Evaluation of plasma polymer-coated contact lenses by electrochemical impedance spectroscopy

The influence of ultra-thin (i.e., 5-50 nm) plasma polymer coatings on silo xane-based hydrogel contact lenses was investigated by electrochemical impe dance spectroscopy (EIS). Impedance measurements as a function of frequency (Bode plots) were taken at regular intervals until steady impedance was ob tained, indicating that the lenses were saturated. Appropriate equivalent c ircuit models were constructed to describe the salt intrusion characteristi cs of the plasma polymer-coated contact lenses. This provided information p ertaining to the resistance and capacitance of interfacial and bulk layers in the plasma polymer-coated lenses. Resistance relates to ion permeability and cayacitance for water uptake. This investigation showed that some of a n ultra-thin layer of plasma polymer applied onto a dry hydrogel remains a contiguous film after the substrate hydrogel swells upon hydration. In some cases, however, the overall impedance of the coated lens is so low that th e state of the plasma polymer layer after the hydration of the substrate hy drogel could not be judged by EIS. (C) 2000 John Wiley & Sons, Inc.