Purpose:
Surface lubricity plays an important role in biomedical device comfort by reducing stresses on tissue. A qualitative manual (finger) rubbing lubricity and a quantitative inclined plane method have been developed to describe contact lens lubricity and friction.
Methods:
Experimental contact lenses with four different surface chemistries were developed. Four lenses of each group were evaluated with qualitative manual rubbing and quantitative inclined plane methods after overnight phosphate buffered saline (PBS) soaking to remove any package saline additives.To determine manual (finger) lubricity values, the lenses were rubbed between the thumb and index finger for approximately 20 seconds. The lenses were ranked according to their lubricity value from LV0 (best) to LV4 (worst). These values are found to be inversely proportional to the crosslink density. The lubricity values were repeatable in a blind test.For the inclined plane, a clean glass plate is adjusted to the desired angle in a PBS bath. The lens of interest is placed at the top of the glass plate, and a 0.8 g weight is placed on the lens to initiate movement. A minimum critical angle is determined, which just maintains lens movement over a distance of approximately 100 mm. The tangent of the critical angle is the critical coefficient of kinetic friction (CCOF). These values were compared with a lotrafilcon B control.
Results:
Different experimental surface chemistries (on the same core lens material) were able to be categorized repeatedly with distinct manual lubricity values (LV, shown in figure 1). The kinetic coefficient of friction as measured by the inclined plane method (CCOF) showed an increase in CCOF consistent with the manual lubricity values. The experimental groups had a lower average CCOF than the control lens (n=4, p<0.05), which has a different core and surface chemistry.
Conclusions:
Two novel techniques (manual lubricity and inclined plane) have been evaluated to rate and quantify the lubricity of contact lenses. This method allows contact lens lubricity to be optimized by adjusting surface chemistry for improved on-eye performance.