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Alison Dunn, Juan Uruena, John Pruitt, Thomas Angelini, W Sawyer; Migrating versus Stationary Pressure Experiments in Lubricity Measurements of Hydrogels. Invest. Ophthalmol. Vis. Sci. 2013;54(15):5457.
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Lubricity has been shown as an excellent predictor of contact lens comfort (Brennan 2009). Soft contact lenses are made from hydrogels, which are poroelastic biphasic materials that can change water content under stress. In lubricity measurements, the influence of the contact area motion (whether stationary (Roba 2011) or migrating (Dunn 2013)) has not been explored or discussed. We hypothesize that such conditions may lead to dramatic lubricity differences because of the propensity of the hydrogels to concentrate polymer at the surface under stationary contacting conditions (Chen 2007). The purpose of this work is to clarify the effects of motion profiles, and demonstrate the sensitivity of hydrogel lubricity to stationary versus migrating contacts.
Polyacrylamide hydrogels were molded into flat sheets and hemispherical tips (Fig 1) in order to test both stationary and migrating contact. Lubricity experiments were performed in ultrapure water under an applied force, Fn = 400 μN. Friction forces were measured over 30 reciprocating sliding cycles for stroke length of 3 mm, and experiments were performed at 2 sliding speeds (V=300 μm/s and 1,000 μm/s). The reported friction coefficients were calculated by averaging the ratio of the friction force to the normal force. Mean values and standard deviation were plotted over the whole experiment (Fig 2).
The dramatic effect of migrating contact area as opposed to stationary contact area is clearly demonstrated in Fig 2. At a sliding speed of V=1,000 μm/s the friction coefficient after 30 cycles is below μ=0.02 for the migrating contact area and approaches μ=0.25 for stationary contact. As sliding speed is reduced to V=300 μm/s both experiments give similar friction values.
The hypothesis that friction coefficient is dependent upon the motion of the contact area was confirmed. We propose that water content of the gel is maintained in migrating contacts over short time scales as occurs in the on-eye situation for the eyelid moving over a contact lens, and that this provides a more relevant measurement of lubricity of a contact lens. Brennan, AAO Annual Meeting, 2009 Roba, et al., Tribology Letters, 2011 Dunn, et al., Tribology Letters, 2013 Chen, et al., Journal of Biomechanical Engineering-Transactions of the Asme, 2007
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