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Thomas Angelini, Ryan Nixon, Alison Dunn, Juan Uruena, John Pruitt, W Sawyer; Viscoelasticity and mesh-size at the surface of hydrogels characterized with microrheology. Invest. Ophthalmol. Vis. Sci. 2013;54(15):500.
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© ARVO (1962-2015); The Authors (2016-present)
Delefilcon A contact lenses contain a water gradient structure that transitions from a low water content silicone hydrogel core to a high water content surface gel. This work explores the properties of the high water content surface gel, designed to mimic the physical properties of the corneal surface, improving contact lens comfort. The ~6 µm thick gels have an average water content that exceeds 80%, yet the properties of the outer layer that makes direct contact with the eye are not known. This study examines the elastic and viscous moduli at the outermost surface of these soft gels to understand the physical interactions between contact lenses and the eye.
Microrheological tests were performed on the surfaces of a standard silicone hydrogel (balafilcon A) and on water gradient contact lenses (delefilcon A). Microspheres (0.5 micron radius) were sandwiched between 3mm sections of lens material and kept in deionized water. Video microscopy was performed at 90x magnification, and the beads were tracked using digital image analysis software.
The mean-squared-displacement (MSD) was computed for each particle, and averaged (N=32 for nelfilcon A; N=80 for balafilcon A). Beads embedded in the surface gel layer of delefilcon A exhibited significant motion, displacing between 50 to 100nm over timescales below two seconds. By contrast, no detectable bead motion above the noise threshold was observed in the balafilcon A system; beads moved only 10nm to 16nm over the same two second period. We compute the frequency-dependent elastic and viscous moduli, G’ and G’’ from the MSD measurement. We find moduli with very weak frequency dependence between 10 and 100 radians per second, and an elastic modulus that varies between 0.3 and 1 Pa. Thus, from elasticity theory of cross-linked flexible polymers, we estimate that the water content at the outermost region of the delefilcon A surface gel layer is above 99%.
Surface gel layers on contact lenses possess a frequency dependence similar to low concentration polymer gels. Remarkably, the modulus at the outermost surface is over 1000 times lower than the mean elastic modulus of the entire surface gel layer. This large modulus change, however, only requires roughly 15% reduction in polymer concentration. This suggests that the polymer concentration marginally drops near the surface as polymer chains and crosslinks become sparse.
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