The interaction of preservatives with polyhydroxyethylmethacrylate (polyHEMA)

The interaction of the four most commonly used preservatives in contact lens solutions (chlorbutol, thiomersal, chlorhexidine gluconate and benzalkonium chloride) with polyhydroxyethylmethacrylate (polyHEMA), has been examined. Benzalkonium chloride and chlorhexidine gluconate show typical high affinity type isotherms. The interaction of benzalkonium chloride with polyHEMA from aqueous solution was reversible whereas that of chlorhexidine was only reversible in the presence of electrolyte or surfactant. Chlorbutol showed a typical reversible linear isotherm. Thiomersal does not interact with polyHEMA above pH 5.0. The extent of chlorhexidine--polyHEMA interactions in increased by the presence of formulatory adjuvants such as electrolyte and hydrophilic polymers. PolyHEMA lenses that apparently have been equilibrated with chlorhexidine gluconate will, on the addition of fresh preservative solution, bind further quantities of chlorhexidine above that which would be predicted from the sorption isotherm.