Asian. No significant changes were observed in rheological properties of both Asian and white lipids when temperature increased from 22.0 to 36.5°C. However, for white lipids, E∞ reduced considerably at temperatures higher than 42.0°C, whereas t remained unchanged. For Asian lipids, both E∞ and t started to decline as temperature increased to 38°C and higher. CONCLUSIONS: Higher elastic modulus of white lipids and elasticity threshold at certain deformations indicate stronger structure and intermolecular interactions as compared with more viscous Asian lipids. The differences in interfacial behaviors between Asian and white lipids may be associated with the differences in their chemical compositions.

Contact lenses wettability in vitro: effect of surface-active ingredients.

PURPOSE: To investigate the release of surface-active agents (surfactants) from unworn soft contact lenses (SCLs) and their influence on the lens surface wettability in vitro. METHODS: Surface tension (ST) of blister pack solutions was measured by pendant-drop technique. STs at the air-aqueous interface and contact angles (CAs) of four conventional and seven silicone hydrogel SCLs were evaluated in a dynamic-cycling regime using a modified captive-bubble tensiometer-goniometer. Measurements were performed immediately after removal from blister packs, and after soaking in a glass vial filled with a surfactant-free solution, which was replaced daily for 1 week. Lens surface wettability was expressed as adhesion energy according to Young equation. RESULTS: STs of all blister pack solutions were lower than the reference ST of pure water (72.5 mN/m), indicating the presence of surfactants. When lenses were depleted of surfactants by soaking, the STs for all studied lenses and advancing CAs of selected lenses increased (p < 0.001). Receding CAs of all studied lenses were 12 degrees +/- 5 degrees and were not affected by the presence of surfactants. For most of the conventional lenses, the surface wettability was largely dependent on surfactants, and reduced significantly after surfactant depletion. In contrast, most silicone hydrogel lenses exhibited stable and self-sustained surface wettability in vitro. CONCLUSIONS: The manufacturer-added surfactants affected wetting properties of all studied SCLs, although to different degrees.

Tear lipids interfacial rheology: effect of lysozyme and lens care solutions.

PURPOSE: To evaluate the interfacial properties of ex vivo tear lipid multilayers with controlled and varying thickness. The influence of lysozyme and surfactant-containing multipurpose lens care solution (MPS) on interfacial rheology of lipids and mixed lipid-protein films were studied. METHODS: Lipids were extracted from lotrafilcon A lenses worn continuously for 1 month. Interfacial properties of the lipids without and with lysozyme in the aqueous phase were examined using tensiometry and step-strain relaxation. Lipid-lysozyme multilayers were exposed to either diluted opti-free express (OFX) or opti-free replenish (OFR) for 30 min, and then MPS was displaced from the bulk phase. Surface tension and rheological parameters before and after MPS exposure were measured and compared. RESULTS: Thick lipid films exerted high surface pressure at the air-aqueous interface, 50 +/- 2 mN/m, with little inter- and intrasubject variability. The rheological storage modulus (E(infinity); 25.3 +/- 2 mN/m) and relaxation time (tau; 87 +/- 25 s) were similar among subjects. Neither lysozyme nor MPS changed the surface tension of the lipid multilayers. Lysozyme adsorbed irreversibly onto multilayers without changing E(infinity), but increased tau 2.5 times. Exposure of mixed multilayers to OFX reduced E(infinity) to less than a half of its original value (13 +/- 4.5 mN/m; p < 0.001), whereas after OFR exposure, small but statistically significant changes in E(infinity) were found (21 +/- 3.0 mN/m; p = 0.0044). MPS-treated mixed films relaxed much slower than an untreated one, tau increased 4.5 times after OFX and 7 times after OFR exposure. CONCLUSIONS: Thick multilayers of the ex vivo tear lipids have exhibited surface tension 15 +/- 1 mN/m lower than reported in the literature for meibomian lipids. The lipid-lysozyme interaction altered the interfacial rheology of the ex vivo lipids. OFX and OFR changed rheological properties of the mixed films to different extents.