Evaluation method for the coverage of a coating on a contact lens surface

A method for determining whether a coated polyvinylalcohol-based hydrogel contact lens is completely covered by a covalently bonded lubricous coating, comprising the steps of:

• providing a coated polyvinylalcohol-based hydrogel contact lens comprising a polyvinyl alcohol-based hydrogel lens body, wherein the polyvinylalcohol-based hydrogel lens body is composed of a polymer comprising at least 50% by mole of repeating units of vinyl alcohol, and the lens body has a lubricious coating covalently attached to the lens body, wherein the lubricious coating comprise a hydrophilic surface lubricity enhancing polymer,
• contacting the coated polyvinylalcohol-based hydrogel contact lens with an aqueous solution of a hydrophilic marker polymer for obtaining a treated contact lens, the hydrophilic marker polymer having reactive groups of
• in which R1 is methyl or ethyl, and R2 is hydrogen or a C1-C4 alkyl, and a dye tag moiety, at a pH of 4.0 or less, for a contacting time to covalently attach the marker polymer onto the coated polyvinylalcohol-based hydrogel contact lens through 6-membered acetal rings,
• determining whether the marker polymer is covalently attached to the surface of the treated contact lens as an indication of whether the coated polyvinylalcohol-based hydrogel contact lens is completely covered by the lubricious coating.