An in vitro methodology to simulate in vivo wearing of contact lenses has been proposed. The results suggest that certain lens materials show increased CoF after ageing, with potential clinical implications. The results indicate that the presence of a persistent wetting agent is of advantage to maintain a low CoF after prolonged wearing.
The coefficient of friction (CoF) has been reported to correlate with clinical comfort of soft contact lenses (SCL). However, a classification in terms of a CoF is not always applicable to soft materials, such as hydrogels, due to the frequently observed nonlinearity between the lateral and the normal forces. An alternative methodology is presented to quantify the tribological characteristics of soft materials under boundary lubrication in terms of average work. Average work was derived from knowledge of the area of contact, the interfacial shear stress, and sliding distance. To illustrate the work concept, three commercially available SCL (n = 10) and rabbit corneas (n = 10) were characterized with regard to lateral force against a biomimetic mucin-coated glass disk in a tear-like fluid, by means of microtribometry. The contact area between the glass disk and the SCL was measured in situ and fitted to an elastic-foundation model of the material. On the cornea, the contact area was observed via the expulsion of a fluorescent marker from the contact region. All SCL materials had significantly (p \ 0.05) different values for average work. Furthermore, the interfacial shear stress on the cornea was found to be at least an order of magnitude lower than on any of the SCL. Average work represents a single figure of merit for the lubricious properties of soft materials, such as SCL, that do not show a linear relationship between lateral and normal forces.
Successful measurement of the fresh human corneal tissue CoF was demonstrated, with values differing in the evaluated buffer solutions, within this limited sample size.
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