Influence of Lipid Coatings on Surface Wettability Characteristics of Silicone Hydrogels

Abstract

Insoluble lipids serve vital functions in our bodies and interact with biomedical devices, e.g., the tear film on a contact lens. Over a period of time, these naturally occurring lipids form interfacial coatings that modify the wettability characteristics of these foreign synthetic surfaces. In this study, we examine the deposition and consequences of tear film lipids on silicone hydrogel (SiHy) contact lenses. We use bovine meibum, which is a complex mixture of waxy esters, cholesterol esters, and lipids that is secreted from the meibomian glands located on the upper and lower eyelids of mammals. For comparison, we study two commercially available model materials: dipalmitoylphosphatidylcholine (DPPC) and cholesterol. Upon deposition, we find that DPPC and meibum remain closer to the SiHy surface than cholesterol, which diffuses further into the porous SiHy matrix. In addition, we also monitor the fate of unstable thin liquid films that consequently rupture and dewet on these lipid-decorated surfaces. This dewetting provides valuable qualitative and quantitative information about the wetting characteristics of these SiHy substrates. We observe that decorating the SiHy surface with simple model lipids such as DPPC and cholesterol increases the hydrophilicity, which consequently inhibits dewetting, whereas meibum behaves conversely.

Dynamics and function of the tear film in relation to the blink cycle

“…First, there may be significant differences between the tear film lipid layer and meibum spread on saline which cause a difference in evaporation resistance. Second, despite the findings of Mishima and Maurice (1961), the tear film lipid layer may have little or no effect on evaporation and tear film thinning, implying that tear film instability may have another cause, e.g., related to the viscoelastic properties of the lipid layer (Bhamla et al, 2014; Georgiev et al, 2014). …”

“…In observations of the lipid layer, it is not uncommon to see more lipid left behind in central regions of the cornea during the upstroke which subsequently spreads. The dynamics and mechanisms of this lipid motion are an area of considerable interest (Aydemir et al, 2011; Bhamla et al, 2014; Bruna and Breward, 2014; King-Smith et al, 2013a; Rosenfeld et al, 2013). …”

“…They may also greatly aid in understanding the function and properties of the lipid layer, as well as testing of multilamellar models of the lipid layer structure (King-Smith et al, 2013a; Leiske et al, 2012b; Rosenfeld et al, 2013). Mathematical models may need to incorporate complex surface rheology of the lipid layer as a result of lipid layer structure (Bhamla et al, 2014; Georgiev et al, 2014), but it is unclear at the time of writing how much of the in vivo elastic properties at room temperature remain at physiological temperatures (Leiske et al, 2012a). …”

Instability and Breakup of Model Tear Films

“…31 Dewetting of the tear film may also transfer the lipid onto the underlying lens substrate, resulting in fouling of the lens surface. This fouling can modify the lens surface wettability 32 with acute ophthalmologic consequences for the lens wearer, including inflammatory disorders, reduced vision, and symptoms of dry eye. 2,8,33 A photograph and schematic of the instrument is seen in Figure 2.…”

“…Past work has shown that the dewetting mechanism results in transfer of the insoluble lipids at the moving contact line and subsequent fouling of the SiHy surface, ultimately affecting the lens wettability. 31,32 The whole dewetting process takes~5 minutes, leaving behind a dry SiHy surface that is fouled with a meibum layer.…”