Ionic conductivity in hydrogels for contact lens applications

Biphasic hydrogel polymers are in the forefront of new extended wear contact lens development. In the biphasic hydrogel the objective is to produce co-continuous domains of siloxane units for high oxygen permeability, coupled with hydrophilic units forming aqueous channels for hydraulic and ion mobility. These are distributed in phase separated nano-scale regions such that the material is optically clear while achieving the required properties to maintain corneal health and lens movement. This paper describes how Impedance Spectroscopy permits a rapid measurement of ion conductivity in a range of silicone and non-silicone hydrogel materials with water contents ranging from 18% to 75% equilibrium water content. For non-silicone hydrogels relative sodium ion conductivity follows a typical percolation curve. However, for silicone hydrogels ion mobility is three orders of magnitude higher than conventional hydrogels of the same equilibrium water content. The influence of electrolyte concentration, interfacial electrode sample contact pressure and temperature are also reported. Paper presented at the Patras Conference on Solid State Ionics — Transport Properties, Patras, Greece, Sept. 14 – 18, 2004.