Characteristics of Pre-Lens Tear Film Behavior in Eyes Wearing Delefilcon A Silicone Hydrogel Water Gradient Contact Lenses.

The pre-lens tear film (PLTF) over (i) delefilcon A silicone hydrogel water gradient (WG; 33-80% from core to surface) contact lenses (CLs) (SHWG-CLs) and (ii) subjects' own non-WG soft CLs (SCLs) (SO-SCLs) was studied in 30 eyes of 30 subjects to assess the hypothesized PLTF stabilization over SHWG-CLs. In both eyes, delefilcon A SHWG-CLs (DAILIES TOTAL1®; Alcon, Fort Worth, TX, USA) or SO-SCLs were worn. After 15 min of wearing each lens, the tear meniscus radius (TMR, mm), lipid-layer interference grade (IG) and spread grade (SG), and non-invasive breakup time (NIBUT, seconds) were evaluated and compared between the SHWG-CLs and the SO-SCLs. The comparison between the SHWG-CL and SO-SCL groups (SHWG-CL and SO-SCL, mean ± SD) revealed that TMRs temporarily decreased and reached a plateau value after 15 min (0.21 ± 0.06; 0.21 ± 0.06) compared to the value prior to CL insertion (0.24 ± 0.08; 0.25 ± 0.08), with no significant difference between the two groups. The NIBUT, IG, and SG values after 15 min of wearing the CLs were (9.7 ± 3.7; 4.7 ± 4.2), (1.0 ± 0.2; 1.8 ± 1.0), and (1.1 ± 0.4; 1.9 ± 1.5), respectively, and all values were significantly better in the SHWG-CL group (p < 0.0001, p = 0.0039, and p < 0.0001, respectively). We found that compared to the SO-SCLs, the maintenance of the PLTF on the SHWG-CLs was supported by the thicker and more stable PLTF.

Dynamic Aspects of Pre-Soft Contact Lens Tear Film and Their Relation to Dry Eye: Basic Science and Clinical Relevance.

Soft contact lens (SCL) perturbs the intimate connection between the pre-lens tear film (PLTF) and the ocular surface in various ways, i.e., (i) decrease in tear meniscus radius and aqueous tear thickness, (ii) attenuation of tear film lipid layer spread, (iii) limited wettability of SCL surface, (iv) increased friction with eyelid wiper, etc. This often results in SCL-related dry eye (SCLRDE) manifested as PLTF instability and contact lens discomfort (CLD). In this review, the individual contributions of factors (i-iv) to PLTF breakup patterns (BUP) and CLD are considered via the tear film-oriented diagnosis framework adopted by the Asia Dry Eye Society from a clinical and basic science perspective. It is shown that SCLRDE (due to aqueous deficiency, increased evaporation, or decreased wettability) and BUP of PLTF classify within the same types as the ones observed for the precorneal tear film. The analysis of PLTF dynamics reveals that the inclusion of SCL enhances the manifestation of BUP associated with (i) decreased thickness of PLTF aqueous layer and (ii) limited SCL wettability as shown by the rapid expansion of BUP area. PLTF thinness and instability result in increased blink-related friction and lid wiper epitheliopathy as major contributor to CLD.

Successful Detection of the Characteristics of Tear Film Breakup Appearing Immediately after Eye Opening by Videokeratography with a Newly-Developed Indicator.

Spot break (SB), a tear film breakup (TFBU) subtype seen in decreased wettability dry eye (DE), is characterized by a spot-like TFBU that appears immediately after eye opening. It is sometimes difficult to detect using currently available devices for evaluating non-invasive TFBU. The purpose of this study was to investigate the effectiveness of using a newly developed videokeratography indicator for detecting SB. The study involved 44 eyes of 44 DE patients (21 eyes with SB (SB group) and 23 eyes with random break in which fluorescein breakup time was ≤ 5 s (s) (RB group)). All eyes were examined using videokeratography, with digital Meyer-ring images being obtained. By calculation of the degree of luminance blur on the cornea in the Meyer-ring images, termed 'disturbance value' (DV), DVs at 0 s (DV(0)]), 2 s (DV(2)), and 5 s (DV(5)) after eye opening, and the changes of DV between each time, were compared between the SB and RB groups. Results: No significant differences in DV(2) and DV(5) and the rate of change between DV(2) and DV(5) were found between the two groups. However, DV(0) and rate of change between DV(0) and DV(2) in the SB group were significantly greater (p < 0.001) than those in the RB group. SB characteristics were successfully detected by videokeratography using a new videokeratography DV indicator.

