Macromolecule release and smoothness of semi-interpenetrating PVP–pHEMA networks for comfortable soft contact lenses

Knowledge about the microstructure and the release rate of hydrophilic macromolecules is required for a rational development of comfortable and safe contact lenses. Semi-interpenetrating networks of poly(hydroxyethyl methacrylate) (pHEMA) with poly(vinyl pyrrolidone) (PVP) were prepared by free radical polymerization of HEMA in the presence of PVP K30 or K90F, under anhydrous conditions or after addition of water, and evaluated in terms of swelling, porosity, PVP release rate, air–water surface tension, and friction coefficient. The greater water content was during polymerization, the higher was the swelling degree and porosity. Microphase separation above a certain volume of water resulted in hydrogels with bumpy surface and interconnected pores. All hydrogels showed a high optical clarity and slowly released PVP (20% after 9 days). In general, the greater the content of PVP or the higher its molecular weight was, the lower the friction coefficients were. In the case of hydrogels prepared with water, the friction was influenced by the balance between the ability to hold water in the network (which contributes to the sliding and PVP release) and the deleterious effect of an irregular surface. Controlled delivery of PVP revealed as a critical factor for improving the frictional behavior of pHEMA contact lenses.