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A. Croll, K. Dalnoki–Varess, L. Liu, H. Sheardown; Lateral Force Microscopy Measurements of Friction on Contact Lenses . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2394.
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© ARVO (1962-2015); The Authors (2016-present)
Friction between the surface of a contact lens and the lid may be an important parameter in predicting lens comfort. However, measurement of friction in a meaningful way under physiologically relevant conditions is difficult. Here we present the results of a study of contact lens friction that highlights some of the different types of friction that can occur in contact lenses under an idealized lacrimal film. Comparisons are made between the 1–Day Acuvue® MoistTM (AVM) contact lenses (J&J Vision Care, USA) lenses that permanently incorporate an internal wetting agent (poly vinyl pyrolidone, PVP) using the proprietary LACREONTM technology and the Focus DAILIES® with AquaReleaseTM (FDAR) (CIBA Vision, USA) lenses are designed to release poly vinyl alcohol (PVA).
Lenses were clamped to clean electronics grade silicon substrates and completely submerged in an aqueous environment. Lateral Force Microscopy (LFM) measurements were made, providing information on frictional force between a nanoscopic tip and the contact lens. To complement these data, force spectroscopy measurements were also made.
Results and Discussion: :
Analysis of the data suggests that different types of friction are present on the different lenses. This seems to be dependent on the lens composition, and makes direct comparison between the lenses difficult. Slip stick friction was observed for AVM lenses while the FDAR lenses showed properties that were more similar to solids. Force spectroscopy measurements also showed significant differences between the lenses. The differences are thought to be related to surface properties. Specifically, it is believed that the slip stick observations with the AVM lenses are the result of the formation of an interfacial polymer layer due to relationship of the wetting agent and the aqueous interface. Work using of modified AFM tips to further examine the nature of the interactions and measurement of friction on polymer surfaces with interfacial brush layers is ongoing.
A novel aqueous technique has been developed for the measurement of friction on the surfaces of contact lenses. The results suggest that there are differences in the absolute friction as well as the nature of the interactions between the AFM tip and the surface which it is thought may play a role in lens comfort.
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