FRICTIONAL BEHAVIOUR OF CONTACT LENSES

Fiona Lydon

Abstract

PURPOSE. One important property for which no currently accepted measurement technologies exist is the frictional and biotribological behaviour of contact lenses and ophthalmic solutions. The aim of this study was to develop a technique and apply it to the study of biotribological phenomena associated with the contact lens care solutions and artificial tears. METHOD. Static and dynamic sliding coefficients of friction were measured with a range of extended wear contact lenses. Three different loading periods were used and the effect on friction was observed.

RESULTS. The results show that when an intact lubricating layer separates the (hydrogel) lens and substrate it is the properties of this solution rather than those of the substrate that govern the value of sliding friction, provided that the lens maintains an adequate hydrodynamic lubrication boundary. The nature of the substrate can however have a marked effect on the stability of the boundary layer under load. Comparison of values for ~{!0~}start-up~{!1~} (static) and steady state (dynamic) friction clearly show the greater level of discrimination in material characteristics afforded by start-up friction measurements, especially on a non-hydrophilic surface. The results show that increasing the loading time does reduce the effectiveness of the layer, thereby causing an increase in start-up friction. This has a direct parallel in front surface dehydration of soft lenses which means that the lens effectively becomes the less hydrophilic substrate, producing an increase in start-up friction between the lens and eyelid.

CONCLUSIONS. It is logical to expect that changes in tear characteristics coupled with lens dehydration will have the dominant effect on coefficients of friction and thus the biotribological behaviour of the lens in the eye. The consequences of these phenomena are not yet fully understood but appear to relate to, for example, ~{!0~}mucin balls~{!1~}, lens binding and end-of-day dryness symptoms.

Details

Year: 2001

Program Number: Poster 26

Author Affiliation: Aston Biomaterials Research Unit

Co-Authors: Beverley Benning, Brian Tighe

Co-Author Affiliation: n/a

Room: Exhibit Hall C