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Angela A Pitenis, Juan Manuel Uruena, Kyle Schulze, Ryan Nixon, Alison C Dunn, Wallace Gregory Sawyer, Thomas Angelini; Contact Mechanics of High Water Content Hydrogels. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):6116.
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© ARVO (1962-2015); The Authors (2016-present)
The relationship between contact lens comfort and the stiffness of the lens surface is not well understood. Recently, sliding glass probes against contact lenses with hydrogel surface gels has shown low friction at low loads and high friction at high loads, indicating gel collapse1. In this work, the contact mechanics of high water content hydrogels poly(N-isopropylacrylamide) (pNIPAM) and polyacrylamide (PAAm) in a Gemini interface2 were explored to elucidate the origins of this transition in lubricity.
To create a Gemini contact, a probe and a disk were made of 7.5% polymer3. The probe had a radius of curvature of 2 mm and the hydrogel disk was 4.5 mm thick and 30 mm in radius. The contact area was determined by in situ microscopy during an indent. Indentations were performed by loading the probe against the disk to 2 mN at a rate of 65 µm s-1 with a microtribometer. Prior to indents, the system was heated above the lower critical solvation temperature (LCST) of pNIPAM. Indents were performed continuously throughout the cooling process until the system reached room temperature.
For high-water content hydrogels in a swollen state we see low stiffness and adhesion and the loading and unloading curves show nearly Hertzian behavior. Hydrogels in a collapsed state exhibit nearly twice the stiffness and an order of magnitude increase in adhesion force and indentation curves followed the Johnson-Kendall-Roberts (JKR) model of elastic contact.
Previous work was performed with glass probes, but a Gemini hydrogel configuration better models the ocular environment. A gel with zero adhesion obeys traditional Hertzian contact mechanics. If adhesion were to occur between a contact lens and a biological surface in the eye, our results suggest that the contact mechanics of this system would be better described by JKR Theory. Acknowledgements: This work was funded by Alcon Laboratories References: 1. Dunn AC, Urueña JM, Huo Y, Perry SS, Angelini TE, Sawyer WG. Lubricity of Surface Hydrogel Layers. Tribol Lett 2013; 49:371-378. 2. Dunn AC, Sawyer WG, Angelini TE. Gemini Interfaces in Aqueous Lubrication with Hydrogels. Tribol Lett 2014; 54:59-66. 3. Pitenis AA, Urueña, Schulze KD, Nixon RM, Dunn AC, Krick BA, Sawyer WG, Angelini TE. Polymer fluctuation lubrication in hydrogel Gemini interfaces. Soft Matter 2014; 10, 8955
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