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Daniel Hook, Sandra Taft, Robert Steffen, Mohinder M Merchea; Comparing the static and kinetic friction of unworn and worn silicone hydrogel contact lenses. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4654.
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© ARVO (1962-2015); The Authors (2016-present)
To assess how the coefficient of friction (CoF) of a contact lens material changes after wearing, using a novel measurement technique that mimics the applied pressure and sliding speeds encountered by a contact lens on the human eye
Coefficient of Friction measurements were performed using a TA Instruments DHR3 stress rheometer outfitted with specially designed tooling to hold and rub a contact lens fully immersed in borate buffered saline at 25°C (Figure 1). A 6.0 mm diameter polished stainless steel ring (Ra = 40 nm) was used as the counter-surface. The lens was conformed to the ring and allowed to relax until the applied axial force reached 0.052 ± 0.02 N. A torque ramp determined the static friction and a controlled rotation speed test yielded the kinetic friction. The CoF was calculated as the ratio of the applied torque, divided by the radius of the ring, to the applied axial force. We report static friction just as the tool begins to spin in a torque ramp of 2 µNm/s and kinetic friction measurements during a constant rotation speed test at 0.25 rad/s (0.75 mm/s linear speed). Measurements were performed on a novel silicone hydrogel lens (samfilcon A), and commercially available lenses: senofilcon A, comfilcon A and delefilcon A. Ten measurements were made using samples of each lens either directly out-of-the package or worn on-eye for 4 hours and stored in Sensitive Eyes saline.
Mean and 95% confidence intervals for static and kinetic friction are presented in Table 1 for unworn and worn lenses. There was no statistical difference for static CoF between samfilcon A, senofilcon A and comfilcon A (p>0.99). Differences were detected between those lenses and delefilcon A and balafilcon A, which demonstrated higher static CoF (p<0.001). Only balafilcon A had a statistically higher slow kinetic CoF for unworn and worn lenses (p<0.001).
Coefficient of friction for samfilcon A was low and comparable to other inherently wettable silicone hydrogel materials. Different lens materials exhibit varying friction responses. It may be useful to define an aggregate measure of friction that combines both static and kinetic responses. Further research is needed to correlate coefficient to friction with clinical comfort scores.
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