Studying the Effect of Tear Fluid on Contact Lenses
The zeta potential indicates changes in the surface chemistry of contact lenses upon wearing and is thus suited to study biocompatible surface coatings.
Soft contact lenses have to comply with different demands regarding oxygen permeability, wettability and wearing comfort. Biocompatible materials such as e.g. phosphorylcholin or polyvinylalcohol should furthermore diminish the formation of biofilms and bacterial adhesion on the surface, in order to reduce the risk of eye infections.
New contact lenses, both for monthly and daily use, show an isoelectric point at pH 4 (IEP = pH value where the zeta potential is 0 mV), which is characteristic for an acrylate-based hydrogel.
Upon wearing monthly lenses for the proposed time, the isoelectric point shifts to higher pH, mainly due to deposition of proteins from the tear fluid such as albumin (IEP 4.7), but also lactoferrin (IEP 8.5) or lysozyme (IEP 11.1).
At higher pH the zeta potential moves to more negative values, which is explained by the deposition of lipids and/or by a decreased swelling, both render the soft contact lens less hydrophilic.
The measuring cell for contact lenses of SurPASS™ 3 accommodates a single soft contact lens, which is simply placed on a "model eye". A proprietary design of the micro-channel surrounding the surface of the contact lens enables a non-destructive analysis of the zeta potential.
G. M. Bruinsma, H. C. van der Mei, H. J. Busscher, Biomaterials 22 (2001) 3217-3224