Dry Eye Subtype Classification Using Videokeratography and Deep Learning.

We previously reported on 'Tear Film Oriented Diagnosis' (TFOD), a method for the dry eye (DE) subtype classification using fluorescein staining and an examination of fluorescein breakup patterns via slit-lamp biomicroscopy. Here, we report 'AI-supported TFOD', a novel non-invasive method for DE subtype classification using videokeratography (VK) and "Blur Value" (BV), a new VK indicator of the extent of blur in Meyer-ring images and deep learning (DL). This study involved 243 eyes of 243 DE cases (23 males and 220 females; mean age: 64.4 ± 13.9 (SD) years)-i.e., 31 severe aqueous-deficient DE (sADDE) cases, 73 mild-to-moderate ADDE (m/mADDE) cases, 84 decreased wettability DE (DWDE) cases, and 55 increased evaporation DE (IEDE) cases diagnosed via the fluorescein-supported TFOD pathway. For DL, a 3D convolutional neural network classification model was used (i.e., the original image and BV data of eyes kept open for 7 s were randomly divided into training data (146 cases) and the test data (97 cases), with the training data increased via data augmentation and corresponding to 2628 cases). Overall, the DE classification accuracy was 78.40%, and the accuracies for the subtypes sADDE, m/mADDE, DWDE, and IEDE were 92.3%, 79.3%, 75.8%, and 72.7%, respectively. 'AI-supported TFOD' may become a useful tool for DE subtype classification.

Surface Chemistry Study of the Interactions of Sesame Oil with Meibomian Films.

A possible approach for the treatment of meibomian gland disease (MGD) can be the supplementation of meibomian gland secretion (MGS) with nonpolar lipids (NPL) rich plant oils. Sesame oil (SO), approximately equal in monounsaturated fat (oleic acid, 40% of total) and polyunsaturated fat (linoleic acid, 42% of total), has shown multiple health benefits due to its anti-inflammatory and antioxidant effects. Thus, the interactions between SO and MGS in surface layers deserve further study. Therefore, pseudobinary films were formed with controlled MGS/SO molar ratios (0%, 10%, 30%, 50%, and 100% SO) at the air/water surface of the Langmuir trough over phosphate buffered saline (pH 7.4) subphase. Surface pressure (π)-area (A) isotherms and Brewster angle microscopy observations showed nonideal interactions where SO aggregates with MGS and complements the NPL stratum of the meibomian layers. The analysis of stress relaxation transients with Kohlrausch-Williams-Watts equation revealed that the supplementation of fixed amount of MGS with excess lipids via SO altered the dilatational elasticity of the films as reflected by the increase of the exponent ß. Thus, SO with its unique combination of high oxidative stability and abundance of long polyunsaturated acyl chains might be a useful supplement to MGS layers.

Correlations between bulk and surface properties of meibomian lipids with alteration of wax-to-sterol esters content.

