Abstract
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.
Highlights
A thin film of lipids, the tear film lipid layer (TFLL), secreted mostly from the meibomian glands in the eyelids, covers the air/tear surface
The =CH resonance region accounts for six of the 50 protons of SQ (Figure 2Aii) that are characteristic of terpenoids (Figure 1)
SQ-meibum interactions could be important to the functions of the TFLL
Summary
A thin film of lipids, the tear film lipid layer (TFLL), secreted mostly from the meibomian glands in the eyelids, covers the air/tear surface. As hMGS is the main constituent of the TFLL, a considerable effort has been made to study its structure and properties at the air/water interface in vitro and in vivo and it was found to form a thick viscoelastic duplex film composed of a monomolecular layer of amphiphilic polar lipids at the aqueous surface and a generally unstructured lipophilic suspension, consisting of lipid lamellar-crystallite particulates immersed in a continuous liquid phase with no long-range order, located on top and facing the air [10,11,12,13]. There is growing evidence that, hMGS lipids are the major ingredient (~90%–95%), meibum is not the only source of lipids in TFLL and admixtures of polar and non-polar lipids may be expected to come from other sources such as aqueous tears (e.g., from the lipocalin bound phospholipid pool) [14,15,16,17,18,19,20] or by eyelid sebum [21,22,23]
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