Friction of contact lenses: Silicone hydrogel versus conventional hydrogel

Abstract

Silicone hydrogel contact lenses can be worn for up to 30 nights without removal, but for some users, wear time is limited due to poor comfort. This lack of comfort may be related to friction between the anterior lens surface and the under-surface of the eyelid. The purpose of the present study was to initiate an investigation of these friction forces. In-vitro experiments were performed to measure friction forces for silicone hydrogel (SH) lenses and for conventional hydrogel (CH) lenses. The SH and CH lenses were represented by Focus Night and Day (CIBA Vision) and by Optima FW (Bausch and Lomb), respectively. Half of the lenses were subject to in-vitro deposition (doping) of both human serum albumin and hen-egg lysozyme proteins while the other half were undoped and only soaked in phosphate buffer saline solution to remove the presence of the packaging fluid. Scanning electron microscopy was used to obtain images of the undoped and doped lenses to provide insight into the differing friction forces that were measured. For the friction experiments, the contact lenses were placed on a silicone eye-form, which approximated the shape and compliance of the human eye. A custom friction testing apparatus was developed in which a stationary lens was loaded against a sinusoidally reciprocating, flat glass plate in the presence of a saline solution. Two normal forces were applied during testing and peak friction forces were measured at cyclic steady state. Scanning electron microscopy imaging involved preparation of protein conjugated gold nano-spheres in order to provide a label for each of the proteins. The measured friction forces increased for increasing normal force and in the absence of protein doping. Although the friction forces measured for the SH lenses were consistently higher than those for the CH lenses, the differences were not statistically significant. The images of doped lenses showed aggregated mounds of both proteins on both lens types but they were more apparent on CH lenses. A monolayer coverage of both proteins was most evident on the SH lenses. After correlating the images with the friction results, it was suggested that early stages of protein deposition could help lubricate the lens-eyelid interface and reduce friction. However, protein deposition patterns (profiles), as observed in the present study, had no obvious influence on the friction forces.