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 siloxane-based hydrogel contact lenses was investigated by electrochemical impedance spectroscopy (EIS). Impedance measurements as a function of frequency (Bode plots) were taken at regular intervals until steady impedance was obtained, indicating that the lenses were saturated. Appropriate equivalent circuit models were constructed to describe the salt intrusion characteristics of the plasma polymer-coated contact lenses. This provided information pertaining to the resistance and capacitance of interfacial and bulk layers in the plasma polymer-coated lenses. Resistance relates to ion permeability and capacitance for water uptake. This investigation showed that some of an 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 the state of the plasma polymer layer after the hydration of the substrate hydrogel could not be judged by EIS.