In a recent study (Ewurum et al., 2021), wax (WE) and sterol esters (CE) from human meibum secretions (MGS) were separated and reconstituted with controlled WE/CE ratios (0%, 20%, 30%, 40%, 50%, 75% and 100% CE weight fractions). It was found that the alterations in the CE content of WE/CE mixtures modified the hydrocarbon chain conformation and packing of the mixture. A major question that emerges is whether the spectroscopic packing parameters determined for bulk meibum translate to a change in the performance of meibomian layers at the air/water interface, as it is the surface film functionality that is crucial for the performance of MGS at the ocular surface. The study of human meibum films with Langmuir surface balance was performed to access the surface properties at blink-like deformations of the film area. Surface pressure (π)-area (A) isocycles and stress relaxations were used to assess the layer's reorganization during area cycling and dilatational elasticity, respectively. The morphology of the films was monitored by Brewster angle microscopy. It was found that the increased order and chain melting temperature of the bulk samples correlated with a raise in the maximum surface pressure attained at minimal surface area and in the transient dilatational modulus of the meibomian layers. Such correlations may allow for development of an improved understanding between the bulk and surface properties of human meibum and of other natural and synthetic tear lipid films.

Editorial of Special Issue "Differential Diagnosis for Dry Eye".

This editorial aims to summarize the scientific papers that contributed to the Special Issue "Differential Diagnosis for Dry Eye" [...].

Interactions of Meibum and Tears with Mucomimetic Polymers: A Hint towards the Interplay between the Layers of the Tear Film.

Recent clinical findings suggest that mucomimetic polymers (MMP) can alter not only the texture of the aqueous tear but also the spreading and structure of the tear film (TF) lipid layer, thereby allowing for their synchronized performance in vivo. Thus, we aimed to evaluate in vitro (i) the capability of pharmaceutically applicable MMP to ensure the formation of post-evaporative ferning patterns (a characteristic feature of the "healthy" tear colloid) and (ii) the MMP interactions with human meibum films accessed in the course of blink-like deformations via Langmuir surface balance and Brewster angle microscopy (BAM). Four MMP were used- hyaluronic acid (HA), cross-linked hyaluronic acid (CHA), carboxymethyl cellulose (CMC) and gellan gum (GG)- at the concentrations of 0.0001%, 0.001%, 0.01%, 0.05% and 0.1%. Significant differences were observed in the MMP fern formation capability: CHA (≥0.001%) > HA (≥0.01%) = CMC (≥0.01%) > GG (≥0.05%). All MMP affected the spreading of meibum, with BAM micrographs revealing thickening of the films. CHA was particularly efficient, showing concentration-dependent enhancement of tear ferning and of meibomian layer structure, surfactant properties and viscoelasticity. Thus, endogenous and exogenous MMP may play key roles for the concerted action of the TF layers at the ocular surface, revealing novel routes for TF-oriented therapeutic applications.

Improving Stability of Tear Film Lipid Layer via Concerted Action of Two Drug Molecules: A Biophysical View.

The tear film at the ocular surface is covered by a thin layer of lipids. This oily phase stabilizes the film by decreasing its surface tension and improving its viscoelastic properties. Clinically, destabilization and rupture of the tear film are related to dry eye disease and are accompanied by changes in the quality and quantity of tear film lipids. In dry eye, eye drops containing oil-in-water emulsions are used for the supplementation of lipids and surface-active components to the tear film. We explore in detail the biophysical aspects of interactions of specific surface-active compounds, cetalkonium chloride and poloxamer 188, which are present in oil-in-water emulsions, with tear lipids. The aim is to better understand the macroscopically observed eye drops-tear film interactions by rationalizing them at the molecular level. To this end, we employ a multi-scale approach combining experiments on human meibomian lipid extracts, measurements using synthetic lipid films, and in silico molecular dynamics simulations. By combining these methods, we demonstrate that the studied compounds specifically interact with the tear lipid film enhancing its structure, surfactant properties, and elasticity. The observed effects are cooperative and can be further modulated by material packing at the tear-air interface.

Contribution of Mucins towards the Physical Properties of the Tear Film: A Modern Update.

