Biophysical properties of tear film lipid layer I. Surface tension and surface rheology

Sex Differences in the Lacrimal Gland: Implications for Dry Eye Disease

Sexual dimorphism significantly impacts the lacrimal gland's structure, function, and ageing processes, playing an important role in dry eye disease (DED) pathophysiology. This multifactorial disorder, characterised by tear film instability, inflammation, and visual impairment, disproportionately affects women, especially after menopause. It highlights the interplay between sex steroid hormones, lacrimal gland function, and environmental factors. Systemic and local androgens are vital for maintaining lacrimal gland health and tear production, while the role of oestrogens remains less clear. Evidence suggests dose and context-dependent effects on inflammation and glandular function. Histopathological and molecular studies reveal significant sex differences in the lacrimal gland, with women exhibiting more pronounced age-related degenerative changes, including fibrosis and acinar atrophy, contributing to their increased susceptibility to DED. Despite these findings, the underlying mechanisms connecting sex steroid hormones, receptor expression, and local tissue regulation to these disparities remain poorly understood, highlighting the need for further research. This review synthesises the current knowledge of sex-specific differences in the lacrimal gland, emphasising the importance of integrating systemic and local biomarkers, histological data, and molecular insights into personalised therapeutic strategies. By tailoring treatments to patients' unique hormonal and molecular profiles, personalised medicine has the potential to transform DED management, addressing unmet clinical needs and improving outcomes.

Changes in the distribution of the tear film lipid layer after intensive pulsed light combined with meibomian gland expression in patients with meibomian gland dysfunction

PURPOSE

This study aimed to investigate changes in dry eye disease (DED) parameters and tear film lipid layer distribution after intensive pulse light (IPL) combined with meibomian gland expression (MGX) in patients with meibomian gland dysfunction (MGD).

METHODS

This retrospective study included 218 patients diagnosed with MGD who underwent IPL combined with MGX. Various DED parameters, including tear film lipid layer thickness (LLT), were measured using a Placido disc tear film analyzer and slit lamp. Inferior corneal images were quantified and divided into upper, lower, nasal, and temporal quadrants, with further subdivision into six parts from top to bottom using Python.

RESULTS

The ocular surface disease index, meibomian gland expressibility, and quality scores significantly improved after three treatment sessions. Slit-lamp-measured DED parameters also improved, excluding the fluorescein-stained tear meniscus height. Redness in the nasal limbal and bulbar conjunctivae significantly decreased. The mean LLT tended to increase after treatment. LLT in the upper half of the images, but not in the lower half, increased significantly, with the difference in LLT between the two halves decreasing significantly from 34.46 ±  15.73 to 30.27 ±  14.63 nm (p =  0.031). When the vertical distribution was analyzed by further subdivision into six equal parts from top to bottom, the average difference in LLT decreased in the uppermost segment after treatment.

CONCLUSION

IPL combined with MGX reduced the vertical distribution of lipids in patients with MGD by reducing lower tear film lipid layer stasis. Thus, the Placido disc tear film analyzer is a useful tool for analyzing lipid layer distribution in MGD.

Lipid-based eye drop formulations for the management of evaporative dry eyes

Dry eye disease is a progressive prevalent ocular surface disorder that arises from various factors and is characterized by insufficient quality and/or quantity of tears. The underlying pathophysiology is intricate and can progress to chronic, difficult-to-treat conditions. Multiple strategies and therapeutic approaches are utilized in its management that target one or more etiopathological components of dry eyes, which may include aqueous tear deficiency or evaporative dry eyes. The primary focus of this paper is on treatment alternatives that utilize lipids for the treatment of evaporative dry eyes. This may arise from either abnormal lipid production or inadequate lipid spreading caused by meibomian gland dysfunction. The hypothesis behind the development of these lipid-containing eye drops is that if they can imitate the lipid layer, they may be able to help in the management of the signs and symptoms of evaporative dry eyes. The lipids used in commercial formulations for dry eyes are mineral oil, castor oil, phospholipids, omega-3 fatty acid, and medium-chain triglycerides. The literature suggests the potential of lipid-containing eye drops to alleviate some of the signs and symptoms and enhance the quality of life for individuals suffering from evaporative dry eyes.

Pulmonary Surfactant: A Mighty Thin Film

Pulmonary surfactant is a critical component of lung function in healthy individuals. It functions in part by lowering surface tension in the alveoli, thereby allowing for breathing with minimal effort. The prevailing thinking is that low surface tension is attained by a compression-driven squeeze-out of unsaturated phospholipids during exhalation, forming a film enriched in saturated phospholipids that achieves surface tensions close to zero. A thorough review of past and recent literature suggests that the compression-driven squeeze-out mechanism may be erroneous. Here, we posit that a surfactant film enriched in saturated lipids is formed shortly after birth by an adsorption-driven sorting process and that its composition does not change during normal breathing. We provide biophysical evidence for the rapid formation of an enriched film at high surfactant concentrations, facilitated by adsorption structures containing hydrophobic surfactant proteins. We examine biophysical evidence for and against the compression-driven squeeze-out mechanism and propose a new model for surfactant function. The proposed model is tested against existing physiological and pathophysiological evidence in neonatal and adult lungs, leading to ideas for biophysical research, that should be addressed to establish the physiological relevance of this new perspective on the function of the mighty thin film that surfactant provides.

