## 4 Discussion
Poloxamer 407 is reported to penetrate in lipid films, but due to its hydrophilic polyoxyethylene chain, it has a high affinity to water and readily gets squeezed out in the aqueous subphase at \(\pi\leq 20\) mN/m in experiments with phospholipid monolayers [30] and also in control experiments with MGS layers. The interaction of menthol with lipid films revealed that menthol could supplement the PL-coated MGS/aqueous interface. The interaction enhances the spread of MGS layers and their dilatational viscoelasticity. The results agree with recent reports on the increased stability of the tear film of dry eye patients upon treatment with menthol-loaded warm compresses [31]. The positive impact of RDA on the spread of meibomian lipids was confirmed by experiments with talcum used as a visualizer. It can be seen that the supplementation of MGS with the RDA oil phase resulted in faster and better spreading at the air/water interface that cannot be readily achieved with simple extra supplementation of MGS amounts. This effect aligns with the classical observation of [PERSON] and [PERSON] [32] that MGS behaves like a \"tracer oil\" that exhibits partial and incomplete spread and may need endogenous (i.e., tear inherent) or exogenous (pharmaceutically supplemented) lipids to enhance the distribution of meibomian lipids at the air/tear surface. Thus, RDA performs like a well-rounded formulation capable to supplement both the NPL and PL strata of meibomian films, and enhancing the TFLL structure, spreading and elasticity (Figure 11). The elasticity of MGS films is an important property as it was found to ensure the structural stability (accessed by image cross-correlation analysis of TFLL patterns at the ocular surface) of TFLL in consecutive blinks which in turn correlates with the overall stability of the TF both in healthy and in dry eye [2, 22, 33, 34]. A number of clinical studies have confirmed the strong correlation between (i) the capability of the tear film lipid layer _in vivo_ to spread rapidly and uniformly and to recover its morphology (i.e., a manifestation of elasticity) between blinks and (ii) the overall stability of the tear film [35, 36, 37]. In particular, it was found that in MGD patients the spread of the lipid layer after eye opening took much longer (3.54 s in MGD vs. 0.36 s in normal) [36] and the lipid layer roughness, i.e.,  
Figure 11: A simplified figure of the redistribution of the spread and elasticity enhancement effect of RDA on MGS films. RDA supplies MGS layers with both polar lipid (shown as individual molecules) and nonpolar lipid-like (shown as bright islands) molecules, which facilitate the uniform distribution and the enhanced thickness of the resultant MGS/RDA layers. the heterogeneity of the spatial distribution of the lipid layer thickness, is significantly increased (lipid map uniformity = 125 nm\({}^{2}\)) compared to healthy individuals (lipid map uniformity = 14 nm\({}^{2}\)) [37]. Excellent agreement is observed between the clinical data and _in vitro_ laboratory studies where meibomian and tear samples from dry-eye individuals are found to have decreased surfactant properties and lack the ability to form continuous duplex films upon compression in contrast to samples from healthy eyes [2]. Thus, the capability of ophthalmic nanoemulsions to enhance the spread and the structure of MGS layers _in vitro_ represents a reliable indication of the clinical promise of the formulations _in vivo_[13; 29].