Instability of the tear film (TF) protecting the ocular surface results in dry eye syndrome (DES), the most prevalent public health ophthalmic disease affecting the quality of life of 10 to 30% of the human population worldwide. Although the impact of the tear film lipid layer (TFLL) and of the aqueous tears (AT) to the TF stability is extensively studied, in contrast the contribution of the secretory mucins (SM) and of the membrane-associated mucins (MAM), i.e., one of the most abundant molecular classes in AT and in the corneal epithelium respectively, remains poorly defined. However, it is well known that in DES both types of mucins are quantitatively or qualitatively deficient. Numerous studies since the 1990s until now have proposed direct involvement of SM and MAM in the material properties (viscoelasticity, hydration, and protection of the ocular surface; synergistic cooperation with the rest of the TF layers; etc.) and stability of TF. These theories will be reviewed here in the context of the classical and modern in vitro and in vivo results that allow their reappraisal and in view of the novel mucin secretion enhancing pharmaceuticals, which have opened innovative routes for the therapy of DES.

Lipid Saturation and the Rheology of Human Tear Lipids.

Elevated levels of acyl chain saturation of meibomian lipids are associated with enhanced tear film (TF) stability in infants to shortened TF breakup time with meibomian gland dysfunction. Thus, the effect of saturation on the surface properties of human TF lipids (TFLs) using a Langmuir surface balance and Brewster angle microscopy was studied. Lipid phase transitions were measured using infrared spectroscopy. The raise in the % of saturation resulted in thicker, and more elastic films at π = 12 mN/m, with the effects being proportional to the saturation level. At the same time, at lower (≤10 mN/m) π, the raise in saturation resulted in an altered spreading and modified structure of TFL layers. The strong impact of saturation on TFL surface properties correlated with a saturation induced increase of the TFL acyl chain order, phase transition temperature, and lipid-lipid interactions. The native TFL order and πmax were significantly greater, compared with native meibum collected from the same individual. Aggregation of lipids on the tear surface due to saturation was not as significant as it was for meibum. Although the surface pressure/area isotherms for TFL were similar for meibum, differences in rheology and phase transition parameters warrant the study of both.

Impact of Air Exposure Time on the Water Contact Angles of Daily Disposable Silicone Hydrogels.

The wettability of silicone hydrogel (SiHy) contact lens (CLs) is crucial for the pre-lens tear film stability throughout the day. Therefore, sessile drop and captive bubble setups were used to study the advancing and receding water contact angles (CA) of four SiHy materials: narafilcon A (TE), senofilcon A (AOD), stenfilcon A (MD), and delefilcon A (DT). TE and AOD have 48% and 38% water content, respectively, and no surface coating. MD (54% water) implements "smart chemistry" with just 4.4% bulk silicone content, while DT has >80% water at its surface. These SiHy were subjected to continuous blink-like air exposure (10 s)/rehydration (1s) cycles for 0, 1, 2, 3, 4, 6, 8, 10, 12, 14, and 16 h. The advancing CA, which measures the rehydration propensity of the CL surface, proved to be the most sensitive parameter to discriminate between the samples. The order of performance for the entire time scale was DT > MD >> AOD ≥ TE. The extended desiccation/rehydration cycling increased the differences between the CA of DT and MD compared to AOD and TE. This suggests that the low Si surface content and the high surface hydration are major determinants of SiHy wettability.

Tear-film-oriented diagnosis for dry eye.

Tear-film (TF) stability protects the ocular surface epithelium from desiccation and is ensured via cooperation between the ocular surface components including constituents of the TF and ocular surface epithelium. Thus, when those components are insufficient or impaired, the TF breakup that initiates dry eye occurs. Recently, new, commercially available eye drops have appeared in Japan that enable TF stabilization via targeted supplementation of deficient ocular surface components. Hence, a new layer-by-layer diagnosis and treatment concept for dry eye, termed tear-film-oriented diagnosis and tear-film-oriented therapy (TFOD and TFOT, respectively), have emerged and become widely accepted in Asian countries and beyond. TFOD is a diagnostic method for dry eye based on the TF dynamics and breakup patterns (BUPs), through which dry-eye subtypes, including aqueous-deficient dry eye, decreased-wettability dry eye, and increased-evaporation dry eye, are diagnosed. BUPs and/or each diagnosed dry-eye subtype can, in a layer-by-layer fashion, reveal the insufficient ocular surface components responsible for the TF breakup. Using these data, the optimal topical TFOT to treat dry eye can be proposed by addressing the TF breakup via the supplementation of the insufficient components. In Japan, TF breakup is now regarded as a visible core mechanism of dry eye, and abnormal breakup time (ie, ≤ 5 seconds) and symptoms are currently considered part of the diagnostic criteria for dry eye. In this review, the importance of TF instability as a core manifestation of dry eye, the molecular mechanism of TF breakup, the concept of TFOD, and the methods for implementing TFOD for TFOT are introduced.