Comparative Biophysical Study of Meibomian Lipids of Wild Type and Soat1-Null Mice: Implications to Meibomian Gland Dysfunction and Dry Eye Disease

Purpose

The biophysical roles of Meibomian lipids (MLs) played in health and meibomian gland dysfunction (MGD) are still unclear. The purpose of this research is to establish the composition-structure-functional correlations of the ML film (MLF) using Soat1-null mice and comprehensive in vitro biophysical simulations.

Methods

MLs were extracted from tarsal plates of wild type (WT) and Soat1 knockout (KO) mice. The chemical composition of ML samples was characterized using liquid chromatography - mass spectrometry. Comprehensive biophysical studies of the MLFs, including their dynamic surface activity, interfacial rheology, evaporation resistance, and ultrastructure and topography, were performed with a novel experimental methodology called the constrained drop surfactometry.

Results

Soat1 inactivation caused multiple alternations in the ML profile. Compared to their WT siblings, the MLs of KO mice were completely devoid of cholesteryl esters (CEs) longer than C18 to C20, but contained 7 times more free cholesterol (Chl). Biophysical assays consistently suggested that the KO-MLF became stiffer than that of WT mice, revealed by reduced film compressibility, increased elastic modulus, and decreased loss tangent, thus causing more energy loss per blinking cycle of the MLF. Moreover, the KO mice showed thinning of their MLF, and reduced evaporation resistance.

Conclusions

These findings delineated the composition-structure-functional correlations of the MLF and suggested a potential biophysical function of long-chain CEs in optimizing the surface activity, interfacial rheology, and evaporation resistance of the MLF. This study may provide novel implications to pathophysiological and translational understanding of MGD and dry eye disease.

A biophysical study of tear film lipid layer model membranes

The tear film lipid layer (TFLL), the final layer of the human tear film is responsible for surface tension reduction while blinking, water evaporation retardation and maintaining the stability of the tear film. The study of the composition-structure-function relationship of TFLL is paramount, as a compromised structure of TFLL leads to the emergence of dry eye disease (DED) which is one the most prevalent ophthalmic surface diseases of the modern world, associated with chronic pain and reduced visual capability. In this model membrane study, a systematic approach is used to study the biophysical properties of TFLL model membranes as a function of composition. Three mixed-lipid model membranes are studied along with their individual components comprising cholesteryl oleate (CO), glyceryl trioleate (GT), L-α-phosphatidylcholine (egg PC) and a free fatty acid mixture. The models become progressively more complex from binary to quaternary mixtures, allowing the role of each individual lipid to be derived. Langmuir balance, Brewster Angle Microscopy (BAM) and Profile Analysis Tensiometer (PAT) are used to study the surface activity and compression-expansion cycles, morphology, and rheological behaviour of the model membranes, respectively. Evidence of multilayering is observed with inclusion of CO and a reversible collapse is associated with the GT phase transition. An initially more coherent film is observed due to the addition of polar PC. Notably, these individual behaviours are retained in the mixed films and suggest a possible role for each physiological component of TFLL.

Effect of Model Tear Film Lipid Layer on Water Evaporation

Purpose

A majority of in vitro models were incapable of reproducing the evaporation resistance of tear film lipid layer (TFLL) in vivo. The purpose of this research is to develop a novel in vitro model to study the effect of TFLL on water evaporation.

Methods

A ventilated, closed-chamber, droplet evaporimeter with a constant surface area has been invented to study the evaporation resistance of TFLL. This evaporimeter ensures a rigorous control of environmental conditions, including the temperature, relative humidity, airflow rate, surface area, and surface pressure, thus allowing for reproducible water evaporation measurements over a time period of only 5 minutes. The volumetric evaporation rate of this droplet evaporimeter is less than 2.7 µL/min, comparable to the basal tear production of healthy adults. Together with direct film imaging using atomic force microscopy (AFM), we have studied the effect of a model TFLL on water evaporation, as a function of the lipid composition and surface pressure.

Results

A model TFLL composed of 40% wax esters, 40% cholesteryl esters, and 20% polar lipids was capable of reducing the water evaporation rate by 11% at surface pressure 47 mN/m. AFM revealed that the model TFLL at high surface pressures consists of discrete droplets/aggregates of the nonpolar lipids residing atop a polar lipid monolayer with phase separation.

Conclusions

The TFLL may resist water evaporation with a combined mechanism by increasing film compactness of the polar lipid film at the air-water surface, and, to a lesser extent, by increasing film thickness of the nonpolar lipid film.