Oxygen and ionic transport in hydrogel and silicone-hydrogel contact lens materials: an experimental and theoretical study Articles uri icon

authors

publication date

  • October 2013

start page

  • 62

end page

  • 72

volume

  • 452

International Standard Serial Number (ISSN)

  • 0376-7388

Electronic International Standard Serial Number (EISSN)

  • 1873-3123

abstract

  • The transport of oxygen, water and naked ions of Na⁺ and Cl⁻ across two kind of hydrogels materials, made of a conventional hydrogel (Hy) based on hydroxyethyl methacrylate (pHEMA) and a silicone hydrogel (Si-Hy) material containing siloxane moieties, was compared between Molecular Dynamics Simulations (MDS) and experimental measurements. Computer assisted simulations were carried out for wet hydrogels at 310 K and equilibrium water uptake in the range from 10% to 40%. Our results show that in Si-Hy materials the aqueous hydrogel and hydrophobic siloxane phases are separated suggesting a co -continuous structure, and oxygen moves predominantly through the free volume of the hydrophobic siffixanc phase. The values of diffusion coefficient of O₂, water and Na⁺ and Cl⁻ ions in Si-Hy was about one order of magnitude higher than in conventional hydrogels when the water content was above 25 wt % up to a critical value of 35 wt% where a percolation phenomenon is observed. The value of the oxygen diffusion coefficient obtained by simulations are roughly similar to that experimentally found using potentiostatic techniques. Values founcl experimentally for Na+ diffusion coefficients are between three or five Limes lower than MDS. For Si-Hy materials with 36 wt% of water the Na⁺ permeability, diffusion coefficient and salt partition coefficient (k(m)=P/D) are 6.7 ± 0.2 x 10⁻⁷ cm²/s, 1.8 ± 0.5 x 10⁻⁶ cm²/s and 0.42 ± 0.13, respectively. For Hy materials of 38.6 wt% the values found were 18.4 ± 1.2 x 10⁻⁷ cm²/s, 5.4 ±1.0 x 10⁻⁶ cm²/s and 0.34 ± 0.09, respectively. The coordination number between the fixed groups (-SiO-) and water in HEMA and the particles (O₂, Cl⁻ and Na⁺) is slightly larger than unity...

keywords

  • hydrogels; silicone-hydrogel; molecular dynamics simulation; gas transport simulation; ionic transport simulation