Tear Film-Oriented Diagnosis and Tear Film-Oriented Therapy for Dry Eye Based on Tear Film Dynamics.

In December 2010 and January 2012, 3% diquafosol sodium ophthalmic solution and 2% rebamipide ophthalmic suspension, respectively, appeared first in Japan as prescription drugs for the treatment of dry eye (DE). Since then, not only the diagnosis and treatment but also the understanding of the pathophysiology of DE have greatly advanced, and a new concept of layer-by-layer diagnosis and treatment for DE, respectively termed "tear-film-oriented diagnosis" (TFOD) and "tear-film-oriented therapy" (TFOT) was born. This new concept is currently in the process of expanding from Japan to other Asian countries. TFOD is the method used for the differential diagnosis of DE, which includes aqueous-deficiency DE (ADDE), decreased wettability DE (DWDE), and increased evaporation DE (IEDE), through the dynamics of tear film (TF) and breakup patterns (BUPs) after the eye is opened. BUPs and/or each diagnosed DE subtype are/is able to distinguish the insufficient components of the ocular surface that are responsible for each BUP in a layer-by-layer fashion. Aqueous fluid, membrane-associated mucins (especially MUC16), and the lipid layer and/or secretory mucins must be insufficient in ADDE, DWDE, and IEDE, respectively, and this allows for a layer-by-layer treatment to be proposed for each BUP via the supplementation of the insufficient components, using the topical therapy currently available. In Japan, TF breakup is regarded as a visible core mechanism for DE, and an abnormal breakup time (i.e., ≤5 seconds) and symptoms are currently used for the diagnosis of DE. Therefore, TFOD and TFOT could be an ideal and practical pathway for clinicians to manage DE.

Effects of Lipid Saturation on the Surface Properties of Human Meibum Films.

Elevated levels of acyl chain saturation of meibomian lipids are associated with vastly different effects: from enhanced tear film (TF) stability in infants to shortened TF breakup time in meibomian gland disease patients. Thus it is important to study the effect of saturation on the surface properties of human meibum (MGS). Therefore, MGS films (1, 2, 3, 4, 5, 10, 25, 50, 67, and 100% saturation) were spread at the air/water interface of a Langmuir surface balance. The layers' capability to reorganize during dynamic area changes was accessed via the surface pressure (π)-area (A) compression isotherms and step/relaxation dilatational rheology studies. Film structure was monitored with Brewster angle microscopy. The raise in the % (at ≥10%) of saturation resulted in the formation of stiffer, thicker, and more elastic films at π ≥ 12 mN/m with the effects being proportional to the saturation level. At the same time, at low (≤10 mN/m) π the raise in saturation resulted in altered spreading and heterogeneous structure of MGS layers. The strong impact of saturation on MGS surface properties correlates with our recent spectroscopy study, which demonstrated that saturation induced increase of MGS acyl chain order, phase transition temperature, and cooperativity.

Structure-function relationship of tear film lipid layer: A contemporary perspective.

Tear film lipid layer (TFLL) stabilizes the air/tear surface of the human eye. Meibomian gland dysfunction (MGD) resulting in quantitative and qualitative modifications of TFLL major (>93%) component, the oily secretion of meibomian lipids (MGS), is the world leading cause of dry eye syndrome (DES) with up to 86% of all DES patients showing signs of MGD. Caused by intrinsic factors (aging, ocular and general diseases) and by extrinsic everyday influences like contact lens wear and extended periods in front of a computer screen, DES (resulting in TF instability, visual disturbances and chronic ocular discomfort) is the major ophthalmic public health disease of the present time affecting the quality of life of 10-30% of the human population worldwide. Therefore there is a pressing need to summarize the present knowledge, contradictions and open questions to be resolved in the field of TFLL composition/structure/functions relationship. The following major aspects are covered by the review: (i) Do we have a reliable mimic for TFLL: MGS vs contact lens lipid extracts (CLLE) vs lipid extracts from whole tears. Does TFLL truly consist of lipids only or it is important to keep in mind the TF proteins as well?; (ii) Structural properties of TFLL and of its mimics in health and disease in vitro and in vivo. How the TFLL uniformity and thickness ensures the functionality of the lipid layer (barrier to evaporation, surface properties, TF stability etc.); (iii) What are the main functions of the TFLL? In this aspect an effort is done to emphasize that there is no single main function of TFLL but instead it simultaneously fulfills plethora of functions: suppresses the evaporation (alone or probably in cooperation with other TF constituents) of the aqueous tears; stabilizes (due to its surface properties) the air/tear surface at eye opening and during the interblink interval; and even acts as a first line of defense against bacterial invasion due to its detergency action on the bacterial membranes. An effort is done to highlight how the concept on the importance of TFLL and TF viscoelasticity transpires from old and new studies and what are its clinical implications. An attempt is made to outline the future hot directions of research into the field ranging from quest for molecules that can significantly alter TFLL properties to addressing open questions on the contribution of TFLL to the overall performance of the TF.

Surface Chemistry Interactions of Cationorm with Films by Human Meibum and Tear Film Compounds.

Cationorm® (CN) cationic nanoemulsion was demonstrated to enhance tear film (TF) stability in vivo possibly via effects on tear film lipid layer (TFLL). Therefore the interactions of CN with human meibum (MGS) and TFLL in vitro and in vivo deserve special study. MGS and CN were spread at the air/water interface of a Langmuir surface balance to ensure a range of MGS/CN oil phase ratios: 20/1, 10/1, 5/1, 3/1, 2/1 and 1/1. The films capability to reorganize during dynamic area changes was evaluated via the surface pressure-area compression isotherms and step/relaxation dilatational rheology studies. Films structure was monitored with Brewster angle microscopy. CN/TFLL interactions at the ocular surface were monitored with non-contact specular microscopy. The in vitro studies of MGS/CN layers showed that (i) CN inclusion (at fixed MGS content) increased film elasticity and thickness and that (ii) CN can compensate for moderate meibum deficiency in MGS/CN films. In vivo CN mixed with TFLL in a manner similar to CN/MGS interactions in vitro, and resulted in enhanced thickness of TFLL. In vitro and in vivo data complement each other and facilitated the study of the composition-structure-function relationship that determines the impact of cationic nanoemulsions on TF.

Classification of Fluorescein Breakup Patterns: A Novel Method of Differential Diagnosis for Dry Eye.

PURPOSE: To investigate the relationship between fluorescein breakup patterns (FBUPs) and clinical manifestations in dry eye cases. DESIGN: Cross-sectional study. METHODS: In 106 eyes of 106 subjects (19 male, 87 female; mean age: 64.2 years), FBUPs were categorized into 1 of the following 5 break (B) types: area (AB, n = 19); spot (SB, n = 22); line (LB, n = 24); dimple (DB, n = 19); random (RB, n = 22 eyes); and dry eye-related symptoms using the visual analog scale (VAS, 100 mm = maximum), tear meniscus radius (TMR, mm), tear film lipid layer interference grade (IG) (grades 1-5; 1 = best) and spread grade (SG) (grades 1-4; 1 = best), tear film noninvasive breakup time (NIBUT, seconds), fluorescein breakup time (FBUT, seconds), corneal-epithelial damage (CED) score (15 points = maximum), ocular surface epithelial damage (OSED) score (9 points = maximum), and the Schirmer 1 test (ST1, mm) were examined and compared between each FBUP. RESULTS: In each FBUP, eye dryness and fatigue were the severest symptoms. Characteristic symptoms were sensitivity to light, heavy eyelids, pain, foreign body sensation, difficulty opening the eye, and discharge for AB, heavy eyelids for SB, and foreign-body sensation for LB. Statistically significant differences were found in TMR (AB-SB, -DB, and -RB; LB-RB), IG (AB-all other FBUP; LB-SB and -DB), and SG (AB-all other FBUPs), FBUT (AB-LB, -DB, and -RB; SB-DB and -RB; LB-RB; DB-RB), and NIBUT (AB-all other FBUPs; SB-DB and-RB, and LB-RB), CED (AB-all other FBUPs; LB-SB, -DB, and -RB) and OSED (AB-SB, -LB, and -DB; LB-SB, -DB, and -RB), and ST1 (AB-SB, -DB, and -LB) (P < .05 in each comparison). CONCLUSIONS: The 5 different FBUPs constituted different groups, reflecting different pathophysi-ologies.

Surface properties and exponential stress relaxations of mammalian meibum films.

The surface properties of meibomian secretion (MGS), the major constituent of the tear film (TF) lipid layer, are of key importance for TF stability. The interfacial properties of canine, cMGS, and feline, fMGS, meibum films were studied using a Langmuir surface balance. These species were selected because they have blinking frequency and TF stability similar to those of humans. The sample's performance during dynamic area changes was evaluated by surface pressure (π)-area (A) isocycles and the layer structure was monitored with Brewster angle microscopy. The films' dilatational rheology was probed via the stress-relaxation technique. The animal MGS showed similar behavior both between each other and with human MGS (studied previously). They form reversible, non-collapsible, multilayer thick films. The relaxations of canine, feline, and human MGS films were well described by double exponential decay reflecting the presence of two processes: (1) fast elastic process, with characteristic time τ < 10 s and (2) slow viscous process, with τ > 100 s-emphasizing the meibum layers viscoelasticity. The temperature decrease from 35 to 25 °C resulted in decreased thickness and lateral expansion of all MGS layers accompanied with increase of the π/A hysteresis and of the elastic process contribution to π relaxation transients. Thus, MGS films of mammals with similar blinking frequency and TF stability have similar surface properties and stress relaxations unaltered by the interspecies MGS compositional variations. Such knowledge may impact the selection of animal mimics of human MGS and on a better understanding of lipid classes' impact on meibum functionality.

Surface Properties of Squalene/Meibum Films and NMR Confirmation of Squalene in Tears.

Squalene (SQ) possesses a wide range of pharmacological activities (antioxidant, drug carrier, detoxifier, hydrating, emollient) that can be of benefit to the ocular surface. It can come in contact with human meibum (hMGS; the most abundant component of the tear film lipid layer) as an endogenous tear lipid or from exogenous sources as eyelid sebum or pharmaceuticals. The aims of this study were to determine (i) if SQ is in tear lipids and (ii) its influence on the surface properties of hMGS films. Heteronuclear single quantum correlation NMR confirmed 7 mol % SQ in Schirmer's strips extracts. The properties of SQ/hMGS pseudo-binary films at the air/water interface were studied with Langmuir surface balance, stress-relaxation dilatational rheology and Brewster angle microscopy. SQ does not possess surfactant properties. When mixed with hMGS squalene (i) localized over the layers' thinner regions and (ii) did not affect the film pressure at high compression. Therefore, tear SQ is unlikely to instigate dry eye, and SQ can be used as a safe and "inert" ingredient in formulations to protect against dry eye. The layering of SQ over the thinner film regions in addition to its pharmacological properties could contribute to the protection of the ocular surface.